David Knapp is with the Energy and Minerals Applied Research Center (EMARC) University of Colorado, Boulder - Department of Geological Sciences.
The resonator used is confocal-confocal. Since the optics used in the this project were donated from a company, the author did not have a choice of mirror curvature. Since it was higher power and not best quality mode that was desirable, particular attention was not paid to resonator optimization. With the High Reflectance mirror radius of curvature of 2.685 m, and Output Coupler radius of 5.072 m, the resonator is in the stable resonator regime of 0.66, as determined by the stability relation:
where L is the cavity length, here 65 cm, and R is the radius of curvature of the mirror.
2.2 Energy Transfer in the Discharge
The most commonly observed laser transitions in the CO2 molecule, barring the use of any frequency tuning mechanisms, are from the CO2 asymmetric stretch transitions, from the 00*1 to the 10*0 10.6 micron and 02*0 9.6 micron states, using the notation v1v2*v3, where v1 refers to the symmetric stretch quantum number, v2 refers to the asymmetric stretch quantum number and v3 refers to the asymmetric stretch quantum number. There are literally dozens of other lasing transitions  which can be easily chosen by employing an intracativity grating. In a CO2 laser, lasing of one vibrational transition precludes the efficient lasing of another, so that lasing lines 'hop' from one to another depending on instantaneous gain medium and resonator conditions.
Any single possible laser line can be forced through the use of an intracavity grating. Rotational structure, having energies clustered very close to one another, may exist at any time. Nonradiative decay to short-lived lower lying states followed by nonradiative decay to the ground state follows. N2 is added to the laser gas to more efficiently transfer energy from electron impact to the CO2 upper vibrational laser level. The glow discharge is a very effective mechanism for vibrational excitation of nitrogen. Since N2 is a homonuclear molecule, dipole radiative de-excitation is forbidden. This allows for long-lived vibrational states which makes excited N2 molecules more readily available for collisional excitation of CO2. De-excitation is only accomplished collisionally with the wall or other gas constituents, the most beneficial of which is the CO2 molecule. The N2 v=2 state is only 18 cm^-1 (2.2 E -3 eV) from the upper laser level of the CO2 molecule. This makes resonant energy transfer between N2 and CO2 more likely. This energy is much smaller than the average kinetic energy of the molecules in the surrounding glow, so vibrational energy can easily be supplied to the CO2 molecules. Energy transfer occurs from vibrational levels up to v=4 in N2, because the ensuing anharmonicity of these states, due to bond stretching, is still well below the average molecular kinetic energy . CO is isoelectronic with N2 and also has vibrational levels easily excited in the glow discharge. Figure 1 Details the more common energy transfer routes in the CO2 laser. Excited N2 and CO transfer vibrational energy through collision to CO2, exciting any of a number of stretch and vibrational modes.
Thermal poisoning can occur, which is a build up of lower lasing level populations in CO2. This results in a reduction in laser output power due to a clogging of the path from the upper lasing level to the ground state, where the CO2 upper lasing level is most efficiently populated through collisions with N2. These lower levels are cooled by the addition of He to the gas mix. Helium energy levels are much higher than the molecular energies of N2 and CO2, above 20 eV. For typical electron energies in the glow discharge of 1 to 3 eV, the discharge is not significantly affected by the addition of He, other than to raise the electron temperature of the discharge ; Since the first ionization level of He is higher than that of the other gas components, high energy impacts (higher "voltage") is required to make it part of the glow conducting path. Only a small amount of energy is lost from the discharge due to inelastic collisions with He and subsequent collisions with the walls. Thermal conductivity in gases is independent of pressure and since thermal conductivity of He is roughly six times that of CO2 and N2, He makes an efficient transporter of waste heat to the walls of the discharge tube. The efficiency of heat transfer resulting from the addition of He to the mixture allows for a higher discharge current before radiation saturation . CO may also be added to the laser mix to improve efficiency, but it does not transfer vibrational energy as efficiently as N2, due to a difference between the CO v=1 level and the CO2 upper lasing level of 170 cm^-1. CO also has a dipole moment which creates a radiative decay channel to depopulate the electron impact excited CO, thus making CO less available for the job of CO2 excitation. CO is also a component in the dissociation equilibrium of CO2, so when using added CO with CO oxidation catalysis, larger concentrations of CO affect the CO2 concentrations, not always in a predicatable manner. With these drawbacks, CO still adds to more efficient CO2 vibrational excitation than electron impact alone. H2O can be added as a heat transfer enhancer but is less efficient at cooling than He. H2O, in small concentrations, also has the beneficial side effect of homogeneous catalytic recombination of the dissociated CO2 products, CO and O . H2O in larger concentrations overwhelms the beneficial catalytic effects and effectively depopulates the upper lasing levels of CO2. The optimum concentration of H2O in the laser gas has been shown to be a function of the laser bore diameter . Xe may also be added to a laser gas mix to effectively cool the electron temperature of the discharge for a given current, thereby reducing the amount of electron impact dissociation of CO2. The prohibitive cost of laboratory grade Xe prevented this investigator from utilizing it.
The laser is a sealed, DC discharge type with attached ballast tank for long gas life. The silver-copper cathode design was used to reduce the amount of gas consumption by sputter pumping through chemisorption and physisorption. Construction of the laser was kept simple to reduce expense and excessive consumption of time. The laser is constructed of Pyrex, fabricated on the Boulder Campus by the chemistry department's Master Glass Blower. See Figure 2.
The design incorporates a laser bore nested in a water cooling jacket, with a feed through the water jacket so that the discharge can go to an external cathode and anode. Having the external electrodes lower than the axis of the laser bore reduces the possibility that sputtering or oxidation products at the electrodes will contaminate the optics. This becomes an important consideration when working with CO2 lasers as intracavity power densities, even in a resonator of this design, can easily exceed 100 W/cm^2, quickly causing thermal damage on the surface of an optic, should a small piece of contaminant land on it. A glass vacuum valve is attached for evacuation and filling of the laser mix. Water inlet and outlets are positioned so that air bubbles which form inside the water jacket are ejected as they rise to the top of the tube. Without this design, air buildup inside the jacket would create a radial temperature differential perpendicular to the optical axis, detuning the resonator and/or thermally poisoning the gain medium.
3.2 Mirror Mounts
Mirror mounts were turned out of scrap rolled aluminum from the Physics Instrument Shop. The mounts are of a simple yet versatile design, incorporating an O-ring which provides both the vacuum seal and the restoring force for the mirror adjustment. The finished pieces were sanded with a very fine grained sand paper, then polished, to reduce high surface area materials in the low pressure environment. This reduces the potential of laser gas poisoning due to adsorbed contaminants. See Figure 3. Since the travel on the mirror for optical adjustments is very small, motion of the mirror on the o-ring for mirror adjustment doesn't break the vacuum seal. Both the high reflectance (HR) mirror, and the output coupler (OC) have mirror mount backings with a depression in them to hold the mirror near the center of the optical axis during assembly. With extra room between the glass end of the laser and the mirror mount shoulder, the mount can be aligned coaxially to the laser bore during assembly and the glass blowing need not obey strict tolerances. The mirror mounts need not be mounted exactly perpendicular to the laser bore either, since even relatively gross alignments can be accomplished with the o-ring backing plate.
The mirror mounts were attached to the glass laser tube with the aid of a mandrel, which aligned the mirror mount coaxially with the optical axis. The mandrel consisted of a piece of Delrin turned so that one end fit snugly into the center of the mirror mount, and the other end fit snugly into the laser bore. This alignment procedure was necessary as tight tolerances were not requested for the glass work in order to avoid excessive cost. The laser tube was tipped on end and the mirror mount brought up from beneath with a small lab jack, which then met the laser tube end. After the mandrel had aligned the mirror mount to the axis correctly, it was removed for the next step. Apiezon W vacuum sealing wax was broken into small chips and dropped along the inside edge of the mirror mount, between the glass and the aluminum shoulder. See Fig 3. A heat gun was used to warm the mirror mount so that the sealing wax melted and flowed to form a positive vacuum seal between the mount and the tube. While Apiezon W makes a suitably clean vacuum seal, it does not have adequate structural strength for this application, so five minute epoxy was added on top of the wax, to enhance the mechanical integrity of the mount. Alignment of the laser using a Helium Neon laser required the mounting of the laser on an optical bench. Since the laser itself had been built into an acrylic box to protect it from breakage, and the user from electrical hazards, the entire box was mounted on an optical rail. The laser was secured in an acrylic box, on acrylic mounts lined with Sorbethane shock absorbing material. Aluminum strips were used to secure the ballast tank to its mounts while acrylic pieces screwed in the laser mounts secure the laser tube. For alignment of the laser resonator, two alignment mandrels were again made, each fitting snugly in the ends of the laser bore, but easily passing through the mirror mount.
Each had a 1 mm hole drilled in the center so that coaxial alignment of the HeNe alignment laser with the CO2 resonator was assured. After the HeNe beam was aligned with the CO2 laser bore, the mandrels were removed and the rear reflector was mounted and aligned so that the beam was reflected back into the HeNe aperture. The output coupler was then mounted then aligned to the HR by eye, since trying to make sense of the multiple reflection of the HeNe beam was futile. A bright, quartz halogen light was directed into the OC so that the image of the light was visible on the HR when looking down the bore through the OC. The OC was then adjusted so that the light source created "hall of mirrors" effect between the HR and OC. With a concave-concave resonator and such a high gain medium as CO2, this type of alignment procedure is more than adequate. A simple evacuation and fill system was assembled using Polyflo tubing and Swagelock fittings which used polyethylene ferrules. Industry standard CO2 laser gas mix consisting of 4.5% CO2, 14% N2 and balance He was used for the fill.
3.4 The Cathode
Cathode material selection in a sealed DC laser is crucial. The abundance of free, ionized oxygen in the discharge rules out any materials which readily oxidize, like tungsten, nickel, aluminum or stainless steels. The abundance of CO in the discharge also limits many otherwise suitable materials since many materials form gaseous arbonyls, removing CO2, and exposing fresh cathode surface for oxidation. These carbonyls can then be transported to the optics and deposited, forming a strong IR absorber. The "Hochuli cathode" was found through a literature search on cathode materials conducted earlier by the author. Professor Urs Hochuli generously donated cathodes for this project. The Hochuli cathode was designed specifically for long life sealed DC CO2 lasers, but was only tested at much lower currents, of about 5ma.  The Hochuli cathode is made of Ag and Cu in a matrix which is internally oxidized. This oxygen equilibrium within the cathode allows the Ag and Cu to not consume oxyge n from the discharge, thereby eliminating the cathode as a chemical sink of oxygen. The materials in the cathode are not readily transported in the discharge, so mirror contamination is not a problem. The cathode is in the familiar hollow cathode design, so sputter pumping of gas through physisorption is less of a problem, although this may be the ultimate source of gas consumption by the cathode. A glass plasma limiting sheath is incorporated in the cathode design to prevent the glow from forming on the outside of the cathode. Should that happen, it would rend the hollow cathode design useless, and sputter pump the heavier gas constituents quite efficiently. The discharge is physically channeled to the inside of the cathode, as the sheath acts as a physical plasma limiter. A small rod of tungsten is used as the anode since it is rather easily fed through the glass using a graded Uranium glass to metal seal. The anode doesn't suffer from the high momentum impacting ions as the cathode does, so it does not act as a sputtering loss element. The cathode is the only electrode which suffers from sputtering, since it is the source of electrons, which requires a large number of ions crashing into its surface to form secondary electron emission. This is the mechanism whereby the cold cathode provides electrons .
3.5 Power Supply
The power supply for the laser is of the simplest design. A 15 kV, 60 mA neon sign transformer is used to obtain the high voltage. The output of this is connected to a high voltage bridge rectifier. Since high voltage bridges are expensive and often difficult to find, the bridge was assembled out of several HV diodes in series, one set of three diodes on each leg of the bridge. The output of the b ridge is connected to a HV filtering capacitor, which has a 30M ohm bleeder resistor across its terminals. Since the output of the transformer is center tap grounded, neither end can be tied to ground. consequently, while in operation, all components of the HV end of the power supply are floating with respect to building ground. 200k Ohms of high power ballast resistance is placed in series with the discharge to limit tube current. The tube current is varied rather crudely, by simply using a Variac to adjust the input voltage to the transformer.
The data in Table 1 were taken over tube current and gas pressure before the laser tube had come to complete equilibrium from the turn-on phenomenon described earlier, and without optimizing the resonator each time. Therefor, the peak power obtained in the data does not correspond to the highest power obtained, of 22 Watts. It is quite clear form the data, however, that an optimum pressure and current does exist for maximum power and efficiency. The optimum pressure was arrived at empirically to be 30 Torr. The optimum current was 9.2 mA. Table 1.
Current (ma) Voltage (kV) Power (W) Efficiency
20 Torr 5.3 9.76 1.1 .021 5.7 9.59 .93 .017 6.1 9.55 .22 .004 6.5 9.36 .99 .016After cycling the laser power and allowing for mixing as described earlier, output power was measured as a function of time. Power fluctuations at turn on were found to be due to mode changes. It appears that as the laser warms up, the resonator alignment changes. This can be verified by simply tweaking the mirrors for the lowest order mode while it is warming up, then watching that mode be detuned. The mode can be brought back by simply adjusting the resonator again. Thermal detuning appears to be a plausible explanation since the glass tubes which feed the discharge out the laser bore go through the walls of the tube which make up the laser resonator. As these feeds heat up on the outside due to no water cooling over them, they expand and create a wedging in the resonator structure. The laser can be tuned for best mode after stabilizing. If the laser is allowed to cool, then is turned back and allowed to restabilize, the good mode that was achieved previously will come back. The sup ports the idea of thermal detuning. Power performance as a function of time after turn on is given in Figure 4.
24 Torr 6.9 10.46 1.42 .020 6.5 10.44 1.75 .026 7.0 10.29 2.07 .029 7.6 10.15 1.26 .016 7.8 10.12 .99 .012 8.1 10.03 .17 .002
26 Torr 7.4 10.57 2.94 .037 7.7 10.78 3.49 .044 8.2 10.45 2.62 .031 8.6 10.35 2.18 .024 8.9 10.71 1.75 .018
28 Torr 8.4 11.13 9.80 .015 8.8 11.03 7.62 .079
30 Torr 9.2 11.40 11.43 .109 8.9 11.37 10.34 .102 8.8 11.28 9.53 .096 9.1 11.25 5.45 .053 9.4 11.08 .99 .009
32 Torr 9.0 11.41 3.75 .037 9.1 11.31 3.43 .033 9.3 11.17 1.64 .016 9.7 11.08 .55 .005
Monitoring the mode periodically during the first few minutes at turn on indicates that the power fluctuations are due to the cavity tuning through different modes. In the data shown, the power fluctuations settle down after about 25 minutes and settle to something other than the best power mode. Further measurement has shown this settling behavior to consistently be the case. The spikes in the data are artifacts of a noisy chart recorder. The thermal inertia of the meter does not allow for such a fast response so the spikes cannot be real. The ordinate is one minute per division. Initial variation of 22% of the peak power is seen as the cavity tunes through different modes and the highest power attained is 16.5 Watts. The resonator does not settle to the highest power mode. Vertical sensitivity is 1.17 Watts/division. Typical best power donut or bulls eye laser modes, (TEM 01* or TEM 10) are shown as contrast enhanced burns in thermally sensitive fax paper and plain white cardboard. Other mode burns in cardboard, taken during cycling of the laser during warm up, over the period of about one minutes, shown evolving symmetries corresponding to lower power.