The format of the laser specifications will follow the general outline given below. (Since most of these are gas lasers many of the entries will be missing for the dye laser.)
The following notation will be used to denote input and output connections:
Duplicating (or improving on) a known commercial design rather than one from a 20+ year old article could very well result in higher efficiency and output power and better beam quality. See the section: Sam's Three Part Process for Getting Your Feet Wet in Gas Lasers for one possible approach to this.
Where possible, an attempt will be made to estimate the expected power (or at least an upper bound) based on tube dimensions, power supply, and other factors. At best this will be a wild guess but may provide some indication of the possibilities for improvement by tweaking the design.
Note that this also means that it is NOT possible to determine the laser safety classification for these lasers. They maybe of very low power but this is not guaranteed. So, treat their output as at least Class IIIb (Class IV for the CO2 laser) until you can be sure that it isn't!
Refer to Typical Home-Built Helium-Neon Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics.
H o----------+ T1 T2 +---------o HV AC
)|| ||=||(
Variac )<--------------+ || ||(
0-110V )|| )|| ||(
5A )|| Neon Sign )|| ||( Well insulated
)|| Transformer )|| || +---+ HV wires to HeNe
+--+ 9KV,18mA )|| ||( | tube electrodes
| )|| ||( |
N o-------+-------------------+ || ||( |
||=||( |
| | +---|-----o HV AC
| | |
G o-----------------------------+---+-----+
_|_
////
A modern tube with a 34 cm discharge length would be rated about 5 mW when run from a normal HeNe laser power supply (DC, constant current).
For an HeNe tube, a wider bore doesn't necessarily permit higher power since the walls of the capillary apparently are important in depopulating higher energy levels. Therefore, I will assume that this is really equivalent to a 1 mm bore in a modern tube.
Loss factor: .5.
Loss factor: .5.
Loss factor: .5.
Loss factor: .5.
Based on these considerations, I would be surprised if the original design produced more than .5 mW. But the good news is that it might be possible to approach 2 or 3 mW without too much effort using a narrower bore, large can style cathode, and modern HeNe power supply.
As described in the article, the procedure is extremely risky. The alignment is performed with the laser powered but presumably not lasing since a 'spoiler glass' - a glass slide - is placed in the optical path. While this probably is fairly reliable for the low gain HeNe laser (but I wouldn't want to bet my vision on it!), many other lasers have high enough gain that the losses introduced by the spoiler would NOT prevent lasing and a beam could appear without warning (once the mirrors are aligned well enough) as the adjustment screws are being tweaked! I would NOT recommend the procedure as described for any laser unless a more reliable method were used of preventing accidental lasing (like the use of a 50 percent neutral density filter).
The description is also somewhat ambiguous. My interpretation is that the mirror closest to the observer is aligned first by centering the cross-hair image and then the other mirror is adjusted to center the 'moon' image. This is then repeated from the other end of the tube.
Also, using red gelatin for the viewing filter would seem to prevent observing anything about the far mirror as this would require passing red light - the same color as what the mirrors are designed to reflect best - through the closer mirror.
I would recommend using one of the other alignment techniques described in "Light and its Uses" or in the chapters in the document on HeNe and Ar/Kr lasers.
You will need the vacuum setup and a source of the HeNe gas mixture, but the serious glass working can be postponed for another day.
The basic idea will be to start off with a laser resonator that once worked (a commercial HeNe tube) using a regular HeNe power supply. Inexpensive HeNe tubes and power supplies are readily available and therefore, much of the uncertainty can be easily eliminated so you can concentrate on the gas and vacuum issues.
I would recommend something in the 5 to 10 mW range - large enough to be interesting but not so long as to possibly require magnets or other special attention to operate reliably.
Note: Don't file all the way through as this will deposit metal particles and who-knows-what-else inside the tube.
WARNING: The anode will be at a KV or more with respect to everything else! Cover, shield, or otherwise insulated it from accidental contact.
Note: For this to work, both mirrors must be planar - otherwise, the added distance between them when mounted externally will mess up the confocal relationship that would have been present where one or both was concave. Alternatively, use mirrors from a *longer* HeNe tube (though I don't know if the mirror coatings are optimized for a particular length discharge or not).
Use Epoxy to attach the windows to the tubes and let them set for a couple of days. Then clean them inside and out with denatured alcohol.
You can use a pair of identical electrodes and the AC power supply described in "Light and its Uses". However, it would also be possible (with just a little more glass-work) to provide a large side-tube (which also provides a much greater gas reservoir) and aluminum (can) cathode as in commercial tubes with a regular HeNe (DC) laser power supply.
Refer to Typical Home-Built Argon Ion Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics.
D1
H o--------+ T1 T2 +--|>|--+------+------+------o HV+
)|| ||=||( 10KV | | |
Variac )<--------------+ || ||( | | |
0-110V )|| )|| ||( | | / R1
5A )|| Neon Sign )|| ||( | _|_ C1 \ 2M
)|| Transformer )|| || +-------|--+ --- 1uF / 3W
+--+ 5KV,30mA )|| ||( | | | 5KV \ 5KV
| )|| ||( | | | |
N o-----+-------------------+ || ||( | | | |
||=||( D2 | | | |
| | +--|>|--+ +---+------+--+---o HV-
| | 10KV | _|_
G o---------------------------+---+------------+ ////
Triggering could be done using any of the pulse type HeNe starting circuits described in the chapter: HeNe Laser Power Supply Design but as above using the external electrode rather than a direct connection to the tube anode. Any similar xenon strobe trigger should also work.
A modern Ar/Kr ion tube with a 50 cm discharge length would likely be rated at over a WATT when run from a high current power supply. Obviously, since this one is pulsed at a very low duty cycle, the power will be much reduced considerably.
For a HeNe tube, a wider bore doesn't necessarily permit higher power since the walls of the capillary apparently are important in depopulating higher energy levels. I don't know how this affects ion laser performance. Therefore, I will assume that this is really equivalent to a 1 mm bore in a modern tube.
Loss factor: .5.
For a 30 mA maximum source, a 1 uF capacitor can charge at 30,000 V/second best case. It is not clear at what voltage the tube will fire when triggered or how this related to actual peak laser tube current. There is no ballast resistor. In 1/120th of a second, the cap will charge to to perhaps 450 V (from a tube cutoff of 200 V).
Best case, if the energy represented by the difference between 200 and 450 V when dumped into the tube with the optimum current of, say 20 A, the pulse width would be about 15 us (very wild guess!). Thus, compared to a modern ion laser run on a DC power supply, the ratio of output power to what is possible is about 1:(1.5E-5 * 120) or about 1:555.
Loss factor: .9982.
Loss factor: .75.
Loss factor: .5.
Here is a set of questions from someone with somewhat grandiose ideas:
You probably won't end up with a sealed tube either - as it will have a very short life.
Then, you need to have a vacuum system, tubing, gauges, the appropriate gasses, a 5 A or more 100 V current regulated power supply, etc.
Light bulb filaments are not designed for emission - they are not coated. You would have to obtain a proper coated tungsten filament. And, of course, a 300 W bulb is only about 2.5 A so even if you were able to do this and sent current from both ends, you are on the hairy edge. A microwave oven magnetron tube filament might work although these are rated more in the 1 A maximum range.
I must say that I encourage you to pursue your interests and passions, but perhaps start with something somewhat less ambitious!
However, if all you want is a different colored laser, green laser pointers are coming down in price as are other Diode Pumped Solid State (DPSS) lasers:
(From: Walter Burgess).
If you want to try a different color, a green DPSS laser might be easier. An 808 nm diode laser emits into a Nd:YAG crystal (substitute Nd:VYO4 for better results). That crystal emits at 1064 nm. This is applied to a KTP crystal which doubles it to 532 nm which is green colored light.
If you decide to attempt the argon ion laser, I would probably recommend using bore closer to 1 mm since this should assure TEM00 mode operation and is more typical of a commercial ion tube. Then, the current thresholds should be similar as well.
Its output is at 10.6 um (10,600 nm) or mid-IR. This wavelength is about 10 to 30 times longer than the other lasers under discussion and often considered a source of a heat beam than a light beam. At this wavelength, normal glass and plastic optics are either too lossy or totally opaque so alternatives must be found both for the end-mirrors and any other mirrors, lenses, or prisms required by the external optical setup. Divergence/diffraction effects are also increased by this same factor so obtaining a collimated beam is also more difficult.
Many common materials including wood, paper, plastics, composites, and properly prepared metal surfaces absorb quite well at this wavelength so the CO2 laser makes an effective marking, cutting, welding, and heat treating tool.
See the chapter: Carbon Dioxide Lasers for more information on the characteristics and applications of these devices.
It is possible for an amateur to construct a working CO2 laser in the 10 to 50 W range without too much difficulty - at least compared to some of the other types. A vacuum system is needed but the range of operating vacuum is modest - 10 to 100 Torr. And while several gasses are needed, the purity of the final gas fill isn't nearly as critical as for, say, the HeNe laser. With a bit of resourcefulness, no fancy glass work is needed. The power supply can be just a neon sign transformer on a Variac.
My only complaint about CO2 lasers in general are that the beam is totally invisible and boring in some ways. :-) Otherwise, it is nearly the perfect choice for a home-built laser - high power, continuous operation, and relative simplicity!
However, constructing a CO2 laser is still NOT easy in an absolute sense and you have to make a considerable commitment of time and effort in locating parts and materials, setting up your vacuum system and gas supplies, fabricating the tube and water jacket, constructing the power supply, and putting it all together, or you will end up with a box full of parts and equipment gathering dust in the attic.... :-(
This includes plans for a HeNe power supply as well as complete ruby/Nd-YAG and CO2 lasers and other interesting stuff. See the section: Iannini CO2 Laser Description for more information.
The summary below is for the CO2 laser from "Light and its Uses". Also see the section: Iannini CO2 Laser Description.
Refer to Typical Home-Built Carbon Dioxide Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics.
CO2 can be easily obtained from dry ice; N2 from filtered air (despite the presense of O2 and other trace gasses) apparently works well enough.
D1
H o--------+ T1 T2 +--------+--|>|--+---o HV+
)|| ||=||( | |
Variac )<--------------+ || ||( | D2 |
0-110V )|| )|| ||( +--|--|>|--+
15A )|| Neon Sign )|| ||( | |
)|| Transformer )|| || +--+ | |
+--+ 12KV,100mA )|| ||( | | | D3
| )|| ||( | | +--|<|--+
N o-----+-------------------+ || ||( | | |
||=||( | | D4 |
| | +--|--+-----|<|--+---o HV-
| | |
G o---------------------------+---+----+
_|_
////
The laser can be operated on the raw output of the neon sign transformer (AC)
but the efficiency and maximum power output may be lower (though it isn't
entirely clear why this should be the case since the discharge current is
varying in the same manner for both cases). With the addition of some
filtering and a regulator and/or ballast resistor, the optimal current could
be maintained continuously boosting efficiency and power output. However, a
starter like a high power version of that used for modern HeNe laser tubes may
be required. See the HeNe power supply information in the sections beginning
with: Starters - Voltage Multiplier, Pulse, Inverter, Piezo.
Although generally similar to the CO2 laser from "Light and its Uses" (see the section: Home-Built CO2 Laser Description, it is more than twice as long and should therefore be capable of much higher power operation. Some glass working is specified (though this could easily be avoided with minimal design changes) and the power supply has more frills (but is only AC instead of DC and therefore will not be as efficient).
My original CO2 laser and power supply is pretty much along the lines as the Scientific American as described in the section: Home-Built CO2 Laser Description
I have recently built a smaller and more compact power supply for it. This consists of a 9,000 volt, 30 mA neon sign transformer, manufactured by the Canadian firm of Allanson, as the basic unit. This is the type of transformer with the two high voltage terminals coming directly out the top of the tar insulation, all enclosed in a somewhat larger metal case. This case left plenty of room for a bridge rectifier, key switch, fuse, 'on' light, milliameter and a 3 amp Variac. A six foot 3-wire appliance cord with IEC input module and two 20 inch high voltage output leads, with color-coded alligator clips complete the package.
This power supply seems to perform as well as the 15,000 volt @ 60 mA unit that I had employed in the past with this modest sized laser. I no longer have a supply of commercial CO2 laser gas and have been using the 'three separate gas' system for some time now with good success. I have been using a commercial cylinder of compressed helium (useful for most other lasers also), nitrogen from the air (bubbled through water) and sublimating dry ice for the CO2 supply. I had bought a small supply of dry ice on Friday which lasted only until Saturday evening, due to its tendency to mysteriously disappear even when stored in an insulated cooler. Out of sheer curiosity today I decided to try a much more readily available source of carbon dioxide gas to operate this laser with. Searching through the cupboards I found some white vinegar and a small box of baking soda. Lo and behold, a small amount of this mixture in a flask produced a fine source of CO2. I was pleasantly surprised to open the needle valve from this flask to the laser and see a fine burn pattern appear on my thermal FAX paper target. No more dry ice! I just thought you may wish to pass this on to the amateur laser community. This really simplifies the gas problem for small CO2 lasers.
I am not sure of the power output I am getting with this system, but it will easily ignite the FAX paper in a matter of five to ten seconds without any focusing optics. The burn pattern is quite large also with this wide bore (1 inch) plasma tube. I have read about your plan for measuring CO2 output power by heating a measured quantity of water for ten minutes. I have yet to try this yet, basically for lack of a suitable reflector to steer this beam into a container. I have a simple thermometer with a flat black penny attached to the stem to see temperature rise from the beam impinging upon said target. This allows some comparison of output power, but is there anyway to translate this to watts? I am getting about a one and one half degree centigrade rise in temperature per second at full power.
(From: Sam).
Yes, by knowing the mass of the penny (about 5 grams I think) and the rate of initial rise in temperature, you should be able to calculate the approximate beam power. What you need is temperature rise/Joule (or calorie) of energy for a standard penny. :-) I say 'initial' since losses due to convection and radiation of the heat energy will be small. This is still going to be tricky without a serious effort to insulate the 'sensor'.
Since water is a good absorber of 10.6 um radiation, a simple 'water heating test' like that used to evaluate the performance of a microwave oven should be able to determine the actual power in the laser beam to a fair degree of accuracy. To do this, provide a way of directing the beam from your CO2 laser downward - a polished copper plate used a mirror, for example. Place 100 ml of water in a small thermos (dewer) and measure its temperature. Run the CO2 laser beam into the water for 10 minutes. Then, the power in the beam is given by: P(beam) = Temperature rise in DegC multiplied by .7. I do not know to what extent the reflectivity of the water will affect these readings but the experiment is easy enough!
I'm considering a project of building a small CO2-laser. I have a fully reflective mirror (germanium with gold coating) and output-coupler (transparent material with gold coating) from a 500 W laser. These mirrors are old but usable according to the person who kindly gave them to me from a company doing laser-cutting.
(From: A. E. Siegman (siegman@ee.stanford.edu)).
The output coupler is possibly zinc selenide (expensive); coating probably is *not* gold, but a gold-colored dielectric coating.
If mirrors are salt (NaCL) (they'll be quite light in weight) keep 'em dry!
(From: Kristian Tapani Ukkonen (kukkonen@epsilon.hut.fi)).
I have plans for a CO2-laser (from "Light and its Uses" but several questions do arise: The article does not really comment the ratio of discharge tube length to the diameter and how to correlate this to optimal performance and pressure and ratio of gasses. They do mention about 18" long and 1" diameter tube with 1-20 torr and 8 parts He, 2 N2 and 1 CO2 while with narrower tube higher pressures apply. Their efficiency is rather low (about 1%) while I've read even 30% can be achieved. Why would this be - does errors with pressures and tube dimensions cause this? They do use 12 kV 0 to 100 mA to drive the tube with <10 W output.
(From: A. E. Siegman (siegman@ee.stanford.edu)).
CO2 insensitive to tube diameter; gain goes up linearly with length. 1" diameter is largish; 1 cm more common; 18" length is quite short; 60 cm to 1 m might be more like it.
Power output per unit length is almost independent of tube diameter (larger tube has more gas, but gas gets hotter because of longer diffusion length to walls, and so you don't get more power).
Optimum pressure, mixture and discharge conditions probably best determined by trial and error. There are probably known recipes for optimum conditions, but still, trial and error is the easiest way to go.
(From: Kristian Tapani Ukkonen (kukkonen@epsilon.hut.fi)).
Sputtering to mirrors. The article has the mirrors directly mounted to the ends with bellows of brass as means to align them. I'd guess that the mirrors get destroyed by sputtering at least at one end?? In order to prevent this I've thought about using bi-sectional tube with middle grounded and either +V or -V at both ends. Would this help? What is it that really is the threat - ionized electrode material? I've planned using stainless-steel. I'd rather than mount mirrors to direct contact to plasma do use some windows at ends and mirrors at some distance away - does this sound reasonable or will I face problems - if not what would be a good material for the windows and do they have to be at brewster angle - I've seen some designs of high-gain lasers that do not use angled windows.
(From: A. E. Siegman (siegman@ee.stanford.edu)).
Be careful!!!! Bellows could be hot electrically, and even if you think bellows are grounded, discharge could jump to mirrors.
I shouldn't give advice on this, because I really don't know, but I have the impression the CO2 laser discharge is a fairly 'soft' glow discharge, and doesn't do a lot of sputtering or damage to mirrors.
CO2 gain fairly low; if not intracavity mirrors, then Brewster windows essential. Large salt flats possible, but fragile; big ZnSe slabs best but pricey, and angle is very steep.
(From: Kristian Tapani Ukkonen (kukkonen@epsilon.hut.fi)).
I do use a 220 VAC:20 KV 7KW externally current-limited transformer for this project. I can control both voltage and current with Variac and adjustable inductance -> 0 to 20 kV 0 to 350 mA
(From: Sam).
That sort of power supply is gross overkill for a small CO2 laser and very dangerous. I would recommend using a luminous tube transformer as suggested in the article - still very dangerous but not quite as instantly deadly.
(From: Kristian Tapani Ukkonen (kukkonen@epsilon.hut.fi)).
Laser cavity shall be constructed using pyrex-tube with stainless-steel ends (electrode/window-holder/adjuster) and plastic water-jacket around the glass tube. Vacuum is maintained with mechanical 2-stage pump.
I can put a focusing lens after completing the laser.
Water
_ In Water Jacket _
Mirror | |_____||____________________v_________| | Mirror
|| | |________________.__._________________| | ||
|| I ________________|__|_________________ I ||
|| | |_________________||___________ _____| | ||
|_| || || |_|
Electrode Ground and Water Electrode
Vacuum Pump Out
The gas-feed is at the end-electrodes, vacuum-pump connected to middle.
(From: A. E. Siegman (siegman@ee.stanford.edu)).
In chemical glassware catalogs you can find water cooled straight distillation columns in meter lengths. These make a good cheap water-cooled CO2 laser tube.
(From: Kristian Tapani Ukkonen (kukkonen@epsilon.hut.fi)).
Any other comments welcome.
Just to limit the amount of safety concerns: I have experience about high voltages (even Tesla-coils) and shall use adequate protection to IR protective goggles and IR camera to view beam-lines. I'm no mad scientist but an amateur with real need to a cutting laser.
The problem is that there are such a wide variation in what it will cost to obtain things like vacuum pumps that it is hard to put prices on a completed laser. Depending on your resourcefulness, scavenger ability, and luck, it could be less than $100 but it could also be a lot more. But, if you haven't anything set up now, there will be serious additional overhead. An adequate vacuum pump could easily blow the budget right there.
(From: Joe or JoEllen (joenjo@pacbell.net)).
Yes it possible to build a medium to high power laser with a limited budget but even the best scrounge would have a tough time doing it for $100! I started building a multi-watt CO2 laser years ago but had a hard time maintaining mirror alignment during pump-down (the corner-cutting design is to use the end mirrors to seal the resonator in lieu of ZnSe brewster windows used in the commercial versions.
BTW, I also started building the argon laser featured in the same book.
(From: Dave (dave-a@li.net)).
I work for a CO2 laser manufacturer (name withheld) and it depends on what you want to cut with it.
You'll need laser gas mix (on the cheap side), three flow meters (expensive), and then either nitrogen or oxygen to cut depending what you are cutting, polarizing mirrors, output mirrors, etc, etc. Its going to be expensive anyway you look at it. What are you trying to cut? And how much wattage?
(From: Christopher R. Carlen (crobc@epix.net)).
On first note, if you are not experienced with electronics, you should know that a CO2 laser power supply is VERY DANGEROUS. The high voltage and high current involved can quickly kill you. There will be absolutely no room for error. Get acquainted with basic electronics, and then electrical safety skills before setting out to build a laser power supply.
My plans for building a CO2 are not really plans, but rather practical engineering formulas that come from a declassified old CO2 laser manual from the U.S. Air Force's early experiments with CO2 and CO laser technology for Star Wars applications.
My purpose for building the thing is, well quite simply, my professor told me to do it. He wants a CO2 on a long rail which will allow the cavity length to be adjusted over a large range to demonstrate laser concepts to optics students. Plus, it should be a whole lot of fun, and a good experience for me.
If you want to cut stuff, I would recommend a commercial CO2 or Nd:YAG laser, from the surplus market, like from MWK Industries. Currently you can get something in the $1500 to $5000 range that will do the job, and you will avoid a great deal of time invested in building your own which might turn out to be disappointing, or worse, deadly if you aren't sure what you're doing.
With respect to the cost of components alone needed to build your own CO2 laser, you must consider:
Well I would be highly surprised if anyone could really build a CO2 for $100 as mentioned at Information Unlimited. You must understand that they are a dealer in some good information, and much highly suspect information.
(From: Dave (dave-a@li.net).
Damn that's cheap! The CO2 (4.5 or 4.8) usually goes for a lot more then that! Better the gas, the longer your optics will last. The N2 and He are the cheap ones (compared to CO2). I just went through this with a customer. He bought 4.0 CO2 and there was a *major* jump in price to scientific grade (4.8 I think it was. 99.998%) The 99.99 wasn't good enough. Though he should get away with premix (Mazack I now uses/used premix).
But you're right, buy a surplus 'head' its cheaper. Plus you now have a working unit that you can modify to tweak up the power :).
Refer to Typical Home-Built Nitrogen Laser Assembly for a simplified diagram of the overall structure and low voltage operated inverter type power supply electronics.
The Blumlein circuit is approximately equivalent to the following:
R1 L1
- o----/\/\------+-------CCCCC-------+--------+
| | |
+--------> <--------+ |
20 KVDC C1 _|_ Laser _|_ C2 v SG1
--- Cavity --- ^ Spark Gap
| | |
+ o--------------+-------------------+--------+
The HV power supply charges both capacitors to nearly 20 KV. When the spark
gap breaks down, C2 is shorted out and the full voltage on C1 appears across
the gap along the length of the laser cavity instantly causing the nitrogen
gas to ionize. (Actually, the voltage pulse hits the center of the cavity
first and spreads to the ends in a couple of nanoseconds!) R1 simply isolates
and limits current from the HV power supply - its value is irrelevant when the
spark gap triggers!
Note: The drawings in "Light and its Uses" are at the very least confusing and may be wrong in some aspects of the description of operation of the Blumlein effect.
You won't find the PCB material at Radio Shack but it should be available at reasonable cost from surplus electronics dealers or PCB fabrication houses. However, tracking down one in your neighborhood may take some finger or leg work. Here are a couple of sources found in the Portland, OR, area, for example:
(From: Scott & Pat Chewning (patch@europa.com)).
WARNING: Handle and dispose of used etching chemicals responsibly - depending on type, they may stain or eat pipes (copper drain lines!!!), masonry and other building and landscaping materials, as well as human flesh. Read and follow all package instructions.
For the Blumlein capacitors as specified for the N2 laser, proceed as follows: Taking care not to penetrate the copper, use a sharpened nail or awl to scribe lines on the *surface* 2 cm from all 4 edges top and bottom, and another pair of lines front to back across the board 5 cm apart on the top only for the gap between the two plates. Then, use an Xacto knife or razor blade to lift the copper from the from the board at a corner or edge. The copper can be peeled from the board to back to these lines and then flexed back and forth to break the copper. I don't know what, if any, latent surface damage may result from this approach. Therefore, the chemical etch is preferred if you can stand the mess!
The puncture voltage for acrylic plastic (Plexiglass) varies between 450 and 990 volts per mil (.001") depending upon the quality of the product. Common 1/8" (.3 cm) thick sheet (assuming the lower value for puncture voltage) should be able to withstand over 55,000 volts; 1/16" sheet (.015 cm) will handle over 25 KV which is still adequate. The problem that you will encounter is the low capacitance obtained with the same surface area of even the thinner Plexiglass material (compared to the .015" (.04 cm) Fiberglass-Epoxy (though as noted above, that thickness may not be quite enough to withstand 20 KV). The dielectric constant for Plexiglass is also lower and with the increased thickness results in much less total capacitance. Stacking additional layers may not be a viable option due to the nanosecond timing requirements!
What about using discrete capacitors? You are, of course, welcome to experiment. However, I might suggest that starting with something that is known to work would be prudent even if it takes a little longer to find the proper materials. Homemade capacitors made from 10 or 100 or whatever of 1 or 2 KV units in series-parallel is asking for trouble especially in a rapid discharge circuit like this. The inductance of the leads alone will probably greatly slow the rate of discharge. Any inequality in capacitance and even how they are connected will result in voltage differences across the capacitors so they may fail immediately if not sooner. Even commercial high voltage capacitors may be inadequate. And, distributing enough of either type of discrete capacitor in just the right locations to simulate the discharge wave of the Blumlein device could prove quite a challenge in its own right! >
(Responses from: placayo@hotmail.com) unless otherwise noted).
I had it in the back of my head that input amperage also mattered somehow. In this case, milliamperes as generated by the usual laser power sources Van de Graafs and Wimshurst machines put out mere microamps. On the other hand, the nanosecond switching in Blumlein's pulsar (such as in the Scientific American column) guarantees an avalanche of current at high voltage in the laser channel. So, input current is not really critical.
Or he could have gone the simpler route and used air as the lasing medium. I understand that air lases (even at normal pressure) when used in a traveling wave laser. It's a more likely scenario. They were tireless and thorough experimenters back then to a degree we rarely find today. Some busy-body, Old Ben perhaps, may have slapped together something resembling the Blumlein switch, just to see what happens, fired it up, watched a curious fluorescence in the long notch on one Leyden jar plate or capacitor plate, packed up and went home. He wouldn't have an idea that he had just invented a laser because the output is UV and invisible.
There'd be no record of it consequently. And history slips through his fingers. If the would-be inventor had been Ben Franklin, the device would look exactly as it does today. At one point, Franklin preferred flat-plate capacitors in his work. He popularized them. They called them "Franklin squares", I think.
The traveling wave laser is one example of a technology that was sought for because it was predicted by established theory. But because of its simple construction, it could just as easily be the sort of thing that might have been invented by accident way before -- and ignored.
Or what old, "obsolete" inventions or long-discarded ideas might be resurrected with the benefit of today's know-how and materials.
(From: Sam).
Of course, in Ben's time, there was no way to realize that laser action was taking place or that such a thing was different than ordinary light anyhow, no obvious way to see UV, or reason to even check for it.
For one to claim that an invention was created they either have to have sought it out before hand or understood the ramifications of the results of a fortuitous experiment. Neither of these were really possible for another couple of centuries. :-)
Charging current is irrelevant except to the extent that it affects the repetition rate and to overcome electrical leakage in the setup (which can be quite minimal).
In fact, in the article, they suggest alternatives - their little transistor ignition coil thing and a neon sign transformer. It just affects the repetition rate. The transistor circuit probably puts out order of 10 uA at 20 KV (they say 1 percent efficiency!). A neon sign setup, perhaps 2000 times this. When the spark gap fires, the current available from the charging source is of no consequence.
It would be interesting to connect a Van De Graff or Wimhurst machine to such a setup.
(From: Chris Chagaris (pyro@grolen.com)).
Well yes, I have fired my nitrogen laser by using my Wimshurst machine as the charging source. I just tried this on a whim, and it worked. It does take a little while to get the capacitors charged to firing potential though. It is a nifty combination of the old and the new.
I followed the development of nitrogen lasers from their beginning. You will find most of the articles in Journal of Physics E Scientific Instruments in about 1968 to 1975 but that was a long time ago. The dates are ballpark. Nitrogen lasers can be used at atmospheric pressure with a helium nitrogen mix. Sulfur hexaflouride changes the near ultraviolet lifetime. The length is limited to about three feet in some designs with power output in the MEGAWATTS. They can be more than a glorified flashtube. The voltages, RF, flash, in short nearly everything about some of the designs can be deadly or permanently maim you. The cavity and capacitor are simple but must be used with the utmost caution. Study in depth first - then build!!!!!!
(From: Jon St Clair (Jon_St_Clair-QJS006@email.mot.com)).
I had a friend that made one of these once, but I never saw it operate.
It was a remarkably simple device and consisted of two sheets of metal separated by a dielectric like a sheet of Plexiglas. The lower sheet, viewed from the end, was shaped like the letter "J" laying on its back. The upper sheet was positioned so that a spark gap formed between the hook of the J and one edge of the upper plate.
The spark gap was covered on top with plastic so that nitrogen could flow along the spark gap. The spark gap was slightly more narrow on one end than the other.
When a DC high voltage charged up what was essentially a parallel plate capacitor, a nitrogen arc forms at one end of the spark gap. At least one photon of UV causes ionization of the nitrogen right next to it line with the spark gap and results in a spreading out of the discharge along the length of the spark gap.
The spark starts at one end of the gap, propagates at the speed of light along the gap adding energy to a coherent packet of photons as they travel along the gap. I am not clear on this, but I think that the pulse width of the laser may be equal to the length of the spark gap divided by the speed of light. This is a very very short period of time, and if the conversion efficiency of the energy stored in the parallel plate into coherent UV is some reasonable amount(?) then the peak power could be huge. x-Watts/second divided by a very small number of seconds!
I think obtaining a source of nitrogen and finding a optically clear (at the UV wavelength) and distortion free lens for one end of the spark gap was the only hard stuff other than building a high voltage DC power supply (and that is not hard).
When you hook up a continuous DC supply, you get a burst, it charges up again, bursts, charge up again etc. I understand it will really light up UV dyes some distance away.
(From: Dr. Peter Schellenberg (peter.schellenberg@physik.uni-ulm.de)).
Among the lasers described in "Light and its Uses" and other Amateur Scientist articles, I guess the nitrogen laser is the cheapest one, since you do not even need mirrors, and can go around with some handmade stuff and school equipment. The electrical discharge is done by a home-made capacitor in a so-called Blumlein arrangement. (Something similar can also be done to build a CO2 laser. However, in this case you need mirrors, preferentially gold mirrors. Don't be shocked, these are not that expensive).
By using a cylindrical lens, you can focus the beam from the N2 laser in a cuvette with almost any dye and get superradiant laser emission from it.
There are also a lot of descriptions for laser construction in the following scientific journals:
I prefer older articles, lets say between 1965 and 1980, some of the lasers at that time were not as complicated (and expensive!) as later designs.
Note that there are only few lasers that can really build with less than a few 100$.
(From: Richard Alexander (RAlexan290@gnn.com)).
Building a good nitrogen laser is almost trivial, and low-cost. All you have to do is send a 100 KV arc across a spark gap in a moderately low pressure (100 torr, a little more than 1/10th atmosphere) nitrogen environment. The output is crude, but $15,000 nitrogen lasers don't make a much better beam than a $200 device (the difference between the two appears to be versatility and electrical noise suppression).
(From: Chris Chagaris (pyro@grolen.com)).
I have just re-built my N2 laser and have substituted a PVC pipe in place of the plexiglass box. This withstands the vacuum much better than the box design. The box would contract whenever the vacuum was applied and was prone to leaks. I have recently obtained a cylinder of compressed nitrogen gas (for free!) to use in this laser. With the leak-proof construction and the pure N2 gas, the output of this laser has increased dramatically. I have no way of measuring the output power, but when the beam strikes a fluorescent piece of paper, it is so bright that it is somewhat uncomfortable to look at. UV protective eye ware is worn at all times, but these are clear to visible light.
I am in the process of building a small dye head to be pumped by the output of this N2 laser. I have heard that many of the dyes will 'lase' superradiantly when pumped by such a source. As these dye heads are of simple construction, I will build some with resonant optics and some without. This should be a very interesting area of experimentation.
(From: Frank Roberts (Frank_Roberts@klru.pbs.org)).
The supply needed to excite a nitrogen laser is basically a capacitive-discharge DC supply. The high voltage AC from the transformer is diode-rectified to DC and then applied across a suitably rated capacitor. Since a 15 kv neon sign transformer is rated at 15 kv RMS (average), the actual peak voltage from the transformer is greater than that by a factor of the square root of two (1.414). This means that a 15kv transformer will charge the capacitor up to 15 KV x 1.414, or over 21 KV. This is plenty of oomph for a Nitrogen laser. Be careful, this voltage applied across a capacitor is deadly! If you need more voltage, two diodes and two capacitors can be connected in the form of a voltage doubler. Your 15kv transformer will now provide 42.42 kilovolts of pump energy. You'll find that these voltages, capacitors and diodes can become expensive. One way around this is to connect your neon transformer to a relatively inexpensive voltage tripler. These are used in some color TV sets to get the 30 to 45 KV needed for the picture tube. These are potted modules containing all the capacitors and diodes in one unit. You can salvage them from broken TVs or buy them new at any electronics parts house.
Boosting the high voltage or capacitor size isn't the way to go. Increase the length of the laser tube itself to a meter or so and then move the Blumlein spark gap to one corner to introduce a traveling wave so the gain follows the coherent light down the tube.
(Replies from: Roland A. Smith (see.www.lsr.ph.ic.ac.uk@web.pages)).
I've a big problem. I have to build a Nitrogen-Laser for school. I got a scheme to build something like that in the Scientific American. It's based on the Blumlein-effect.
+7kV ______ooooo______
___________|____> L <____|__________________ Spark gap
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx '
-----------------------------------------------------+-------------
0V
xxx is Epoxy-circuit-board --- is copper
> L < is laser chamber ooooo is inductor
Sorry for that terrible scheme! :)
Ok, but now the PROBLEM!!
If i'm firing up the system, it doesn't want to create any kinds of laser radiation. I'm despairing!! PLEASE HELP!!"
I have built a few of these in my time (and have run this project for first year undergrad students), and there are some critical points to watch out for. First off, SAFETY, these things can kill you if you get a good solid shock from them, so take care and follow all the usual high voltage handling guidelines. If you or your supervisor haven't done HV work before, then I strongly suggest you get someone who has to look over your working practices etc. Make very sure the PCB capacitor and power supply are properly discharged before you go near it. Remember that 7 KV can jump a fair way across surfaces to fingers etc.
Ok, with that bit out of the way, you need to check the following points.
(From: (gary@astro.as.utexas.edu)).
Subject: 1 MW N2 laser for sale $1 per pound.
Date: 08/25/97
I have this mondo 1 Mw peak N2 laser that weighs in at 1200 pounds. It is in plug-and-play condition complete with documentation. All you need is a source of nitrogen. I have a dewer and bought some LN2 and used the boil off to run the laser. Most impressive!! It made everything that was even remotely fluorescent in my garage glow intensely. I had planned to use it to pump some fluorescent plastic for a down converter to coherent visible light but I have lost momentum and want my garage back.
I want to sell the laser for $1,200. I can supply copies of the manual and photographs to anyone that is interested. It has a rather small vacuum pump that will allow 5-10 Hz rep rate but it can sustain a much higher repetition rate with a larger vacuum pump. The laser was manufactured my Molectron and is a model UV1000.
There are some other chuckles along the same lines awaiting you as links from its parent page as well.
Serious note (sort of): Aside from the obvious danger involved in obtaining the needed power supply, this is not likely to work in any case. Due to the self limiting behavior of the nitrogen laser emission characteristics, the electrical discharge would need to be precisely timed to travel along the laser tube at roughly the speed of light as in the normal Blumlein power supply rather than simply blowing a modest size crater in a one location. The required Lightning Bolt Control board (RISC microprocessor based, of course) would be one of the many major challenges of this design.
(Portions from: Brian (LOGICWIZ@prodigy.net)).
I have an idea for building a GigaJoule laser out of simple materials. The power source might seem outlandish at first but very viable in the end. This laser would be based upon the nitrogen laser. It would be constructed out of a glass or PVC pipe 4 inches x 100 feet evacuated to 25 Torr filled with nitrogen gas from probably LN or a decomposition reaction and the electrodes will be made out of band saw material (because it stands heat real well). The laser would be operated in a thunderstorm. You already know how to attract lightning using wires attached to rockets. I estimate its power to be equal to about 14.8 GigaJoules based on the charge and voltage of your typical lightning bolt and the expected (very precisely computer) efficiency of a nitrogen laser:
Refer to Typical Home-Built Helium-Mercury Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics.
D1
H o--------+ T1 T2 +--------+--|>|--+--+-------+---o HV+
)|| ||=||( | | | |
Variac )<--------------+ || ||( | D2 | | |
0-110V )|| )|| ||( +--|--|>|--+ | /
3A )|| Neon Sign )|| ||( | | _|_ C1 \ R1
)|| Transformer )|| || +--+ | | --- 10nF / 100M
+--+ 9KV,20mA )|| ||( | | | D3 | 15KV \ 5W
| )|| ||( | | +--|<|--+ | | 20KV
N o-----+-------------------+ || ||( | | | | |
||=||( | | D4 | | |
| | +--|--+-----|<|--+--+-------+--o HV-
| | |
G o---------------------------+---+----+
_|_
////
Refer to Typical Home-Built Helium-Mercury Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics which should be similar except for the dimensions.
(Specifications from: Garrett Hansen (gldhansen@worldnet.att.net)).
D1 Spark Gap
H o--------+ T1 T2 +--------+--|>|--+--+-------+--> <---o HV+
)|| ||=||( | | | |
Variac )<--------------+ || ||( | D2 | | |
0-110V )|| )|| ||( +--|--|>|--+ | /
15A )|| Neon Sign )|| ||( | | _|_ C1 \ R1
)|| Transformer )|| || +--+ | | --- 20nF / 100M
+--+ 15KV,60mA )|| ||( | | | D3 | 35KV \ 25W
| )|| ||( | | +--|<|--+ | | 35KV
N o-----+-------------------+ || ||( | | | | |
||=||( | | D4 | | |
| | +--|--+-----|<|--+--+-------+--------o HV-
| | | D1-D4: HVC1200
G o---------------------------+---+----+
_|_
////
I am just about to finally complete a mercury vapor laser as per the SciAm plans. All that I have left to do is polish flat the brewster angle cuts at either end of the plasma tube so my quartz windows will fit snugly. I am looking forward to many hours of exciting experimentation with this rare type of laser. I will get some pictures of the completed system to you as soon as I can.
I gather from the "Light and its Uses" HeHg laser description that while the laser produced 'intense pulses of green and red light', the average output power wasn't more than a mW or so. Even a 1 mW of 567 nm green would appear quite intense - 4 or 5 times brighter than 1 mW of 632.8 red.
Also see the section: Comments on HeHg Lasers.
However, a tube full of mercury vapor at a couple of Torr is not much material so a drop of mercury should go a long way. In addition to losses out the vacuum system and by condensation on the tube walls, some may form an amalgum with the metal of the electrodes or their lead-in wires (depending on the metals involved), be adsorbed by the tube walls or Epoxy seals (if used), or be buried under sputtered electrode material.
Also see the section: Comments on HeHg Lasers.
Just off the top of my head, I'd say the problem with it is the mercury. That is toxic stuff, and it goes right through your skin. Make it a vapor and you would breath it. The less you fool with it, the better off you are.
Another problem might be that mercury is expensive. A pint jar of mercury would be worth hundreds or thousands of dollars (well, a lot, anyway). The same amount of copper would be worth only a few dollars.
(From: David Demmer (ddemmer@physics.utoronto.ca)).
Nah, in an earlier life I was a chemist, and demonstrated a physical chemistry lab that used a Toppler pump. For the uninitiated, this consists of some ingenious glassware and about 500 mL of mercury. The stuff is really pretty cheap when bought in quantity: actually about $100/kg for reagent grade from Aldrich. Of course a kg is a smallish volume because it's surprisingly dense stuff.
As for a laser like a helium-mercury vapour laser, the amount of mercury necessary is actually pretty small. And besides, expense doesn't stop anybody using a gold vapour laser. The cost of the fill is trivial compared with capital and other operating costs.
Toxicity, too, never stopped anybody from using HF, F2, etc. in their excimer lasers.
So, while not knowing the answer to the fellow's question, I guess I'm saying that I don't agree with yours.
(From: Richard Alexander (RAlexan290@gnn.com)).
Well, it looks like you pretty well stopped my argument for the expense, but I think toxicity is still in play. In defense of that argument, I present my belief that excimer lasers are used mostly in industrial settings, away from most people (ok, I know there are medical excimers that offer some interesting promise).
Maybe it's not too late for me to agree with the other poster that the wavelength put out by a mercury vapor laser is better produced by another type of laser. (I've had this discussion before, several years ago. It's just taking time for me to remember what I was told.) It seems like it isn't worth the bother of making the laser when HeNe and Argon Ion cover whatever the Hg vapor laser will do (ok, I admit I don't remember what lasers cover the Hg vapor laser wavelengths. You get the gist of my hazy memories.)
(From: Herman de Jong (h.m.m.dejong@phys.tue.nl)).
Mercury is used in home used thermometers, in thermostat switches for heating the home, in tilting switches, in high power reed switches, in TL end related light tubes etc.
The mercury laser uses maybe a few milligrams/year and it could be trapped in an amalgam with silver, copper, zinc, not iron, about the only exception. this effectively blocks mercury from reaching the pump.
A thermometer contains maybe a few hundred milligrams this is no problem.
(From: Herman de Jong (h.m.m.dejong@phys.tue.nl)).
I agree with you David.
If I remember right the CW laser wasn't peculiarly intense but it had low (single ended) optical feedback and the length of the vapour was a few feet. and it was a DC discharge at a few KV with a neon transformer. It needs a Helium flushing bottle and a vacuum pump. At that wavelength it probably can't compete with the more powerful Ar+ lasers (514 nm, some are sealed) and the much simpler green HeNe(sealed). The Neon transformer (6 KV, 100 mA?) suggests also very poor efficiency. But for your home built laser it is a nice project and even the vacuum pump can be very cheap by using the compressor from a discarded air conditioner, refrigerator, or freezer.
The expensive setup would only be acceptable it the power could be high. I suspect the population of the concerning state densities to be low although they have very high absorption/stimulated emission probabilities. This means the Sc.Am. is probably operating near saturation and it is hard to increase output energy. More length will have a linear effect, higher vapour pressure (T) is complicated. Maybe a different kind of discharge favors the population of concerning states but it could be a lot more complicated then this.
I did a bit of a literature search on the helium-mercury laser.
The Scientific American design is gain limited, but the power scales with tube diameter up to say 2 inches, and gain scales pretty much proportionally to length as well. The laser not only emits in the green and red, but has many lines in the IR around 1.2 to 1.8 microns. It has lased continuously on the visible lines in a hollow cathode type tube. It is also super-radient. It lases after the excitation pulse has almost died out (lases in afterglow). For all intents and purposes it seems like a ideal low cost laser and would probably be useful to someone.
However I've noted two things, No one ever seems to have measured the output power, save for one citation listed below and two, papers stopped appearing on it after 1970 or so. One wonders if it does not have some major fault, as it was discovered by one of Spectra-Physics chief scientists, and yet nobody makes it commercially?
The above book has plans for a two meter long Scientific American style design and a hollow cathode design excited by a thyratron based pulser, it does 10 W pulsed on the red line.
This still leaves the question, Why is it not in Use?
(From: David Demmer (ddemmer@physics.utoronto.ca)).
Well done!
The large bore size and scaling sound a lot like a copper or gold vapour laser, i.e. unstable or planar multimode resonator. This would make it lose out to, say a Kr ion laser for beam quality, and a copper/gold vapour for raw power in the green or red.
On a different note, I don't know that the phrases "ideal low cost" and "thyratron based" should appear in the same discussion ;->.
(From: Sam).
Aside from the cost issue of a thyratron pulsar, the need to keep fiddling with the gas pressure/fill to maintain laser action in an efficient and stable manner could be a factor in the lack of commercial viability. Unlike a fluorescent or high pressure discharge lamp using mercury vapor, the HeHg laser would appear to operate at a very low pressure (1 or 2 Torr) where a stable gas fill could not easily be achieved. Therefore, some possibly complex and costly feedback mechanism would be needed to produce a viable product (possibly along the lines of a low flow CO2 laser). This may push it over the cliff so to speak when compared to alternatives like those mentioned above. Then again, perhaps it was a simple business decision to go with Ar/Kr ion rather than HeHg and now that any patents have run out, they are just keeping its potential benefits and ease of construction quiet. :-)
Since, the lifetime of the separate atoms is short, this must be repeated for each activation of the laser. This makes the power supply design a bit more interesting than the run-of-the-mill neon sign transformer! Also they are pulsed lasers but at a high enough repetition rate, the output will appear continuous.
This can be accomplished by a simple but brute-force motor driven distributor somewhat like that used in an automotive ignition or by a fancy sophisticated solid state power supply. The former looks and sounds like something out of a bad Sci-Fi movie (or nightmare, take your pick) but works!
Like the N2 laser, CuCl and CuBr lasers do not need a resonator to operate. The design in "Light and its Uses" uses a mirror at one end and an *unsilvered* piece of glass at the other (output) end. (Of course, plain glass will act as a partial reflector - reflecting about 4% at zero-degree incidence).
(From: Steve Quest (Squest@galileo.cris.com)).
Most lasers need to resonate to build up emission. However, copper vapor lasers would damage themselves if they were allowed to resonate. There is so much photonic emission you only need one mirror, the back mirror in the cavity of a Cu laser. One pass through the cavity is enough, Cu lasers produce hundreds of watts per pulse!