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    Carbon Dioxide Lasers

    Sub-Table of Contents

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    Introduction, Links to Other Information

    High Power Lasers

    Carbon Dioxide (CO2) lasers make all the types discussed in previous chapters seem like LEDs compared to xenon arc search lights! Where common diode, HeNe, and even most Ar/Kr lasers put out mW of optical power, nearly all CO2 lasers are rated in WATTs or KILOWATTS. Even the smallest sealed CO2 laser looking much like a HeNe tube, will produce 10s of W continuously. Large CO2 lasers used in the metalworking industry for welding, cutting, annealing, and so forth may have continuous outputs of up to 30 KW or more.

    The distinguishing characteristic of the CO2 lasing process that makes these sustained power levels possible is its relatively high efficiency - at least compared to most other common gas lasers. The typical electrical power in to optical power out efficiency of a CO2 laser may be anywhere from 5 to 20 percent or more (compared to less than .1 percent for a HeNe or Ar/Kr ion laser).

    Unlike the other lasers producing visible or short near-IR light, the output of a CO2 laser is medium-IR radiation at 10.6 um. It is the classic heat ray of science fiction. I have no doubt that the Martians in H. G. Wells' "The War of the Worlds" used CO2 lasers powered by cold fusion generators (probably with superconducting electrical backup storage) for their directed energy weapons. :-) (Chemical lasers would have required bulky reactant storage tanks to achieve the number and length of blasts and none were visible!)

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

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

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

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

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

    Links to Information on CO2 Lasers

    For an introduction to CO2 laser technology, see:

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

    Types and Excitation

    Types of CO2 Lasers

    The most common types of excitation are a direct electrical discharge (usually DC) and radio frequency (RF).

    Sealed CO2 Lasers

    Commercial sealed CO2 lasers have much in common with sealed HeNe lasers. Like HeNe lasers, they use DC electrical current excitation requiring a very high voltage for starting and somewhat less (but still high) voltage while operating. However, specific requirements differ: Therefore, requirements for the power supply to be used with sealed CO2 and HeNe lasers are very similar. However, due the significantly larger negative resistance characteristic of the CO2 laser, there is more incentive to push part of the effective ballast resistance into the control of a switchmode inverter rather than a simple power wasting resistor.

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

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

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

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

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

    RF Excited CO2 Lasers

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

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

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

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

    Gas Fill

    Composition and Pressures for CO2 Lasers

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    Why is There Carbon Monoxide in Some CO2 Lasers?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


    A Discussion on CO2 Laser Optics

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    (From: Sam).

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

    What Passes or Block the CO2 Laser 10.6 um Wavelength?

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

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

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

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

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

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

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

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

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

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

    Burning and Cutting Lasers, Costs

    Small Wood Burning Lasers

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

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

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

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

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

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

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

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

    (From: Steve Roberts (osteven@akrobiz.com)).

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

    Large Wood Cutting Lasers

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

    I am currently operating a 1500 W Amada LasMac 1212 Pulsar Laser. I have been able to cut 1/2 plywood at 350 inches a minute at 1/4 power (Power=700 Frequency=1000 Duty =35% Assist gas=3Kg shop air

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

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

    This guy Randy is full of s**t. cutting wood with shop air is a fire risk for one, and two wood has physical properties that make it very undesirable to cut. Cutting with oxygen is a no no and nitrogen makes it cut slow, about 10 ipm at 2600 watts, that is slower than it will cut 1/2 inch stainless steel. To suggest you can cut 1/2 inch wood with 375 watts is a tale only an Amada salesman could dream up.

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

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

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

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

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

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

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

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

    Cutting Printed Circuit Boards

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

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

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

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

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

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

    Large Cutting Laser - Not Quite a Basement Project!

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    Continue Dreaming

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

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

    (From: Tjpoulton (tjpoulton@aol.com)).

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

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    Miscellaneous CO2 Laser Tidbits

    CO2 Laser Communications

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

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

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

    Novel Idea for CO2 Laser

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

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

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

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

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

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

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

    Too bad glass is opaque to 10 micron radiation.

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

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

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

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

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

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

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