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    Carbon Dioxide Lasers

    Sub-Table of Contents



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  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Introduction, Suitability, Links to Other Information

    High Power Lasers

    Carbon Dioxide (CO2) lasers make all the types discussed in previous chapters seem like LEDs compared to xenon arc search lights! Where common diode, HeNe, and even most Ar/Kr lasers put out mW of optical power, nearly all CO2 lasers are rated in WATTs or KILOWATTS. Even the smallest sealed CO2 laser looking much like a HeNe tube, will produce a few watts continuously. Large CO2 lasers used in the metalworking industry may have continuous outputs of up to 30 kW or more. And special purpose CO2 lasers have been constructed that produce 10 or 20 MEGAWATTS CW.

    The distinguishing characteristic of the CO2 lasing process that makes these sustained power levels possible is its relatively high efficiency - at least compared to most other common gas lasers. The typical electrical power in to optical power out (wall plug) efficiency of a CO2 laser may be anywhere from 5 to 20 percent or more (compared to less than 0.1 percent for a HeNe or Ar/Kr ion laser). The only well developed laser technology which has a higher conversion efficiency than the CO2 laser is the high power IR laser diode, where a wall plug efficiency of greater than 50 percent is possible.

    Unlike the other lasers producing visible or short near-IR light, the output of a CO2 laser is medium-IR radiation at 10.6 um. At this wavelength, normal glass and plastics are opaque, and water completely absorbs the energy in the beam. The 10.6 um energy is ideal for cutting, engraving, welding, heat treating, and other industrial processing of many types of materials including (as appropriate): metals, ceramics, plastics, wood, paper, cardboard, fabric, composites, and much much more.

    Needless to say, 10.6 um is totally invisible to the human eye and conventional solid state sensors are blind as a post. Therefore, thermal approaches are generally used to measure beam power or determine beam profile. Companies like Macken Instruments, Inc. sell low cost CO2 viewing plates, power meters, and spectrum analyzers.

    The CO2 laser represents the classic heat ray of science fiction. I have no doubt that the Martians in H. G. Wells' "The War of the Worlds" used CO2 lasers powered by cold fusion generators (probably with superconducting electrical backup storage) for their directed energy weapons. :-) (Chemical lasers would have required bulky reactant storage tanks to achieve the number and length of blasts and none were visible!)

    The output power of most CO2 lasers is between a few W and a few kW (CW or average if pulsed). The smallest of these look a lot like helium-neon lasers and also used 'brick' type power supplies. Such a 5 W CO2 laser would be similar in size to a 5 mW HeNe laser though forced air cooling might be required. However, some amazingly high power CO2 lasers have been constructed. The largest one might be at the Troisk Institute for Thermonuclear Research (in Troisk, about 80 miles outside of Moscow, Russia). This is claimed to be a 10 MegaWatt laser but that might be a slight exaggeration but not by much. It is truly a CW laser though and would run for as long as power and cooling were supplied. I don't know the exact size of the laser but the room it is in rivaled that of the NOVA pulsed laser at the Lawrence Livermore National Laboratory. (I don't know if it is still in operation.)

    For high peak power, there are Q-switched CO2 lasers though they probably aren't that common. One example is the "GEM Q-Switched" from Coherent, Inc.. (Go to "Laser and Other Related Products", "Laser Systems", "CO2", "GEM Q-Switched".)

    CO2 lasers are also among the easier types to construct (in a relative sort of way) so they make decent condidates for home-built lasers. See the chapter: Home-Built Laser Types and Information.

    CO2 Laser Safety

    (Portions from: Leonard Migliore (lm@laserk.com).) Laser safety is something that must always be taken seriously. CO2 lasers are more dangerous in many ways but less of a hazard with respect to some. The combination of high output power and an invisible beam AND high voltage power supplies represent a very real possibility of fire/heat damage from the direct or reflected beam as well as a serious shock hazard. On the other hand, the 10.6 um wavelength doesn't pass through the cornea, lens, and vitreous of the eye to be focused on the retina. Therefore, damage to structures in the back of the eye is unlikely although the front can still be burned to a crisp!

    Virtually all CO2 lasers are Class IV devices. The safety discussion below assumes a relatively 'small' DC or RF excited CO2 laser, perhaps up to 200 W of beam power. Such a laser could conceivably be constructed at home or acquired surplus. However, keep in mind that the term 'small' here is only relative to a high power industrial laser. A 100 W unit can cut a reasonable thickness of sheet steel and certainly set a big fire at 100 yards. I guess a 100 kW laser would be more dangerous, but few hobbyists have one. :-)

    Safety precautions are extremely important for even small CO2 lasers for a variety of reasons:

    Some general considerations when working with CO2 lasers: Acrylic, polycarbonate and glass are all quite opaque to CO2 laser light. However, if plastic is hit with enough of the beam, it will burn through. You might not think this would happen unless you decide to put your window directly in the beam path, but I've seen plenty of windows damaged by the beam reflecting off the work or from fixturing.

    Speaking of beam blocks and burning through things.... Take the following with a grain of possible exaggeration. :)

    (From: Steve Roberts.)

    I have a friend who was working on a 3 kW CO2 laser at a job shop, they were running the optics train in test mode with the interlocks defeated and left the folding mirror assembly off, so the beam shot out and quickly went through a steel door, two cement/fiberboard walls, and partially through a cinder-block wall before they realized their mistake. He even claims it lased quite well when a factory tech left a large monkey wrench inside the resonator. I wouldn't have believed it till I heard a second tech back up his story. Needless to say I never turn my back on him while he's in my lab!

    It must be nice to have that kind of single pass gain, because the argon lasers I work on can wink out if you even just lean on the unit and slightly stress the resonator or have even a invisible amount of crud on the optics.

    (From: Leonard Migliore (lm@laserk.com).)

    That doesn't sound quite right. Even a 3 kW beam has to be focused to get through steel. Cement is even more resistant. Did it take them an hour to realize their mistake?

    (From: Steve Roberts.)

    It's your typical plasterboard wall. I couldn't remember the word 'plasterboard' late at night when I wrote that. I have watched a 60 watt doubled Q-switched YAG go through them unfocused about a foot from the wall, so I can see a hot CO2 beam going through it easily enough. The laser in question is a Mitsubishi workstation with recirculating flowing gas.

    (From: Leonard Migliore (lm@laserk.com).)

    The monkey wrench part is pretty reasonable. When I was at Spectra-Physics, we made this great big 5 kW laser called a 975 (the head weighed 9000 lbs). People were always dropping pens, tools and hardware into the cavity, which was such that you didn't want to take it apart enough to get the stuff out. This never seemed to bother the things.

    One day, we sold our lab laser to a French customer (Yes, we told them it was used). This required disassembling the laser to put in 50 Hz blower motors. When we did this, we found several pounds of garbage sitting in a quart of vacuum pump oil (pump failure some time before). The laser had been operating perfectly, delivering 6 to 7 kW maximum power with a swamp under the resonator.

    This is a righteous piece of industrial hardware.

    Reasons to Choose a CO2 or Nd:YAG Laser

    (From: Leonard Migliore (lm@laserk.com).)

    There are major performance differences between Nd:YAG and CO2 lasers. One reason is that Nd:YAG light is emitted at a wavelength of 1.06 microns in the near infrared, while CO2 light is emitted at 10.6 microns. The material interactions at these wavelengths differ. Most organics don't absorb 1 micron light very well, while they absorb 10 micron light. So, non-metal processing is generally a CO2 application. Metals are more reflective at 10 microns than at 1 micron, so CO2 lasers only weld effectively in the "keyhole" mode, where the irradiance is high enough to generate a vapor channel in the workpiece. Once you get into keyhole mode, the high average power of CO2 lasers makes high speed welding possible. For small spot welds, Nd:YAG lasers are far more controllable.

    Also, since there are a lot more Nd atoms in a YAG rod than there are CO2 atoms in laser gas, Nd:YAG lasers can deliver much higher peak powers than CO2 lasers. This makes them better for drilling. Conversely, since it's hard to cool a solid rod, Nd:YAG lasers have problems with high average powers. You can build a CO2 laser with very high power; Convergent has commercial 45 kW units, and much bigger ones have been built.

    It's usually pretty clear if a given application is best done with Nd:YAG or CO2. There are a few areas where either ( or neither) are equally good, but, in general, the application areas are quite separate.

    Commercial Nd:YAG lasers are available with powers up to 4 kW (continuous) or so. Pulsed Nd:YAG lasers have lower average powers but have much higher peak powers.

    CO2 lasers are generally used for cutting materials like stainless steel because they can, in general, be focused to smaller spots, which improves cut quality. You can focus a 1 kW CO2 laser to a 100 micron spot. A 1 kW YAG is generally used with fiber optics for beam delivery and can't be focused smaller than 400 microns or so.

    You might expect that since the output beam from a Nd:YAG laser has a wavelength 1/10th that of a YAG laser, a YAG will focus to a smaller spot. However, this assumes equivalent beam quality - which the YAG does not have. At the power levels required for material processing (note that the context here is metal cutting), YAG lasers have terrible beam quality (M2 can be 50 or 70), so they can't be focused as well. The culprit is the YAG rod, which is heated by the pump lamps and exhibits thermal lensing.

    Gas lasers don't have as big a problem with thermal lensing, so you can make them real big and still get good beam quality. Self-focusing of a CO2 beam is also seldom seen. What is common in the beam path is defocusing (a beam goes into a pipe 20 mm in diameter and comes out 200 mm wide) and mirages (a beam goes into a pipe round and comes out semicircular).

    There's no problem with absorption of light from either a Nd:YAG (1.06 microns) or CO2 (10.6 microns) laser in nitrogen, or in air without contaminants. In practice, air has variable percentages of CO2 and water vapor and also tends to contain hydrocarbons, all of which absorb 10.6 um light.

    Both Nd:YAG and CO2 lasers are used for welding stainless steel, with CO2 lasers being used for high speeds welds.

    BTW, you might come across the term: "fluence" in conjunction with materials processing lasers. Fluence is used to characterize pulsed laser processing, and is the energy of a single pulse divided by the area being affected. Thus, if you have a 20 joule pulse focused to a 1 mm spot, the fluence is 20/0.78 or 25 J/mm2, except that everyone uses J/cm2.

    Comments on CO2 Laser Efficiency

    Until the advent of the diode laser, the CO2 laser was by far the most efficient laser in existence. Why?

    (From: Professor Siegman (siegman@stanford.edu).)

    It's been quite a while since I looked at this, but as I recall:

    1. Plasma discharge (or any form of heat input) quite efficient at setting N2 molecules in He-N2-CO2 gas mixture vibrating.

    2. Vibrating N2 molecules are symmetric, hence can't radiate their vibrational energy away effectively, hence these vibrations have very long lifetime.

    3. Vibrational frequency of N2 molecules coincides very closely with ground to upper laser level (E1 -> E3) transition frequency in CO2 molecules; hence N2 molecules transfer very large fraction of their vibrational energy to pumping CO2 molecules in the laser gas mixture up to upper laser level E3 via simple collisions.

    4. Leaving aside complexities of molecular spectra, CO2 molecule is simple "pump on E1->E3, lase on E3->E2" system -- and pump frequency (= N2 vibration frequency) is only about twice the CO2 lasing frequency

    (From: Harvey Rutt.)

    Well Tony may not have looked at it for a while, but thats an excellent summary!

    Some extra points of detail: Re point one, an important aspect is that the cross section for exciting N2 by electron impact is high at electron energies which can be obtained in a nice stable discharge, & that at those energies very little else gets excited. (Look at a multi-kW CO2 laser discharge - it is a pretty faint glow; very little goes into useless electronic states.) CO2 & N2 molecules that do end up in higher vibrational states (and lots do; these modes have temperatures of a few thousand K) tend to cascade down very efficiently & with minimal energy loss into the upper laser level. The collision cross sections are such that vibration of the N2/CO2 coupled modes can be at several thousand K while rotation and translation can stay not far above 300K; the electron energy is funneled to where you want it.

    And re point 3, that the lower laser level, E2 in the above can be rapidly depopulated, partly because you have two levels in Fermi resonance close together + a third nearby, partly because two of these three are 'overtones' of the bending vibration, & that (plus helium collisions) is a fast 'route down'.

    Another useful fact is that CO2 lasers have reasonably high gain, so that efficiency is not too sensitive to small losses, and that the system is essentially free of parasitic losses such as excited state absorption (ESA.) In fact the gain is somewhat anomalous, because this is a 'difference transition' in which *two* states change their quantum numbers, & generally such transitions are quite weak, this one being stronger than the usual rules of thumb would suggest.

    It also happens that CO2 is 'almost' mono-isotopic ~99% 12C16O2 (13C, 17, 18O are quite rare, less than ~1%) which also helps. And because 16O has no nuclear spin, half the rotational levels are absent, which ~ doubles the gain compared to if 17O happened to be the common isotope.

    Overall, no one single factor; a combination of helpful circumstances come together in one molecule. The spectroscopically closely related N2O laser is far less efficient for example. (Although N2O is NNO, not NON, which reduces the symmetry.)

    I think you will find the *original* CO2 laser (CKN Patel?) had no N2 in it and was pretty pathetic.

    Comparison of CO2 and Nd:YAG Lasers for Industrial Applications

    (From: Richard W. Budd (rbudd@ibm.net).)

    Basically you have a choice of 2 basic families of lasers: CO2 or Nd:YAG (usually shortened to just YAG). For the time being only these 2 families of lasers have the efficiency and output power to do large-scale material processing e.g., cutting, cladding, drilling, surface hardening, welding, etc. There are major differences between the two types and both have advantages and disadvantages that must be considered based on the job to be performed or the materials being processed.

    CO2 lasers and YAG's produce very different wavelengths and beam shapes. CO2 lasers are gas lasers that use carbon dioxide as the lasing medium. YAG's are solid-state lasers that use the element Neodymium (the Nd) diffused in a crystal of Yttrium-Aluminum-Garnet (YAG) as the lasing medium. CO2 lasers emit at a wavelength of 10.6 microns (far-infrared), while YAG's emit at 1.06 microns (near-infrared, just below the visible red). Because of these different wavelengths some materials are better absorbers (or reflectors) of the 2 different beams. Aluminum is fairly highly reflective to the CO2 beam and requires almost 40% more power to cut as opposed to the beam from a YAG - to a beam from a YAG aluminum is almost a perfect absorber. On the other hand, most carbon steels and stainless-steels absorb CO2 and YAG beams pretty much the same - very well.

    Beam shape. Here there is basically no comparison: Virtually all CO2 lasers produce a beam that is far, far, more symmetrical and even than ANY industrial-class YAG laser.

    The beam shape is important: Think of a lathe. If you use a fine pointed cutter you produce finer, more precise cuts with less force needed to cut in to a given depth. If you use a bull-nose bit you take out a much larger piece of metal with a corresponding drop in the depth you can achieve with a specified amount of force.

    Peak and average power. CO2 lasers are usually operated in continuous mode while YAG lasers are pulsed.

    Beam delivery. Here there are very significant practical differences:

    Operating considerations. As was said before, CO2 lasers are gas lasers and YAGs are solid-state lasers. Gas lasers are very rugged - the material that actually makes the beam is a gas and therefore cannot be damaged. Solid-state lasers use a crystal to generate the beam. These crystal rods are very expensive - several thousand dollars for an industrial size laser. If the laser is improperly tuned or operated the crystal can be almost instantly destroyed. Overall efficiency (wall plug to optical output) differs greatly:

    Safety considerations. Electrically, both types of lasers can be EXTREMELY dangerous. Both use high-energy, high-voltage circuits. Servicing must be done by qualified personnel. However, there are significant differences with respect to optical hazards:

    Operational Hazards with CO2 Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    One big area of hazard is that CO2 lasers are used for material processing. They do a great job of cutting plastics. Unfortunately, the by-products of laser processing of organics generally include very hazardous materials. Carcinogens such as benzene and PAH's (polycyclic aromatic hydrocarbons) are typically generated in reasonably significant quantities. Certain materials have even better surprises: You get cyanide out of Kevlar and HCl out of PVC. It's hard to handle this stuff, and the associated solids tend to clog filters. Don't cut plastic (any kind) without a fully-enclosed system that exhausts into scrubbers.

    Metal cutting has some other hazards, although nothing is as bad as plastic. Cutting stainless steel generates carcinogenic Cr (VI), generally in amounts greater than allowed by OSHA. Most particles generated in gas-assisted laser cutting have diameters around 1 micron, which is the worst size for your lungs as they can get all the way to your alveoli and clog them.

    Welding (you can make a nice weld with a 200 W CO2) generates fumes too, and also generates UV light from the weld plasma. Your glasses can protect you from the CO2 light but pass enough UV to give you a burn (This is, of course, a much bigger problem with multi-kW lasers. I have gotten sunburned skin from a 5 kW welder)

    Another issue is that, if you sell a laser, you must comply with the FDA's regulations as specified in Title 21, Code of Federal Regulations, Subchapter J because you have become a "system supplier". Sell a system without the right paperwork and the Feds can drag you away in chains. If you just build something and use it yourself, you don't need to follow these regulations. It is safer, however, if you do make an effort to comply with the physical requirements of the code such as beam guards, warning lights and safety interlocks.

    Detecting or Locating CO2 Laser Beam

    (From: Larry (lhh@nac.net).)

    There are several ways to find out where your CO2 laser beam is:

    The simplest is to use Scotch Tape or a piece of paper in the beam. The laser is turned on for an appropriate length of time, and the burn pattern tells you where the beam is. This is also quite useful for locating the focus point of a lens illuminated with a CO2 laser.

    The second way is to use phosphor plates manufactured by Optical Technology. A UV light illuminates the phosphor, which is coated on a metal plate. The phosphor glows in the visible. However, where the 10.6 micron CO2 laser light strikes the plate, the phosphor is deactivated. So the position of the beam appears as a dark spot on a glowing plate. Phosphor plates of different sensitivity are available.

    The last way is to use a co-axial red alignment laser. The Synrad CO2 laser that I use routinely has one of these, and it makes life much simpler when you are aligning systems of mirrors, etc. However, one caveat. Zinc selenide is commonly used for CO2 laser optics - lenses, beamsplitters, beam combiners, etc. It has the advantage that it passes both CO2 laser light and the red light of the alignment laser. But the dispersion of zinc selenide is different at the two wavelengths, so if you are off-axis, the focal point of the red laser and the CO2 laser will be slightly different.

    (From: Jeff Glee.)

    If it is possible to lower the beam power to the single watt range, a Post-it makes a great beam locator. The dye used to color the paper is temperature sensitive, and darkens when hot. Depending on the intensity used, it may be necessary to attach the post-it (I used 3M spray adhesive) to a thin aluminum plate (less than 0.1") to help dissipate excess heat and prevent flames. Different color Post-it's react differently, but the best I have found is the bright pink color. Visually, it is about the same effect as you see on commercial fluorescent plates. I have yet to try it at higher than about 2 watts with a 5 mm beam, but given sufficient thermal dissipation (thicker backing, or copper instead of aluminum), higher intensities should be viewable without burning.

    On-Line Introduction to CO2 Lasers

    There are a number of Web sites with laser information and tutorials.

    Links to Information on CO2 Lasers

    For an introduction to CO2 laser technology, see:



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Types and Excitation of CO2 Lasers

    Basic Principles of Operation

    (Portions from: David Crocker.)

    The physical arrangement of most CO2 lasers is similar to that of any other gas laser: a gas filled tube between a pair of mirrors excited by a DC or RF electrical discharge. Metal coated mirrors (e.g., solid molybdenum or a gold or copper coating on glass or another base metal) may be used for the high reflector (totally reflecting mirror). However, at the 10.6 um wavelength, a glass mirror cannot be used for the output coupler (the end at which the beam exits) as glass is opaque in that region of the E/M spectrum. One material often used for CO2 lasers optics is zinc selenide (ZnSe) which has very low losses at 10.6 um. Germanium may also be used but must be cooled to minimize losses for high power lasers. Other materials that may be used for CO2 laser optics are common substances like NaCl (rock salt!), CaCl, and BaFl (but these are all hydroscopic - water absorbing - so moisture must be excluded from their immediate environment).

    Many details differ between a 50 W sealed CO2 laser and a 10 kW Transverse Excited Atmospheric (TEA) flowing gas laser machining center but the basic principles are the same. While HeNe lasers are based on excited atoms and ion laser use ions, CO2 lasers exploit a population inversion in the vibrational energy states of CO2 molecules mixed with other gases.

    Additional gases are normally added to the gas mixture (besides CO2) to improve efficiency and extend lifetime. The typical gas fill is: 9.5% CO2, 13.5% N2, and 77% He. Note how He is the largest constituent and CO2 isn't even second! (This also means that leakage/diffusion of He through the walls and seals of the laser tube may be a significant factor is degradation of performance and/or failure of a sealed CO2 laser to work at all due to age.)

    The CO2 laser is a 3-level system. The primary pumping mechanism is that the electrical discharge excites the nitrogen molecules. These then collide with the CO2 molecules. The energy levels just happen to match such that the energy of an excited N2 molecule is the energy needed to raise a CO2 molecule from from the ground state (level 1) to level 3, while the N2 molecule relaxes to the ground state. Stimulated emission occurs between levels 3 and 2.

    The metastable vibrational level (level 2) has a lifetime of about 2 milliseconds at a gas pressure of a few Torr. The strongest and most common lasing wavelength is 10.6 um but depending on the specific set of energy levels, the lasing wavelength can also be at 9.6 um (which is also quite strong) and at a number of other lines between 9 and 11 um - but these are rarely exploited in commercial CO2 lasers.

    Here are some of the more subtle details. (Skip this paragraph if you just want the basics.) As well as the 3 energy levels of CO2 I referred to, there is actually a 4th involved, about midway between the ground state and level 2. After emitting, the CO2 molecules transition from level 2 down to this 4th level, and from there to the ground state (because a direct transition from level 2 to the ground state is forbidden by quantum rules). Level 2 is actually a pair of levels close together, which is why there are 2 separate frequency bands that a CO2 laser can operate on, centred around 9.4 um and 10.4 um (i.e., just above and just below 30 THz). Each of these bands is actually composed of about 40 different vibration/rotation transitions with frequencies spaced about 40 GHz apart. The strongest transition is the one called 10P(20), which is about 10.6 um, so a CO2 laser with no tuning facilities normally operates at this wavelength. It is possible to select a particular transition (and hence frequency) using a diffraction grating instead of one of the mirrors. The exact transition frequencies were known to an accuracy of about +/-50 kHz back in 1980.

    The helium in the mixture serves 2 purposes: (1) He atoms collide with CO2 molecules at level 2, helping them relax to the ground state; (2) it improves the thermal conductivity of the gas mixture. This is important because if the CO2 gets hot, the natural population in level 2 increases, negating the population inversion.

    Cooling of the gas mixture is critical to achieveing good power output. The gas at the centre of the tube is hottest and loses heat by thermal conduction through the surrounding gas to the walls. As the gas pressure increases, the thermal conductivity gets worse. So with a smaller tube, the gas pressure can be higher. This is why the power available from a properly-designed CO2 laser depends on the length of the tube but not the diameter (i.e., smaller diameter tube = higher pressure = greater density of CO2, which compensates for the smaller diameter).

    Types of CO2 Lasers

    Although there are many types of structures for CO2 lasers, the most common are probably:

    The most common types of excitation are a direct electrical discharge (usually DC) and radio frequency (RF). Either current control or pulse width modulation can be used for power control (since this does make sense for a CO2 laser - output power is directly related to tube current).

    To boost power output and provide some redundancy, some CO2 laser have multiple separately excited discharge tubes which are optically combined. For example, Synrad Model 48 Seris, a 50 W sealed tube CO2 laser, actually has a pair of 25 W tubes and each tube has 2 RF drivers. Thus, a failure of a single tube or driver will result in at most a 50 percent drop in output power but not total failure.

    Detailed specifications, mechanical drawings, and product manuals, may be dowloaded from Synrad's Product Page.

    Flowing Gas CO2 Lasers

    This was the way the earliest CO2 lasers were constructed. The tube was not sealed but required an active pumping system and gas supply to operate. Such lasers are very easy (in a relative sort of way) to construct (see the chapter: Home-Built Carbon Dioxide Laser). Many older medical lasers of this type are now becoming available surplus at attractive prices. These have maximum power outputs in the 20 to 100 W range. See the section: Descriptions of Typical Small Flowing Gas CO2 Lasers for some examples of commercial units.

    More modern CO2 lasers in this power class - and up to about 500 W - feature a low or zero maintenance sealed tube. However, very large (e.g. kWs to 10s of kW) still use a flowing gas design.

    (Portions from: Flavio Spedalieri (fspedalieri@nightlase.com.au).)

    Controlling the output power of a flowing gas CO2 laser is done with a combination of discharge voltage and gas flow. As the voltage increased, the gas flow must also be increased and visa-versa.

    In commercial lasers, the tube voltage is either mechanically or electronically coupled to the flow valve or ('witty valve' as it is known). As you increase the voltage, the flow valve is opened in proportion to the power setting.

    Sealed CO2 Lasers

    Commercial sealed CO2 lasers have much in common with sealed HeNe lasers. However, you can't just take an axial flow CO2 design, seal it up, and expect the laser to work for more than a few minutes. The discharge process breaks down the CO2 to produce CO and O2 which quickly poison the lasing process. There ARE a number of solutions to this problem including the addition of other gases like H2 or H20 (water vapor) to the gas mix to react with the CO and O2 to regenerate CO2 or the use of a high temperature (300 °C) cathode to act as a catalyst to stimulate recombination.

    (From: Dr. George Wood.)

    "There is a catalyst specfically developed by NASA for ambient temperature conversion of CO to CO2 during laser operation. This catalyst, available from STC Catalysts, Inc., has reduced operating costs as much as 75% in some applications."

    Therefore, requirements for the power supply to be used with sealed CO2 and HeNe lasers are very similar. However, due the significantly larger negative resistance characteristic of the CO2 laser, there is more incentive to push part of the effective ballast resistance into the control of a switchmode inverter rather than a simple power wasting resistor.

    For example, the dissipation in a 300K ballast resistor at 10 mA, would be 30 W. Depending on your actual needs, this may still be acceptable since it should simplify the power supply design not to have to deal with the negative resistance in the current regulator feedback loop itself. However, at the high end of the range where an 800K ballast is required at 20 mA of operating current, the corresponding power dissipation would be - ready? - 320 W! This is probably a bit more than is desirable. :-)

    Finding inverter schematics for HeNe lasers is tough enough. Finding them for C02s is virtually impossible. Most of the CO2 power supply schematics of any kind I have are based on neon sign transformers for use with home-built CO2 lasers. See the sections beginning with: Introduction to Home-Built CO2 Laser. They have no regulation but may be an alternative at least for initial testing.

    However, there is a description of one, the Universal Voltronics BRC-30-25-S, in the section: Typical Power Supply for a Sealed CO2 Laser.

    The good news is that if you were to design an inverter type of power supply with an oversize or multiple flyback transformers, I think you would find that it inherently had a high effective series resistance - possibly enough so only a minimal external ballast would be needed.

    The starting voltage is no problem - that can use any of the approaches for starting higher power HeNe tubes.

    Here are specs for a couple of larger sealed CO2 laser tubes. These and similar internal mirror tubes have been appearing on the surplus market and via eBay recently, supposedly originally for a medical application.

          Maximum               Supply
          Output    Beam        Voltage        Tube       Tube Size
          Power   Diameter   Start Operate    Current  Diam/DLgth/TLgth
         ---------------------------------------------------------------
           35 W   1.5-2 mm   30 kV  10 kV     7-25 mA   2.35"/24"/31"
          120 W    ??? mm    40 kV  25 kV    10-30 mA   3.20"/52"/60"
    
    (DLgth is discharge/gas reservoir length corresponding to the rather large diameter listed; TLgth is total length which includes the much narrower tube extensions and mirror mounts.)

    Varying the tube current controls output power (I don't know whether the lower values are at threshold or the relationship is more or less linear). Both these tubes are water cooled, either via a chiller/recirculator or straight from the tap.

    One thing that is significantly different for a CO2 laser compared to the HeNe variety is efficiency: The electrical to optical efficiency of a typical small sealed CO2 laser is around 5 to 8 percent compared to less than .1 percent for a HeNe laser. However, the efficiency of large (flowing gas) CO2 lasers can exceed 20 percent.

    Some photos and specs of typical larger sealed CO2 laser tubes (those for which specs are listed above and others) can be found in the Laser Equipment Gallery (Version 1.68 or higher) under "Assorted Carbon Dioxide Lasers".

    For more information on sealed CO2 lasers and a home-built design, see: Plans for a Sealed CO2 Laser.

    See the section: Power Supplies for CO2 Lasers for additional comments.

    (From: David Crocker.)

    The best efficiency is only achieved when tube diameter and gas pressure are optimal. For example, the optimum gas pressure for a sealed CW CO2 laser using an 8 mm inside diameter glass tube (cooled to room temperature by a surrounding water jacket) is about 14 Torr.

    For tubes of different diameters, the plasma scaling laws for CO2 laser operation work as follows, where D is the tube diameter or waveguide cross section:

    This assumes that the temperature of the inside wall of the tube is the same regardless of D. In practice, small glass tubes perform worse because there is less surface area (but constant power), hence more temperature drop between the inside and outside of the wall (another reason why ceramic waveguides work better then small diameter glass tubes).

    Transverse Excited Atmospheric (TEA) CO2 Lasers

    One problem with electrically excited axial CO2 lasers is that there is an upper limit to the gas pressure at which a discharge can be maintained. This is well under 100 Torr. Unlike helium-neon and many other gas lasers, the CO2 laser will operate up to atmospheric pressure (and probably beyond) with gain/power output proportional to pressure. Thus, 10 to 20 times more power would be possible running at 1 atm.

    Most high power (kWs) CO2 lasers are constructed using TEA (Transverse Excited Atmospheric) designs. Rather than having a long tube with a electrical discharge along its length and the CO2 gas mixture flowing from end to end, a series of electrodes and gas inlets are spaced along the tube. In this manner, higher pressures can be used since the electrical discharge doesn't need to go the full length of the tube - only across it. And, fresh gas can flow to all parts of the tube and doesn't get depleted along the way.

    TEA lasers must operate pulsed (a continuous discharge can't be maintained at higher pressures) and due to the shape of the cavity, the output beam has a rectangular cross-section. Gas flow is a major design consideration since the intense electrical pulses are very effective at destroying and rearranging chemical bonds.

    For more information on TEA CO2 lasers, see the CO2 laser manufacturers links in the section: Laser and Optics Manufacturers and Suppliers.

    Also see the comments on home-built TEA CO2 lasers in the section: Home-Built Transverse Excited Atmospheric CO2 Lasers.

    (From: Paul M. Brinegar, II (montyb@pulsar.hsc.edu).)

    Basically, you have a chamber with CO2 laser gas in it. The gas is at atmospheric pressure (hence the "A" in "TEA"). Two electrodes run down the length of the chamber, one on the left side, one on the right. These electrodes are connected to a capacitor bank which, when discharged, sends a spark/pulse across the gap inside the chamber. The pulse excites the CO2 laser gas, which then begins spontaneous emission. A reflector and an output coupler outside the chamber form the resonating cavity. The "TE" in "TEA" comes from the fact that the excitation is performed transverse to the direction of the laser output, as opposed to down the length of the tube like in the CO2 lasers being built by folks on this mailing list.

    The problem with TEA lasers is that the high voltage pulse causes the CO2 gas in the laser gas mixture to dissociate into CO and O2. If you don't flow new gas into the chamber or recirculate the dissociated gas through a catalyst to recombine the molecules, you limit the pulse rate of the laser. For high rep-rate TEA lasers, it is not uncommon for the gas to flow at many many many meters per second through the chamber.

    I have worked with two TEA CO2 lasers. The largest one had pulses that were about 1-2 microseconds long, and the rep-rate was about 10 pulses per second. It was tunable to any one of about 69 CO2 emission lines centered around 10.6 microns. The strongest lines had output energies of about 150-200 millijoules per pulse. The output cross section was square instead of round since the laser used two parallel (or almost parallel) electrodes. After passing through some collimation optics, the beam diameter was about 3 centimeters, and the energy density was low enough (measured using calibrated instrumentation) that you could safely put your hand into the beam when it was firing single pulses. It felt like a mild electric shock when the beam struck your hand. When the beam was focused to a point in space, it would cause a spark to appear in mid-air. I suppose the electric field from the beam exceeded the breakdown voltage of the atmosphere and resulted in a bit of ionization. When focused onto a black anodized surface, the laser pulse would vaporize the anodized coating, resulting in little jets of flame.

    (From: Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

    CO2 TEA lasers operated with a normal gas mix typically produce a 'spike and tail' output, with ~~1/3 of the energy in the ~~100nS spike, and 2/3 in the few hundred ns to 1 us 'tail'. The 'spike' is a gain switched feature - the 'tail' from v-v transfer from the N2.

    If the laser is operated with reduced nitrogen the tail goes away; you throw away about 2/3rds of the energy. If the laser has a short cavity, and is fairly heavily coupled, the spike shortens.

    The exact numbers are very system dependent of course.

    It is quite easy to get approximately 50 ns pulses with no tail. The small Edinburgh Instruments commercial mini TEA laser does just that (we have one).

    Slab Type CO2 Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    The big problem with getting power out of CO2 lasers is that when the gas mix gets hot, the power goes away. Hence, fast-axial and transverse-flow lasers, which use mechanical blowers to run the gas through heat exchangers.

    The fundamental concept of a slab CO2 laser is that you can extract heat from the laser gas without any moving parts by having a pair of planar water-cooled electrodes separated by a small gap. The laser gas is RF-excited by these electrodes.

    This is swell for exciting the gas mix but miserable to extract power from; lasers want to have round beams, not slits (except diodes, but their beams aren't any good anyway).

    To get around this, the Rofin-Sinar lasers use a Tulip resonator (named for its inventor, not its shape) which has an unstable resonator in one axis and a waveguide resonator in the other (!!). Power is extracted off of one side of the slab. The actual output that comes off of this is strangely shaped, but external correcting optics and knife edges produce something that looks and works amazingly like a Gaussian beam. The Slab series is described on Rofin-Sinar's CO2 Laser Products page. There are nice cutaway drawings of the lasers showing the way they work under the "Principle" links.

    The smaller (50 to 500 watts) Coherent Diamond(tm) lasers use the same configuration and are completely sealed. I'm not sure why the big Rofin units need the external gas fill since the Diamonds seem to run for years before they lose power. Maybe it's harder to seal a big cavity,

    Waveguide CO2 Lasers

    (From: David Crocker.)

    Instead of using a tube with slightly concave mirrors to form the cavity, an alternative is to use a waveguide and flat mirrors. This works better than a tube if you are using small diameters. The waveguide is made from a ceramic such as beryllium oxide (highly toxic!), alumina, or hexagonal boron nitride, with a round or square shaped hole for the gas. For example, we used a 300 mm long waveguide of hexagonal boron nitride, bore 1.4 mm square, total gas pressure between 50 and 220 Torr, current between 2 mA and 4 mA. The discharge was in 2 sections, driven in parallel from a 20 kV power supply. We also made a waveguide laser using a length of precision bore glass tubing.

    The whole point for us was to be able to tune the lasers a little (about each of the 80 or so centre frequencies) which is possible when running at higher pressures.

    RF Excited CO2 Lasers

    CO2 lasers can be operated using radio frequency (including microwave) excitation instead of a direct electrical discharge but this results in more complex resonator/electrode configurations, more complex driving electronics and additional safety issues. Depending on the size and type of laser, maximum achievable power output and efficiency may be lower as well.

    However, There ARE many efficient, compact RF excited lasers on the market. For example, the Coherent Gem Select 100 is a water cooled sealed tube RF excited waveguide CO2 laser using a folded rsonator. Its rated output power is 100 W but typically produces 120 W. The laser is only about 7" (H) x 8" (W) x 31" (L) and weighs under 52 pounds including power supply, and runs on 200 to 240 VAC at 14 A max.

    (From: Leonard Migliore (lm@laserk.com).)

    It is possible to make an RF-excited CO2 laser with the electrodes in the form of broad plates that are closely spaced. With such a configuration, the gas mix can be efficiently cooled by the electrodes. This is diffusion cooling.

    Of course, the discharge area is poorly shaped for power extraction by stable resonators, so these lasers use complex resonator designs. The Tulip resonator, used by Rofin-Sinar and Coherent, has a waveguide mode in one axis and an unstable resonator in the other.

    (From: David R. Whitehouse, manager, Laser Advanced Development Center Raytheon Co., Waltham, MA).)

    Although the CO2, N2, and He discharge can be operated either with DC, AC, RF, or pulses, it appears that the maximum average output power can be achieved either with DC or low frequency AC applied directly to the electrodes. The power is less with RF, probably because it is hard to keep a long length of the discharge uniformly excited. Where the discharge must be pulsed, the average power may be only 1/10th to 1/8th of the DC value. Also, the optimum pressure and output coupling conditions change."

    Gas Dynamic CO2 Laser

    (From: Mike Poulton (tjpoulton@aol.com).)

    A gas dynamic CO2 laser essentially uses a rocket engine as the exciter. :) The propellants are burned to create a very high velocity, high pressure, high temperature stream of gases (nitrogen and CO2) in a lase-able ratio. As the exhaust expands through a nozzle, the temperature of the gases drop very rapidly. The lower energy levels are rapidly and selectively depopulated (aided by trace chemicals introduced into the combustion chamber exclusively for that purpose) while the upper energy levels (populated because of the high temperature of the gases in the chamber) remain energized. Presto - an instant, complete population inversion. After a time delay determined by the relaxation time of those upper energy states, the stimulated emission begins. The optical resonator cavity is positioned a certain distance beyond the throat of the nozzle (determined by exhaust gas velocity), so as to initiate the lasing process at the proper time.

    These lasers are quite difficult and expensive, but produce tremendous beam power. I know this information because I have talked with an engineer who worked on the design and construction team for several of these ranging from 1 kW up to 100 MW CW for the Star Wars project. He said building a huge rocket engine and 100 million watt laser at the same time was the most fun he's ever had. :)

    Comparison of DC and RF Excitation

    (From: David Toebaert (olx08152@online.be).)

    One of the reasons RF is promoted for high-power fast-flow CO2 lasers is that you don't have internal electrodes that tend to sputter and contaminate the resonator optics. In the begining, DC fast-flow lasers were, due to the large pressure drop resulting from the inlet section design, necessarily equipped with modified roots blowers, which were not very leak-tight, contaminated everything with oil, gave pressure pulsations. And RF was more efficient because they could use "standard" turbines which run smoothly and are oil-free. Nowadays, turbines are also available with increased volume flow at higher pressure ratios, and so they are used for DC lasers, eliminating this contamination problem. A good design of cathode also reduces sputtering contamination a lot, and there you have it: DC excited lasers are at least as efficient (>20% HVDC in to optical power out). So now another matter arises: HVDC is WAY cheaper and more efficient (electrical to electrical) than RF.

    One unavoidable loss with DC excitation is the cathode fall - the voltage drop at the cathode which results in heat dissipation and doesn't contribute to the discharge. However, there is also a sheath region for RF excitation and it's distributed over the entire outer surface of the discharge, not localised at a single electrode. It shrinks with rising RF frequency, but then the oscillator becomes more expensive. Anyhow, for DC, the cathode fall is only a few hundred volts for a discharge voltage of many kV, so it's not that significant.

    (From: David Toebaert (olx08152@online.be).)

    There is a large difference between fast-flow, slow-flow, and sealed off operation of CO2 lasers. If you operate sealed off (as I guess Peter Laakmann does), then a very complicated plasma chemistry takes place involving dissociation and recombination, interaction with wall and electrode materials etc. Almost all sealed off lasers use at least a quarternairy mix, or even one with five constituents (H2O or H2 , CO , or Xe are mostly added). That way, after a series of burn-ins, bake-outs, refills etc one ends up with a tube that can have up to 10,000 hours lifetime. But it also reduces the gain because the mix is not optimised for maximum output, but for long life.

    I don't think there is a fundamental reason why RF should lead to less efficient laser operation (at MHz frequencies, the vibrational kinetics can't follow the electric field anyhow). If you compare apples with apples, e.g., a fast-flow 3 kW RF excited Trumpf with a DC excited 3 kW laser - Rofin, WB, mine :), the total discharge volumes are comparable. If you insist on getting as much power as possible out of a given total discharge volume, RF might be a little favoured because you can go higher in power input per cm3 discharge without deteriorating the glow discharge. Gain will drop because of higher average temperature, but at first this will be over-compensated by the increased input. But it won't do any good to the beam quality, so for cutting applications it's a no-go.

    I guess what I'm trying to say is that it's necessary to first decide what your laser specs are to be (what are the crucial things? Beam quality? Power? Compactness? ....) before choosing RF or DC, sealed or flowing gas, etc.

    Power Supplies for CO2 Lasers

    While there is little information on power supplies for helium neon lasers in the public domain, there is even less for CO2 lasers.

    Some (probably older) commercial designs have used the ultimate low-tech approach of a neon sign (luminous tube) transformer and Variac! See the section: Description of Typical Small Flowing Gas CO2 Lasers. These are simple and robust, but don't make optimal use of the available power. Phase control of the primary (in place of the Variac or even a motor driven Variac!) could be coupled with tube current sensing to provide regulation.

    For testing of DC excited sealed CO2 laser tubes up to perhaps 2 feet in length, an oil burner ignition transformer (typically 10 kV at 15 to 20 mA) or neon sign transformer (12 to 15 kV at 15 to 30 mA) is more than adequate. Even running half wave rectified with a few (depending on peak voltage) microwave oven HV rectifiers, or 30 or 40 1N4007s in series, will produce a substantial fraction of rated power. I tested a 14 W tube using a 10 kV, 20 mA oil burner ignition transformer and a pair of microwave oven HV rectifiers. The laser produced at least 8 W of beam power. Note that for this low current, the discharge is not very bright. If you're used to HeNe lasers, don't be fooled into thinking the tube isn't working as it burns a hole through your wall and the neighbor's. :)

    Except for smaller sealed CO2 lasers where only a few mA are required, linear approaches are probably way too inefficient to be practical. (See the section: Sealed CO2 Lasers), Scaling up a HeNe laser power supply design for use with a CO2 tube requiring 100 mA at 10 kV isn't realistic. A linear pass-bank would need to use multiple 1,000 V MOSFETs or 1,000 V deflection type bipolar transistors and dissipate 100s of watts to achieve an adequate compliance range with all sorts of fault sensing to protect them from excessive current or short circuits. However, as noted, for small sealed CO2 lasers, this approach could be used.

    An inverter is the most viable option for higher power CO2 lasers. Regulation is provided by PWM (Pulse Width Modulation) of the switchmode transistor drive. This permits the ballast resistance to be much smaller as the loop response provides most of this function.

    Like most other gas discharge devices, the CO2 laser tube exhibits a negative resistance when excited directly (e.g., DC or low frequency AC). Therefore, a higher starting voltage (perhaps 10 to 30 kV) is required to ionize the gas and then a lower voltage (perhaps 1 to 3 kV) will sustain the discharge. A variety of approaches can be used to provide the starting voltage including: a voltage multiplier and capacitor discharge into a pulse transformer or other pulse technique. An external RF source (even if it isn't used for the main excitation, see below) or even ultra-violet (UV) light can reduce the starting voltage requirements. With DC excitation, starting is only required once. However, where line frequency AC is used for the main supply, starting must take place on each half-cycle resulting in an output power spike followed by a period of normal output 120 (or 100) times a second.

    Where DC excitation is used with a flowing gas CO2 laser, if the starting voltage is insufficient, the pressure can be reduced until the gas breaks down, and then increased for operation.

    RF and microwave excitation can also be used for excitation and can be very efficiently coupled to the discharge. No additional means for starting is required for these.

    And, of course, chemical reactions for the gas dynamic type. :)

    One starting point for finding more information on CO2 laser power supplies would be a patent database. Search for major CO2 manufacturers like Synrad or keywords like "CO2 laser power supply" or just "CO2 laser". While you won't find complete plans in most patents, some have a remarkable level of circuit detail.

    If anyone has any CO2 laser power supply schematics (beyond the basic manually regulated neon sign transformer variety), I would be happy and eager to add them to this document. Please send me mail via the Sci.Electronics.Repair FAQ Email Links Page.

    Typical Power Supply for a Sealed CO2 Laser

    The Universal Voltronics BRC-30-25-S power supply is designed to drive CO2 laser tubes of up to about 35 W of optical output requiring up to 200 W input. The unit provides a starting voltage of more than 30 kV with an operating voltage of around 20 kV at up to 25 mA of tube current. I imagine that a power supply for larger tubes (up to 120 W or more) would have a similar design. It is a fairly simple switchmode type that appears to be based on the AC line front-end of a PC power supply, jumper selectable for either 115 VAC or 230 VAC (the typical doubler/bridge configuration). A half-bridge MOSFET chopper provides 300 V p-p to a high voltage transformer that looks sort of like a large TV flyback (though its internal construction is unknown). Voltage/current control is via pulse width modulation using an SG3525 chip coupled to the gates of the MOSFETs via a drive transformer with two output windings driven on opposite phases of the PWM waveform.

    The output of the high voltage transformer feeds a rectifier and then the CO2 laser tube without any additional components. So, there is no filtering or ballast resistance.

    There are separate control loops for voltage and current:

    The two loops are virtually identical and include an op-amp buffer, error amplifier (which is basically an integrator), and reference with potentiometer for setting the reference sensitivity to external input. The outputs of the loops are ANDed together (with diodes) and feed the pulse width control input of the SG3525. Whichever reference setting/input is lower takes precedence. Thus, the power supply will maintain the tube current at the value selected by the current select input (which controls the reference) with the constraint that the power supply output voltage doesn't exceed some selected maximum value or vice-versa. At least, that's they way it appears. :)

    There is also an overload protect circuit which disables the SG3525 (shutdown input) upon detection of a fault (presumably, a short circuit).

    There are inputs for controlling tube current (0 to 10 V or an external 5K ohm pot) and for remote turn-on/pulsed operation (5 VDC).

    A photo of the BRC-30-25-S can be found in the Laser Equipment Gallery (Version 1.68 or higher) under "Assorted Carbon Dioxide Lasers".

    Someday, I may get around to entering the schematic for all to enjoy. :)

    Electrical Modulation of a CO2 Laser

    Low frequency modulation can be achieved by pulsing or chopping the electrical power to the discharge. As the frequency is increased, the effect of the varying input decreases and above a few kHz, disappears entirely. The output of a DC or RF excited CO2 laser are both CW beams.

    (From: David Toebaert (olx08152@online.be).)

    This remark really holds for any kind of CO2 laser (the effect gets worse at higher pressure). It's just nature: it takes time for the molecules to 'meet' one another causing the delay. For a laser at 100 mbar (around 76 Torr) and a typical gas mix, the cut-off frequency is about 3 kHz. Above that the modulation of the input power is strongly damped and hardly visible anymore in the output power. Simply think of the discharge as a low pass filter for the input power, no matter how you excite the discharge. Of course, it's possible to modulate the input power at much higher frequencies (e.g. an RF supply can easily be modulated up to 100 kHz, that is, the Mhz signal is modulated at 100 kHz), but from the point of view of wanting to modulate output power, it makes no sense. Maybe it's beneficial for other reasons (e.g., discharge stability).



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Gas Fill

    You might think that a CO2 laser uses, well, CO2. However, a CO2 laser using only CO2 would be very inefficient. Other gases, mostly nitrogen (N2) and helium (He) are required to achieve any decent level of performance. The N2 molecules are raised to a high vibrational state by electron collisions (in electrically excited CO2 lasers). Collisions between the N2 molecules and ground-state CO2 molecules then excite the CO2 molecules to the upper lasing level. The He serves as a buffer gas and provides for cooling, principally achieved by collisions with tube walls, which must be maintained at no more than a modest temperature, often by flowing water.

    Composition and Pressures for CO2 Lasers

    (From: Paul (paulfr@lineone.net).)

    I am bulding a low flow axial type CO2 laser and I have a question about the lasing gas. It is a mixture of 9.5% CO2, 13.5% N2, and 77% He.

    Would an error in the mixture of the gas cause a lower output power or would it stop lasing? What tolerance do I need on the mixture? Can I increase power by increasing voltage and/or gas flow rate of the system?"

    (From: John Szalay (john.szalay@postoffice.worldnet.att.net).)

    Yes, mixture is very important. It takes very fine tweaking.

    In our slow flow system, power is a function of current, not voltage. Starting voltage is 25 kVDC and drops to 13 to 15 kV once laseing is steady. We run both fast and slow flow systems. Slow flow takes pains to set mixtures while fast flow system uses a preset mixture from the factory and is not normaly adjusted in the field.

    (From: Richard. A. Kleijhorst (r.a.kleyhorst@student.utwente.nl).)

    The power of the laser is dependent of several things. First thing is the current and the gas mixture. The higher the current, the higher the power (up to the saturation point). You can check it out yourself : why do you need just these three gases (CO2, N and He)to make the CO2 laser work. The energy of the discharge is first absorbed by the Nitrogen (that's also the "pink" discharge color which you see in the tube), the Nitrogen transfers the energy to the CO2 molecule and the heat that arises is carried to the "wall" by the Helium. Second thing, where the voltage comes in, is the length of the discharge. The longer the discharge length, the more power, but also the more power you need to start the discharge (ignition). Further is the gas flow also of importance. The more "fresh" gas is present and "old" gas renewed, the more power (if possible you could consider building a fast flow CO2 laser). Also other factors like the gain, inversion population, resonator design, cooling and the output coupler mirror have to be taken in account when you want your laser to give the most power. All these things are to read in more then enough available books.

    (From: Leonard Migliore (lm@laserk.com).)

    Gas mix has a tremendous effect on power. That looks like too much CO2 to me. You would probably get more power with 7% CO2 and 18% N2, but you generally have to establish the optimum CO2: N2 ratio by tweaking anyway. I usually start at low CO2 and increase it. The power goes up until a saturation point. Beyond that, the excess CO2 absorbs photons and the power drops. Increasing nitrogen increases the voltage. You get more power with more voltage until the discharge breaks down.

    (From larkinsg@solix.fiu.edu (Dr Grover Larkins):

    CO2 lasers operate at reduced pressures with a CO2, N2, and He gas mixture. It's been a while but I seem to recall that 1:2:3 CO2:N2:He worked OK at roughly 1/2 an atmosphere (Ratios are MOLAR not Pressures!!!!). They can also work at higher pressures but tube limitations (strength and window mounting) play a role - 4 Watts is plenty of power to damage eyes, etc. Caution is advised!!!

    (From: David Knapp (david@stella.Colorado.EDU).)

    DC discharge CO2 lasers run 30 to 80 torr (1 torr = 1/760 of an atmosphere, one atm is 14.7 psig). RF driven CO2 lasers can run from 30 torr to over 120 Torr for waveguide operation.

    Also, PV=nRT, and for any gas, we have 22.4 liters/mole so number density is proportional to pressure anyway.

    (From: David Toebaert" (olx08152@online.be).)

    Fanuc uses a strange gas mix of 5:40:45 (CO2:N2:He) in their fast-flow lasers. I don't understand it entirely, it seems an awful lot of N2, far above the optimum CO2/N2 ratio. Of course, it would allow to couple very much energy into the gas, so maybe the greater input over-compensates the lower efficiency? However, it should be of no use in a slow-flow laser (to little cooling capacity). Better stick to a lot of He for those.

    Why is There Carbon Monoxide in Some CO2 Lasers?

    (From: Ngiam Shih Tung (stngiam@pacific.net.sg).)

    My company recently bought some Lumonics lasermark lasers which are supposed to be CO2 lasers, but I was surprised to see that it contains half as much carbon monoxide as carbon dioxide. Specifically, the Lasermark IV premix gas contains 4% CO and 8% CO2 in He and nitrogen.

    Why would Lumonics include CO in what is supposed to be a CO2 laser? This is a continuous flow laser, and as far as I know, lasing only takes place at the CO2 wavelength. I've flipped through a number of laser textbooks, but none of them mention anything about deliberately adding CO to CO2 lasers. I did find out that CO2 will dissociate to CO during the discharge, and that CO lasers exist, but no mixed CO/CO2 lasers were mentioned.

    (From: Andrei Romanov (anrom@aha.ru).)

    It is well-known problem: dissociation of CO2 to O2 and CO in lasers with closed chamber. Some devices have special regenerators of mixture. But I believe that Lumonics added CO to gas mixture especially to make stable chemical composition of mixture and, hence, to make stable output power. So there is also reaction CO + O2 = CO2.

    (I worked in 1982-1990 in Gas Lasers Laboratory of the P. N. Lebedev Physics Institute, USSR Academy of sciences, Moscow.)

    (From: Harvey N Rutt (hnr@ecs.soton.ac.uk).)

    It is quite common to include some CO in CO2 lasers, athough 50% CO is higher than I've seen before.

    As you point out, in the discharge a chemical equilibrium is set up: CO2>2CO+O2.

    If you add CO you push the equilibrium to the left, less CO2 >dissociates, you keep more of the active laser gas in its correct form, so to speak. It happens that the CO vibrational level is close to the N2/CO2 sym. stretch level, & can fullfill a similar role to N2 in the >mix (not quite as well.) Oxygen on the other hand tends to lead to discharge instability & more problems with electrodes.

    So many CO2 lasers work better with a little CO in the mix; just how much depends on the details of the laser. It wd *probably* work without any CO; up the CO2 & the N2 a bit to compensate; but you wd loose some power, & might have discharge stability problems.

    CO of course is very toxic, & cummulative over many hours, so some people dont like using it.

    (From: Andrei Romanov (anrom@aha.ru).)

    In the discharge there are a lot of other processes. There are also other componets: molecules N2O, O2, CN etc, ions O3(-), O(-), CO3(-) etc., excited molecules and atoms. To calculate all processes is not possible in theory. The chemical composition is defined by experimental investigation only.

    I remember when a group in our laboratory tryed to improve CO-laser by adding small quantity of N2O in 1989. Theorists had some ideas about it. They did not receive good result in CO-laser but suddenly they obtained a very high power lasing in this mixture on levels of N2O molecules (10,8 microns). The output power of the laser was of the same order that usual CO-laser has.

    (From: Harvey N Rutt (hnr@ecs.soton.ac.uk).)

    While I would agree these other processes certainly *exist*, they are of very minor importance compared to the basic CO2 chemical equilibrium, and have relatively little effect on the laser. It happens that in the past I made extensive measurements of nitrogen oxides etc in big CO2 lasers; their main effect related to discharge stability & electrode corrosion processes, they are too low level to have much effect on the laser kinetics.

    The situation in CO lasers is very different to that in CO2; a low gain laser, notoriously touchy on gas purity etc, with a quite different pumping mechanism (anharmonic collisional up-pumping & direct e impact as opposed to v-v resonant transfer).

    So while I wouldnt disagree with what is said above, it does not alter the basic reasons your mix includes CO!

    Incidentally, just to further complicate things, a very few CO2 lasers actually add O2 to the mix; again it suppresses the dissociation of CO2 and production of CO, the odd thing is you'd think it would wreck discharge stability! For completeness, Xe is often added to small sealed CO2 lasers (not big, costs too much.) Basically it tailors the electron energy distribution in the discharge & improves the pumping efficiency.

    (from: Leonard Migliore (lm@laserk.com).)

    When I was at Spectra-Physics, we added oxygen to the gas mix of DC-excited transverse flow lasers. These had big, water-cooled copper pipes for cathodes, and the oxygen slowed down the formation of oxides on the copper. I am not sure if anyone really knew the mechanism of this effect, but it did work.

    About the High Cost of Sealed CO2 Laser Refills

    This is a somewhat different situation than for, say argon/krypton ion or helium-neon lasers where a large part of the cost is due to the incredibly high level of purity and elimination of all traces of residual gases and any other contamination.

    (From: Steve Roberts.)

    They have a right to their trade secrets. A figure like $800 to reprocess a CO2 laser tube is cheap, especially when you consider a sealed CO2 has a very tricky gas mix, often containing Xe, CO, NO2, H20 and half the rest of the chemical alphabet. A sealed CO2 laser needs a complex mix in order to recatalyze the CO2, which breaks down into CO and C during the discharge. If it's a few tenths of a percent off, the laser quickly dies. Often when refilling a tube, they also pop on a new front optic, something not easy to do when you need new indium seals.

    You didn't pay literally millions of dollars to develop those lasers, therefore you are only entitled to learn from published documents, patents, talking to others and whatever reverse engineering you can do, and even then possibly only for your own use. The high cost is because a company needs to make a profit to exist and improve, and people need to eat and pay bills.

    (From: Sam.)

    For some information on sealed CO2 laser construction and the catalyst issues, see U.S. Patent #4,756,000: Discharge Driven Gold Catalyst with Application to a CO2 Laser.

    CO2 Chemical Laser

    Here's an interesting twist on the usual boring CO2 laser. The following is the abstract of the paper entitled "CO2 Laser Using Electrochemical Transformation of Organic Compounds" by K. Midorikawa, H. Tashiro, and S. Namba, Applied Physics Letters 44 (4), 15 February, 1984:

    "A novel CO2 laser using electrochemical transformation of organic compounds (ECTO) has been developed. CW CO2 laser action was obtained by applying an electric discharge to the air, which contained organic vapors such as alcohols and benzene derivatives. The CO2 molecules produced by the chemical reaction in the laser tube and the N2 molecules contained in the air was excited by the discharge. The ECTO CO2 laser produced an output power of 7 W, which was comparable to the power obtained from a conventional CO2 gas mixture."

    WARNING: Before you try this experiment at home, don't forget that certain concentrations of organic vapors and air are just a bit explosive! At least, go read the entire paper first. :)



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Optics

    A Discussion on CO2 Laser Optics

    (All questions from: Ray Abadie (rabadie@bellsouth.net).)

    "I am planning the conversion of a medical 35 W CO2 laser system for light duty CNC cutting of wood up to 1/8" thick. The unit is complete and functional except it has no final optics. The plan as it stands is to eliminate the articulated arm and remount the now vertical tube (bean up) vertically (beam down) or horizontally. The unit is of the flowing gas, water cooled variety. I have the design of the electronics for coupling of the drive subsystem of the movable table to the pulsing of the laser pretty well down but the optics are giving me fits (guess I should have paid more attention to college physics). The laser is to be stationary, mounted either vertically (beam down) or horizontally with a 45 degree mirror "bend".

    First a general question. Generally speaking are laser tubes particular about their orientation?"

    (From: Leonard Migliore (lm@laserk.com).)

    Tubes don't care but some other components, notably the cooling system, often do. I don't like vertical tubes in a flowing gas laser because dirt collects on the bottom mirror.

    (From: David Knapp (david@lolita.colorado.edu).)

    In general (IMO) yes. If you mount the optical axis vertically you risk having particulates fall onto the bottom optic. Some lasers are designed to have less of a problem with this (RF excited have less problems with "junk" in them).

    "I understand that for minimal loss the front surface mirror at the 45 degree bend should be of a material appropriate for far-IR operation. I have heard copper, is this correct?"

    (From: Leonard Migliore (lm@laserk.com).)

    Copper is highly reflective at 10.6 microns but the normal bend mirror is coated silicon. With the right coating, you get much better reflectivity than copper and it doesn't tarnish in air.

    (From: David Knapp (david@lolita.colorado.edu).)

    AR coated copper can be excellent, so can protected gold and dielectric enhanced silver on silicon, the latter of which should be your cheapest bet.

    "The desired cutting action should produce as vertical a cut as possible (i.e., minimum "coning" of the beam) of the minimum width practicable. To minimize the beam diffusion effect of the smoke I intend to use a vacuum table for workholding (which should suck away some of the smoke) and experiment with a Nitrogen shield. What final optics would yield the desired results?"

    (From: Leonard Migliore (lm@laserk.com).)

    That depends on the initial beam characteristics of the laser. If the beam waist diameter, beam waist location and beam divergence are known, then the focus spot and Rayleigh range may be calculated for any focal length lens. Unfortunately, the smaller the spot, the greater the divergence. For 1/8" wood, you can generally get a 0.01" kerf that's pretty straight with an f/8 or so lens.

    "I have read that optics for use in this region of the spectrum and at these power levels must be specially coated (ZnSe?)."

    (From: Leonard Migliore (lm@laserk.com).)

    Zinc selenide is a substrate rather than a coating. Very few materials transmit 10.6 micron light and ZnSe is one of the best. Since its index of refraction is quite high, it has to be coated with stuff like thorium flouride to be useful.

    (From: David Knapp (david@lolita.colorado.edu).)

    Coating is "insurance" that you pay to keep damage possibilities lower and to make your optics cleanable. They do not *need* to be coated, and ZnSe is generally used an tramissive optical element for 10 microns. I'm not sure what the state of the art is in coating dielectric materials. GaAs maybe?

    Your biggest challenge is going to be supplying clean, dry air to your delivery optics to keep them from getting munged.

    "Finally, does anyone know of an affordable source for the optics (and possibly tje mirror)."

    (From: Leonard Migliore (lm@laserk.com).)

    CO2 optics tend to be expensive because of the materials required. One rlatively inexpensive source is Directed Light in San Jose. I'm reading this at home and I don't have their phone number, but it should be easy to get.

    Email me with any other questions that come up as you proceed with this project and I'll try to answer them.

    (From: David Knapp (david@lolita.colorado.edu).)

    Edmund Scientific sells some. Check out Laser Focus World at your library. There are many companies advertising for Mid/Far-IR optics.

    (From: Sam.)

    Leonard Migliore (lm@laserk.com) is with Laser Kinematics provides consutling services in the areas of cutting, welding, and heat treating.

    What Materials Pass or Block the CO2 Laser 10.6 um Wavelength?

    "I need information on plastics which will pass and block 10,600nM far infra red from a CO2 laser. Personal experience shows Perspex (Methyl Methacrylate) blocks that wavelength extremely well and is ideal for a viewing port to watch the laser at work cutting wood, etc. But I also need a plastic (or easily machinable glass) that I can use to pass the IR with minimal attenuation so I can keep smoke and spatter off the ZnSe lenses."

    (From: Leonard Migliore (lm@laserk.com).)

    Every plastic or glass that I know of has significant absorption at 10.6 um. The only classes of materials that have good transmission at that wavelength are semiconductors such as ZnSe and ionic crystals like KCl. International Crystal Laboratories sells a product they call "Lens Saver" which appears to be a salt (KCl) window. Their phone number is 1-973-478-8944.

    You may experience some loss of focus quality if you put a window in front of your lens. Most CO2 cutting systems incorporate some form of air shield to keep smoke off the lens, even if they don't use it as an assist gas.

    (From: Neil Main (neilmain@micrometric.demon.co.uk).)

    As far as I know, there are no cheap materials.

    ZnSe is very good. NaCl, sodium chloride is often used as an anti-spatter window in welding. Some of the other alkali/halides also work. The advantage is that they are cheaper than ZnSe (but still not cheap), the disadvantage is that they are hygroscopic.

    The other technique is to use air pressure / vacuum to blow/suck the fume away before it hits the lens. Air knives (laminar flows of high velocity air) are good positioned just below and across the front of the lens.

    (From: Chris Chagaris (pyro@grolen.com).)

    I think the only material you will find that will pass this radiation and is inexpensive will be salt windows. Janos Technology sells disposable salt windows for just such an application.

    Required Mirror Diameter for CO2 Laser Beam Steering

    (From: Leonard Migliore (lm@laserk.com).)

    You usually want the clear aperture to be at least 1.5 times the beam diameter. And, if you're bending the beam 90 degrees (using a 45 degree mirror), you have an increase of 1.4X because of the angle of the mirror. So, the mirror should be at least twice the diameter of the beam.

    The smallest diameter mirrors I know of for CO2 lasers are 1", although you can order smaller ones, I suppose. So, even if your beam starts out at only 1.5 mm, you still need a minimum of 3 mm and might as well use 1" (25 mm) mirrors unless space is critical.

    Another thing that needs to be taken into account is that while the beam may exit the laser at 1.5 mm (in this example), it won't stay that small for long. If the laser is TEM00, it will have a divergence of 9 milliradians, so the diameter will increase to 9.1 mm at a distance of 1 meter. If the initial beam diameter is 15 mm, the divergence will be 1/10th as much or less than 1 mR. Then after 1 meter, its diameter still be less than 17 mm.

    Reflective Optics for CO2 Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    Reflective optics are very common for high-power CO2 applications. There are a lot of 5 kW and larger lasers doing production welding. If you use lenses in these things they don't last long because of weld spatter. Once anything gets on the lens, the high power beam heats it and the lens is garbage. So, you just diamond-turn a paraboloid and it focuses the beam quite well, although they are very sensitive to angular misalignment. The combination you describe sounds like a welding head with a flat mirror and then a 90 degree deviation paraboloid; the flat is just there so the beam is collinear with the incoming beam, making it possible to focus without moving the location of the spot.

    The paraboloids are generally made of copper, which is easy to turn, reflects 10.6 micron light real good, and can be cleaned with a shop rag and metal polish when the weld debris gets too thick (I said this was production welding). I once had a few made with a sputtered molybdenum coating. Those things were really tough. You couldn't scratch them or burn them but bare copper is nearly as good. I've seen gold-coated copper but I don't know why anyone would use it, as the coating can get damaged and soon you're down to copper.

    Polarizing Optics for CO2 Lasers

    (From: Juozas Reksnys (rexnys@uj.pfi.lt).)

    Polarizers for CO2 laser can be made from:

    1. Two Ge plates V-placed (polarization degree aproximately 98%, losses less than 5 percent, angle of view 4 degrees). The same with ZnSe plates can be achieved. Using several pairs of plates extinction can be increased till 500:1 for ZnSe or 3000:1 for Ge.

    2. Plates of NaCl, KCl, BaF2 or others low refractive index material X-configured and coated with Ge film. Using multibeam interference polarization degree in such device 99.8% can be achieved.

    3. Polarising prisms can be made from Cinnabar (HgS) or GaSe (very expensive) birefringent crystals. Both crystals have absorption less than 0.07 cm-1 and are not fit for a high power CW beam.
    Furthermore polarizers can be air or water cooled for HP applications. 0.1 mrad is small quantity and therefore is necessary to avoid noises of radiation in beam , beam displacement on detector etc.

    CO2 Optics Cleaning

    Since CO2 lasers operate at a wavelength 10 times longer than most other common lasers, there are a variety of materials used in their fabrication including some that would be affected or even dissolved in water or other common solvents normally used for the cleaning of hard coated glass optics. Not only may there be soft coatings, but the substrate may be easily damaged as well. Therefore, it is essential to be know exactly what is safe for your optics. The best source for this information is obviously the manufacturer of the laser or optical components in question.

    Cleaning is most critical for higher power lasers. Whereas a dirty HeNe optic will just result in reduced or no lasing, in a 1 kW CO2 laser, a dirty window or mirror could actually be destroyed by the absorption of the beam energy. And, inside the resonator, IR flux may be several times higher than in the beam delivery system.

    Detergent and water may be acceptable for metal or metal coated mirrors but pure alcohol, acetone, or other anhydrous solvent would be needed for soft coated optics (e.g., the HR or beam delivery mirror) or those fabricated from a water soluble or hydroscopic material (e.g., salt windows).

    Laser Beam Products has some general info on Cleaning of Lenses and Mirrors for CO2 (and other high power) lasers.

    Many of these materials are also substantially more fragile and susceptible to damage than normal optical glass. For example, Zinc Selenide (ZnSe) is a crystal very commonly used for CO2 laser lenses and windows. Great care must be exercised in its handling, mounting, and cleaning. Apply uniform pressure when handling/mounting. Tools like tweezers must be avoided because this material easily scratches, cracks, and chips. Latex gloves or finger cots should be worn for handling and cleaning to avoid contaminating the substrate or coating. (Paraphrased from the Edmunds Scientific Industrial Optics Catalog.)

    (From: Chris Chagaris (pyro@grolen.com).)

    The best method of cleaning any optic............ is to avoid contamination in the first place. Dust should be simply blown off with a jet of compressed dry air or nitrogen. If the optic requires further cleaning the 'drop and drag' method would be recommended, using a high quality lens tissue or lint free cotton swabs made especially for this purpose (which I prefer) and spectroscopic grade methanol. The solvent soaked swab is dragged under it's own weight ONLY, slowly across the optic. The solvent at the leading edge will dissolve the dirt; the trailing part of the swab will absorb the resulting solution back off the optic. This should be repeated several times. It is important that the solvent is absorbed back onto the swab and not allowed to dry into a tide mark of concentrated dirt. Change the swab every time, as fresh solvent will absorb the contamination better and particles picked up in the swab cannot be repeatedly dragged back across the optic fingerprints on any of your optics, as these can be particularly damaging to the coatings and are very difficult to remove. Avoid ANYTHING that may scratch your optic!!!



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Marking, Burning and Cutting Lasers, Costs

    Marking/Engraviing Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    If you want to make white marks on black or color anodize, a low-power (like 10 watts) CO2 laser does a good job. You get better control if the laser can be pulsed but it's possible to make it work CW.

    If you're bleaching the anodize only, not much power is required. If you want to mark the aluminum itself, you'll want about 104 W/cm2 of Nd:YAG.

    Small Wood Burning Lasers

    "I'm looking for a laser which is suitable for engraving wood under the control of a PC. I have some of my own bas-relief sculptures scanned in 3D that I would like to transfer to wood furniture panels as a serious hobby.

    What type of laser would be appropriate? What power level must I consider? What cost can I expect? Where do I start looking for such a beast on the used market?"

    (From: Leonard Migliore (lm@laserk.com).)

    Companies such as Laser Machining, Inc. (now Preco, Inc.), Laser Cut, Inc., and Jamieson Manufacturing (jamieson.mfg.co@snet.net), all make systems that will cut plywood, but they vary in table size, laser power and cutting performance.

    (From: Ron Wickersham (rjw@crl.com).)

    A CO2 laser is most commonly used for wood cutting and decorating.

    I suggest that minimum 100 watts be considered, certainly no less than 25.

    A used machine may not really be the best for you. In the last year or so, sealed-off lasers in the 100 watt range have become available with lifetimes of approx 10,000 hours. If you go with a used laser that has to be pumped down and supplied with CO2, N2, and He then you get into a lot of auxiliary equipment that will be priced in addition to the cost of the bare laser. Additionally, the size of the used laser, power supply, etc will be huge compared to a new one which will be compact and light.

    You can then consider moving the laser head itself around under computer control to do the wood burning. With a larger used laser, you will have to buy additional beam-delivery optics that are also expensive and will require extremely critical alignment. As a first-time builder of a machine you will have a very high learning curve and make a lot of costly mistakes if you go that route.

    Another thing that you must consider. The laser itself will have a beam that may be around 1/4 to 1/8 inch diameter. To get to the tiny, hot spot that will do the cutting, you use a lense to focus the energy. But the smoke from the burning wood will ruin the lense in a few seconds so it has to be encased in pressurized chamber with a tiny exit hole that blows a compressed gas out the same hole the nearly focused beam emerges from, mthus keeping the smoke away from the lense. The depth of focus is small if you use a short focal-length lense and the power density is not as high if you use a long focal-length lense. So you may need to use the applications department of your supplier to help you with your first machine. Or work with someone who has an existing machine and learn everything about it before you undertake to build a system from scratch.

    (From: Steve Roberts.)

    Some place around two watts of visible light would be a good start for wood cutting if you only want a pinhole. The only problem is that unless you have a thin sheet of wood, a tightly focused beam, and assist gas of some sort, you can end up with a charred edge very quickly. I have actually used a 2x4 as a beam stop for a 10 watt argon ion laser. You get about 30 seconds of smoke and fire and then a nice deep pile of charcoal forms and the burning stops. When a 1 kW CO2 is used for engraving wood, it leaves a clean slightly fused edge in cuts up to 1/8th inch deep in the factory I toured. If you want a pinhole all the way through a 2x4, you are going to need serious power, a variable depth zoom and focus and after a heck of a lot of trying, you'll quickly go buy a thin drill bit. :-) Keep in mind that lasers don't cut a perfectly straight edge, they cut a tapered hole because of focusing. But 10 to 15 watts of CO2 would be good for cutting model airplane parts out of balsa wood.

    Depending on the assist gas, you can get wonderfully clean cuts in wood with only a faint char layer along the cut. The char often doesn't look black, it's more of a cherry brown color, very distinct. Surgical CO2s work well for this, considering they have a nice delivery system in the form of the surgical arm. Just remember to do 2 things: (1) Add an assist gas jet to the delivery tip and (2) set up a cross wind to blow the reaction products out of the work area. Otherwise you can expect to be blinded and poisoned by a noxious high velocity cloud of smoke and tar, which is what happened to me the first time I cranked the power up. At low power (few watts) this wasn't a problem but when I dialed up 30 watts and focused into some pine, I got nailed by a hot jet of nasty combustion products.

    You really need a needle valve on the assist. High pressure air works well as a start.

    I have a beautiful commercially laser engraved sailboat on my desk, burned 3/16" deep by an industrial CO2 laser. The image was created by an etched brass mask in sitting in front of the wood. The wood passed through the beam, which was focused into a line, on a moderately fast conveyer belt. I still love the looks serious woodworkers get on their faces when they see it has no tool marks. :)

    Model airplanes are a booming business, with balsa wood job shops even offering to take CAD drawings and cut parts for a very reasonable fee. There even is an off-the-shelf CO2 laser cutting system that does this for sale for about $15,000. And there is one heck of a market for this now that modern mass production has got the RC radios down to $200 or less and the engines to $69, or even better, the electric motors and rechargeable batteries now available. With balsa, you can go from CAD to aerodynamic simulation to production in a day or two using modern PCs and a laser.

    Large Wood Cutting Lasers

    CO2 lasers can be used in a factory under controlled conditions for production wood cutting. However not everyone agrees on their capabilities. Here is a short discussion:

    (From Randy (xoxthorxox@aol.com).)

    I am currently operating a 1,500 W Amada LasMac 1212 Pulsar Laser. I have been able to cut 1/2 plywood at 350 inches a minute at 1/4 power (Power: 700, Frequency: 1,000, Duty: 35%, Assist gas: 3 kg shop air).

    Works great. I experimented on a cut out of the Harley Eagle. It is very detailed and I was able to maintain razer sharp corners of up to 120 degree planes from the zero point. It is possible to cut virtually anything on the right laser. but the key there is "the right laser" I would recommend a CO2 laser of 1000 watts or better for optimum performance. But since you are talking about a quarter of a million dollar machine, you might want to look around for other advice. Good luck to you :).

    (From: Master Elf (helper@toontown.com).)

    This guy Randy is full of s**t. Cutting wood with shop air is a fire risk for one, and wood has physical properties that make it very undesirable to cut. Cutting with oxygen is a no no and nitrogen makes it cut slow, about 10 ipm at 2,600 watts. That is slower than it will cut 1/2 inch stainless steel. To suggest you can cut 1/2 inch wood with 375 watts is a tale only an Amada salesman could dream up.

    You can put a etch in it about .01 inches deep at 350 ipm.

    (From: Ray Abadie (rabadie@bellsouth.net).)

    I guess we are all out to lunch in the model airplane field where balsa and plywood are cut everyday at speeds upwards of 100 ipm by CO2 lasers in the 100 watt range and shop air to assist the vacuum chucks in clearing the smoke. Hum...

    (From: Master Elf (helper@toontown.com).)

    I thought we were talking about 1/2 inch wood, not .090 balsa or .090 plywood there is a big difference, and we were talking about assist gas, which follows the beam through the cut to remove the vaporized material. It is pretty obvious it's not a hazard to vacuum the smoke away duh...

    Seriously though 1/2 inch wood is about the break point for lasers regardless of power. You can cut it, but not efficiently.

    (From: Steve Llewellyn (stevell@indlaser.com).)

    Industrial lasers up to 2 kW are used effectively in cutting wood to 3 inches thickness in the furniture business. Steel-rule dies are cut up to one inch thick with 2 kW lasers in very common use. Shop air - clean and dry is used as an assist gas in all of those examples. The edge produced is square, with a dark grey to black color and the carbon is 0.005 to 0.010" thick and easily removed with a sandpaper rub. Using nitrogen as an assist gas would clean the surface a little but would be expensive.

    Cutting Printed Circuit Boards

    "Does anyone know if you can laser cut printed circuit boards, i.e. a copper clad glass/epoxy composite? I'm looking at thicknesses up to 12 mm. If so, what type of laser, what kurf sizes to expect, what power levels, what is the process (i.e. ablate, melt and/or burn), and what kind of cutting rates can I expect?"

    (From: Leonard Migliore (lm@laserk.com).)

    CO2 lasers are often used to cut glass/epoxy boards. They don't do so well if there's copper on them. You can probably get through a copper-clad board with an excimer, but the process rate would generally be unacceptable.

    When cutting fiberglass-epoxy, a CO2 laser melts the glass, which burns the epoxy. There is always some char on the edge from the decomposed epoxy.

    (From: Jef Falk (jlfalk@pacbell.net).)

    I've seen a 2000 W laser cut copper sheet but it had to be sanded to remove the reflectivity from the surface. It was VERY slow (read: expensive), required a full time eye on the beam, and puts a lot of heat into the material.

    Plastic Cutting Lasers

    "I would like to cut through thin (~2 mm) styrene plastic. It is thin enough to cut with an exacto knife. (It is usually scored with an Xacto knife and snapped out.)

    I would spend around $600 for a laser that did the job. What type of power would I need. I would like to be able to do a clean cut, but scoring would probably be adequate. What would a 20 W laser do? How much would they cost with power supply, and other neccessary gear?"

    (From: Mike Poulton (tjpoulton@aol.com).)

    The biggest problem is thermal conductivity of the material being cut. If the total irradiance is too low, the heat will be conducted away and the beam scattered before it can penetrate the material -- regardless of how small the beam is. Theoretically, a 1 W laser cannot be focused any better than a 1000 W laser of the same wavelength. In reality, it can -- but only moderately. This extra focusing, however, will not make up for the lack of power -- the larger laser will have a much higher power density In the end, total power is more important (and cheaper to obtain) than extremely small spot size.

    The other important thing to take into account is the transparency of the material being cut. If this is clear or white plastic, for example, a visible laser (e.g., doubled YAG) will be highly inefficient. 100 W of 532 nm in a .1 mm beam will almost certainly cut clear or white styrene, but not efficiently. A CO2 laser, on the other hand, will efficiently cut almost anything but rock salt -- styrene included. I would say that CO2 is your best bet, regardless of the color or lack thereof of the plastic. My best estimate is that 10 W would cut at a manageable rate, and 20W would cut as fast as your X-Y table can move. 1 W would not cut it at all. You may be able to find a used sealed-tube 10 to 20 W CO2 laser for under $1000, but it may require some work and possibly regassing.

    Try looking for used medical lasers -- they have articulated arms which allow easy positioning of the beam.

    Cutting Styrofoam with a CO2 Laser

    The traditional method of cutting styrofoam sheets (up to a few inches thick or more) is to use a hot wire on an electronically controlled XY table (like a flat bed plotter). One problem with this somewhat low-tech approach is that the path of the hot wire must be continuous so certain shapes like rings or anything with interior holes cannot be cut in a single operation with a wire that is attached above and below the styrofoam sheet.

    (From Leonard Migliore (lm@laserk.com).)

    Some basic process questions first:

    How fast do you need to cut, and what is the foam density? How narrow a cut width do you need, and how straight must the sides be?

    The generation of a narrow beam reasonably well collimated beam can be accomplished with a single lens of the appropriate focal length. In general, you don't gain anything by using a pair of lenses. If, for example, you were using a Synrad Model 48 laser (3.5 mm beam, 4 mR divergence) and focused the beam with a 5" lens, you would have a focus spot 0.020" in diameter, and it would only increase to 0.024" at the edges of a 2" thick part.

    The cut width in styrofoam is much greater than the diameter of the laser beam. The laser vaporizes the foam and the hot gases cut the material beyond the beam. The cut tends to be V-shaped and widest at the top. The best results are accomplished by going slowly to decrease the rate of gas evolution. The actual cutting speeds you get vary greatly with the density of the foam and even more with the allowable edge quality.

    Are you looking at building your own system? There are many cutting systems on the market with low-power CO2 lasers. If any of them meet your needs, it will be a lot cheaper than building your own because you generally have to hire people like me to help you and I cost a lot.

    By the way, you get terrible choking fumes doing this. You will need a first-rate exhaust system to keep from dying.

    Cutting Polycarbonate with a CO2 Laser

    (From: Leonard Migliore (lm@laserk.com).)

    Any time you laser-cut plastic (at least with a CO2 laser, which is the usual tool) you get ugly fumes. Most plastics generate some benzene, which is generally considered to be carcinogenic. A lot of them also form PAHs (I think that's polycyclic aromatic hydrocarbons) which are bad for you too. Some special favorites of mine are Kevlar (cyanide!) and PVC (hydrochloric acid; gets you and the machine too).

    PMMA cuts by melting and vaporization, leaving relatively unaffected material at the cut edge. If you don't rile the cut with a lot of assist gas, the material on the edge solidifies smooth, giving you a "fire-polished" edge. Polycarbonate decomposes rather than melts, so it leaves a tarry brown residue on the edge. You can push most of this out by using a lot of assist air, but then the edge gets rough from turbulence. I've seen pretty good edges on polycarbonate 1 mm or less thick. Any heavier and it's visibly darkened.

    Cutting Thin Mylar Film with a CO2 Laser

    (From: Leonard Migliore (lm@laserk.com).)

    CO2 is the best choice for Mylar (polyester) film because it's absorbed well and the watts are cheap. You can also cut Mylar with UV but you end up paying a lot more per watt. The only reason this might make sense is that you can focus UV a lot smaller, like if you need 10 micron slits or something.

    Wattage depends on speed. You won't need much. 30 watts cuts it at about 1 meter/second. It's hard to find CO2 lasers with less than 10 watts. I don't really know the minimum power needed to cut 50 micron Mylar since most folks want to cut fast.

    You don't need an assist gas to cut material this thin, so you could use a galvanometer scanner. You must, though, focus the beam to a rather small spot like 100 microns or less. This tends to make your scanner expensive so it's cheaper to use an XY stage and a focusing lens.

    New 30 watt CO2 lasers are about $4000. Used and smaller lasers should be less; folks on this group probably have some lying around.

    Aluminum Cutting Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    Aluminum is second only to graphite-Epoxy as a miserable material to cut with a laser. We usually use 2 kW of CO2 power to go after 1/8" aluminum. You could probably do it with a little less. Now, diodes (say 808 nm) couple a lot better to aluminum than the 10.6 micron CO2 light, but the beam out of diodes is lousy so you can't get a decent focal spot. I'd guess that 1 kW of diode light focused to a 200 micron spot would do fine. There are, to my knowledge, no such animals but there might be soon. Expect the laser to cost $80,000. It would cut wood too.

    Aluminum Welding Lasers

    (From: Neil Main (enquiries@micrometric.co.uk).)

    It depends on the application of course but laser welding aluminum often puts less total heat into the part to achieve the same weld as TIG.

    People are now welding 10 mm and thicker aluminum in a single pass with high power CO2 lasers and achieving e-beam like key-hole welds, but less width to depth ratio. Lack of distortion for sandwich structures can be "good enough to justify the extra cost" - quote from someone doing it.

    People also use pulsed YAG for welding aluminum electronic packages. Here the requirement is to not get critical components hot - often 80degC. The electronic bits are obvious but glass-to-metal feed-throughs (a metal rod passing through a metal tube with the space between filled with glass) are also heat sensitive. These feed-throughs are a few mm diameter and located on the wall a few mm from the bottom of the lid weld - and they must not die!

    Cutting Copper with a CO2 Laser

    (From: Neil Main (neilmain@micrometric.co.uk).)

    It is possible to cut thin copper with a CO2 laser.

    You need enough power, we use a 2.7 kW Bystronic and always use full power. The lens should be good quality and short(ish) focal length - we use a lens with a 5 inch focal length. The assist gas should be oxygen and the pressure should be high. Focus on the surface.

    What you are trying to do is couple into the copper surface with as high a peak power density as possible and cause melting quickly. Molten copper has a higher absorption of laser light than cold metal.

    If you cut slower than maximum speed the risk of miss-cuts is reduced and back reflections minimized. This dramatically improves the life of nozzles, lenses, bellows and anti-reflection optics.

    It is possible to cut 2 mm copper with only a small burr on the underside at speeds of 1 meter/minute.

    (From: Leonard Migliore (lm@laserk.com).)

    Your reference to anti-reflection optics is important; it's not a good idea to cut copper without them.

    Steel Cutting Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    These are typically medium to large CO2 lasers. Laser diodes don't have enough brightness for metal cutting. High-power Nd:YAG lasers have pretty lousy beams too. Some typical power requirements:

    High-speed cutting is done CW, fine contouring is done pulsed.

    Stone Cutting Lasers

    "I'm interested in finding out about using a laser for cutting stone. I know the basic physical principals but know little about laser cutting technology. I figure natural stone is a combination of materials including quartz, feldspar, and various metal salts. Can one type of laser be used to cut these various materials all at once? How much power and time is required? How narrow is the beam and what is its divergence?"

    (From: Leonard Migliore (lm@laserk.com).)

    It depends what you want to do. If you want to slice through a foot of granite, lasers won't do it unless you work for the Air Force. Lasers do a pretty good job of etching marble (like for tombstones).

    You need a lot of laser power focused to a small spot to cut refractory materials such as stone. I am not aware of laser cutting of minerals more than 1/8" thick. This takes a laser with 3 kW or so of power focused to a spot around 0.01" in diameter. Travel speeds are low, around 10 IPM or so, varying greatly with the material. A lot of stone is sensitive to thermal shock, so it cracks when you cut it. My experience with rock is that the response to laser light varies greatly, but I am unaware of any systematic studies of the effectiveness of lasers for cutting a variety of types of stone.

    The 10.6 micron light emitted by carbon dioxide lasers is absorbed by most minerals. These lasers are available commercially with power outputs up to 60 kW, although units with more than 6 kW output are uncommon and expensive. I believe that most stone cutting has been attempted with carbon dioxide lasers.

    Your question about beam diameter and divergence implies that you want to cut through a significant thickness of rock, as in a mining operation. This is not possible with commercial lasers since they won't remove enough rock to make it worthwhile. For any laser, beam diameter and divergence are inversely related. As an example, the beam from a certain large CO2 laser is 20 mm in diameter at its waist and has a divergence of 3 mr. This can be converted with optics to any combination yielding a product of 60 mm-mr. Different lasers will have different diameter-divergence products.

    (From: Steve Roberts.)

    I don't think you'll do well using a laser to cut stone, but Q-switched YAG marking systems do a wonderful job of lightly engraving the surface and discoloring it at the same time from oxidation. However effects vary from stone to stone. We did a friends wedding present that way, a custom engraved slab of marble.

    (From: Anonymous (localnet1@yahoo.com).)

    Contrary to what everyone else has to say, some stone would seem rather easy to cut by laser unless you are looking for high cut rates. Using a laser engraver (e.g., a Lumonics 50 W YAG), with a fairly large aperture (so probably putting out 30 W or so), I have seen deep deep engravings done in marble and granite. Not only were these engravings deep, but they were large, in less than a minute I would venture to guess that at least a few cubic centimeters of granite was removed by scanning the beam repeatedly over the same spot on the desired engraving pattern. Although the mark took a fairly long time, if you think about the volume of several cubic centimeters, and then were to have such a volume again, but in a narrow, wide, deep mark (i.e., through a relatively small block of material, no bigger than a few inches, to accommodate for the depth of focus of the laser) you would cut through your small granite block in a rather short time. Now if you are wanting to go through large amounts of material, you may again have a problem (i.e., mining operations), but if you are looking to make small intricate parts out of something like granite and a saw will not suffice, use a Q-switched Nd:YAG laser. They seem to work fairly well.

    Fabric Cutting Lasers

    (From: Leonard Migliore (lm@laserk.com).)

    I usually use scissors.

    If one wants to build a laser system for cutting fabric, the laser of choice is CO2. The power level, and consequently the cost of the laser, is a function of the material and the desired travel speed. The system cost is a function of work envelope and travel speed.

    Some curtains are glass fiber. This is hard to cut and needs higher laser power than organic fibers.

    If I was building a fabric cutter, I'd probably use a 100 watt laser (about $20,000 new) and put it on a 4' x 8' gantry with speeds around 1,200 inches per minute. System cost: about $300,000.

    That's why I use scissors. :)

    Focus Position and Cutting Speed

    (From: Leonard Migliore (lm@laserk.com).)

    You get the maximum cutting speed when the greatest irradiance is delivered to the bottom of the material. This usually occurs with the focus slightly below the surface. If the focus is on the surface, the kerf on top is small but the beam diverges as it gets deeper. As you drop the focus, the spot size at the bottom is smaller, so the irradiance is higher. This effect has to be balanced by the greater loss from the out-of-focus beam cutting at the top.

    A lot of times, you're more concerned about edge straightness than maximum speed. A lot of work has been done on die board cutting, and there the focus is raised above the surface by several mm in order to get a straight kerf at the expense of cutting speed.

    I don't know of any formal work on focus position in wood and acrylic. Most operators set it where it does what they want. John Powell's book "CO2 Laser Cutting" has a lot of information on wood and plastic (along with a lot of information on everything else you can cut).

    Large Cutting Laser - Not Quite a Basement Project!

    "I am looking for a schematic for a laser that can cut anything."

    (From: Dave (dave-a@li.net).)

    Its a little more complicated then that. Tee CO2 laser is the workhorse of lasers. Cuts most anything (though Copper and stainless steel are a bit funky since it reflects the light easily. You'd need anti-back reflctor mirrors) For a CO2 laser you need CO2 (very high grade, 5.0) Nitrogen (5.0) and helium (5.0), -20 kV DC @ 81 mA, a turbo or roots blower capible of circulating the gas at around 400 MPH (not sure of the metric eqiv), a vacum pump to bring the chamber down to 150 millibar and keeping it there, glass tubes, heat exchangers, etc, etc. It's a bit more involved then you'd think.

    You'd be bettor off buying on then building one. (you'll save money too)

    (From: Junius Hunter (junius@bellsouth.net).)

    Well, yes and no. Usefull wattage for cutting metal would typically be at a minimum of approximately 400 watts of CO2 laser power. A simple slow-flow design could be managed by someone with good mechanical skills. The cost would be equal between the resonator structure, power supply and optics. You would most likely want to have at least a 100 ma power supply. The laser gas could be obtained easily enough in a pre-mix format from an industrial gas supplier. The driving question is this; is this laser to be used in any sort of continuous manner, such as production. In that case, you may be better off buying a used laser at the desired rating. The cost of doing this in the basement could easily run several thousand dollars. This is probably still cheaper than buying a used laser, but then again, buying a used laser will normally get you a working and ready to go unit.

    (From: Dave (dave-a@li.net).)

    Yup. (I goofed on the current. For got we are pumping 81ma per tube).

    Premixed gas is the easy way out, sorta. It gets tough to control voltage when you can't adjust the nitrogen and or helium. Then again some people may not want to anyway. :)

    (From: Junius Hunter (junius@bellsouth.net).)

    One more thing, DC excited CO2 lasers at this power level (and lower for that matter) are lethal. Not so much from the laser beam, but from the high voltage needed for operation. The laser beam may burn, but the voltage can kill. Keep that in mind.

    (From: Dave (dave-a@li.net).)

    Yup. We had one customer, who thought he knew what he was doing, kill himself. Another got a real nice 'zap' but lived to talk about it. We have the customers sign a waiver if they want to go into the HV section. I still feel uneasy and always will.

    P.S. I still think it's easier and cheaper to buy used/surplus in the long run with the mistakes, and extra crap you will need to keep the stuff going.

    Continue Dreaming

    Well all of us had fantasies at one time or another! :-)

    "I am conducting a laser experiment for my High School Science Fair. I would need a fairly high powered laser which could cut things like wood, cardboard, cloth, and metals. If anyone has such a laser and has no use for it at the present time I would be willing to buy it at a reasonable price or if you would like I could pay for shipping and use it then send it back. We could figure out the details of that situation at a later time. Serious offers only."

    (From: Mike Poulton (tjpoulton@aol.com).)

    Serious offers only is right! You probably need a 30W or greater CO2 laser (30W, when focused, will cut cardboard, wood, cloth, sheet metal, and plastic -- see note). A used 30W CO2 laser, with a sealed tube and working PSU, will run around $20,000. If you can do without a sealed tube (which means you eed to do some serious maintenance and have the laser gas available) it may only cost $7000. A non-operational unit may only be $800. To cut wood, ardboard, or many types of cloth, you will need to use an "assist gas" this can be compressed air for sheet metal (although oxygen would improve it a great deal), but it must be something inert (or at least non-oxidizing -- CO2 works well) for cardboard, wood, and cloth. The purpose of this is to blast away the burned material and to prevent a fire. For sheet metal, the oxygen will enhance the burning of the metal and make it cut faster and more easily. Also, I hope you are aware of the dangers of these devices. A 30W CO2 laser does not give you a second chance in terms of eye damage -- any reflection (from a mirror or anything else somewhat reflective) will instantly cause irreversible eye damage. You cannot see the beam (IR), so you must be extremely careful. Always wear safety goggles when the laser is operating. BTW, you will need a focusing lens for this (it must be made from a special material -- the beam will not pass through glass or plastic) which will be another $200+.



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Miscellaneous CO2 Laser Tidbits

    Steve Discovers the Fun of CO2 Lasers

    Also see the section: Description of Typical Small Flowing Gas CO2 Lasers

    (From: Steve Roberts.)

    I recently have been buying used lasers from a medical surplus place and reselling them, and a few flowing gas CO2s show up from time to time. The medical types all want sealed tube systems so the flowing gas units are 500 pound useless dogs from the medical places' point of view. My main interests are visible lasers and laser rebuilding. I also do light shows. However, after playing with a medical CO2 and carving my name into glass bottles with it, I have decided I must have one. :-) It's also nice to not need nearly opaque safety glasses for a change!

    CO2 has to be the easyest to use funnest laser I have worked on in a long time, with just a few watts coming out of the surgical arm, we engraved wood, brick, glass, then cranking up the power, cut a wine bottle in half. Signing your signature in glass while wearing perfectly transparent safety goggles is not something you can do with a fussy argon by a long shot. And with the argon you'd be scared to death that the light would leak around your nose. The CO2 is a laser that is serious about lasing. The whole thing ran off 115 VAC single-phase.

    This was a sealed mirror tube, like a HeNe. Only you hooked it up to a bottle of lasing gas neatly left 3/4 full by its previous owner and it had a water to air heat exchanger. Its only fault was a blocked cooling line as it kept overtemping itself off. BTW, CO2 does not lase well as it gets warm - it almost falls off linearly to the temperature of the colling water. We could see water around the tube but it was not flowing fast enough so it would start at 21 watts and fall down to zero in about 30 seconds and shut itself off. And unlike the fussy argon, all the vacuum, gas, and water lines were cheap plastic. They weren't too worried about leaks either. Non megadollar Swagelock connectors and joints on the gas system - just off the shelf pneumatics.

    CO2 Laser Communications

    (From: Charles P. McGonegal (CPMCGONE@uop.com).)

    If I remember correctly, the atmosphere is quite transparent to the CO2 laser line (give or take water scattering). Also, they are very easy to modulate when RF driven instead of DC driven. The Melles Griot catalog lists a drive frequency of 28 to 30 MHz, and built in modulation capability (just plug in your signal) to 50 kHz (don't quote me, this is from memory)

    I think, that detection would be the biggest hurdle for use in communication (outside the need for water cooling and gas sources - not all these things are sealed units like HeNe lasers.

    Novel Idea for CO2 Laser

    Perhaps another use for old microwave ovens or a variation on "Hey Mon, I put my sneakers in the microwave." :-)

    (From: Scott Stephens (stephens@enteract.com).)

    A problem with atmospheric - pressure lasers is creating a stable, diffuse excitation discharge. This may be created using a common, cheap microwave oven(?). But what type of optical cavity?

    I recently read an interesting article, "Experimental Investigation of Large Volume PIA Plasmas at Atmospheric Pressure" by Brandenburg and Kline in the IEEE Transactions on Plasma Science, vol 26, April, 1998.

    It detailed the generation of a continuous plasmoid volume in a microwave oven. The oven was modified by (1) installing a 1/4" bolt-antenna, (2) a spark plug in the bottom center of the oven, and (3) mounting a 1/8" gas feed tube 1" next to it, and (4) surrounding the modifications with a 3"diameter. glass or plexiglass kerosene lamp chimney tube.

    The plasmoid was spawned in the oven by activating the spark plug, which irradiated the microwave antenna with UV, decreasing the ionization threshold and creating a spark at the antenna. When the gas flow volume is right (1.2 liters/minute) a large volume, persistent plasma vortex forms in the chimney tube. If the flow rate is too slow, the plasmoid rises, if its too fast, the antenna arcs. Having the gas flow offset from the chimney/plasmoid axis is necessary to insure stability via vorticity of the plasmoid.

    Instead of a 1.2 liter/minute airflow, how about CO2? And how 'bout replacing the glass chimney with an optical cavity? What I have in mind is a copper sphere; Copper being a good reflector of IR. The sphere would have sections removed from the top and bottom for gas flow. The sphere around 2"diameter, could be tuned by inserting a ceramic or glass slug at top and bottom. The plasmoid vortex axis would be pinned in the sphere. a pinhole in the sphere side allows the beam to exit, out a mm diameter hole in the front of the oven.

    Too bad glass is opaque to 10 micron radiation.

    The sphere could be electro-formed (plated) from a graphite-painted balloon, but how do you polish the inside?

    An appropriate diameter (tuned) copper chimney pipe is another thought. This would require a slit in the pipe to release the IR. It would be nice to create a linear array of holes, spaced to form a focusing diffraction grating.

    Could this line of holes be generated using a computer & printer, reduced photographically and etched lithographically in the copper? Would the plasma & radiation quickly deteriorate a fragile foil structure? Any better idea's (buying a ready-made laser notwithstanding)?

    (From: Dr. Mark W. Lund (mlund@moxtek.com).)

    My colleague Mike Lines and I tried to make a CO2 laser with a magnetron a couple of years ago. After solving a lot of problems with microwaves and waveguides and coupling we couldn't get any lasing because there was no way to cool the plasma in our configuration. Mike changed the design to a gas discharge and got it to lase. Then he got married, which ended our experiments for now :(

    The configuration that you propose will probably have the same problem. Your idea has a new twist because we relied on reducing the pressure to get a plasma discharge.

    The World's Most Inefficient CO2 Laser

    A commercial CO2 laser using a 4 foot tube with a 1" bore powered by a 15 kVAC, 60 mA neon sign transformer can generate 50 to 100 W. A determined amateur can build a similar CO2 laser in their basement. See the section: Description of Typical Small Flowing Gas CO2 Lasers. Compare that with the following (followed by some additional comments):

    (From: Robin Stoddart (stoddart@interlog.com).)

    A while back, I decided to go to the Ontario Science Centre. They have a "high power" CO2 laser there. It is about 40 to 50 feet long with a bore diameter of about 6". The power supply is rated at 20 kVAC @ 15 A (yes, AMPS). The OC is about 6" in diameter and has a hole about 1" in the centre for the beam. The approximate output (are you ready?): 100 W!!!! HA!!! 100 measly watts!!!! What really gets me is the inefficiency of it now: 300 kW input, 100 W output! That thing should have been able to cut through buildings and other large objects. It was a gift from France in the 60's or 70's. HAHAHAHAHA!!! 100 W!!! No joking! :)

    Regrettably, the laser is now nowhere to be found. The Ontario Science Centre had to get rid of it because of electrical problems. (The thing was almost always NOT working. Very annoying, as that was one of the best demos at the place.) Anyway, they say that they have acquired a krypton ion laser to show the people, but I don't think that it will be quite as impressive. So, who's up for building a twin of THAT beast? You would need a small power plant, a whole lot of glass, and some 6" diameter resonant optics. Come on, you know you want to! How 'bout it? See the chapter: Home-Built Carbon Dioxide Laser for instructions. :)

    (From: Sam.)

    I can't imagine that any sort of modern commercial laser will be nearly as impressive and educational as that beast. Everything is likely to be hidden behind protective panels and safety interlocks. Sure, laser light is laser light but the laser technology would be less visible. If they wanted a replacement to burn things, maybe one of those older flowing gas CO2 lasers could be substituted - similar 100 W power output, smaller package, lower electrical bills. Not quite the same but frees up valuable floor space. Any why a krypton ion laser? Who's going to maintain that? They mmight as well display an external mirror helium-neon laser (or HeNe laser of any kind!). At least you can leave the covers off and see the discharge tube and mirrors!

    (From: Steve Roberts.)

    Having seen the same unit, I'd like to interject that it probably did much more when it was built as I saw it blow through a brick when I was a kid. Museums don't usually have technicians that specialize in lasers enough to know how to spot a bad optic. Those guys are usually sharp generalists who can maintain anything - a sort of jack of all trades but master of none. Or, the operator that was trained on it, retired or moved to hopefully better things. Nor would it surprise me if it were merely misaligned. 100 watts is still an impressive setup - that was probably a kW or more once. Is it on their webpage?

    CO2 Laser for Tree Pruning?

    This question was actually asked on a laser related discussion group by someone who shall remain nameless and probably had had too high a dose of Sci-Fi! He figured it would be easier to aim a laser at a high branch than climbing a ladder and would be a good excuse to build a laser! After all, CO2 lasers may be used for production wood cutting. (See the section: Large Wood Cutting Lasers.)

    Tree pruning is probably not a good application for a CO2 laser. Think of trying to cut branches with a propane torch. It is easy to start fires, limbs would come crashing down uncontrollably, the speed of anything but a 10,000 W, $250,000 industrial laser would be disappointing compared to a saw, aiming and focusing the beam - and maintaining it would be quite a challenge, and CO2 lasers aren't exactly portable. In addition, pilots of low flying aircraft would not generally appreciate having holes punched in their wings or elsewhere by accident (no beam stop after all!):

    Besides, even a bushel of chain saws and ladders is going to be a lot less expensive than ANY cutting laser! :)

    Keep it low tech!

    And, no, lasers aren't appropriate for mowing grass either despite the fact that there is such a thing as laser hair removal and hair does share some similarity to grass! :)

    CO2 Laser for Paint Removal

    Yeah, hmmm, grrr. ;) I sort of think laser paint removal runs in the same category as laser tree trimming - more trouble than it is worth. Even if a laser could do this, it would need to be HUGE and probably require much more power than a sander or heat gun, not to mention cost and portability.

    A 1,500 W electric paint stripper or 1,200 W sander uses 1200 W and puts out nearly that much in heat or work. A CO2 laser to produce the same beam power would require 5 kW or greater input! The laser's effectiveness would also be dependent on the color, texture, age, and type of the paint and thus one type of laser probably wouldn't be useful for all situations. Other types of lasers might be even less efficient. And, of course, there would always be the issues of safety and risk of property damage: "Oops, left it in one spot too long, sorry about that string of holes clear through to the far wall". :)

    If You Have the Space and Power

    Here's a little item that might make you the envy of the neighborhood:
    Plasmatronics, Inc. 10 Kilowatt CO2 Gas Laser System. Built in 1984 for General Dynamics for research in the Star Wars program. Original cost was $1,500,000.00 paid for by you, the taxpayer!

    Laser comes complete with control cabinet and power regulator system. The entire system was gone through and updated just before it was taken out of commission.

    Price: $15,000.00.

    Check out some pictures of this beauty at: Laser Surplus Sales while it lasts!

    High Power Compact CO2 Laser?

    The authors of the information at the following Web site: Super Laser Page claim to have invented a CO2 laser that (according to them) will be significantly smaller and lighter than conventional CO2 lasers of similar output power. They haven't completed a prototype as yet (December, 1999) but the site links to a couple of patents. Of course, we all know what that means. :)

    Like the respondent below, I haven't studied the Web site or the referenced patents in detail but from what I have seen, there is more left out than included. And, it is in those details where viability will be determined. It's real easy to propose bucket loads of new and improved laser designs. They're a dime a dozen. :) Making such systems work, and then taking them successfully to market, is real the challenge!

    (From: Anonymous (localnet1@yahoo.com).)

    I must admit that I did not have time to thoroughly look into your plans, but your approximations for weight and size are in error, by orders of magnitude. First you overlook the fact that lasers with many multiple bounces per cavity round trip have much lower gain than 'single shot' resonators. This is due to the fact that a mirrors efficiency is not perfect, and many cumulative bounds leads to a lower cavity finesse. Therefore, your laser would have lower efficiencies than normal. Let's say that average CO2 efficiency is 15%. That would mean that you would have to dissipate tens of thousands of watts in your 3,500 W laser head. This would be a feat, to say the least to accomplish, and certainly would not be done in a 2 kilogram apparatus. Affects of thermal gradients are also ignored in your proposal. Although it is good to an extent to quickly cool the laser gas, when you do this too fast, it leads to thermal lensing and other optical problems. It is for this reason that some YAG lasers, and other high power systems, are run a bit on the hot side at elevated temperatures higher than what the cooling system is capable of handling. Finally, there would be a very complex optical mechanical system needed to support this assembly, again, mainly due to the very long effective bore length, and the high number of bounces. I think you have some interesting ideas, but get someone working with you that knows a lot more about CO2 laser design!

    Frequency Multiplication of CO2 Laser?

    With the popularity of frequency multiplication (2X, 3X, 4X) of YAG and other solid state lasers, how about doing this for a CO2 laser? Well, the first problem is that few materials are known that would be suitable. And, the efficiency is probably terrible. And, you would need 16X or so to have a visible wavelength. By the time you did all that, your 100 W CO2 laser might produce about as much 662 nm red light as a $5 laser pointers. :)

    (From: Kenneth Ferree (lasersnow1@aol.com).)

    You can actually buy a frequency doubling crystal for 10.6 um. They are made in Russia: AgGaSe2, 5x5x25 mm, (better be sitting down) $5,550 (USD) each. Type 1, theta = 52 deg, phi = 45 deg, SHG of 10.6 um AR/AR coatings at 10.6/5.3 um.

    Laughing Gas (N2O) Laser

    (From: David Crocker.) When added to a normal CO2 laser mix, even 0.1 percent of N20 would significantly reduce the gain and affect the electrical characteristics of the discharge as noted in the paper:

    But CW CO2 lasers can also be made to work on N2O (laughing gas) - replace the CO2 by N2O. I don't have a record of the exact wavelength(s), but it is somewhere in the 9 to 11 um region because we were able to use the same diffraction grating and other optics.



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Small Commercial Sealed CO2 Lasers

    Coherent Xanar XA-20 Medical CO2 Laser

    This system is/was used in a number of medical/surgical applications. Although the systems presently have the Coherent name, the actual laser is built by Laakman/Synrad. See Industrial Laser Solutions - Synrad Celebrates 20 Years for some more info.

    The XA-20 uses an air-cooled RF-excited sealed CO2 laser head and mating RF power supply which runs on 28 VDC at about 15 A max. Its rated output power is 20 W, though a new system will do somewhat more. The laser can be modulated (on/off) at up to a rate of to at least 10 kHz so that variable power control can be implemented via Pulse Width Modulation (PWM). If recycled as a marking or cutting laser, this permits precise control of laser output on the fly.

    The laser head enclosure is about 36.5 inches in length with the rectangular all metal sealed RF-excited tube being about 29 inches long. The remaining space is for a cooling fan at the HR-end and the delivery and aiming system at the output-end. There appear to be three-screw adjustable mirror mounts on the tube. The RF enters via a pair of coax cables. The only other connection to the head required for running the laser is 115 VAC for the fan. And there is a thermal protector that can be included in an interlock circuit to shut down the system if the baseplate gets too hot.

    I was given a Xanar XA-20 head (minus the delivery and aiming system) and RF power supply, and picked up a 28 VDC power supply on eBay. The only thing required to run the laser beyond that is a normally open switch to turn it on via the modulation input on the RF power supply. After some quick testing using a pushbutton switch, I constructed a little control box providing for adjustable power via PWM of about 1 percent to 99 percent via a knob (along with a full on override). I don't know what variable power capability was provided in the original system. It runs at about 6.5 kHz since these types of lasers are supposedly most efficient between 5 and 10 kHz. Something about gas circulation/cooling. And, running at 95 percent may actually produce a slightly higher power output than 100 percent but I have not confirmed this with measurements as I still have to set up a power meter that won't be melted by this laser. :)

    The beam close to the laser head aperture is between 1 and 2 mm in diameter. When set at 5 W, a red construction brick begins to glow. At 20 W, the spot becomes white hot incandescent and almost too bright to watch in a fraction of a second. Little black crators are left behind along with a fine black halo 3 or 4 mm in diameter surrounding eash spot. I don't know if the halo is simply some artifact of the thermal event or in the actual beam profile.

    With a wavelength of 10.6 um, the divergence is rather high but suitable optics can be added to control this. The components of the original hand-piece would perhaps be of use in this regard. I was given a short focal length germanium lens to try but need to arrange for a stable mount. At these power levels, the consequences of an optic accidentally shifting while the beam is on could be, shall we say, unfortunate. So, one doesn't use duct tape and bailing wire. ;-)

    If anyone has more information on the XA-20 like detailed specifications and/or an operation and service manual, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Synrad 48-1 and 48-2 RF Excited CO2 Lasers

    These use tubes that appear similar to the one in the Xanar, above. But the tube itself is of all aluminum construction except for viton seals for the mirror mounts and gas-fill/RF ports. The tube box structure and electrodes are extruded while the end-plates are machined and welded in place. The mirror mounts are simple circular disks with three screws bearing on the periphery for adjustment, which are sealed with a dab of Epoxy after alignment. While not being hard-sealed, the tubes have a typical shelf life in excess of 4 years, and 10 years or more with little degradation is not unheard of. The 48-1 requires one RF driver while the 48-2 requires two RF drivers and is essentially a pair of 48-1 cavities in series. Also unlike the Xanar which has an RF power supply remote from the laser head, the RF driver(s) of Synrad 48 laser are normally close-coupled to the tube using the internal capacitance of the electrodes in conjunction with an internal tuning inductor as part of its resonant tank circuit, resulting in a very compact lightweight and efficient system.

    Here is a summary of the specificatiohns:

      Specification                    48-1              48-2
    -------------------------------------------------------------------
      Nominal Power Output             12 W              27 W
      Minimum Power Output            9.5 W              23 W
      Guaranteed Power Output         7.5 W              20 W
      Wavelength                          10.5 to 10.7 um
      HR Mirror                           Si, 3 meter RoC
      OC Mirror                             ZnSe, Planar
      OC Reflectance                   95%               92%
      Beam Diameter                            3 mm
      Beam Divergence                          5 mR
      Transverse Mode        TEM00, rect. at exit, circ. in far field
      Polarization                Linear, vertical, 50:1 minimum
      RF Frequency                          45 to 46 MHz
      RF Power                        120 W            120 W x 2
      RF Drive Circuit              Single transistor oscillator
      Power Control           Pulse width modulation 0 to 100 percent
      Modulation Frequency                  0 to 10 kHz
      Optimal PWM               5 to 10 kHz at 95% for maximum power
      Cooling                          Forced air or water
      Tube Power Diss. Max            110 W             220 W
      DC Input                   28 VDC at 11 A    28 VDC at 22 A
      Total Power Diss. Max           310 W             620 W
       (RF driver and tube)
      Tube Dimensions                2"x2"17"          2"x2"31"
      Laser Dimensions WxHxL       2.8"x3.89"17"    2.8"x3.89"31"
       (Includes RF driver)       (Add 0.25" for top cooling fins)
      Laser Weight                   9 Pounds         18 Pounds
    

    Two of the relevant patents are:

    Applications include small to medium size laser engraving and marking systems such as those from Epilog Laser.

    Hughes 3800H Waveguide CO2 Laser

    The 3800H is a very small and very simple DC-excited self-contained air-cooled waveguide CO2 laser dating from 1984. There were other Hughes waveguide CO2 lasers based on the same technology with the 3800H probably being the lowest power. The model 3802H had two tubes like the one in the 3800H joined end-to-end and 4 HV power supplies. Versions with a "-T" (e.g., 3802H-T) had circuitry in the laser head to regulate the tube temperature by varying fan speed. Adjusting the temperature would allow some control of cavity length to select any one of the lasing transitions around 10.6 um. (Or at least, that's what the manual says!) Those versions without this option had a simple fan on/off switch. (Water cooling could also be used since there were plumbing connections to the baseplate.)

    The 3800H and 3802H are called "waveguide" lasers NOT because they use RF but because the bore is quite narrow (less than 1 mm) and designed to act as a waveguide for the 10.6 um lasing wavelength. The mirrors are probably planar. See the section: Waveguide CO2 Lasers.

    Photos of both of these lasers, as well as one of their predecessors, can be found in the Laser Equipment Gallery (Version 2.24 or higher) under "Hughes CO2 Lasers".

    The only major components inside the toaster oven-size enclosure of the 3800H are the laser tube on top and a pair of high voltage power supplies below deck that look similar to large HeNe laser power supply bricks, except that these produce 6.6 kV at 2 to 4 mA (adjustable). There is a PieZo Transducer (PZT) at the HR-end of the tube for cavity length tuning, but using it requires a separate "dither" controller, the 3870H. In "Automatic" mode, it applies a low frequency signal to slightly change cavity length and locks on to a peak of the gain curve based on current feedback from the dischage (which correlates with maximum output power). There are also "Manual", "Ramp", and "External" modes.

    The tube is all metal-ceramic (BeO!) with a large "gas can" in the center and the high voltage applied to the ends. The maximum output power listed on the safety sticker is only 5 W for the 3800H. But its tube is only about 10 inches in total length. I would have assumed it to have dual anodes with a center cathode, but measurements of the voltages when turned on seem to be the exact opposite. Normally, the cathode should be away from the mirrors to allow for better heat dissipation (which is mostly at the cathode) and to prevent sputtering damage to the mirrors. However, this sample of the 3800H laser seems to have it backwards. But, the power supplies are simply marked "6.6 kV at 2-4 mA" with no polarity indication and the color code (red) would normally imply positive output. At first I thought that perhaps the power supplies somehow failed in s peculiar way resulting in reverse polarity (as impossible as this could be). However, there is a current meter in the return circuit and that reads the correct direction. So, it's still a mystery. Here is the HV circuit:

         Black +-------+ Red      -12 kv (open circuit)
      +--------| PSU 2 |------------------------------------+
      |        +-------+                                    |
      |                                                     |
      |  Black +-------+ Red      -12 kv (open circuit)     |
      +--------| PSU 1 |------------+                       |
     _|_       +-------+            |         +---+         |
    ////                         ___|___      |   |      ___|___
                                |    |  ------|   |------  |    |
                          PZT ---||   ========     ========    | ====> Out
                                |____|__------|   |------__|____|
                                              |   |
                                              +-+-+
                                                |
                                            - +-+-+
                                              | M | 10 mA full scale
                                            + +-+-+
                                               _|_
                                              ////
    

    Measurements were made with a Simpson 260 with HV range extending resistor (read: poor excuse for HV probe) and meter polarity was confirmed with a low voltage DC power supply. Although not entirely conclusive, there isn't any evidence that either the power supplies have been replaced or that the meter had been rewired.

    The unit I have is apparently quite dead. Although the tube draws current, it is erratic - more of a stuttering than a continuous flow - and there is no detectable output. I don't want to power it for any length of time until the polarities have been confirmed since I'd expect incorrect polarity to totally ruin the mirrors in a very short time.

    If anyone has more information on this or similar Hughes CO2 lasers, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    VINCA Mini TEA CO2 Laser

    See Patrick's VINCA Mini TEA CO2 Laser for a series of photographs and some description. (Also linked from Patrick's Homepage).

    Here is some basic info:

    At the Lasers & Laser Applications Division within the Department of Physical Chemistry of the Vinca Institute of Nuclear Sciences, which has long time experience in R&D of the molecular gas lasers and their components, a mini TEA CO2 laser head has been developed with the following characteristics:

    The mini TEA CO2 laser head operates in a slow axial flow regime. The typical gas mixture is CO2/N2/He. Typical consumption of the gas mixture is less than 0.4 l/min. The laser can also operate in the fundamental mode (TEM00) by inserting an aperture (about 6 mm in diameter) into the resonator. Under these conditions the output energy is 1/3 of that obtained in multimode operation. Diameter of the laser beam in TEM00 mode is about 6 mm and beam divergence is of the order of several mR. Under special conditions with a nonconventional CO2/N2/He/H2 gas mixture the specific laser output energy is approximately 30 J/l. In this case the extracted laser output energy is 440 mJ (from only 16 cm3 of lasing volume).

    Laser Photonics CO2 Laser Tube

    This is a small metal-ceramic tube with dual anodes and a center cathode. Other than the following photos, I do not have much more information on it except that the price was around $4,000!

    A similar tube is used in the Laser Photonics model:CT-55-A000, circa 1989. It's good for at least 3 W. Here are a couple photos (courtesy of Matthew Dickson).

    If anyone has more information on this or similar Laser Photonics CO2 lasers, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.



  • Back to Carbon Dioxide Lasers Sub-Table of Contents.

    Descriptions of Typical Small Flowing Gas CO2 Lasers

    Commercial Versions of the Type of CO2 Laser You Could Build

    These are the type of CO2 lasers used for various medical/surgical applications (we won't get into the gory details!). Photos of typical units can be found in the Laser Equipment Gallery. The first one described below is a self contained unit only requiring 115 VAC power on a roll-around base (like all that other medical equipment you know and love). The plasma tube along with its HeNe pointing laser was mount on the side vertically with the articulated beam feed head exiting from the top. These are both basically the commercial versions of the sort of CO2 laser you would build at home.

    Also see the section: Steve Discovers the Fun of CO2 Lasers.

    Here are various assorted notes and comments on various models. (All of these so far are from: Steve Roberts.)

    Coherent 541

    Gas flow path: Premix tank at 2,200 psi, normal tank regulator set to 50 psi, then a solenoid valve, then a fine adjust regulator at 0 to 10 psi, a sensitive pressure gauge, a check valve followed by a needle valve, then a Tee with a solenoid valve to atmosphere followed by the tube. The fine adjust regulator was a critical part of setting up the system for repeatable adjustements. The vacuum pump and a dirt trap were on the other end of the tube. When the laser is shut off, the gas solenoid valve closes shutting off gas to the tube. The other solenoid valve opens venting the tube to atmosphere, thus preventing the pump oil from backflowing into the system. A 0 to 40 Torr mechanical gauge was on the high (feed) side of the tube.

    From the user manual for the 541, the gas mix should be: 4.5% CO2, 13.5% N2, and the balance (82%) He. From the pressure chart on the side of the unit: 0 to 15 watts, set needle valve for 20 Torr; for 16 to 40 watts, set it for 25 Torr.

    The tube was made of Pyrex with a half inch bore and cooling jacket. The jacket was put on using a glass lathe. (Something no well equipped shop should be without - it looks something like a machine lathe but with its headstock and tailstock designed to hold glass pieces up to perhaps a foot in diameter while they are rotating with multiple gas torch heads on movable swivels along the sides.) The distance between the electrodes was 24" with 31" between the mirrors. Two salt windows of some sort were carefully epoxied to the tube with TorrSeal. A plastic piece with O-rings held the tube into the mirror mounts and sealed the salt windows from the atmosphere. The HR (High Reflector) mirror was silicon, the OC (Output Coupler) was ZeSe, I could not measure the radiuses without loosing alignment, something I was unwilling to do on a customer's laser.

    The PSU was a phase controller of some sort in front of the commercial neon sign transformer they used to power the unit. A 15 KC, 30 mA transformer did 24 watts maxed out on the power meter. They didn't rectify the AC.

    Compared to the argon lasers I usually work on, the CO2 requires much looser tolerances. All the vacuum hoses were low cost plastic and not much attention was placed on the hose fittings. With the exception of the Swagelock needle valves, all the fittings were hardware store stock - something I would never dream of doing on an HeNe or ion laser. In fact, leaks seemed to be tolerated well.

    The power measurement on these units is usually nothing more then a Peltier cooler element in reverse hooked to a water cooled heat sink with an adsorber on on one side. Anybody know what coating they use on the adsorber? When I tried my carbon dust visible adsorber on the CO2 it burned the PM sensor. The Peltier based PMs can be calibrated by winding a Nichrome loop around the sensor disk, it's 10% accuracy at best, but thats not bad for a home made system. You are measuring the heat flow passing through the peltier, and are not too concerned with the heat sink temperature on its back side.

    Cooling was REALLY critical. The customer's laser had a blocked cooling loop. It would start but then the controller would shut it down. It measured water temp using a thermister in the return line from the cooling jacket. As you pinched the cooling tubing, you can watch the power shoot down as the gas heated up. The heat exchangers were small 3 layer radiators about 1 foot square with a 6" muffin fan behind them and the single coolant pump was rated for 22 liters a minute. Kinda like the postal truck heater cores in surplus catalogs.

    Sharplan 733

    . Electrodes are nickel tubing, 0.6" diameter, .8" long, with beam passing through them (collinear with bore). Electrodes have 1.6" dia 1.8" long section of cooling jacket around them. The remainer of cooling jacket is 0.870" diameter. Bore (best eyeball measurement) is 0.3" diameter with a 3 mm long gap between the electrodes and the start of the bore. The electrodes are surrounded by the water jacket wall with a 0.1 millimeter gap between them the two.

    The distance from inside edge of electrode to inside edge of other electrode is 22 inches.

    A 1.25" diameter 2" long section at each end of the tube has a plastic fitting glued to it with TorrSeal and an O-ring on the face of the butts up against plastic in the mirror mounts that hold the mirror. A clamp slides over the tube and screws into the mount pressing the face of the fitting into the plastic of the mirror mount for a gas tight seal using the oring.

    In an unusual twist, the plastic of the mounts has a metal cooling jacket around it, to chill the plastic and the optics.

    One third of the way down the tube from the anode-end is a metal clamp around the tube about two inches long with a 110 °F Klixon switch on it - the overtemp shutdown sensor. The back side of the clamp is made of PVC to accommodate expansion.

    Gas flow in and out of the tube is right at the outside edge of each electrode, so the gas is flowing through the electrodes, but yet is 2.5" from the optics at each end. Water and gas flow is via 1/4" OD glass tubes sealed to the cooling jacket and bore. these are bent back at right angles to make the tube assembly more compact.

    1 inch diameter optics were used, a Si mirror at the HR, and a ZnSe OC. One the unit I have, someone misconnected the water cooling hose and the ZnSe optic dissolved. :(

    The power supply is a 5.3 kV, 150 mA heavy duty custom wound transformer followed by a full wave bridge and two .1 uF, 10 kV filter caps. At the cathode-end, an Eimac/Varian 3-400Z vacuum tube was used as the current regulator. The card that drove the tube was fried beyond belief or I'd trace the schematic and post it. I suspect this one did much more then 24 watts. :-)

    Sharplan 1060

    The main transformer is 5.3 kV at 220 mA. There are three wires going to the CO2 laser tube, one to the middle one on each end. There are a pair of Eimac 8163/3-400Zs power triode tubes in the high voltage power supply. The two 3-400Zs are ran as grounded grid current sources, two HV transistors for each tube drive the cathodes with something like -12 V. Watch out for the nasty voltage doubler capacitors on the HV tranny! Hams love to buy those 3-400Zs and their sockets if you ever scrap it. The tubes actually act as a constant current circuit and do the overdrive square wave pulses for the "bone digging mode". BTW, stay out of pulse mode if you like to keep the thing running.

    If this is the flowing gas generation, Power is varied by the gas pressure knob (labeled power) with tube current held constant.

    Usually where they die is the Teflon block under the tube that holds the vacuum hoses and gas feed, it cracks from stress and the tube doesn't quite get enough vacuum to start. It should have about 13.8 KV in the HV supply and half that across each part of the tube when no plasma lit.

    If it's a flowing gas laser, change the oil on the vacuum pump immediately and run flushing oil followed by a good grade of vacuum pump oil. They never bother to change that oil and its hard on the pump!

    
                     HV+
                      o
                      |  
              |=======^=======|  Laser Tube
             /                 \
     anode  [+]               [+]  Anode
     HVgnd [---]   3-400Zs   [---] HV Gnd
     -12 V  [=]               [=]  -12 V Constant current control
    
    

    Merrimack 850

    Everybody wants more then 50 watts. Well, here it is. And, compact and light weight too.

    I got a call from the gent whom I buy my used lasers from today, a rather cryptic "Just get up here and bring your tools, Drop whatever your doing, and bring a extra set of safety goggles". He's a man of few words, and that speaks volumes. So I drove the hour and a half. It was worth it. I finally got in something that doesn't weigh 160 pounds for the head alone and is much less then 6 feet long. And - does much more then 50 watts out of a 37 inch tube. It was however expensive. Much more expensive then the scrap Sharplans I've been getting.

    What was almost better then getting the unit was the fact that it came with complete electrical blueprints, the operators manual, and the full engineering and repair data in a 3 ring binder with multiple copies of the 100 or so foldout pages. :-) The critical control circuits are quite simple: opamps and SCRs. It uses a filtered 60 Hz line waveform and comparators to drive 4 SCRs to phase control the transformer primaries for a constant tube current in one leg of the tube with the others track the monitored one.

    What he wanted me to do is fire it up for him, he's never seen a working laser, but the mix tank was empty. :-( I did try to get the tank filled while I was there but the local place wanted $167 dollars for the fill and $2.50 a day for tank rental. As the company that filled this tank was 125 miles away and is not a national brand, the local place would not take the tank in trade or swap it. $170 for an hour of lasing wasn't worth it. Besides, if he knew it works, it would have cost more, if he would have sold it at all. The customer just wanted the electronics and controls. Eventually I'll fire up one for him.

    Alas, although the head and arm was available, the only thing I was allowed to take from the supply was the vacuum pump and the needle valve assembly, the later fortunately being in a remote box for controls that was not sold. I got a empty CO2 tank for my troubles, and a full nitrogen tank. The unit only has 299 hours on it being the prototype for a new medical protocol and thus was not allowed to be used for anything other then the microsurgery study it was designed for. But I also got the pressure versus power data for several tube lengths.

    The design is well, bizarre.

    It's a Merrimack 850.

    There are 3 anodes, 2 cathodes and gas flows from both cathodes and out the center anode. Two transformers power an anode at the end of the tube, and sharing a anode in the middle. The power is rectified and filtered DC, one bridge and filter cap for each transformer. The cathodes are centered between the anodes. Each anode is limited to 25 mA. 4 LEDs in series with one of the the ballast resistors to indicate current via a fiber optic link to a light meter. The supply is CONSTANT current, they vary the gas pressure from 5 torr (starting) to 85 torr, at 65 watts. The power control knob is literally a geared down needle valve with limit stops.

    A array of solenoid valves purges the tube, backfills it with mix before starting, and bypasses the power control valve during starting. It also repressurizes the tube when shutting down and protects it from overpressure. A complex circuit monitors tube pressure and remaining gas, warning when the tank is "down to 200 psi and approximately 30 minutes of lasing at full power remains" and it shuts down at 10 psi above what the regulator is set, about 12 psi. There is a pretty complex array of ballast resistors to equalize the currents. The transformers are 18 kV, drop across the tube is 5 kV starting and 8.8 kV at max current. The total tube current is 85 mA max.

    Tube details:

    Mirrors on flanges with soft indium metal gaskets. No bellows are required as they compress the indium wire to change the mirror position. There are no external mirror mounts - it's supported by two thin plastic blocks, so more like an internal mirror HeNe laser tube. Made in 1984. It has a ceramic bore, internal diameter unknown at this time, surrounded by a large diameter cooling jacket. The outer diameter of the bore looks like 3/16". The mirrors are cooled before the water jacket, a pair of thermisters maintains constant temperature by turning on and off the pump.

    The bore is glassed into the jacket with a glass to ceramic seal. There are several bore sections, probably aligned on a tensioned tungsten wire while the glass was sucked down around the bore on a glass lathe using a weak vacuum and a swivel fitting. there is a diverging lens at the output end, followed by a collimating lens before it goes into the arm, this indicates a high divergence. Its probably a waveguide laser, using the bore walls as bounce paths to extract every possible bit of power, but till I can run a gas that won't lase down it and see what the spot looks like, I wont be able to measure the diameter (WARNING: Do this wearing laser safety goggles, many gases may lase besides CO2 mix!)

    The 37" long tube produces 65 watts when powered by two 20 mA discharges. It will run quite well off two standard 15 kV, 60 mA neon transformers. Power is varied by adjusting the gas flows with a needle valve assembly. The head Includes a power meter sensor. It's a waveguide tube with mirrors sealed on with replaceable indium seals and looks sort of like a 37" long water cooled HeNe laser tube weighing in at only about 2 pounds. (However, it isn't sealed and does need a tank of CO2 laser mix gas and a vacuum pump to run - see below). Laser head is a light weight 4 foot 6 inch long by 6 x 5 inch grey metal case.

    The recommended CO2 laser mix is (listed on side of tank): 8.704% Co2, 15.21% Nitrogen, balance helium. Spec in book is 9:15:balance.

    From the book:

       Tube length (in)    Max Pressure   Start pressure   Pwr-Max     Pwr-Out
      --------------------------------------------------------------------------
             37             98-104 Torr       17 Torr      65 Watts    58 Watts
             44            105-110            13           88          68  
             24             81-87             10           47          38 
             16             79-85              7           31.5        26
             16-low pwr       80               7           27          24 
    
    Pwr-Out is what is delivered to the articulated arm.

    The OC is ZnSe (transmission not specified, no other details available.)

    HR is silver coated germanium with a overcoat on the silver to protect it. Doesn't say the radius, but specifies it as 99.4 % reflective.

    Arm Optics Cleaning (from the Merrimack manual):

    Copper Mirror:

    "Cover the reflecting surface with a mild household detergent (JOY) rub the mirror surface with a clean finger, rinse with distilled water, dry with filtered blown air. Check the surface for spots, discoloration of the mirror will not necessarily impair its reflectance."

    Finger, NOTTTT!!! more like sterile chemwipe!

    "Silicon mirror has soft coating on surface - DO NOT use water. Cover the reflecting surface with absolute (pure) alcohol and drag a lens tissue moistened with alcohol over the surface. Let the mirror dry by evaporation."

    The Coherent Laser Group has a better explanation of drop-and-drag cleaning. Search for "optics cleaning".



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