Reality → Tech → Laser → How a laser works
To initiate and sustain a lasing process, a significant input of high-intensity energy is required [1] . The key is to create an excited state for the larger part of atoms (see population inversion). While the full process details are dauntingly complex, the design of a basic solid-state laser can be remarkably simple, that is if the right materials and dimensions are chosen. The following main components are involved:
Thermal equilibrium is the normal state of matter and its molecules and atoms. To excite atoms to a higher energy level, optical or electrical 'pumping' of energy from an external source is necessary. Overall efficiency rates are normally below 30 %, with most of the input energy converted into waste heat.
A flashlamp spiraled around the lasing medium, as realized with the first laser, is but one way (and not the most efficient one) of transferring the atoms of the lasing medium into an excited state.
The lasing medium consists of a dopant (normally less than 1%) evenly distributed in the host material (normally more than 99%). The dopant is the critical component in which stimulated emission of the laser radiation is generated. Chromium was the dopant in the monocrystalline synthetic ruby rod of the first laser.
Two opposed mirrors at the ends of a reflecting cylinder surrounding the pump source and lasing medium act as an optical resonator for the first laser. The mirrors bounce the photons emitted by stimulated emission (as well as other photons) back and forth through the lasing medium, causing additional stimulated emission with each passage until the lasing threshold is reached and a laser pulse exits the resonator.
The partially reflective mirror (output coupler) is coated with a partially reflective material. The output coupler of the first laser was 95 % reflective and 5 % transparent.