What is the difference between solid-state (SS), semiconductor and gas lasers? I know SS lasers are optically pumped (generally), while semiconductor and gas lasers are electrically pumped (laser diode). The active mediums are also unique. Beyond this though, I'm not entirely sure about any other differences. I've read about efficiencies for example, but usually it says, 'this laser is more efficient than this laser, or this laser is more efficient than that laser.' Nothing that gives general information, i.e. 'SS lasers are less efficient than semiconductor lasers'.