Semiconductor Lasers

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A semiconductor laser is a type of laser wherein the active medium is a semiconductor material, such as a laser produced by a p-n diode junction.

Laser operation requires three things: 1) a medium for amplification; 2) a resonant cavity for feedback; and 3) a form of stimulation. In a laser diode, light emission occurs when holes and electrons recombine in the diode's junction, with the optical amplification of this emission achieved by a population inversion of the charge carriers (as explained below).

A semiconductor laser's resonant cavity consists of an optical waveguide constructed on the crystal, which confines the light to a narrow path. The ends of the optical waveguide are partially transparent so that photons traveling along the waveguide may be reflected several times at the end faces before they are finally emitted.

The source of stimulation or excitation for a laser diode is the electric current flowing through the diode's junction.

Figure 1. Photos of a laser diode component and a semiconductor laser

When light is produced by the junction and the wave travels through the optical waveguide, it is amplified by stimulated emission. Stimulated emission is a process whereby a photon hitting an electron causes the latter to emit a second photon of the same phase, after which the electron drops to a lower-energy state.

The optical gain achieved by this amplification process is diminished (if not negated) by optical losses resulting from photons being absorbed by the material and imperfect reflection of the light at the cavity ends. Thus, the lasing action of the semiconductor material only starts when the light amplification achieved by stimulated emission exceeds the light losses from absorption and incomplete reflection.

Silicon, an indirect bandgap semiconductor, is not commonly used as laser diode material because the process of stimulated emission is much less efficient in indirect bandgap semiconductors than in direct bandgap semiconductors.