It's funny that the most powerful laser generally available is one of the easiest to build for the hobbiest. Since almost all CO2 tubes are sealed these days (up to 1KW or so) it's just a matter of building or purchasing a power supply for the tube, provide some water cooling and viola, a metal blasting, eye melting good time. Not bad for a gas laser. Unfortunately, if you want to do 100W you can figure the tube to be a good four feet long. The only easier gas laser to build is a nitrogen laser if you like MW pulses. These operate in the UV range and produce exceedingly short pulses (hence the MW power output).
Solid state lasers are a blast to build (literally). Unfortunately, they require many hours of mirror alignment. Any misalignment and the cavity does not resonate. You are left with a YAG crystal that fluoresces nice but does not lase. Fortunately, you can buy resonators ready to go for very little money such as the SSY1 from the range finder of the M1 tank. The U.S. Army replaced the YAGs with Er:Glass if I remember correctly as these are more eye safe. With SSY1 in hand it's just a matter of building a circuit to energize and trigger the flashlamp. I built one using electronic parts around the shop. It was not pretty but it worked. The capacitors I used were huge. I blew out the passive q-switch on the second shot. No worries though, it's easily removed.
Here's a video of it:
[media]http://www.youtube.com/v/vJGxT2E1Bhk&hl=en[/media]
They are fun and pretty inexpensive.
