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* Lasers: Safety, Diode Lasers, Helium-Neon Lasers, Drive, Info, Parts *
* *
* A Practical Guide for Experimenters and Hobbyists *
* *
* **** Version 2.62 **** *
* *
* Copyright (C) 1994,1995,1996,1997 *
* Samuel M. Goldwasser *
* Corrections or suggestions to: sam@stdavids.picker.com *
* *
* --- All Rights Reserved --- *
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* Reproduction of this document in whole or in part is permitted *
* if both of the following conditions are satisfied: *
* *
* 1. This notice is included in its entirety at the beginning. *
* 2. There is no charge except to cover the costs of copying. *
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TABLE OF CONTENTS
Introduction - Scope of this document, related information.
Laser Safety - Hazards to vision, other issues, 100W light bulb versus
1 mW laser, safety classifications.
Diode Lasers - Basic considerations, visible and IR (e.g., from CD player)
types, testing, visibility, collimation.
Diode Laser Power Supplies - Drive requirements, modulation, sample circuits
for low and high power devices.
Helium Neon Lasers - Theory (simple), operation, sealed HeNe tubes, testing,
problems, collimation, recharging.
Helium Neon Laser Power Supplies - Tube requirements, PS approaches,
regulation and modulation circuits.
Complete HeNe Laser Power Supply Schematics - 7 AC line and 3 inverter types,
most you can build.
Items of Interest - General Laser Information - Laser power meters, speckle,
Fabry-Perot and DFB, more.
Laser Information Resources - Books, magazines, links.
Laser Parts Sources - Walk-in, mail order, high quality, surplus.
SCOPE OF THIS DOCUMENT:
Both laser diodes and helium neon (HeNe) lasers are popular hobbyist projects.
This document includes information on both - hopefully it will grow in the
future.
Our emphasis is on the care and feeding of these types of lasers. Thus, you
will not find much information on the design of laser shows or holography
experiments. I leave these to the many excellent books and articles that have
been published over the years. However, on-line resources for driving laser
diodes and powering helium neon lasers seem to be scarce. Some of those that
exist are incorrect and potentially dangerous (or at least destructive).
This document was written in the hopes of rectifying this situation.
In addition to basic information, there are 6 sample circuits for driving
laser diodes, 10 *complete* schematics for helium neon laser power supplies,
as well as simple modulators and other useful goodies. Most of these have
been tested and/or came from working commercial designs.
There are also pointers to other web resources, mail order suppliers of laser
parts and equipment, and references on lasers in general.
Contributions are always welcome.
Note: where another 'document' title is referenced without identification of
a specific book or paper, it will be located at the Sci.Electronics.Repair FAQ
web site:
http://www.paranoia.com/~filipg/REPAIR/.
DISCLAIMER:
This document is still under development. Many of the circuits have been
reverse engineered - traced from various schematics or actual hardware. There
may be errors in transcription, interpretation, analysis, or voltage or
current values listed. They are provided solely as the basis for your own
designs and are not guaranteed to be 'plans' that will work for your needs
without some tweaking. We are not responsible for damage to equipment, your
ego, blown parts, county wide power outages, mini black holes, planetary
disruptions, or personal injury that may result from the use of this material.
ACKNOWLEDGEMENTS:
Thanks to Don Klipstein (email: don@misty.com) for his comments and additions to
this document. His Web site (http://www.misty.com/~don/) is a valuable resource
for information relating to lighting and related technology in general.
RELATED INFORMATION:
* See the document: "Notes on the Troubleshooting and Repair of Compact Disc
Players and CDROM Drives" for more info on how the laser diodes in CD players
and CD ROM drives worked originally.
Where the manufacturer and part number for your laser diode are known, by all
means take advantage of the extensive applications information that is likely
to be available. Driving laser diodes without blowing them out is often not
easy - even for an experienced design engineer!
* See the document: "Various Schematics and Diagrams" for a variety of circuits
that may be useful in generating the high voltage for belium neon lasers (in
addition to those found in the chapter: "Complete Helium Neon Laser Power
Supply Schematics".
* See the chapter: "Laser Information Resources" for books, magazine articles,
and links to other laser related web sites.
LASER SAFETY
You only received one set of eyeballs?
Lasers have tended to be high glamor devices popular with with hobbyists,
experimenters, entertainers, and serious researchers alike. However, except
for very low power lasers - those with less than a fraction of a mW of beam
power - they do pose some unique hazards particularly with respect to instant
and permanent damage to vision.
There are several reasons for this even for lasers which do not represent any
sort of burning or fire risk:
* The output of many lasers is a parallel - collimated - beam which means
that not only is the energy concentrated in a small area but the lens of the
eye will focus it to a microscopic point on the retina instantly vaporizing
tissue in much less than the blink of an eye. A collimated beam represents
the rays from an object at infinity so if your eye is focused for distance,
the laser will be in focus as well - to a microscopic point.
The output of a laser pointer or helium neon laser is a collimated beam.
Even at power levels considered relatively safe, one shouldn't deliberately
stare into the beam for any reason. For these relatively low power lasers,
permanent eye damage is not that likely but why take chances? For these
lasers, viewing the spot projected on a white surface is perfectly safe.
* An output of 1 mW may not sound like much compared to a 100 W light bulb
but consider:
A 100 W light bulb puts out about 2 or 3 W of visible light (the rest is
mostly IR and heat) more or less uniformly distributed in all directions.
However, at any reasonable distance from the light bulb, the power density
(e.g., W/sq. mm) is much lower than for a collimated laser beam of even very
low power. And, it takes significant effort to produce any sort of truly
collimated beam from such a non-point source such as is present with even
the filament of a clear light bulb.
For example, at 10 cm from a 100 W bulb (which would be a very uncomfortable
place to be just due to the heat), the power density assuming 3 total watts
of light would be only about .025 mW/sq. mm. At 1 m, it would be only
.00025 mW/sq. mm or 250 mW/sq. m. Based on this back-of-the-envelope
calculation, a 10 mW laser beam spread out to a circular area .2 m in
diameter will be brighter than the 100 W light bulb at 1 m! And, close to
the laser itself, that beam may be only 1 *mm* in diameter and 40,000 times
more intense!
A popular graveyard joke in the laser industry is: "Do not stare into the
beam with your remaining good eye". Nonetheless, laser safety is no laughing
matter.
The most common types of lasers generally available to hobbyists - CD laser
diodes, visible laser diodes, laser pointers, and small HeNe lasers, are all
rated Class II or IIIa. See the section: "Laser safety classification".
Class II lasers should be relatively low risk if even minimal precautions are
taken. However, Class IIIa lasers must be taken much more seriously if the
beam is collimated - as it would be from a laser pointer or HeNe laser tube.
In addition, with helium neon lasers, high voltage power supplies are involved
so there is the added shock hazard resulting from touching or accidentally
coming in contact with uninsulated connections. See the document: "Safety
Guidelines for High Voltage and/or Line Powered Equipment" before working on
any type of equipment which uses line voltage or produces high voltage. Most
of these are quite low power so the actual risk of electrocution from the high
voltage side is relatively small but there may be AC line voltage involved and
there can be collateral damage from a reflex response to the shock. In
addition, a homemade power supply, in particular, may use components which are
grossly oversized for the application (due to low cost availability) like a
15,000 V, 400 W neon sign transformer even though only under 10 W of power is
actually needed (we definitely do NOT recommend this approach).
Furthermore, you may come across a truly high power CO2 or argon ion laser, or
even a 50 mW helium neon tube. These, rated Class IIIb or Class IV, represent
much more significant risks of both instant permanent eye damage even from
momentary reflections from shiny (specular) surfaces as well a very real fire
hazard. In addition there is a very real danger of electrocution from the high
voltage high current power supplies used to power these beasts. Since this
document does not deal with these types of lasers, the essential additional
precautions that must be taken are not covered. However, you must handle them
properly for your own safety and the safety of others around you and your
surroundings.
The following very large number is designed to impress: The power density
of a 1 mW laser beam when focused to a spot of around 2 um (which isn't
difficult with a simple convex lens) is around 250,000,000 W per square meter!
Be Extremely Careful When Working with any laser!
LASER SAFETY CLASSIFICATION:
(From: Richard Trotman (trotman@udel.edu)).
I'm paraphrasing from "Introduction to Lasers", C.O.R.D., 1990:
Class I -- EXEMPT LASERS, considered 'safe' for intrabeam viewing. Visible
beam.
Maximum power less than 0.4 uW.
Class II -- LOW-POWERED VISIBLE (CW) OR HIGH PRF LASERS, won't damage your
eye if viewed momentarily. Visible beam.
Maximum power less than 1 mW for HeNe.
Class IIIa -- MEDIUM POWER LASERS, focused beam can injure the eye.
HeNe power 1.0 to 5.0 mW.
Class IIIb -- MEDIUM POWER LASERS, diffuse reflection is not hazardous,
doesn't present a fire hazard.
Visible Argon laser power 5.0 to 500 mW.
Class IV -- HIGH POWER LASERS, diffuse reflection is hazardous and/or a fire
hazard.
The classifications depend on the wavelength of the light as well.
DIODE LASERS
INTRODUCTION:
Note: throughout this document, we will use the terms "laser diode" and "diode
laser" somewhat interchangeably.
Diode lasers use nearly microscopic chips of Gallium Arsenide or other exotic
semiconductors to generate coherent light in a very small package.
Laser diodes are solid state devices not all that different from LEDs. The
first laser diodes were developed quite early in the history of lasers but it
wasn't until the early 1980s that they became widely available - and their
price dropped accordingly. Now, there are a wide variety - some emitting
*watts* of optical power. The most common types found in common devices like
CD players and laser pointers have an output in the 3 to 5 mW range.
However, unlike LEDs, laser diodes require much greater care in their drive
electronics or else they *will* die - instantly. See the sections on CD and
visible laser diodes, below, before attempting to power or even handle them.
In their favor, laser diodes are very compact - the active element is
about the size of a grain of sand, low power (and low voltage), efficient
(especially compared to the gas lasers they replaced), rugged, and long
lived if treated properly.
They do have some disadvantages in addition to the critical drive requirements.
Optical performance is usually not equal to that of other laser types. In
particular, the coherence length and monochromicity are likely to be inferior.
This is not surprising considering that the laser cavity is a fraction of a
mm in length formed by the junction of the III-V semiconductor between cleaved
faces. Compare this to even the smallest common HeNe laser tubes with about
a 10 cm cavity. Thus, most laser diodes would not be suitable light sources
for holography or interferometry, for example.
However, for many applications, laser diodes are perfectly adequate and their
advantages especially small size, low power, and low cost - far outweigh any
faults. In fact, these 'faults' can prove to be advantageous where the laser
diode is being used as a light source as unwanted speckle and interference
effects are greatly reduced.
The most common types on the planet by far are those used in CD players and
CDROM drives. These produce a (mostly) invisible beam in the near infrared
part of the spectrum at a wavelength of 780 nm. The optical power output
from the raw laser diodes may be up to 5 mW but once it passes through the
optics, what hits the CD is typically in the .3 to 1 mW range.
Visible laser diodes have replaced helium neon lasers in supermarket checkout
UPC scanners and other bar code scanners, laser pointers, patient positioning
devices in medicine (i.e., CT and MRI scanners, radiation treatment planning),
and many other applications. The first visible laser diodes emitted at a
wavelength of around 670 nm in the deep red part of the spectrum. More
recently, 650 nm and 635 nm red-orange laser diodes have dropped in price.
Due to the nonuniformity of the human eye's response, light at 635 nm appears
more than 4 times brighter than the same power at 670 nm. Thus, the newest
laser pointers and other devices benefitting from visibility are using these
newer technology devices. Currently, they are substantially more expensive
than those emitting at 670 nm but that will change as DVDs become popular:
Laser diodes in the 635 to 650 nm range will be used in the much hyped DVD
(Digital Video - or Versatile - Disc) technology, destined to replace CDs
and CDROMs in the next few years. The shorter wavelength compared to 780 nm
is one of several improvements that enable DVDs to store about 8 times (or
more - 4 to 5 GB per layer) the amount of information or video/audio as CDs
(650 MB). A side benefit is that dead DVD players and DVDROM drives (I cannot
wait) will yield very nice visible laser diodes for the experimenter :-).
HOW DO I USE A VISIBLE LASER DIODE?
The quick answer is *very carefully* for two reasons:
I am assuming this is a typical 3 to 5 mW visible laser diode probably
emitting at a wavelength in the 635 to 670 nm range.
1. You can easily destroy the typical laser diode through instantaneous
overcurrent, static discharge, probing them with a VOM, or just looking
at them the wrong way :-).
By far the easiest way to experiment with these devices is to obtain
complete laser diode modules. Versions are available with both the drive
circuitry and (adjustable) collimating optics. They are more expensive
than raw laser diodes but are also virtually foolproof. Inexpensive laser
pointers are one source for similar devices which may be adequate for your
needs but modifying them is probably difficult. See the chapter: "Parts
Sources" for suppliers of both raw laser diodes and laser diode modules.
2. Any time you are working with laser light you need to be careful with
respect to exposure of a beam to your eyes. This is particularly true
if you collimate the beam as this will result in the lens of your eye
bringing it to a sharp focus with possible instantaneous retinal damage.
Typical currents are in the 30-100 mA range at 1.7-2.5 V. However, the power
curve is extremely non-linear. There is a lasing threshold below which there
will be no coherent output (though there may be LED type emission). For a
diode rated at a typical current of 85 mA, the threshold current may be 75 mA.
That 10 mA range is all you have to play with. Go to 86 mA (in this example)
and your laser diode may be history in the blink of an eye.
This is one reason why most applications of laser diodes include optical
sensing to regulate beam power. As the temperature of the laser diode changes
(heats with use), the current requirements change as well.
The third lead is for an optical sensing photodiode used to regulate power
output when used in a feedback circuit which controls your current. This
is very important to achieve any sort of stable long term operation.
You can easily destroy a laser diode by exceeding the safe current even for
an instant. It is critical to the life of the laser diode that under no
circumstances do you exceed the safe current limit even for a microsecond!
Laser diodes are also extremely static sensitive, so take appropriate
precautions when handling and soldering. Also, do not try to test them with
an analog VOM which could on the low ohms scale supply too much current.
It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed or have a laser power meter at your disposal,
you can easily exceed the ratings before you realize it.
You might hear someone bragging "I have driven thousands of laser diodes by
just connecting them to a battery and resistor and never have blown any".
Sure, right. While it is quite possible that the susceptibility to instant
damage due to overcurrent varies with the type of laser diode, unless you know
the precise behavior, you must err on the side of caution. Some designers
have gone to extremes, however. See the section: "Laser diode power supply 2"
for a design with 5 levels of protection!
For testing, see the section: "Testing of low power laser diodes".
For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heatsink if you do not already have the
laser diode mounted on one. See the chapter: "Laser Diode Power Supplies".
The raw beam from a laser diode is generally wedge shaped - 10 x 30 degrees is
a typical divergence. You will need a short focal length convex lens to produce
anything approaching a collimated beam. The optics from a dead CD player (even
though CD players and CDROM drives use infra-red laser diodes, the optics
can likely still be used with visible laser diodes), a low to medium power
microscope objective, or even an old disc camera can provide a lens that may
be entirely suitable for your needs.
CD PLAYER LASER DIODES
The major difference between these and the visible laser diodes discussed
in the section: "How do I use a visible laser diode?" is that the output is
near IR - usually at 780 nm (wavelengths from 400 to 700 nm are generally
considered the visible portion of the electromagnetic spectrum). Therefore,
you must use an IR detector device to even confirm laser emission.
Thus, they make truly lousy laser pointers or laser light shows as the emission
is just barely visible in subdued light. If you hoped for a Star Wars type
laser beam, better go hunting for a 25 W argon laser :-). However, for data or
voice communications, various kinds of scanning or sensing, and electro-optic
applications where visibility is not needed or not desirable, these low cost
sources of coherent light are ideal.
Similar types are found in CDROM drives and CD-R recorders, Minidisc equipment,
newer laserdisc players, magneto-optical drives. Other optical storage
technology uses laser diodes as well. WORM drives, in particular, may use
devices with higher power output - 30 mW or more. High resolution laser
imagers, typesetters, and plotters may use laser diodes producing 150 mW or
more. Take additional precautions if you have a laser diode from one of these
(or don't really know where yours spent its earlier life). There are laser
diodes with optical output measured in watts, though these will not be what
you would call tiny and probably require buss bars for electrical power and
plumbing for cooling!
CD laser diodes are infrared (IR) emitters, usually 780 nm, with a maximum
power output of around 5 mW. There is also a very slightly visible deep red
emission from all those I have seen. This may be a spurious very low power
line in the red part of the spectrum or your eye's response to the near IR
appearing red and about 10,000 times weaker than the actual beam. Despite
what the EM spectrum charts show, the eye's response does not drop off to
zero at exactly 700 nm so there decreasing sensitivity out to 800 nm or beyond
depending on the individual. The main beam is IR and invisible. Take care.
A collimated 5 mW beam is potentially hazardous to your eyes. Don't be misled
into thinking the laser is weak due to the weak appearance of the beam. It is
not supposed to be visible at all!
Typical CD laser optics put out about .3-1 mW at the objective lens though
the diodes themselves may be capable of up to 4 or 5 mW depending on type.
If you saved the optical components, these may be useful in generating a
collimated or focused beam. The aspheric objective lens will be optimized
for producing a diffraction limited spot about 1 to 3 mm from its front
surface when the optical system is used intact.
The optics may include a collimating lens, diffraction grating (to produce the
three beams in a three beam pickup), beam splitter prism or mirror, turning
mirror (for horizontally mounted optics), and focusing (objective) lens.
Older pickups tend to have larger and more substantial sets of optics. Despite
their small size and low cost, these are very high quality optical components.
However, depending on design, some of the parts may be missing or combined
into one component. For example, many Sony pickups do not appear to use a
collimating lens. For pickups with a collimating lens, if the objective
lens is removed, you should get a more or less parallel main beam and two
weaker side beams. Mix and match optics for your needs (if you can get it
apart non-destructively). Where there is no collimating lens, the objective
lens may be used for this purpose if positioned closer to the laser diode.
WARNING: A collimated 5 mW beam is hazardous especially since it is mostly
invisible. By the time you realize you have a problem it will be too late.
The coils around the pickup are used for servo control of focus and tracking
by positioning the objective lens to within less than a um (1/25,400 of an
inch) of optimal based on the return beam reflected from the CD. See the
document: "Notes on the Troubleshooting and Repair of Compact Disc Players
and CDROM Drives" for more information on optical pickup organization and
operation.
Typical drive currents are in the 30 to 100 mA range at 1.7 to 2.5 V. However,
the power curve is quite non-linear (though perhaps not as extreme as the
typical visible laser diode). There is a lasing threshold below which there
will be no coherent output (just IR LED emission). For a diode rated at a
nominal current of 50 mA (typical for Sony pickups, for example), the threshold
current may be 30 mA. This is one reason why most applications of laser diodes
include optical sensing (there is a built in photodiode in the same case as
the laser emitter) to regulate beam power. You can easily destroy a laser
diode by exceeding the safe current even for an instant. It is critical to the
life of the laser diode that under no circumstances do you exceed the safe
current limit even for a microsecond!
Laser diodes are also supposed to be extremely static sensitive, so use
appropriate precautions. Also, do not try to test them with an analog VOM
which in particular could on the low ohms scale supply too much current.
It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed, you can easily exceed the ratings.
For testing, see the section: "Testing of low power laser diodes".
For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heatsink if you do not already have the
laser diode mounted on one. CD laser diodes are designed for continuous
operation. See the chapter: "Laser Diode Power Supplies".
TESTING OF LOW POWER LASER DIODES
If you have pinouts and specifications for your laser diode, these procedures
can be greatly simplified. The following assumes you know nothing about your
device other than that it is a 3 to 5 mW laser diode.
The first step is to identify which pair of terminals are the laser diode and
photodiode. Your laser diode assembly will be configured like one of the
following:
LD LD LD LD
+--|>|--o A +--|>|--o A +--|<|--o A +--|<|--o A
| | | |
C o--+ C o--+ C o--+ C o--+
| PD | PD | PD | PD
+--|>|--o B +--|<|--o B +--|>|--o B +--|<|--o B
If you are leaving the photodiode installed in the optical block, also see the
section: "Reasons to leave the CD laser diode in the optical block" for
sample connections.
The photodiode's forward voltage drop will be in the approximately .7 V range
compared to 1.7-2.5 V for the laser diode. So, for the test below if you get a
forward voltage drop of under a volt, you are on the photodiode leads. If your
voltage goes above 3 V, you have the polarity backwards. Warning: Some laser
diodes have very low reverse voltage ratings and will be destroyed by modest
reverse voltage. Check your spec sheet. However, the laser diodes found inn
CD players seem to be happy with 4 or 5 volts applied in reverse. Of course,
a shorted or open reading could indicate a defective laser diode or photodiode.
The metal case is often one of the terminals, probably C but not always.
If the laser diode is still connected to its circuitry (probably a printed flex
cable), it is likely that the laser diode will have a small capacitor directly
across its terminals and the optical sensing photodiode will be connected to a
resistor or potentiometer. In particular, this is true of Sony pickups and
may help to identify the correct hookup.
Either of the circuits below can be used to identify the proper connections and
polarity and then to drive the laser diode for testing purposes.
* One approach that works for testing is to use a 0 to 10 VDC supply with a
current limiting resistor in series with the diode:
R1 100 ohms 1 W
+ o--------/\/\--------+-----------+--------+
| | |
Power supply C2 + _|_ C2 _|_ __|__ LD1
0 to 10 VDC 10 uF --- .01 uF --- _\_/_ Laser diode
(No overshoot!) - | | |
| | |
- o--------------------+-----------+--------+
If your power supply has a current limiter, set it at 50 or 60 mA to start.
You can always increase it later.
* Alternatively, a fixed supply with a potentiometer can be used:
R2 100 1 W
+ o-----------+ +----/\/\------+-----------+--------+
| | | | |
10 VDC / ^ | C1 +_|_ C2 _|_ __|__ LD1
Power supply \<----+ R1 10 uF --- .01 uF --- _\_/_ Laser diode
(No overshoot!) / 100 ohms - | | |
| 2 W | | |
- o-----------+--------------------+-----------+--------+
R2 limits the maximum current. If you know the specs for your diode, this
is a good idea (and to protect your power supply as well). You can always
reduce its value if your laser diode requires more than about 85 mA (with
R2 = 100 ohms).
The two capacitors provide some filtering to reduce the risk of a transient
blowing the laser diode. C2 should be mounted close to the laser diode.
Before attempting to obtain lasing action with either of these circuits,
monitor the voltage across what you think is the laser diode as you slowly
increase the power supply or potentiometer.
* If you guessed correctly (or have the pinout diagram from the spec sheet
or determined from its former life), the voltage will increase until around
1.5 to 2 V and then climb more slowly. Don't push your luck unless you are
also monitoring the laser diode current and optical output.
* If you are across the laser diode or photodiode in the reverse biased
direction, the voltage will continue to climb above 2 V without slowing.
Don't push your luck here - the breakdown voltage of the laser diode may
be only a little more than this and - you guessed it - exceeding this is
not healthy for the laser diode either.
* If you are on the photodiode in the forward direction, the voltage will get
stuck around .7 V.
Once you have identified the correct connections, monitor the current through
the laser diode as you check for a laser beam.
* For IR laser diodes, you *must* use an IR detector circuit, card, video
camera or camcorder (with the requisite 3 hands) to monitor for an actual
IR laser beam.
Note: If you are trying to use a video camera or camcorder as an IR detector,
confirm its sensitivity to near IR by looking at an active IR remote control
through its viewfinder. It may have a built in IR blocking filter which will
prevent it from being sensitive to IR. This may be removable.
* For visible laser diodes, you can use your eyeballs or any more sophisticated
detector as desired. Look from an oblique angle or better yet, place a white
card a couple of inches in front of the laser diode. Even a 1 mW laser diode
is an intense source of light - there will be no doubt when lasing begins.
Some typical operating currents for laser diodes of various wavelengths are
listed below. THESE ARE JUST EXAMPLES. Your laser diode may have a lower
operating current than the ones listed here! The lasing threshold may be as
little as 5 or 10 mA below the operating current and the operating current may
be 5 mA or less below the maximum current.
Wavelength Operating Current
808 nm 60 - 70 mA
780 nm 45 - 55 mA
670 nm 30 - 35 mA
660 nm 55 - 65 mA
650 nm 65 - 85 mA
640 nm 70 - 90 mA
Of course, if you inherited a bag of identical laser diodes and can afford to
blow one: (1) I could use a few before you do this :-) and (2) you probably
could fairly accurately characterize them by testing one to destruction.
For a current below the lasing threshold for your laser diode, there will be
some emission due to simple LED action. As you slowly increase the current,
at some point (if the laser diode is good) as you exceed the threshold current,
the character of the emission will change dramatically and a very slight
increase laser diode in current will will result in a significant increase in
intensity. Congratulations! The laser diode is lasing.
Caution: unless you have a laser power meter, don't push your luck. The
maximum safe current may be as little as 5% above the lasing threshold. Go
over by 6% and your diode may be history. The exponential power curve seems
to be steeper with visible laser diodes but there is no way to be sure without
specifications. It is all too easy to convert laser diodes into extremely
useless DELDs - Dark Emitting Laser Diodes - or very expensive LEDs.
I have used this approach with laser diodes from dead CD players without
difficulty. In the case of many of these, the operating current is printed
on a sticker on the optical block, often as a 3 digit number representing
the current in 10ths of mAs. Typical values are 35 to 60 mA (350 to 600).
Sony pickups typically average around 50 mA. Without this information, the
best you can do is to estimate when it is lasing at the proper intensity by
comparing the brightness of the 'red dot' one sees by looking into the lens
from a safe distance at an oblique angle. However, this is not very reliable
as the optical power at the objective lens depends on the particular CD player.
REASONS TO LEAVE THE CD LASER DIODE IN THE OPTICAL BLOCK
There are several good reasons to leave your CD laser diode installed in the
optical block assembly even if you are not going to use it with the objective
lens and focus and tracking actuators:
1. The pickup block provides the very important heat sink which is necessary
for continuous operation.
2. There is less risk of damaging it through careless handling and ESD.
3. There may be a collimator lens in there - probably the first or second
optical element in front of the laser diode. It may be combined with the
laser diode in its metal barrel. If there is a collimator, you should be
able to get a nice nearly parallel beam without much work. At most, a
small lens will be needed to optimize it.
Remove the objective (front) lens and its associated coils unless you
require them for a short range application. They will likely come off as
a unit without too much effort. However, try not to destroy this assembly
as you never can tell what might be needed in the future.
4. The multisegment photodiode sensor and focus and tracking actuators may be
useful for a variety of applications.
While there are many variations on the construction of optical pickups even
from the same manufacturer, they all need to perform the same functions so the
internal components are usually quite similar.
Here is the connection diagram for a typical Sony pickup:
_
R1 +---|<|----o A | +----o F+
+-/\/\---o VR | PDA | (
PD1 | | +---|<|----o B | ( Focus
+---|<|--+---+----o PD (sense) | PDB > Focus/ ( coil
| +---|<|----o C | data (
| LD1 | PDC | +----o F-
+---|<|--+--------o LD (drive) +---|<|----o D _|
| _|_ | PDD _ +----o T+
| --- C1 +---|<|----o E | (
| | | PDE > Tracking ( Tracking
+--------+--------o G (common) +---|<|----o F _| ( coil
| PDF (
Laser diode assembly | +----o T-
+----------o K (Bias+)
(includes LD/PD and Focus/tracking
flex cable with C, R). Photodiode chip actuators
The laser diode assembly and photodiode chip connections are typically all on
a single flex cable with 10 to 12 conductors. The actuator connections may
also be included or on a separate 4 conductor flex cable. The signals may
be identified on the circuit board to which they attach with designations
similar to those shown above. The signals A,C and B,D are usually shorted
together near the connector as they are always used in pairs. The laser
current test point, if present, will be near the connections for the laser
diode assembly.
It is usually possible to identify most of these connections with a strong
light and magnifying glass - an patience - by tracing back from the components
on the optical block. The locations of the laser diode assembly and photodiode
array chip are usually easily identified. Some regulation and/or protection
components may also be present.
Note: There are often a pair of solder pads on two adjacent traces. These
can be shorted with a glob of solder (use a grounded soldering iron!) which
will protect the laser diode from ESD or other damage during handling and
testing. This added precaution probably isn't needed but will not hurt. If
these pads are shorted, then there is little risk of damaging the laser diode
and a multimeter (but do not use a VOM on the X1 ohms range if it has one) can
be safely used to identify component connections and polarity.
See the document: "Notes on the Troubleshooting and Repair of Compact Disc
Players and CDROM Drives" for additional information on construction and
testing of optical pickup assemblies.
LASER DIODE LIFE
For all intents and purposes, laser diodes in properly designed circuits do
not degrade significantly during use or when powered on or off. However, it
doesn't take much to blow them (see the sections: "How do I use a visible
laser diode?" and "CD player laser diodes"). I have seen CD players go more
than 10,000 hours with no noticeable change in performance. This doesn't
necessarily mean that the laser diode itself isn't gradually degrading in some
way - just that the automatic power control is still able to compensate fully.
However, this is a lower bound on possible laser diode life span.
Some datasheets list expected lifetimes for laser diodes exceeding 100,000
hours - over 12 years of continuous operation. Of course, I trust these
about as much as the latest disk drive MTBFs of 1 million hours :-).
Laser diodes that fail prematurely were either defective to begin with or,
their driver circuitry was inadequate, or they experience some 'event'
resultling in momentary (greater than a few microseconds) overcurrent.
As noted elsewhere, a weak laser diode is well down on the list of likely
causes for CD player problems.
Of course, in the grand scheme of things, even LEDs gradually lose brightness
with use.
HOW SENSITIVE ARE LASER DIODES, REALLY?
Not all laser diodes are created equal and their susceptibility to damage
through improper handling or improper drive likely varies widely. Here is
a discussion of some of the issues:
From: Eric Rechner (erechner@jetstream.net)).
"Does anyone have any experience with Hitachi laser diode HL7843MG 5 mW 780nm?
I find this diode to be possibly extremely sensitive (ESD??), more so than
any other 780nm laser diode. Does anyone know if there are problems with
Hitachi MQW type diodes? Are MQW diodes more sensitive to ESD than Double
Heterojunction diodes? Does anyone have info on possibly 'bad' or defective
lasers out there?"
(From: Jon Elson (jmelson@artsci.wustl.edu)).
Strange. I think I've used some of these.
I hear everybody babbling about extreme static sensitivity on these devices,
yet I've never had a failure, and I've been using just the usual minimum
precautions with any semiconductor device. I suspect that people may be
exceeding the optical power MAXIMUMS on the devices. I've been very
conservative on that, since the devices only carry an optical maximum, and
don't have that correlated to forward diode current (difficult, because it
varies strongly with temperature). I try to run them at a good bit less than
rated power, maybe 2-3 mW optical output. I'm using a diode sold by Digi-Key
for $19.00, just because it is cheaper than the Panasonic in the 5.4 mm case.
I think the manufacturer is NVG or something like that. I've got 10 of them I
am working with, designing a closed-loop driver for a photoplotter, which
pulses the lasers on and off as fast as 10 uS on, 10 uS off. It is working
pretty well now. I included a series resistor (as well as the control
transistor), so that if the loop becomes unstable or the sensing diode gets
disconnected, it won't fry the laser diode.
(From: Dr. Mark W. Lund (lundm@xray.byu.edu)).
The babbling starts here: You don't have to be a total idiot to blow these
things, in fact I have blown a few myself. Identifying the source of the
trouble is extremely costly and difficult because it only takes a spike of a
few nS to to the damage. I would say that 99.9999% of the time it is the
power supply. Either it spikes on turn-on, turn-off, or at random. We used
to toast lasers with a $5,000 laser diode power supply that would spike every
time you sent certain signals on the IEEE 488 control line. This was a tough
one to figure out, I can tell you. In the process we tried to damage one
using static to try to get a handle on the sensitivity, but were not able to
get a catastrophic failure this way (we may have induced some latent failures,
however). Other laser diodes may vary.
(From: Jon Elson (jmelson@artsci.wustl.edu)).
Ah! This is good anecdotal evidence! I've often suspected that there might
be more of this going on, and instead of examining the drivers, people just
attribute problems to an invisible gremlin! I sure can see how a closed
circuit driver can oscillate or overshoot on transients, and there could be a
situation where some percentage of drivers will be less stable due to
component tolerances. Unless you rigorously test a good batch of your
drivers, you could have this sort of thing and not know it. (Of course, any
time you put a computer in the loop, especially one that is canned inside
an instrument, then the probability of unanticipated gremlins increases
dramatically!).
Of course, I was designing a fixed-purpose driver to be used in a specific
application, inside an instrument, so I had it easier than the guys designing
a lab-quality pulser for who knows what application. So, I could put in a
resistor, which will limit current to some 'safe' level, even if the loop is
unstable, which it certainly was when I was tuning up my driver.
I DO use generally sound anti-static precautions, almost subconsciously, to
protect all semiconductor devices. But, I am aware that I have occasionally,
by accident, touched a cable going to the laser diode before I was grounded,
and I have never noted a catastrophic failure.
I will have to go through some rigorous life-testing to make sure I'm not
causing latent failures, but I've run these diodes for quite a few hours while
testing things, and nothing of note has turned up yet.
By babbling, I meant some items in print media, as well as a lot on this and
other newsgroups, indicating that if you even touch one lead of a diode laser,
it is ABSOLUTELY destroyed, with a probability of 1.000! Obviously not true!
Your comments are well reasoned, and indicate real experience. Others have
also written that only a huge corporation, with millions in test equipment,
could ever make their own laser diode driver. Now, clearly, the nanosecond
multi-watt pulsers ARE much more difficult to do right, fast risetimes without
overshoot is tricky. But, I did it in my basement with just over $1,000 in
test equipment, mostly a decent oscilloscope. I also had the confidence that
if I DID blow a few diodes, it wasn't so painful at $19 each.
So, now, I'm babbling!
IR DETECTOR CIRCUIT
This IR Detector may be used for testing of IR remote controls, CD player
laser diodes, and other low level near IR emitters.
Component values are not critical. Purchase photodiode sensitive to near IR
(750-900 um) or salvage from opto-coupler or photosensor. Dead computer
mice, not the furry kind, usually contain IR sensitive photodiodes. For
convenience, use a 9V battery for power. Even a weak one will work fine.
Construct so that LED does not illuminate the photodiode!
The detected signal may be monitored across the transistor with an
oscilloscope.
Vcc (+9 V) o-------+---------+
| |
| \
/ / R3
\ R1 \ 500
/ 3.3K /
\ __|__
| _\_/_ LED1 Visible LED
__|__ |
IR ----< _/_\_ PD1 +--------o Scope monitor point
Sensor | |
Photodiode | B |/ C
+-------| Q1 2N3904
| |\ E
\ |
/ R2 +--------o GND
\ 27K |
/ |
| |
GND o--------+---------+
_|_
-
DIVERGENCE OF LASER DIODES
(Portions from: Mark W. Lund (lundm@physc1.byu.edu)).
The divergence specification for laser diodes is measured to the half power
points. T full width at the 10% level may be more like 70 or 80 degrees than
the 30 degrees in the specifications.
A simple short focal length lens will collimate the beam. However, laser
diodes tend to be astigmatic which means that you will have one axis
collimated at a different focus than the other. A typical value for this
astigmatism is 40 microns. A cylindrical lens in addition to the spherical
collimating lens or a special lens designed for this purpose can correct this
but may not be needed for non-critical applications.
Any camera lens will be able to produce a reasonably well collimated beam
(subject to the astigmatism mentioned above). Put the laser diode it at the
focal point of the lens. If you want the type of narrow beam produced by a
HeNe laser, you need a short focal length lens, such as a microscope
objective. A good compromise between cheap and short focal length would be
an old disk camera lens. These cameras can be found at thrift shops, garage
or yard sales, and flea markets for a couple dollars or less.
The longer the focal length the larger your beam will be, but the less effect
the astigmatism will have. The diameter of the beam will be the size of the
aperture of the lens (in which case you are throwing away light) or the size
of the beam at the distance of one focal length, whichever is less.
WHY CAN'T AN LED BE FOCUSED LIKE A LASER DIODE?
The cheap laser diode from a CD player can be focused to a spot less than
2 um in diameter. Why is this not possible with an LED?
The quick answer is that an LED does not appear as a point source and has
as effective emitting area which is huge compared to a laser diode. Even
though the emitting area of a laser diode is not a point, due to the way the
laser beam is generated - collimation wise - it appears as a point source.
And, a point source can be focused to another point.
The effective emitting area of an LED is perhaps .25 x .25 mm. To focus
an incoherent source like this to a 2 um spot with imaging optics would
require a ratio of distances of roughly 125:1 for the LED-to-lens compared
to the lens-to-image plane.
With any kind of real world optics, you will get a vanishingly small amount
of power at the image plane. Similarly, an LED beam cannot be cleaned up
with a spatial filter (pinhole) as very little of the beam will make it
through.
The laser diode is coherent and monochromatic (enough) that relatively simple
optics can be used to focus it to a spot smaller than 2 um. While the
dimensions of the laser diode chip are not all that much different from the
LED, the characteristics of the laser emission makes such focusing a
relatively easy task.
Consider that the beam from a HeNe or ruby laser doesn't come from point
source either. The beam can be sharply focussed because it is very well
collimated.
The availability of relatively cheap laser diodes really was the enabling
technology for the CD revolution.
(From: Steve Nosko (q10706@email.mot.com)).
If a beam of light has nothing but *precisely* parallel rays, it can be
focused to a point. Also, if the beam originated from a point, a lens will
focus it to a point.
An LED has neither of these. First, it is an area source and light coming
from that surface is not parallel. It would also be called a diffuse source,
meaning light from all places on the surface travels in many directions. This
kind of source can not be focused to anything but a smaller image of itself.
The shorter the focal length of the lens, the smaller the image - but it is
still an image of the source, not a spot. It is because of these rays,
traveling in different directions, that a lens can't focus them all to the
same point. If you draw the side view of a lens and trace rays this all
should be obvious.
The gas laser, on the other hand, has rays which are much much closer to being
parallel. The diode laser has rays which appear to come from an apparent point
inside the diode.
There are two more subtle effects. One effect is the relatively wide range of
wavelengths in the LED versus the narrow range of a laser. Simple optics don't
focus all wavelengths at the same focal length. So the wide bandwidth of the
LED causes a little trouble. There is another effect having to do with the
size of the lens (diffraction limit) and the wavelength, but this is also
secondary to an understanding of the *primary* reason why an LED can't be
focused. I'll only talk about the largest effect due to the extended, non
collimated source.
One thing to note is that the laser diode actually has two apparent point
sources. One for the wide axis of the beam and another for the narrow axis.
This means that the lens must be more like two crossed cylindrical lenses with
different focal lengths. There are different types of laser diodes with
varying degrees of this so that some are easier to to design lenses for.
There probably are types, by now, where there aren't two.
I think of it like this (right or wrong). The astigmatism has two components.
One is the difference in divergence between the two axes. I think this can be
even if there is ONLY one apparent point source. It is just a point source
with an oval aperture letting light through. The other is the different
apparent point sources for the two axes.
COMMENTS ON DRIVING LASER DIODES WITHOUT OPTICAL FEEDBACK
(From: Dwight Elvey (elvey@civic.hal.com)).
If you intend to use the laser without the feedback, one has to realize that
there are a number of problems. One is that as the temperature goes down, the
laser efficiency goes up. This tends to cause the laser diode to destroy itself
at lower temperatures while running that same current that was OK at some
higher temperature. Generally, if the temperature doesn't vary to much, one
can use something as simple as a limiting resistor and not run the laser at its
highest output. I once made a burn-in driver for some power lasers that used
constant current sources that had no feed back but I had to preheat the diodes
to 100 degrees C before using that high a level of current. The level of
current used would have wiped the diodes out at room temperatures.
The hardest part of the whole thing was making the circuit to have controlled
levels of current during power on and power off. Most things like op-amps are
not specified under these conditions. My first attempt wiped out 10 diodes :-(
when I turned the power on.
To run the diodes at there maximum light out safely, requires using the feed
back photo diode.
VISIBILITY OF NEAR-IR (NIR) LASER DIODES
The following describes an interesting and convincing experiment. I would
tend to believe these results concluding that the visible light from a CD
laser diode is probably a spurious emission rather than the human eye's
weak sensitivity to 780 nm radiation. The fact that the red emission was
undiminished even after the laser diodes were damaged by overcurrent is
further confirmation of these conclusions. If the red is a spurious
(LED-like) emission, it should appear below the laser threshold suggesting
another test.
(From: Kjell Kraakenes (kkraaken@telepost.no)).
I once used 780 nm laser diodes similar to the types used in CD players, and
something that puzzled me was that I was able to see some red radiation from
these diodes. I used a microscope objective to focus the light on a wall a
few meters away, and when properly focused, a red spot was visible to the
naked eye. I had a piece of black card board on the wall, and there was no
specular reflection. I used an IR viewer of the type sold by Edmund
Scientific (Find-R-Scope), and if I looked at the spot with this IR viewer
the beam appeared defocused. By adjusting the distance between the laser diode
and the microscope objective, the spot (as it appeared through the IR viewer)
could be brought to a better focus. The red, visible light was then so much
defocused that it was no longer visible to the naked eye. From these
observations, I assumed that the spot I saw through the IR viewer was the
laser emission at 780 nm, and that the visible light was some weak emission
at a shorter wavelength. Because of the chromatic aberrations in the
microscope objective these two wavelength could not be expected to be in focus
simultaneously. I did not notice whether the distance between the laser diode
and the microscope objective was increased or decreased when shifting between
the focus of the visible and the IR light, but since I did not know the
chromatic aberrations of the microscope objective this information would not
help me.
I damaged a few of these laser diodes. Probably by burning one of the facets
such that the lasing threshold was increased. Electrically they were OK, and
the visible output appeared as intense as before, but the total output was
only a few microwatts.
I therefore believe that the light people see from NIR laser diodes is
spurious emission within the visible band, and not intense NIR radiation.
(From: Don Klipstein (Don@Misty.com)).
Some nominally IR wavelengths are indeed very slightly visible. In favorable
conditions (mainly isolating from more visible wavelengths) I have seen with
my own eyes:
1. The 766.49/769.9 nM potassium lines, as a contaminant in high pressure
sodium lamps.
2. The 818.3/819.5 nM sodium lines in the spectra of high pressure sodium
lamps.
3. The 762.1, 759.4, and 822.85 nM earth atmospheric absorption lines in the
solar spectrum. (Usually with the sun somewhat low.)
4. The output of a laser diode in my CD player is visible at eye-safe
intensities (half a meter from a source with a beam covering nearly a
steradian for a few seconds). I have seen the spectrum of this along with
that of a neon lamp placed next to it, and verified that what I saw was
the laser line, with a wavelength around 800 nM. It could be as low as
around 780 nM.
According to the C.I.E. "Y" or visibility function (or extrapolation thereof),
the visibility of these lines is impressively low. However, considering the
wide dynamic range of the human eye, these wavelengths are visible at eye-safe
levels.
CAUTION: there is no advance warning of having exceeded eye-safe exposure to
slightly visible wavelengths normally considered IR. You may permanently toast
part of your retinas duplicating the above unless you verify retinal exposure
below the Class I laser exposure limit.
I recently got a laser pointer with a wavelength of 660-661 nm or so and
(guesstimated) 2 mW of output power.
I discovered that if I shine the beam through one of those dielectric
interference bandpass filters, I got some weak beam output at other
wavelengths. So, I investigated further.
About (very roughly estimated from standard issue eyeballs) .2 percent of the
beam is spurious radiation with a continuous spectrum. I don't yet know well
what it does at longer wavelengths, but a majority of the short wavelength
side of this is in the few tens of nm below 660 nm. Slight traces exist down
to 540 nm. With two 532 nm filters, I could stare into the beam and see a dim
point of light. With a 570 nm filter, it was slightly bright to stare into
and I could see the beam VERY DIMLY on a wall in a dark room. With a filter
around 630 nm, I could easily see the beam on a wall in a dark room. I used
my diffraction grating to verify that most of this was continuous spectrum in
the passband of the filter.
The spurious radiation takes the same path that the laser radiation does.
With no filter, I could not see any continuous spectrum with my diffraction
grating. The laser line was so much stronger.
As for IR lasers? If the spectrum is just a long-shifted version of what my
visible laser does, the most visible part of the laser output would be the
laser line. Having a wavelength 100 nm closer to visible increases its
visibility only by about a factor of 1,000 and the total spurious output was
(roughly) 1/1,000 of the laser line output. The wavelength of the bulk of
this was nowhere near 100 nm shorter.
Although I can't be sure this would always be the case, the only spectrum
components I could see using a diffraction grating with my CD player
laser was the laser line at about 800 nm.
I suspect different IR laser diodes may have greatly different ratios of laser
and LED output. If the LED output is only a fraction of a percent of the laser
output, the visible output would be mainly the slightly visible laser line. If
the LED output is equal to a few percent or more of the laser output, then it
may be more visible than the laser line.
WHEN WILL WE SEE GREEN AND BLUE DIODE LASERS?
(Portions from: Adam Cohen (adc20@eng.cam.ac.uk)).
Blue and green has been widely demonstrated by SHG (second harmonic generation
a.k.a. frequency doubling) in nonlinear crystals (lithium niobate, KTP et
al.), organic nonlinear materials, etc. etc.
The direct emission from a semiconductor has been the Holy Grail for several
years. The semiconductor materials available with a sufficiently wide
band-gap are notoriously difficult to deposit and cleave....But several groups
are close to a commercial device now. In Japan, Nichia Chemicals, Sony,
Pioneer and Toshiba (see p26 of Laser Focus World, March 1997) are all working
on GaN-based devices (active layer in the Toshiba device is actually InGaN). I
think 3M and some other US firms were concentrating on ZnSe, which emits at a
slightly longer wavelength (more blue-green than blue)....
LASER DIODE POWER SUPPLIES
LASER DIODE DRIVE REQUIREMENTS
The following must be achieved to properly drive a laser diode and not ruin
it in short order:
* Absolute current limiting. This includes immunity to power line transients
as well as those that may occur during power-on and power-off cycling. The
parameters of many electronic components like ICs are rarely specified
during periods of changing input power. Special laser diode drive chips
are available which meet these requirements but a common op-amp may not be
suitable without extreme care in circuit design if at all.
* Current regulation. Efficiency and optical power output of a laser diode
goes up with decreasing temperature. This means that without optical
feedback, a laser diode switched on and adjusted at room temperature will
have reduced output once it warms up. Conversely, if the current is set up
after the laser diode has warmed up, it will likely blow out the next time
it is switched on at room temperature.
Note that the damage due to improper drive is not only due to thermal effects
(though overheating is also possible) but due to exceeding the maximum optical
power density at one of the end facets - and thus the nearly instantaneous
nature of the risk.
Many semiconductor manufacturers offer laser driver chips. Some of these
support high bit rate modulation in addition to providing the constant current
optically stabilized power supply. Other types of chips including linear
and switching regulators can be easily adapted to laser diode applications
in many cases:
* Maxim (http://www.maxim-ic.com/).
The MAX3261 (1.2 Gbps) and MAX3263 (155 Mbps) laser driver driver chips
are two examples of their highly integrated solutions.
* Linear Technology (http://www.linear-tech.com/).
App Note AN52 (and probably others) includes a sample circuit using their
one of their chips (not necessary dedicated laser drivers) for powering
laser diodes. In AN52, the LT1110 Micropower DC-DC converter is used as
the current regulator for operating from a 1.5 V battery.
* Both Sharp and Mitsubishi manufacture IC's for driving laser diodes. Most
will maintain constant power. Some require two voltages, others just one.
These circuits will drive the common cathode lasers, or the Sharp "P" or the
Mitsubishi "R" configuration which has the laser's cathode connected the the
anode of the photo diode. The Sharp IR3C07 is a good for CW or analog
modulation, and the IR3C08 or IR3C09 will allow digital modulation to 10
MHz. These parts are quite inexpensive.
* Analog Devices (http://www.analog.com/) has several laser diode drivers
including the AD9660 and AD9661 both of which provide for full current
control using the photodiode for feedback and permit high speed modulation
between two power levels.
* Burr-Brown (http://www.burr-brown.com/):
(From: Steve White (stevew@hitl.washington.edu)).
We are using the OPA 2662 (Burr-Brown) for this. It is an OTA with 370MHz BW,
59mA/ns SR, and can source/sink 75mA of current per channel (two channels per
chip which may be paralleled quite easily). The part provides the emitter of
the current source to an external pin (programming side of an internal
current mirror), so that a single resistor sets the voltage-current transfer
characteristic. Watch out for the dependence of the harmonic distortion specs
upon the supplied current and frequency though...if this will be a problem
for your particular application that is (didn't matter much for mine).
VISIBLE LASER DIODE POWER SUPPLIES (reverse engineered from commercial units)
These circuits were traced from commercial CW laser lights (these were used
for positioning in medical applications). Errors may have been made in the
transcription. The type and specifications for the laser diode assembly (LD
and PD) are unknown. The available output power of both of these lasers is
about 1 mW but the circuits should be suitable for the typical 3 to 5 mW
visible laser diode (assuming the same polarity of LD and PD or with suitable
modifications for different polarity units.)
If you do build these or any other circuits for driving a laser diode, test
them first with a combination of a visible LED and silicon diodes (to simulate
the approximate expected voltage drop) and a discrete photodiode to verify the
current limited operation. Them with the laser diode in place, start with a
low voltage supply until you have determined optimal settings and work up
gradually. Laser Diodes are NOT very forgiving.
LASER DIODE POWER SUPPLY 1
This one runs off a (wall adapter) power supply from about 6 to 9 V.
D1
Vcc o-----|>|-------+------------+-----------------+-----+--------+
1N4001 | | | | |
Rev. Prot. | | Power Adjust | _|_ __|__
| / R3 10K (2) | PD /_\ LD _\_/_
| R2 \ +----+ | | |
| 560 / | | +-----|---||---+
| \ +---/\/\--+-------+ C4 |
| | | | .1 uF |
| | | +----||----+ /
+_|_ | | __|__ C2 (1)| \ R4
C1 --- | | E / \ 100 pF| / 3.9
10 uF - | +-----|------' Q1 '-------+ |
| |R | BC328-25 (5) | |
| +---+ | (PNP) | |/ Q2 (5)
| | _|_. | +---| BD139
| VR1 +-'/_\ | | |\ (NPN)
| LM431 | | C3 +_|_ E|
| 2.5 V | | 10 uF --- |
| (3) | |X - | |
R1 3.9 | | |Y | |
GND o----/\/\/\-----+------------+-----+--------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
Notes:
1. Capacitor C4 value estimated.
2. Potentiometer R3 measured at 6K.
3. LM431 shunt regulator set up as 2.5 V zener.
4. Supply current measured at 150 mA (includes power on LED not shown).
5. Transistor types do not appear to be critical.
LASER DIODE POWER SUPPLY 2
This one, from the same manufacturer as the one described in the section:
"Laser diode power supply 1", seems to be an improved design including a
soft-start (ramp-up) circuit and an inductor in series with the laser diode.
Otherwise, it is virtually identical and runs off of a 6 to 9 V DC source.
Since both units were from the same company, I assume that these refinements
were added as a result of reliability problems with the previous design - in
fact, I have recently discovered that the unit from which I traced that
schematic is not as bright as the one below!
Interestingly, there is no longer any reverse polarity input protection - I
don't know why that would have been removed! C1 and Q1, at least, would
likely let their smoke out if this circuit was hooked up backwards.
2SC517 (NPN) (6)
Vcc o-----+--. Q1 .---+---------+---------------------+-----+--------+----+
| _\___/_E | | | | | |
| | | | | _|_ __|__ \ R5
R1 \ | | | | PD /_\ LD _\_/_ / 1K
3.3K / | | / | | | \
\ | | R2 \ | | | |
| | | 390 / R3 +-----|---||---+----+
| | | \ +---/\/\------+-------+ C4 (2) |
+-----+ | | | 2.2K | 10 pF +
| | | | +----||----+ )
| +_|_ C2 | | __|__ C3 (1) | ) L1
| --- 33 uF | | R4 E / \ 47 pF | ) (3)
| - | +-----|--/\/\----' Q2 '-------+ +
| | |R | 220 BC328-25 (6) | |
C1 +_|_ | +---+ \ (PNP) | |/ Q3 (6)
1 uF --- | | _|_. /<-+ R6 +---| BD139
- | | VR1 +-'/_\ \ | 10K | |\ (NPN)
| | LM431 | | | Power Adjist C5 +_|_ E|
| | 2.5 V | +--+ (4) 10 uF --- |
| | (5) | |X - | |
| | | |Y | |
GND o-----------+------+---------+-----+------------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
Notes:
1. Capacitor C3 was marked n47 and very small. Guessing 47 pF.
2. Capacitor C4 was marked 10n and very small. Guessing 10 pF.
3. Inductor marked Red-Black-Black-Silver ??.
4. Potentiometer R6 setting not measured.
5. LM431 shunt regulator set up as 2.5 V zener.
6. Transistor types do not appear to be critical.
LASER DIODE POWER SUPPLY 3
This one runs off of a (wall adapter) power supply from about 10 to 15 V
(12 V nominal).
It was apparently designed by someone who was totally obsessed with protecting
the laser diode from all outside influences - as one should be but there are
limits :-). This one goes to extremes as there are 5 levels of protection:
1. Input C-L-C filter.
2. Soft start circuit (slow voltage ramp up).
3. LM7810 voltage regulator.
4. LT1054 DC-DC voltage converter.
5. Optical power based current source.
The first part of the circuit consists of the input filter, soft start circuit,
voltage regulator, and DC-DC voltage converter. Its output should be s super
clean, filtered, despiked, regulated, smoothed, massaged source of -10 V ;-).
MPSA13
L1 D1 C E I +--------+ O -10 V out
+12 o--+--CCCC--+--|>|--+--. Q1 .---+--| LM7810 |--+-------+ o
| |1N4002 | _\___/_ | +--------+ | | C5 |
| | R4 / | | C| | | +------+
| | 10K \ | | | | 8| 7 6 5| 180 |
| | / | | | | +-+--+--+--+-+ uF |
+_|_ C10 +_|_ C11 | | +_|_ C8 | C7 _|_ | |16 V|
--- 2.2 --- 2.2 +-----+ --- .22 | .1 --- | LT1054 | +_|_
- | uF - | uF | | - | uF | uF | | | ---
| | +_|_ _|_ | | | +-+--+--+--+-+ - |
| | C9 --- --- C6 | | | 1 2| 3| 4| C3 |
| | 4.7 - | | .047 | +------+-------------+--|--||--+
| L2 | uF | | uF | | C4 |+ - |.01 uF|
Gnd o--+--CCCC--+-------------+------+-------+ 180 uF +-|(--+------+
16 V
It was not possible to determine the values of L1 and L2 other than to measure
their DC resistance - 4.3 ohms. The LT1054 (Linear Technology) is a 'Switched
Capacitor Voltage Converter with Regulator' running at a 25 KHz switching
frequency. A full datasheet is available at http://www.linear-tech.com/.
The input to the LM7810 ramps up with a time constant of about 50 ms (R4
charging C9). This is regulated by the LM7810.
The LT1054 takes the regulated 10 V input and creates a regulated -10 V output.
There is no obvious reason for using this part except the desire to isolate
the laser diode as completely as possible from outside influences. Like the
use of an Uninterruptible Power Source (UPS) to protect computer equipment from
power surges, a DC-DC converter will similarly isolate the laser diode circuit
from any noise or spikes on its input.
The second part of the circuit is virtually identical to that described in
the section: "Laser diode power supply 1":
Gnd o--------+------------+-----------------+-----+--------+
| | | | |
| | Power Adjust | _|_ __|__
| / R2 20K | PD /_\ LD _\_/_
| R1 \ +----+ | | |
| 470 / | | +-----|---||---+
| \ +---/\/\--+-------+ C2 |
| | | | /
+_|_ | | __|__ \ Rx
C1 --- | | E / \ C /
10 uF - | +-----|------' Q1 '-------+ |
| |R | PN2907 | C|
| | \ (PNP) | |/ Q2
| _|_. / R3 +---| PN2222
| VR1 '/_\ \ 1K | |\ (NPN)
| LM385 | / C1 +_|_ E|
| Z2.5 | | 10 uF --- |
| | |X 16 V - | |
| | |Y | |
+V o---------+------------+-----+--------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
I suspect that there are additional components inside the laser diode assembly
itself (like the hypothetical Rx, probably a few ohms) but could not identify
anything since it is totally potted.
LASER DIODE MODULATION
Neither of these designs can be modulated at any reasonable rate without
modifications to reduce the heavy filter capacitance at multiple locations.
However, in principle, this should be straightforward. Since both the
following affect the optical feedback, attempt at your own risk.
A bi-level modulation scheme could be easily implemented by connecting a
general purpose NPN transistor across an additional resistor (at point XY).
Then, full power will be achieved with the transistor turned on and reduced
power with it turned off. Select a value for R2 that will still maintain
the current above the lasing threshold - 1K is just a start.
X
o
|
+-----+
|C | Typical transistors: 2N2222, 2N3904.
R1 |/ /
TTL Input o---/\/\---| Q1 \ R2
1K |\ / 1K
|E |
Y o------------+-----+
Here is another circuit which should achieve somewhat linear control of laser
power since optical power output should be proportional to photodiode current.
Resistor values shown are just a start - you will need to determine these for
your specific laser diode and operating point.
R
o
|
\ R1 X
/ 10K o
\ |
C1 10 uF | |/ C
o------)|----+--------| Q1
- + | |\ E
Line level | 2N3904 |
audio / /
R2 \ R3 \
o 10K / 1K /
| | |
Y o-----------+------------+----------+
SIMPLE LASER DIODE POWER SUPPLY:
(From: Brian Mork (mork@usa.net)).
Best circuit I've found:
In +-------+ Out 18 ohm*
(+) o-----+---| LM317 |-------/\/\/\----+-----+------o LD anode
| +-------+ | |
_|_ | Adjust | _|_ __|__
22 uF --- +---------------------+ --- 1 uF _\_/_
| | |
| |
(-) o-----+-----------------------------------+------o LD cathode
* Note: Resistor value depends on your specific laser diode current
requirements. Discussion below assumes a laser diode with a 72 to
100 mA drive range --- sam.
Power is 5.5 to 9 VDC. I use a 9 volt battery.
Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic
transistor type case) and the LM317T (TO-220 7805-type case), the pins are,
left to right, Adjust-Output-Input.
For the resistor, I use a small carbon 10 ohm in series with a precision
10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.
On initial power on, use three garden variety diodes stacked in series
instead of the laser diode. Put a current meter in series with the diode
stack and adjust the precision resistor for 50-60 mA. Disconnect power and
replace the diode stack with the laser diode. Connect up power again, still
watching on the current meter. The diode will probably initially glow
dimly. I use a diode that lases at about 72 mA, and has a max rating of 100
mA. I use about 85 mA for normal ops.
Turn up the current, never exceeding your diode's max limit. The dim glow
will increase in intensity, but at some point, a distinctive step in
intensity will occur. Your diode is lasing. Remove the current meter as
desired. Enjoy!
CONSTANT CURRENT SUPPLY FOR HIGH POWER LASER DIODES:
(From: Winfield Hill (hill@rowland.org)).
The schematic in the section: "Laser Diode power supply" is the standard
circuit for making a constant current source from an LM317 or LM338 (e.g. see
The Art of Electronics, fig 6.38). The problem with this circuit is that for
large currents (the only currents for which it has good accuracy, and is a
serious part saver) it's hard to make the current variable.
For example, for a 3.5 A current source, the resistor value is 0.357 ohms,
and if you then want a 3.1 A current you've got to unsolder it and replace
it with a 0.403 ohm resistor. Bummer.
One option would be to put a low value pot across the sense resistor and
connect its tap to the voltage regulator common/adjust terminal. This will
work reasonably well for a modest current range - perhaps up to 2:1 as shown
below - but runs into difficulties where a wide range of control is desired.
In +-------+ Out R1 1.01 ohm
Vin o----| LM317 |---+-----------/\/\----+----o 1.25 to 2.5 A current source
+-------+ | |
| Adj. +---/\/\-----/\/\---+
| R2 ^ R3
| 100 ohms | 100 ohms
+----------------------+
The reason is that this arrangement can only *increase* the current from the
nominal I = 1.25V/R. So, for example, to get a 10:1 range, the voltage across
the sense resistor would be 12.5 V for the 10x current! In general this is not
attractive for the high current condition because not only have you required
a higher supply voltage, at the maximum current, but the power dissipation in
the sense resistor is also quite high (more like HUGE --- sam).
Let me offer the following simple circuit, which I just created and haven't
tried but 'oughta work' as a solution to this problem.
By contrast, this circuit can only *decrease* the current from the 1.25V/R
value, but it easily handles a 10:1 range (or even much more) and the voltage
across the sense resistor is never more than 1.25V, allowing low supply voltage
(e.g. 5 V) and keeping the dissipation low.
In +-------+ Out R1 .25 ohms
Vin o----| LM338 |-------/\/\/----+-----o 0 to 5 A current source
+-------+ |
| Adj. +----+
| cw | |
| 1K ^ / _|_,
+-------------->\ '/_\ LM385-1.2
/ |
| |
+----+
|
+------------------------+
| I = 0.5 to 1.5 mA sink |
+------------------------+
_|_
-
The 1K pot selects a portion of the floating 1.23 V reference voltage, and
tricks the LM317 or LM338 into correspondingly reducing the voltage across
the 0.25 ohm current-sense resistor. The pot is conventional and may be
panel mounted. It should be possible to nearly shut off the LM338 (a
minimum quiescent current will still flow). The current sink, I, which
powers the floating 1.23 V reference, is not critical and may be a simple
current mirror (sorry to see the TL011 gone!), or even a resistor to
ground or any available negative voltage, depending upon the desired
current-source voltage-compliance range. That's it!
HELIUM NEON LASERS
INTRODUCTION
A helium neon (henceforth abbreviated HeNe) laser is basically a fancy neon
sign with mirrors at both ends. Well, not quite, but really not much more
than this. The gas fill is a mixture of helium and neon gas at low pressure.
A pair of mirrors (one totally reflective, the other partially reflective at
the wavelength of the laser's output) complete the resonant cavity. This is
called a Fabry-Perot cavity (if you want to impress your friends). The
mirrors may be internal (common on small and inexpensive tubes) or external.
Electrodes sealed into the tube allow for the passage of high voltage DC
current to excite the discharge.
I remember doing the glasswork for a 3 foot long HeNe laser which included
joining side tubes for the electrodes and exhaust port, fusing the electrodes
themselves to the glass, preparing the main bore (capillary), and cutting the
angled Brewster windows (so that external mirrors could be used) on a diamond
saw. I do not know if the person building the laser ever got it to work but
suspect that he gave up or went on to other projects (which probably were also
never finished).
Some die-hards still construct their own HeNe lasers from scratch. Once all
the glasswork is complete, the tube must be evacuated, baked to drive off
surface impurities, backfilled with a specific mixture of helium to neon at a
pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760
Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly
positioned and aligned. Finally, the great moment arrives and the power is
applied. You also constructed your high voltage power supply from scratch,
right? With luck, the laser produces a beam and only final adjustments to the
mirrors are then required. All sorts of things can go wrong. With external
mirrors, the losses may be too great resulting in insufficient optical gain in
the resonant cavity. The gas mixture may be incorrect or become contaminated.
Seals might leak. It just may not be your day! Nonetheless, if you really
want to be able to say you built a laser from the ground up, this is the
approach to take.
However, for most of us, 'building' a HeNe laser is like 'building' a PC:
An inexpensive HeNe tube and power supply are obtained, mounted, and wired
together. Optics are added as needed. Power supplies may be home-built as
an interesting project but few have the desire, facilities, patience, and
determination to construct the actual HeNe tube itself.
The most common sealed HeNe laser tubes are between 6" and 14" (150 mm to 350
mm) in overall length and 1" to 1-1/2" (25 mm to 37.5 mm) generating optical
power from .5 mW to 5 mW.
Slightly smaller tubes (less than .5 mW), somewhat larger tubes (up to 20 mW),
and much larger tubes with internal or external mirrors (a *meter* or more in
length generating up to 250 mW of optical power), are also available and may
turn up on the surplus market. Specialized configurations - a triple XYZ axis
triangular cavity laser in a solid glass block for an optical ring laser gyro,
for example - also exist but are much much less common - you probably won't
find one of these at a local flea market!
Manufacturers include Aerotech, Melles-Griot, Siemens, Spectra-Physics, and
many others.
HeNe lasers used to be found in all kinds of equipment including early laser
printers, laserdisc players, small laser shows, optical surveying and tunnel
boring systems, medical positioning systems, and supermarket checkout UPC and
other barcode scanners. (You can tell if you local ACME supermarket uses
a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm
wavelength beam from a HeNe laser is noticeably more orange than the 670 nm
deep red from a typical laser diode type.)
Nowadays, these applications are likely to use the much more compact lower
(drive) power solid state laser diodes. Thus, a 5 mW laser pointer complete
with batteries can conveniently fit on a keychain and generate the same beam
power as a HeNe laser half a meter long!
So why bother with a HeNe laser at all? There are several reasons:
* For many applications including holography and interferometry, the high
quality stable beam of a HeNe laser is unmatched (at least at reasonable
cost, perhaps at all) by laser diodes. In particular, the coherence length
and monochromicity of even a cheap HeNe laser are excellent and the beam
profile is circular (laser diodes usually have some amount of astigmatism)
so that simple spherical optics can be used for beam manipulation.
* As noted in the chapter on laser diodes, it is all too easy to ruin them in
the blink of an eye (actually, the time it takes light to travel a few feet).
It would not take very long to get frustrated burning out $50 diodes. So,
the HeNe laser tube may be a better way to get started. They are harder to
damage through carelessness or design errors. Just don't get the polarity
reversed or exceed the tube's rated current for too long - or drop them on
the floor! And, take care around the high voltage.
* Laser diode modules at a wavelength of 635 nm may be somewhat more expensive
than surplus HeNe tubes with power supplies. However, with the introduction
of DVD players and DVDROM drives, this situation probably will not last long.
HeNe LASER SAFETY:
As with *any* laser, proper precautions must be taken to avoid any possibility
of damage to vision. The types of HeNe lasers dealt with in this document are
classified as type II, IIIa, or the low end of IIIb (see the section: "Laser
safety classification". For most of these, common sense (don't stare into
the beam) and fairly basic precautions suffice since the reflected or scattered
light will not cause instantaneous injury and is not a fire hazard.
However, unlike those for laser diodes, HeNe power supplies utilize high
voltage (several KV) and some designs may be potentially lethal. This is
particularly true of AC line powered units since the power transformer may
be capable of much more current than is actually required by the HeNe laser
tube - especially if it is home built using the transformer from some other
piece of equipment (like an old tube type console TV or that utility pole
transformer you found along the curb) which may have a much higher current
rating.
The high quality capacitors in a typical power supply will hold enough charge
to wake you up - for quite a while even after the supply has been switched off
and unplugged. Unless significantly oversized, this isn't usually a lethal
amount of energy but can still be quite a jolt. The HeNe tube itself also
acts as a small HV capacitor so even touching it should it become disconnected
from the power supply may give you a tingle. This probably won't hurt you
physically but your ego may be bruised if you then drop the tube and it
shatters on the floor! Use an insulated 1 M, 2 W resistor to drain the charge
before touching anything.
See the document: "Safety Guidelines for High Voltage and/or Line Powered
Equipment" for detailed information before contemplating the inside or HV
terminals of a HeNe power supply!
COMMENTS ON HeNe LASER SAFETY ISSUES
(Portions from: Robert Savas (jondrew@mail.ao.net)).
A 10 mw HeNe laser certainly presents an eye hazard.
According to American National Standard, ANSI Z136.1-1993, table 4 Simplified
Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective
eyewear with an attenuation factor of 10 (Optical Density 1) is required for a
HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to
10 seconds, the time in which they eye would blink or change viewing direction
due the the uncomfortable illumination level of the laser. Eyeware with an
attenuation factor of 10 is roughly comparable to a good pair of sunglasses
(this is NOT intended as a rigorous safety analysis, and I take no
responsibility for anyone foolish enough to stare at a laser beam under any
circumstances). This calculation also assumes the entire 10 milliwatts are
contained in a beam small enough to enter a 7 millimeter aperture (the pupil
of the eye). Beyond a few meters the beam has spread out enough so that only
a small fraction of the total optical power could possible enter the eye.
INSTANT HeNe LASER THEORY
The term laser stands for "Light Amplification by Stimulated Emission of
Radiation". However, lasers as most of us know them, are actually sources of
light - oscillators rather than amplifiers. (Although laser amplifiers do
exist in applications as diverse as fiber optic communications repeaters and
multi-gigawatt laser arrays for inertial fusion research.) Of course, all
oscillators - electronic, mechanical, or optical - are constructed by adding
the proper kind of positive feedback to an amplifier.
All materials exhibit what is known as a bright line spectra when excited in
some way. In the case of gases, this can be an electric current or (RF) radio
frequency field. In the case of solids like ruby, a bright pulse of light
from a xenon flash lamp can be used. The spectral lines are the result of
spontaneous transitions of electrons in the material's atoms from higher to
lower energy levels. A similar set of dark lines result in broad band light
that is passed through the material due to the absorption of energy at specific
wavelengths. Only a discrete set of energy levels and thus a discrete set of
transitions are permitted based on quantum mechanical principles (well beyond
the scope of this document, thankfully!). The entire science of spectroscopy
is based on fact that every material has a unique spectral signature.
The HeNe laser depends on energy level transitions in the neon gas. In the
case of neon, there are dozens if not hundreds of possible wavelength lines of
light in this spectrum. Some of the stronger ones are near the 632.8 nm line
of the common red-orange HeNe laser - but this is not the strongest:
The strongest red line is 640.2 nm. There is one almost as strong at 633.4
nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak
in an ordinary neon spectrum, due to the high energy levels in the neon atom
used to produce this line.
There are also many infra-red lines and some in the orange, yellow, and green
regions of the spectrum as well.
The helium does not participate in the laser (light emitting) process but is
used to couple energy from the discharge to the neon through collisions with
the neon atoms. This pumps up the neon to a higher energy state resulting in
a population inversion meaning that more atoms in the higher energy state than
the ground or equilibrium state.
It turns out that the upper level of the transition that produces the 632.8
nm line has an energy level that almost exactly matches the energy level of
helium's lowest excited state. The vibrational coupling between these two
states is highly efficient.
You need the gas mixture to be mostly helium, so that helium atoms can be
excited. The excited helium atoms collide with neon atoms, exciting some of
them to the state that radiates 632.8 nm. Without helium, the neon atoms
would be excited mostly to lower excited states responsible for non-laser
lines.
A neon laser with no helium can be constructed but it is much more difficult
without this means of energy coupling. Therefore, a HeNe laser that has lost
enough of its helium (e.g., due to diffusion through the seals or glass) will
most likely not lase at all since the pumping efficiency will be too low.
There are many possible transitions from the excited state to a lower energy
state that can result in laser action. The most important (from our
perspective) are listed below:
Wavelength Color
543.5 nm Green
593.9 nm Yellow
611.8 nm Orange
632.8 nm Red-Orange
1152.3 nm Near Infra-Red
3391.3 nm Mid Infra-Red
While we normally don't think of a HeNe laser as producing an infra-red (and
invisible) beam, the IR spectral lines are quite strong. In fact, the first
HeNe laser operated at 1152.3 nm. HeNe lasers at all of these wavelengths are
commercially available but those operating at 632.8 nm are by far the most
common and least expensive.
When the HeNe gas mixture is excited, all possible transitions occur at a
steady rate due to spontaneous emission. However, most of the photons are
emitted with a random direction and phase, and only light at one of these
wavelengths is usually desired in the laser beam. At this point, we have
basically the glow of a neon sign with some helium mixed in!
To turn spontaneous emission into the stimulated emission of a laser, a way
of selectively amplifying one of these wavelengths is needed and providing
feedback so that a sustained oscillation can be maintained. This may be
accomplished by locating the discharge between a pair of mirrors forming
what is known as a Fabry-Perot resonator or cavity. One mirror is totally
reflective and the other is partially reflective to allow the beam to escape.
These mirrors are normally made to have peak reflectivity at the desired laser
wavelength. When a spontaneously emitted photon resulting from the transition
corresponding to this peak happens to be emitted in a direction nearly parallel
to the long axis of the tube, it stimulates additional transitions in excited
atoms. These atoms then emit photons at the same wavelength and with the same
direction and phase. The photons bounce back and forth in the resonant cavity
stimulating additional photon emission. Each pass through the discharge
results in amplification - gain - of the light. If the gain due to stimulated
emission exceeds the losses due to imperfect mirrors and other factors, the
intensity builds up and a coherent beam of laser light emerges via the
partially reflecting mirror at one end. With the proper discharge power, the
excitation and emission exactly balance and a maximum strength continuous
stable output beam is produced.
Spontaneously emitted photons that are not parallel to the axis of the tube
will miss the mirrors entirely or will result in stimulated photons that are
reflected only a couple of times before they are lost out the sides of the
tube. Those that occur at the wrong wavelength will be reflected poorly if at
all by the mirrors and any light at these wavelengths will die out as well.
MODES OF OPERATION
The physical dimensions of the Fabry-Perot resonator impose some additional
constraints on the resulting beam characteristics.
While it is commonly believed that the 632.8 (for example) transition is
a sharp peak, it is actually a gaussian - bell shaped - curve. In order for
the cavity to resonate strongly, a standing wave pattern must exist. This
will only occur when an integral number of half wavelengths fit between the
two mirrors. This restricts possible axial or longitudinal modes of
oscillation to:
L * 2 c * n
W = --------- or F = ---------
n L * 2
where:
* L is the distance between the mirrors (m).
* W denotes the possible wavelengths of oscillation (m).
* n is a large integer (order of 948,000 for W around 632.8 nm, L = .3 m).
* F denotes the possible frequencies of oscillation (Hz).
* c is the speed of light (approximately 300 million m/s).
The laser will not operate with just any wavelength - it must satisfy this
equation. Therefore, the output will not usually be a single peak at 632.8 nm
but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer
cavities result in closer mode spacing and a larger number of modes since the
gain won't fall off as rapidly as the modes move away from the peak. For
example, a cavity length of 150 mm results in a longitudinal modes spacing of
about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral
lines in the output will be nearest the combined peak of the lasing medium and
mirror reflectivity but many others will still be present. This is called
multimode operation.
Think of the vibrating string of a violin or piano. Being fixed at both ends,
it can only sustain oscillations where an integer number of cycles fits on the
string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5
(harmonics or overtones). Due to the tension and stiffness of the string,
only small integer values for n are present with a significant amplitude. For
a HeNe laser, the distribution of the selected neon spectral line and shape
of the reflectivity function of the mirrors with respect to wavelength
determine which values of n are present. For a typical HeNe laser tube,
possible values of n will form a series of very large numbers like 948,123,
948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4 :-). For example:
I| .
| | | |
| | | | | |
| | | | | | | |
_______|______.__|__|__|__|__|__|__|__|__|__._______
n=948,125 -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
This also means that as the tube warms up and expands, these spectral line
frequencies are going to drift downward (toward longer wavelengths). However,
since the reflectivity distribution of the mirrors remains constant, new lines
will fill in from above so the overall shape of the output doesn't change.
Thus, a typical HeNe laser is not monochromatic though the effective spectral
line width is very narrow compared to common light sources. Additional effort
is needed to produce a truly monochromatic source operating in a single
longitudinal mode. One way to do this is to introduce another adjustable
resonator called an etalon into the cavity. Then, only modes which are the
same in both resonators will produce enough gain to sustain laser output. By
adjusting the etalon, one spectral line can be selected resulting in single
mode operation.
Lasers can also operate in various transverse modes. Laser specifications
will usually refer to the TEM00 mode. This means Transverse Electromagnetic
Mode 0,0 and results in a single beam. The long narrow bore of a typical HeNe
laser forces this mode of oscillation. With a wide bore multiple sub-beams
can emerge from the same cavity in two dimensions. The TEM mode numbers
(TEMxy) denote the number (minus one) of such sub-beams:
O OO OOO Each 'O' represents
O OO O OO OOO is a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
Other (non-cartesian) patterns of modes may also be possible depending on
tube dimensions and operating conditions.
EARLY VS. MODERN HeNe LASERS
In the first HeNe lasers (see the diagram below), exciting the gas atoms to
the higher energy level was accomplished by coupling a radio frequency (RF)
source (i.e., a radio transmitter) to the tube via external electrodes.
Modern HeNe lasers almost always operate on a DC discharge via internal
electrodes.
Bellows Bellows
/\/\/\ Discharge tube with external electrodes /\/\/\
|| \________________________________________________/ ||
|| | | | | | | ||===> Laser
|| ___ __|_|________________|_|______________|_|__ ||===> Beam
|| / || | | | \ ||
\/\/\/ || | o | \/\/\/
Adjustable || +-----------o RF exciter o----------+ Adjustable
totally || partially
reflecting ||<-- to vacuum system reflecting
mirror mirror
Early HeNe lasers were also quite large and unwieldy in comparison to modern
devices. A laser such as the one depicted above was over 1 meter in length
but could only produce about 1 mW of optical beam power! The associated RF
exciter was as large as a microwave oven. With adjustable mirrors and a
tendency to lose helium via diffusion under the electrodes, they were a
finicky piece of laboratory apparatus with a lifetime measured in hundreds
of operating hours.
In comparison, a modern 1 mW sealed HeNe laser tube can be less than 150 mm
(6 inches) in total length, may be powered by a solid state inverter the size
of a stick of butter, and will last more than 20,000 hours without maintenance
of any type or a noticeable change in its performance characteristics.
HeNe LASER OPERATION:
The following applies to most of the inexpensive sealed low to medium power
(.5 to 10 mW) HeNe tubes available on the surplus market and describes the
actual laser tube which may be enclosed inside a laser head depending on the
original application:
This fabulous ASCII rendition of a typical small HeNe laser tube should make
everything perfectly clear :-).
____________________________________________
/ _________________ \
Anode |\ Helium+neon, 2-5 Torr Cathode can ^ \ |
.-.---' \.--------------------------------------. '-'---.-. Main
<---| |:::: :======================================: :::::| |===> beam
'-'-+-. /'--------------------------------------' .-.-+-'-'
Totally | |/ Glass capillary ^ _________________/ | | Partially
reflecting | \____________________________________________/ | reflecting
mirror | | mirror
| Rb + - |
+---------/\/\---------o 1.2 to 3 KVDC o-----------+
(Note: the main beam may emerge from either end of the tube depending design,
not necessarily the cathode end as shown.)
* The anode (+) end is simply a small cylindrical metal electrode with a
mirror fused or glued to its end.
The discharge at this end produces little heat or damage due to sputtering.
* The cathode (-) end is also a cylindrical metal electrode with a mirror
fused or glued to its end but in addition, there is a large 'cylindrical
can attached to the cathode and extending about half the length of the tube.
The discharge at this end is distributed over the entire area of the can
thereby spreading the heat and minimizing damage due to sputtering which
results from positive ion bombardment.
* These mirrors are not silvered or aluminized (metal coated) but are a type
called 'dichroic'. They are made by depositing many alternating layers of
hard but transparent materials having different indexes of refraction. The
thickness of each is precisely 1/2 the wavelength of the laser light (632.8
nm being the most common for a HeNe laser). This results in reflection by
interference with very high (>99.9 %) efficiency - much greater than for
even the best metal coated mirrors.
* One of the mirrors will be nearly totally reflecting and the other will only
be partially reflecting at the laser wavelength. Since the reflection peaks
at a single wavelength, these mirrors actually appear quite transparent to
other wavelengths of light. For example, for common HeNe lasers tubes, the
mirrors transmit blue light quite readily and appear blue when looking
through an unpowered (!!) tube.
* The mirrors will likely not have any 'user' adjustments. However, the
cylindrical end pieces are mounted by thinner sections of metal tubing so
that some slight changes to alignment may be possible with appropriate
fixtures. Don't be tempted: (1) grabbing the high voltage electrodes is
not likely to be pleasant and (2) it is too easy to break the seal if you
get carried away. There should be no reason for the alignment to have
changed unless you whacked the tube - it was set at the factory.
* The main beam will emerge from the partially reflecting mirror but this may
be at either end of the tube depending on model. For example, where the
tube is enclosed in a metal barrel, the HV connections will be to the anode
end and the beam will exit from the cathode end. With this arrangement, the
positive output of the power supply and ballast resistor can be very close
to the tube anode and the entire barrel can be connected to the negative
output of the power supply and earth ground.
* Since the mirrors are not perfect, there will be a weaker beam visible from
the other end if that mirror is not covered (blocked or painted over). One
use of this is to permit monitoring of laser power for purposes of optical
power regulation or other closed loop applications.
* The major discharge is forced to take place inside a thick glass capillary
tube with an inner bore of roughly 1 mm. This concentrates the discharge
forcing operation in the most common and desirable TEM00 mode.
* The cheap surplus HeNe tubes do not generally produce a fixed polarized beam.
The polarization will either be random or slowly changing as the tube heats.
Tubes with a specified polarization are also available but are generally more
costly. Lasers with external mirrors and Brewster windows will be linearly
polarized and really pricey (and more finicky).
* These tubes are nearly always operate in multimode (longitundial) with a
TEM00 beam profile. See the section: "Instant HeNe laser theory" for more
info.
* Power for a HeNe laser is provided by a special high voltage power supply
(see the section: "Basic HeNe power supply considerations" and consists
of two parts (these maximum values depend on tube size (typical 1 to 10 mW
tube is assumed):
- Operating voltage of 1,000 to 3,000 DC at 3 to 10 mA.
Like any discharge tube, the HeNe laser is a negative resistance device.
As the current *increases* through the tube, the voltage across the tube
*decreases*. The incremental magnitude of the negative resistance also
increases with descreasing current.
- Starting voltage of 5 to 12 KV at almost no current.
In the case of a HeNe tube, the initial breakdown voltage is much greater
than the sustaining voltage. The starting voltage may be provided by a
separate circuit or be part of the main supply.
See the chapter: "Helium Neon Laser Power Supplies" for more information and
complete circuit diagrams.
* With a constant voltage power supply, a series ballast resistor is essential
to limit tube current to the proper value. A ballast resistor will still be
required with a constant current or current limited supply to stabilize
operation. The ballast resistor may be included as part of a laser head but
will be external for a bare tube.
In order for the discharge to be stable, the total of the effective power
supply resistance, ballast resistance, and tube (negative) resistance must be
greater than 0 at the operating point. If this is not the case, the result
will be a relaxation oscillator - a flashing or cycling laser!
* Every HeNe tube will have a nominal current rating. In addition to excessive
heating and damage to the electrodes, current beyond this value does not
increase laser beam intensity. In fact, optical output actually decreases
(probably because too high a percentage of the helium/neon atoms/ions are in
the excited state). You can easily and safely demonstrate this behavior if
your power supply has a current adjustment or you run an unregulated supply
using a Variac. While the brightness of the discharge inside the tube will
increase with increasing current, the actual intensity of the laser beam
will max out and then eventually decrease with increasing current. (This
is also an easy way of determining optimal tube current if you have not data
on the tube - adjust the ballast resistor or power supply for maximum optical
output and set it so that the current is at the lower end of the range over
which the beam intensity is approximately constant.)
* These may be 'bare' tubes or encased in a cylindrical or rectangular laser
head - or something in between.
- Bare tubes require clip-on connections to the power supply or high voltage
connector and an external ballast resistor.
Advantages: Less expensive, discharge is fully visible resulting in an
interesting display.
Disadvantages: Fragile, exposed high voltage terminals, need to provide
your own mounting, wiring, and ballast resistor.
- Laser heads will usually come with an internal ballast resistor (though
you may still need additional resistance to match the tube to your power
supply). The high voltage cable will likely use an 'Alden' connector.
The Alden connector is designed to hold off the high voltages with a pair
of keyed recessed heavily insulated pins. This is a universal standard for
small-medium HeNe laser power supplies (the longer fatter pin is negative).
Advantages: High voltage safely insulated, wiring is already done for
you, relatively robust, easily mounted.
Disadvantages: More expensive, discharge not readily visible, repairs
to wiring (unlikely to be needed) difficult.
* The operating lifetime of a typical HeNe laser tube is greater than 15,000
hours when used within its specified ratings. Therefore, this is not a
major consideration for most hobbyist applications. However, the shelf life
of the tube depends on its construction. There are two types of (sealed)
HeNe tubes:
- Most better HeNe tubes (possibly all tubes manufactured in the last 10
years) are 'hard sealed' - the mirrors are fused to their respective
electrodes by a low temperature glass 'frit' - sort of like solder for
glass! These do not leak - at least not on any time scale that matters.
Shelf life is essentially infinite.
- Older tubes have their mirrors just glued - Epoxied to the end electrodes.
This adhesive leaks and such tubes have a shelf life of a few years - they
fail by just sitting around doing nothing. This means that a bargain tube
may not be such a bargain if it is beyond its expiration date (yes, just
like dates on milk containers) as it may have a very limited life, be hard
to start, weak or erratic, or may not work at all. You probably won't see
any of these - at least not in a working condition.
* The efficiency of the typical HeNe laser is pretty pathetic. For example,
a 2 mW HeNe tube powered by 1,400 V at 6 mA has an efficiency of less than
.025 %. More than 99.975 percent of the power is wasted in the form of heat
and incoherent light (from the discharge)! This doesn't even include the
losses of the power supply and ballast resistor.
* The most common HeNe lasers by far produce light at a wavelength of 632.8 nm
in the red-orange part of the visible spectrum - well into the region of the
human eye's high sensitivity (but not as good as green). Thus, a 1 mW HeNe
laser will appear brighter than a 4 mW laser diode operating at 670 nm.
* Green, yellow, and orange HeNe lasers are also available but are not nearly
as 'efficient' as the common red-orange type. Thus, they are also much more
costly for the same power since the spectral lines that need to be amplified
are weaker at these wavelengths and therefore, the tubes must be larger.
Typical maximum output available from 'small' sealed tubes for various
colors: green - 2.0 mW, yellow - 7.0 mW, orange - 7.0 mW, and red - 15 mW.
IR (infra-red) HeNe laser tubes are also available. However, an invisible
beam just doesn't seem as exciting!
* The width of the beam as it emerges from the tube is typically between .5 mm
and 1 mm - the inside bore diameter of the capillary discharge tube.
* The beam from a HeNe laser is already very well collimated even without
external optics (unlike a laser diode which has a raw divergence measured
in 10s of degrees). The divergence measured in milliradians (1 mR is less
than 1/17th of a degree) is usually one of the tube specifications. A small
HeNe tube may have a divergence of 1 to 2 mR. Divergence is affected mostly
by beam (exit or waist) diameter (wider is better). Other factors like the
ratio of length to bore diameter (narrower is better) may also affect this
slightly. The equation for a plane wave source is:
Wavelength * 4
Divergence angle in radians = --------------------
pi * Beam Diameter
So, for an ideal HeNe laser with a 1 mm bore at 632.8 nm, the divergence
angle will be about .806 mR. Note that although a wider bore should result
in less divergence, this also permits more not quite parallel rays to
participate in the lasing process. Also see the section: "Improving the
collimation of a laser".
TYPICAL HeNe TUBE CHARACTERISTICS
(Portions from: Steve Nosko (q10706@email.mot.com)).
The following are typical of small (bare) red-orange (632.8 nm) HeNe tubes:
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
0.5 mW .9-1.0/7 KV 3.2-4.5 mA 1.1-1.3 KV 25/150 mm
1 mW .9-1.0/7 KV 4.5 mA 1.1-1.3 KV 25/150 mm
1-2 mW 1.0-1.4/8 KV 4.5-6.5 mA 1.4-1.9 KV 30/260 mm
2 mW 1.4-1.5/8 KV 6.5 mA 1.9-2.0 KV 30/260 mm
3-4 mW 1.9/10 KV 6.5 mA 2.4 KV 37/350 mm
4-5 mW 1.9/10 KV 6.5 mA 2.4 KV 37/350 mm
Melles Griot, Uniphase, Siemens, PMS, and Aerotech all show similar values.
Note that for a given optical power level, there can be substantial variation
in the tube size. Typically, longer tubes will require higher start and
operating voltages. And no, you cannot get a 3 mW tube to output 30 mW - even
instantaneously - by driving it 10 times as hard!
HeNe tubes of other colors exist but are probably rare on the surplus market.
They are not that common to begin with and are more expensive when new since
for a given power level, the tubes must be larger and thus have higher voltage
and current ratings due to their lower efficiency (the spectral lines being
amplified are much weaker than the one at 632.8 nm).
There are infrared HeNe tubes as well. Yes, you can have a HeNe and it will
light up inside (typical neon orange glow), but if there is no output beam
(at least you cannot see one), you could have been sold an infrared HeNe tube.
The IR may be visible with a video camera (assuming it doesn't have an IR
blocking filter) or bu using one of the IR detector circuits or an IR detector
card as discussed with respect to IR laser diodes.
As a side note... It is strange to see the orange glow in a green HeNe laser
tube but have a green beam emerging. A diffraction grating or prism really
shows all the lines that are in the glow discharge. Red through orange, yellow
and green, even several blue lines!! The IR lines are present as well - you just
cannot see them.
HOW CAN I TELL IF MY TUBE IS GOOD?
A variety of problems can prevent a HeNe tube from lasing properly or make it
hard to start.
* Physical damage. Obviously if the tube arrived in pieces, this is a
shipping, not a technical problem :-).
* Misaligned mirrors. Using the tube as a hammer might bend the mirror mount
at one end or the other. There are ways of determining and adjusting this
alignment but they require some optical components and special jigs. Without
these, adjustments are hit or miss (mostly miss) and will more likely result
in a broken seal than anything else :-(. Problems with mirror alignemnt are
very unlikely to occur with these tubes unless you were working hard at it!
* Loss of gas fill. This may result in a total inability to start or sustain
the discharge. There is usually a metal sealing nipple at one end. This
might be damaged.
* Incorrect gas fill. There may be a glow but the laser output will likely
be weak or non-existent. The normal color of the discharge is whitish
red-orange - a somewhat unsaturated version of the red-orange glow of a
(true) neon sign.
- Loss of helium (from diffusion through the glass or seals) will result in
the glow becoming deeper red-orange and less white. There will be little
or no emission at the wavelength of helium's spectral lines. It probably
isn't worth the effort to refill but see the section: "Recharging HeNe
tubes".
- A leak which has allowed some air to enter (but where it is not totally
up to atmostpheric pressure) will result in a glow with a white or pink
color. Depending on the actual pressure, the intensity will vary as well.
If you can sustain a discharge but it is the wrong color (weak or white/pink
color), you may have one of those really old Epoxy sealed tubes that leak
and air has leaked in. Again, probably not worth repairing.
* Damaged electrodes or mirrors due to running with the power supply polarity
reversed or greatly excessive current for a prolonged period of time. I
don't know exactly what the physical effects might be but I would suspect
metal sputtering from the negative electrode coating the mirror at that end
of the tube. Buy another tube.
IMPROVING THE COLLIMATION OF A HeNe LASER
The following applies to any laser which outputs a substantially parallel
beam but is written specifically for HeNe lasers. Collimation of laser
diodes require a slightly different approach - see the section: "Divergence
of laser diodes".
Although the divergence of a HeNe laser is already pretty good without any
additional optics, the rather narrow beam as it exits from the tube (typically
1 mm) results in a beam with a typical divergence between 1 to 2.5 mR (order
of 1 mm per meter or worse) if no other optics are used.
As noted in the section: "HeNe laser operation", beam divergence is inversely
proportional to the beam diameter. Thus, it can be reduced even further by
passing the beam through a pair of positive lenses - one to focus the beam to
a point and the second to collimate the expanded beam.
A small telescope can be used to collimate a laser beam and will be easier to
deal with than individual lenses. (This is how laser beams are bounced off
the moon but the telescopes aren't so small.) Using a telescope is by far
the easiest approach in terms of mounting - you only need to worry about
position and alignment of two components - the laser tube and telescope.
If you want to use discrete optics:
* The focusing lens should have a short focal length (F1) such as a microscope
or telescope eyepiece (e.g., F1 of 10 mm) or low power microscope objective
(e.g., 10X). Note: the objective lens from a dead CD player has an ideal
focal length - about 4 mm - but is aspheric and would probably not be that
great but give it a try!
This will focus the laser beam to a (diffraction limited) point F1 in front
of the lens from which it will then diverge.
* The collimating lens should be a large diameter medium focal length (e.g.,
15 mm D2, 100 mm F2) lens placed F2 from the focus of the first lens.
For optimal results, the ratio of collimating lens diameter to focal length
(D2/F2) should greater than or equal the ratio of HeNe beam diameter to
focusing lens focal length (D1/F1). This will ensure that all the light
is captured by the collimating lens.
The beam will be wider initially but will retain its diameter over much longer
distances. For the example, above, the exit beam diameter will be about 10 mm
resulting in nearly a 10 fold reduction in divergence.
Adjust the lens spacing to obtain best collimation. A resulting divergence of
less than 1 mm per 10 meters or more should be possible with decent quality
lenses - not old Coke bottle bottoms or plastic eyeglasses that have been used
for skate boards :-).
HeNe LASER TUBE LIFE
Neon signs last a long times - years - how about HeNe laser tubes?
Lifespans of HeNe laser are long - 20,000 or more operating hours are typical.
Shelf life of non-hard sealed tubes is limited by diffusion of the Helium
atoms. Helium atoms are slippery little devils. They diffuse through almost
anything. In the case of HeNe tubes, diffusion takes place mostly through the
Epoxy adhesive used to mount the mirrors in non-hard sealed tubes (not common
anymore) and through the glass itself but at a much much slower rate.
The gas doesn't 'wear out'. However, excessive or reverse current can degrade
performance after a while. Electrode material may sputter onto the adjacent
mirrors or excessive heat dissipation may damage the electrodes themselves.
A HeNe tube, when properly connected has much of its heat dissipated by the
bombardment of positive ions at the cathode (the big can electrode) which is
made large to spread the effect and keep the temperature down. Hook a tube up
backwards and you may damage it in short order!
RECHARGING HeNe TUBES
"I have two large tube green HeNes and and a 1 micron IR HeNe that are dead
from obvious low helium pressure (spectrum from grating shows only weak He
lines) has anyone had any success with putting tubes in a pressure chamber
filled with Helium so it diffuses the other way?"
HeNe tubes which do not laser well or at all due to loss of helium can
sometimes be rejuvenated by soaking them in helium at normal atmospheric
pressure for a few days or weeks.
The point to realize is that it is the partial pressure of each gas that
matters. Neon is a relatively large atom and does not diffuse through the
tube at any rate that matters. However, helium is able to diffuse even
when the pressure difference is small. Even for a HeNe tube at 2 Torr, the
partial pressure of helium is much greater than its partial pressure in the
normal atmosphere. So, helium leaks out even though the total pressure
outside is several hundred times greater. Conversely, soaking a HeNe tube
in helium at 1 atmosphere will allow helium to diffuse into the tube at several
hundred times the rate at which it had been leaking out. Thus, only a few
days of this treatment may be needed if the problem is low helium pressure.
This hardly seems worthwhile for a $25 1 mW HeNe tube but it is something to
keep in mind for other more substantial types.
(From: Mark W. Lund (lundm@xray.byu.edu)).
I have rejuvenated HeNes with low Helium pressure. Since the partial pressure
of 1 atmosphere helium is much higher than inside the tube you don't really
need to use high pressure, or even increased temperature. I just put them in
a garbage bag and blasted some helium into it from time to time. The length
of time necessary in my case was a few days, but depending on the glass type,
thickness, and sealing method this may vary. It would be good to test the
power every couple of days so you don't overshoot too much.
One warning, helium has a lower dielectric strength than air, so don't try to
operate the laser in helium, it may arc over.
HELIUM NEON LASER POWER SUPPLIES
BASIC HeNe POWER SUPPLY CONSIDERATIONS
You will need 1,100 to 2,400 VDC at a few mA and an 5 to 12 KV starting voltage
(but almost no current). Precise values depend on the size of the tube. This
assuming something in the .5 to 10 mW range - not a 250 mW monster. Unlike
laser diodes, the HeNe drive is not nearly as critical to performance and tube
life :-). Therefore, even your tube did not come with a datasheet, you can
probably guess fairly closely as to its requirements.
There are any number of ways of constructing these supplies - over a dozen
sample circuits are provided in the chapter: "Complete Helium Neon Laser Power
Supply Schematics". However, you don't need to buidl your own:
* If you just want a working laser, buying a power supply may be worth the
money - these can be had for about $25 (typical for 1 to 2 mW tubes) to
$100 depending on size. These are available both new and surplus.
With these, at most you will have to add a ballast resistor and power line or
battery connections. Models are available that run off either low voltage
DC (regulated is desirable) or 110 or 220 VAC.
See the section: "Examples of the use of commercial power supply bricks".
* Kits are also available but may not be any cheaper and are not necessarily
as well designed as surplus commercial units.
* The complete laser and optics assemblies from supermarket checkout UPC
scanners and other similar devices are often available at very reasonable
prices - $50 to $100 - which includes the HeNe laser tube, power supply,
various lenses and mirrors, scanner motor or galvo, and other neato stuff.
* HeNe tubes and power supplies are quite frequently offered by people posting
on the sci.electronics hierarchy, sci.optics, alt.lasers, or other USENET
newsgroups. Prices are often very low but of course you may have no way
of knowing who you are dealing with.
If you still want to build your own, there are basically two approaches for
the operating voltage (AC line operated or high frequency inverter) and three
approaches for the starting voltage (diode/capacitor multiplier, pulse trigger
circuit, or high compliance design).
Here are six options for providing the operating voltage. These examples
assume an output of at least 1,800 VDC:
1. Use a 1,300 VRMS power transformer. Then, all that is needed is a rectifier
and filter capacitor.
2. Use a 700 VRMS power transformer with a 2 diode 2 capacitor voltage doubler.
3. Use a lower voltage power transformer and a multi-stage voltage multiplier.
Up to 6 stages should be reasonably easy to construct.
4. Build a low voltage input inverter using a flyback transformer from a small
B/W or color TV computer monitor, or video terminal but running at lower
voltage than normal. These usually have a built in HV rectifier but you
will need a HV filter capacitor and ballast resistor. While rated at only
a mA or so for the CRT HV, more current should be available at reduced
voltage. With proper design, it is possible for there to be enough voltage
compliance to be self starting. See the section: "Sam's inverter driven
HeNe power supplies".
5. Build a HV inverter based on any of a number of simple DC-DC converter
topologies. See the document: "Various Schematics and Diagrams" for ideas.
You will probably need to modify these mostly by increasing the number of
turns on the output windings of the inverter transformer - which in itself
may be a challenge to maintain low capacitance and high voltage insulation.
See the section: "Simple inverter type power supply for HeNe laser" for one
modified for driving a HeNe laser tube.
6. Build a HV inverter of the type discussed above using a PWM controller
integrated circuit - Linear Technology, Maxim, Motorola, National
Semiconductor, Unitrode, and others have suitable DC-DC controller chips.
I would recommend (1) or (2) if portability is not a big issue and you can
locate a suitable transformer. These are virtually foolproof (well, at least
as long as you don't fry yourself from the high voltage). For small tubes,
the design described in the section: "Edmund Scientific HeNe power supply"
is about as simple as possible.
See the sections: "AC line operated power supplies" and "Inverter type power
supplies" for more discussion of features, basic operation, and design issues.
A typical configuration is shown below. As noted, the starting circuit may
be omitted in a high compliance design. The regulator is desirable but the
location shown (low-side series) is just one option. Without a regulator,
tube current will need to be set by controlling the power input and/or
selecting the ballast resistor.
+----------+ +------------------+ HV+ Ballast Resistor
o------| |-----| Starting Circuit |--------------/\/\----+
Input, | Main | +------------------+ Rb |
AC line | Power | |
or DC | Supply | +-----------+ Tube- +-----------+ Tube+ |
o------| |-----| Regulator |----+----|-| -|-------+
+----------+ HV- +-----------+ _|_ +-----------+
- HeNe tube
Depending on the open circuit voltage of your power supply, a ballast resistor
in the 30K to 150K ohm range will be essential to limit the current to the
value specified for your particular tube. A higher voltage supply and larger
ballast resistor will be more stable if there is no built-in regulation. This
results from the fact that the voltage drop across the tube is relatively
independent of tube current so it subtracts out from the supply voltage. What
is left is across the ballast resistor and changes by a proportionally greater
amount when the line voltage varies. The smaller the ballast resistor, the
more this will affect tube current.
You really do want the current to be close to that recommended for your tube.
This can be accomplished by adjusting either the input voltage to the power
supply or the output current and/or by selecting the value of the ballast
resistor. Excessive current is bad for the tube and will actually result in
decreased optical output. It is not possible to pulse a HeNe laser for higher
power. Without any regulation, the value of the ballast resistor is more
critical and power line fluctuations will significantly affect tube current
though such variations may not matter for non-critical applications. Note that
since the tube itself provides a relatively constant voltage drop around its
nominal current, a small change in l