Editing Laser diode (section)

=== Optical cavity and laser modes ===
As in other lasers, the gain region is surrounded by an [[optical cavity]] to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal's surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a [[Fabry–Pérot]] resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they exit. As a light wave passes through the cavity, it is amplified by [[stimulated emission]], but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to ''[[lasing threshold|lase]]''.

Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, the light is contained within a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the transverse direction, if the waveguide is wide compared to the wavelength of the light, then the waveguide can support multiple [[transverse mode|transverse optical modes]], and the laser is known as ''multi-mode''. These transversely multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small [[Diffraction limited beam|diffraction-limited]] TEM00 beam, such as in printing, activating chemicals, microscopy, or [[laser pumping|pumping]] other types of lasers.

In applications where a small, focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single transverse mode is supported and one ends up with a diffraction-limited beam. Such single-spatial-mode devices are used for optical storage, laser pointers, and fiber optics. These lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously. The wavelength emitted is a function of the bandgap of the semiconductor material and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the bandgap energy, and the modes nearest the peak of the gain curve will lase most strongly. The width of the gain curve will determine the number of additional ''side modes'' that may also lase, depending on the operating conditions. Single-spatial-mode lasers that can support multiple longitudinal modes are called Fabry-Pérot (FP) lasers. An FP laser will lase at multiple cavity modes within the gain bandwidth of the lasing medium. The number of lasing modes in an FP laser is usually unstable and can fluctuate due to changes in current or temperature.

Single-spatial-mode diode lasers can be designed so as to operate on a single longitudinal mode. These single-frequency diode lasers exhibit a high degree of stability, and are used in spectroscopy and metrology and as frequency references. Single-frequency diode lasers are classed as either distributed-feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers.