Laser

Stimulated emission alone is not sufficient to produce laser output. The emission from the atoms occurs in all directions; to produce the highly directional output that makes the laser such a useful tool, the light must be trapped in an optical cavity that provides feedback, i.e., forces the light to travel in a desired direction. An optical cavity is free conduction band electrons recombine with holes in the valence band, emitting photons equal in energy to the band gap of the semiconductor. The only suitable semiconductors are the so-called "direct gap" materials, such as gallium arsenide and indium phosphide. By using alloys of different compositions, the laser bandgap can be engineered to produce light over a wide range of wavelengths (426 nm-1,550 nm). Silicon, having an indirect gap, is not suitable for diode lasers.

Diode lasers are typically very small (the laser chip, with wires running into it, is mounted on a copper block only 6 mm wide). The cavity is formed by cleaved crystal facets that act as mirrors. The cavity length is typically between 100 microns and 1 mm, and the emitting volume has a cross-section of about 1 micron high by 1-200 microns wide. A diode laser can produce power in the range 1 mW to 1 W, depending on the size of the active volume. Standard semiconductor processing techniques can be used to form arrays of individual lasers in order to generate higher powers: an array with an emitting area of 1 cm X 1 cm can produce several kW of optical power.