Define the necessary elements of a laser cavity.
A laser generates and amplifies light. This is achieved using an optical gain medium, a resonant optical cavity, and pumping (electrical or optical).
The material of the gain medium absorbs radiation in the desired wavelength. However, when the material is pumped by electrical or optical energy, the material is excited to a non-equilibrium energy level. In the state of non-equilibrium, the incident radiation is amplified instead of absorbed. As the non-equilibrium state electrons enter a de-excitation state, radiation is generated.
Inside the resonant cavity, standing waves or modes form at wavelengths of multiples of twice the cavity length. As the gain of a mode overcome its losses, that mode is emitted.
Very often a single mode is desired. This is achieved using the mirror design of the cavity, spectral filtering elements in the cavity, and the spectral response of the gain medium.
Types of lasers include the diode/semiconductor laser, fiber lasers, gas lasers, solid-state lasers, and dye lasers.
Diode lasers can be pumped directly using electrical current and are more efficient. Efficiencies of diode lasers are in the range of 50%.