Build a Cutting Laser

Cutting lasers are already used by the US Government; in the event of a nuclear missile attack, a plane equipped with such a laser would, in theory, be launched to destroy the missile. Here is how to build it.

Steps

  1. Choose the type of laser you want, there are multiple variations of the CO2 laser. The slow flow, fast flow and pulsed (which can be a slow or fast flow). The type of laser you will be able to build at home is going to be a slow flow continuous wave. Even a slow flow laser is going to cost a lot of money even if you are the best scrounger around. Mirrors alone can run $200 each. Suitable vacuum pumps run about 300-400 used. Tubes, machine costs and glass work can cost many hours unless you can do this yourself. Neon sign transformers are expensive for 30-60ma outputs like you will want, although you can usually find used ones for fairly cheap. Gas bottle rental and gas is also expensive. It can be done though and being known as the guy/girl who built a laser that can light things on fire from across the room can be fun.
  2. You will need laser gas A average home built laser known as the slow flow laser and will have only about 4-5% CO2, 14-15% N2 and Helium Balance. A commercial high power laser will run the higher CO2 levels to increase the power. This works in a high power laser because they pump the gas through the tube at high speed and cool it externally. A roots type blower is usually used to recirculate the gas in an industrial laser. Laser gas is going to be premixed high grade gas, slow flow mixes are not as common as fast flow designs so you will probably have to get them to mix it for you. When starting with a laser do yourself a favor and start with premixed gas. With all the variables involved trying to custom mix gas is a pain in the butt. I used an industrial gas mix of 3.5% Co2 15% N2 and Helium Balance. This is not ideal but it works and was a lot cheaper than a custom fill.
  3. Next you will need a glass tube. You are going to fill this tube with laser gas and suck it down to a vacuum of about 10-30 torr, for which you will need a vacuum pump of good quality. The faster your pump can suck out the gas you put in and maintain the 10-30 Torr the more power you will be able to get out of the laser. I used a rotary vane type pump and it works very well.
  4. Neon sign glass is usually leaded glass is not ideal for a CO2 laser and if you are going to try to build one of these fork out and buy borosilicate glass which will not crack from thermal stresses from your local glass blowing shop, or even better get them to build you a cooling chamber around it with some ground glass joints for connections to some custom mirror mounts you can make. Using a all glass chamber and cooling jacket with glass hose connections will save a lot of time and probably money, making the cooling chamber insulated from the mirror mounts can be troublesome at best. Electricity at this voltage will jump through your cooling water instead of the tube without a problem.
  5. Lasers require an electron source, you can use a Neon sign transformer or a couple sign transformers paralleled up to generate the power. If you are aiming for a 100W expect about 10% efficiency so buy a 1Kw power supply. To keep an arc initiated you are going to have to use around 15kV for a 1.2M discharge tube. If you build a smaller laser you can drop it to a 12kV. Using DC power is preferred because if you use AC the laser output will pulse at 60Hz. Try taking a piece of paper and quickly move it through the beam, you will see dots as the AC waveform increases and decreases. If you want to run DC you will need to buy a high voltage rectifier and capacitor filter, which is going to run a lot of money.
  6. You will need laser mirrors to make this work. CO2 lasers emit light at 10,600uM which is far into the infrared spectrum and invisible to the human eye. This wavelength will not pass through materials like glass and water and are essentially opaque. There are two mirrors on a CO2 laser the rear mirror which is called the TR (Total Reflector) and the OC (Output Coupler) The total reflector will reflect all light which hits it and is usually made from a copper or silver material. The OC is made from a special material usually Zinc Selenide (ZnSe) which is very transparent to the CO2 wavelength. This mirror is partially coated with a reflective coating so 75% of the light will bounce back and 25% will escape. This bouncing back and forth of the light causes the Laser (Light Amplified by Stimulated Emission of Radiation) to gain power.
  7. You will need a cooling system. Get some liquid nitrogen, and a some copper tubing. If you can't get liquid Nitrogen, cold tap water is usable if you keep the laser at low enough temperatures.
  8. Connect the liquid nitrogen source and get it pumping through the tubing.
  9. Note: Most Lasers you would build at home and even high powered ones in industry do not use liquid nitrogen for many reasons. Cost, the temperature gradient between the glass and plasma is going to cause tube cracks without the utmost care, and it is dangerous to work with not to mention at best efficiency of 20% the Liquid Nitrogen is going to boil vigorously even at high flow rates. Most lasers cool the tube with cold water through a recirculating pump and a radiator. Most high power cutting lasers are called fast flow lasers they recirculate the gas through the plasma tube and cool it external to the tube in a heat exchanger. CO2 Lasers work best at cool plasma temperatures so the colder you can keep the gas the more power you can dump in and the more output you get.
  10. You will need to align your laser, this can be very fun the first time you do it. You will need a diode laser some tape and patience. To align a laser you need to put the OC in place and remove the TR. Align the diode so it passes directly down the center of the tube parallel to the laser bore. The diode laser should have a piece of tape in its path with a small hole in the tape. You want to adjust the OC so the reflection off the OC on the tape is reflected directly back on the source. Once you have the OC aligned you install the TR and adjust it until you see rings all aligned on each other. The rings will show because the TR is usually concave. I used a 10M focal length for mine. The ideal focal length is going to based on tube length and bore size. There are calculations on the internet for this. Look at SAMS laser FAQ.
  11. Fire up the laser. You should see anything in front of the laser burst into flames. The first time I ran my laser I thought I screwed up because there was no things bursting into flames. Even after alignment you will have to fine tune the mirrors to get anything useful. Use a focusing mirror positioned the correct distance (focal point) from a piece of carbon. Carbon tends will glow very brightly even will small amounts of heat. The focusing lens will focus the power so you can tell if you are getting anything. Unfortunately unless you have a camera which can pick up far IR, which is unlikely you are going to have to rely on burning things to tell if it is working.
  12. Once you have output and have tuned the mirrors for maximum brightness from the carbon you can go ahead and have fun lighting things on fire. The carbon will be so bright I needed welding goggles to protect my eyes.

Tips

  • You can experiment with different power levels and gases.
  • CO2 pulse lasers can handle power levels in the GW.

Warnings

  • Laser gases can cause asphyxiation if vented in a closed room, vent your pump outside and run a fan.
  • This project uses extremely high DC or AC voltages at currents which will kill you in a split second. CO2 laser light will not pass through clear safety glasses in any significant amount for lasers under 100W, the lenses burn pretty quick though so don't stay standing there. Always use safety glasses and never touch the laser while power is on. If you actually build one of these you will have do a final tune of the mirrors with power applied, use a special tool which is insulated for the voltages we are talking about and only if you have insulated mirror mounts. Do not allow yourself to become a current path, especially through your heart.
  • The laser beam is infrared, so it is invisible.
  • Most importantly, if someone steps in front of the beam, the skin in the path of the beam will be burned. Make certain that all humans and animals are behind the laser and that you have sufficient shielding in place to prevent the laser from continuing on and hurting someone.
  • The radiation emitted by the laser beam is extremely harmful to your eyes. You will need special safety glasses. CO2 Lasers will not damage your retina but will burn the surface of your eye, this can cause permanent damage, depending on power levels and focus you may notice the heat before damage occurs, if your lucky.
  • You will probably want to install an on/off switch.
  • Do not have anything in front of the laser you don't want destroyed.

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