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What issemiconductor laserThe device?

The semiconductor laser was successfully excited in 1962 and achieved continuous output at room temperature in 1970. Later, after improvement, a double heterojunction laser and a laser diode with a strip structure were developed, which are widely used in optical fiber communication, optical discs, laser printers, laser scanners and laser pointers (laser pointers).

in the basic structure,semiconductor laserIt belongs to the P-N interface of semiconductors, but the laser diode is a "double heterojunction structure", in which the metal cladding sandwiches the light-emitting layer (active layer) from both sides. In laser diodes, the interface acts as a transmission mirror (resonator). As the material used, there are gallium (Ga), arsenic (As), indium (In), phosphorus (P) and the like. In addition, Ga · Al · As is also used for the multiple quantum well type. For the stripe structure, even a small current will increase the electron number inversion density in the active region. The advantage is that the excitation form is simple, and the service life can reach 100 to 1 million hours.

Semiconductor lasers are sources of coherent radiation. For it to produce a laser, three basic conditions must be met:

Gain conditions: establish the inversion distribution of carriers in the laser medium (active region). In a semiconductor, the band representing the energy of an electron consists of a series of nearly continuous energy levels. Therefore, to achieve population inversion in semiconductors, the number of electrons at the bottom of the high-energy conduction band must be much larger than the number of holes at the top of the low-energy valence band between the two energy band regions. This is accomplished by adding a positive bias to the homojunction or heterojunction and injecting the necessary carriers into the active layer. Electrons are excited from the lower energy valence band to the higher energy conduction band. Stimulated emission occurs when a large number of electrons in the population inversion state recombine with holes.

In order to truly obtain coherent stimulated radiation, the stimulated radiation must be fed back many times in the optical resonator to form a laser oscillation. The laser resonant cavity is formed by using the natural cleavage surface of the semiconductor crystal as a reflector. Generally, the non-light-emitting end is coated with a highly reflective multilayer dielectric film, and the light-emitting surface is coated with an antireflection film. For F-p cavity (Fabry-Perot cavity) semiconductor lasers, it is convenient to use the natural cleavage plane of the crystal perpendicular to the p-n junction plane to form the F-p cavity.

In order to form a stable oscillation, the laser medium must be able to provide sufficient gain to compensate for the optical loss caused by the resonator and the loss caused by the laser output at the cavity surface, and to continuously increase the optical field in the cavity. This requires a sufficiently strong current injection, I .e. a sufficient population inversion. The higher the degree of population inversion, the greater the gain obtained, I .e. a certain current threshold condition must be fulfilled. When the laser reaches the threshold, light with a specific wavelength can be resonated and amplified in the cavity, and then a laser is formed and continuously output. It is shown that in semiconductor lasers, the dipole transition of electrons and holes is the basic light emission and light amplification process. For new semiconductor lasers, it is recognized that quantum wells aresemiconductor laserThe basic driving force for the development of the device. The topic of whether quantum wires and quantum dots can take full advantage of quantum effects has been extended to this century. Scientists have tried to make quantum dots with various materials with self-organizing structures, and GaInN quantum dots have been used in semiconductor lasers. In addition, scientists have also created another quantum cascade laser with stimulated emission process, which is based on the transition from the sub-energy level of the semiconductor conduction band to the low-energy state of the same band. Because only the electrons in the conduction band participate in this process, it is a unipolar device.