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Principle of a Laser
Excerpt from Field Guide to Lasers
In order to understand the basic principle of a laser, it is instructive to first consider a passive resonator ("cavity"), such as an arrangement of mirrors that creates a closed path for a light beam. The simplest configuration is made with only two mirrors, one being flat and one being curved. Due to that curvature, a light beam with a suitable beam radius can circulate around the resonator without getting wider and wider each time. However, its optical power will decay, as some energy is lost in every resonator round trip.
A so-called gain medium can now be inserted that, when supplied with energy ("pumped") in some way, amplifies the light in each round trip. If the gain g is lower than the resonator losses l, the power decay is only slowed down. For g = l, the optical power stays constant; and for g > l, the power rises with each round trip. The latter condition can not be maintained forever; sooner or later, the high intracavity intensity will saturate the gain. In the steady state, as reached after some time, the gain will be exactly sufficient to compensate for the resonator losses. We then have continuous-wave laser operation with constant optical power and g = l.
For extracting a laser beam as a useful output of the device, the left mirror, for example, acts as an output coupler, transmitting some percentage (say 10%) of the intracavity power. The output coupler transmission for optimum output power depends on the available gain and on other optical losses in the resonator.
As mentioned above, the gain medium needs to be pumped (i.e., supplied with energy). In most cases, a laser-gain medium is pumped either electrically (e.g., with an electric current through a semiconductor structure) or optically (e.g., with light at a typically shorter wavelength than the laser light being absorbed in the gain medium).