types of laser

There are three types of lasers: solid state, gas and liquid. While all work according to the same general principles, they are differentiated on the basis of the medium they employ to create the laser action.
In solid state lasers, an electrical current pumps electrons into the laser medium – typically a semiconductor – exciting electrons that are fixed in the medium. Driven into higher energy states, a condition known as a population inversion, the excited electrons quickly decay back into lower energy states, releasing the excess energy as photons. Carefully positioned mirrors bounce photons hitting them at 90 degree angles back and forth, in turn stimulating other excited electrons to emit photons with identical wavelengths, directions of propagation, and polarizations; this is a process called amplification. Because the mirrors are of unequal reflectivity, the photons are eventually able to escape and their output constitutes the laser action.