Laser Restoration

Aside from interest in things that go boom, I also have a bit of fascination with lasers. Not the common semi-conductor variety that you see in laser pointers and DVD players, but noble gas lasers. Last night I picked up an old Spectra-Physics helium-neon laser from a friend of mine. It’s nothing really powerful. A bit more powerful than your standard laser pointer, but it’s not a Class IV laser that can punch holes throughi solid steel.

Unfortunately, the thing didn’t work, probably because the gas inside has become too impure. The composition of a laser tube isn’t all that different from a neon sign. You have a cathode and anode, and a high voltage power source pumping the electrons of the He and Ne atoms into an excited state. But it takes more than this to get the gas medium to lase.

To accomplish that, you have to pump a large number of atoms into an excited state, and establsih population inversion. When these exited atoms’ electrons return from their excited state to the ground state, they emit a photon. In a laser configuration, you have two reflective mirrors on either side of the tube. One mirror will be highly reflective, and the other semi-reflective. This forms an optical catvity, or resonator along the axis of discharge. What’ll end up happening is you’ll have photons moving back and forth in the cavity hundreds of times, where they’ll interact with other excited atoms, casuing them to emit photons themselves. This process is called “stimulated smission”. Every photon produced through stimulated emissions will be of the same wavelength and move in the same direction as the stimulating photon. Once you build up enough light radiation within the reasonator cavity, some photons will begin to escape from the slightly less reflective mirror, producing a coherent beam of light that most people are familiar with. Helium neon lasers produce a nice, bright pink/reddish light. You can get other colors using other gases as your lasing medium. Laser pointers use a solid state laser, which operates a bit differently than this, but the principle is basically the same.

But the laser I got didn’t work. Just like a neon sign, after a while you start introducing impurities into the tube, both from the glass, the sealants on the tube, and from the operation of the cathode and anode. Most gas lasers have something in them called a “getter”, which is a device, usually heat activated at the time of manufacture, which sucks impurities out of the tube. There are different ways to heat up the getter to reactivate it, in an attempt to restore the laser function, but the easiest way is just to remove the tube from its power source and microwave the thing for a few seconds until you notice the tube start to come back to life. The microwaves induce a current in the metal part, heating it enough to activate. Sadly, this didn’t work for me. The light show inside the tube was impressive, but still no lasing when I reconnected the tube to the high voltage power supply. Sad.

You can actually build your own Class IV lasers, which can actually cut and burn things. There are kits and plans out there if you look. Generally, it’s CO2 lasers that make the best implements of destruction. The cool thing is, unlike guns, lasers are pretty much unregulated. I will post about that a little later in the day.