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Semiconductor lasers, also known as laser diodes, are lasers that use semiconductor materials as working materials. It has the characteristics of small size and long lifespan, and can use simple injection current to pump its working voltage and current compatible with integrated circuits, so it can be monolithically integrated with it. Due to these advantages, semiconductor diode lasers have been widely used in laser communications, optical storage, optical gyroscopes, laser printing, ranging and radar.
The laser must meet the following conditions: First, the population reversal; Second, There must be a resonant cavity, which can play an optical feedback function and form a laser oscillation; the formation of various forms, the simplest is the Fabry-Parrot resonant cavity. Third, the laser must meet the threshold condition, that is, the gain must be greater than the total loss.
In order to form a stable oscillation, the laser medium must provide a sufficiently large gain to compensate for the optical loss caused by the resonant cavity and the loss caused by the laser output from the cavity surface, and continuously increase the optical field in the cavity. This requires a sufficiently strong current injection, that is, enough population inversion, the higher the population inversion degree, the greater the gain obtained, that is, a certain current threshold condition must be met. When the laser reaches the threshold, the light with a specific wavelength can resonate in the cavity and be amplified, and finally form a laser for continuous output.
To actually obtain coherent stimulated radiation, the stimulated radiation must be fed back multiple times in the optical resonator to form laser oscillation. The resonant cavity of the laser is formed by the natural cleavage surface of the semiconductor crystal as a reflection mirror, which usually does not emit light. The end of the lens is coated with a high-reflective multilayer dielectric film, and the light-emitting surface is coated with an anti-reflective film.
The inversion distribution of carriers in the lasing medium (active region) is established. In semiconductors, the energy of electrons is represented by an energy band composed of a series of energy levels that are close to continuous. Therefore, in order to achieve population inversion in semiconductors, it must be in the high-energy state conduction band between the two energy band regions. The number of electrons at the bottom is much larger than the number of holes at the top of the low-energy state valence band. This is achieved by applying a forward bias to the homojunction or heterojunction and injecting necessary carriers into the active layer. Excite electrons from the lower energy valence band to the higher energy conduction band. When a large number of electrons and holes in a state of population inversion recombine, stimulated emission occurs.
