Green diode lasers are a lot different than those common red lasers you see all the time.
In a 640nm red laser pointer, there's a red-emitting diode and a lens to collimate (focus) the beam.
In a 532nm green laser (pointer or larger size), there's a BIG infrared laser diode that generates laser light at 808nm, this is fired into a crystal containing the rare-earth element "neodymium". This crystal takes the 808nm infrared light and lases at 1064nm (yes, deeper in the infrared!). This 1064nm laser light comes out of the NdYV04 (neodymium yttrium vanadium oxide) crystal and is then shot into a second crystal (containing potassium, titanium, & phosphorus, usually called KTP) that doubles the frequency to 532nm - the bright green color you see. This light is then collimated (focused) by a lens and emerges out the laser's "business end". Just before the lens, there's a filter that removes any stray IR (infrared) rays from the pump diode and the neodymium crystal. You don't want that stuff in your green beam, trust me. /ubbthreads/images/graemlins/smile.gif
This is why green diode lasers are so much more expensive than red ones. Lots of itty bitty parts, and they all need to be aligned by hand. If the polarisation is "off", one or both crystals need to be turned. With red diode lasers, you just slap in the diode and slap a lens in front of it.