Semiconductor lasers: types, device, principle of operation, application

Semiconductor lasers are quantum generators based on a semiconductor active medium in which optical amplification is produced by stimulated emission in the quantum transition between energy levels at a high concentration of charge carriers in a free zone.

Semiconductor laser: principle of operation

In the ordinary state, most electrons are located at the level of valence. When photons supply energy exceeding the energy of the discontinuity zone, the electrons of the semiconductor come to a state of excitation and, crossing the forbidden zone, pass into the free zone, concentrating at its lower edge. At the same time, the holes formed at the valence level rise to its upper boundary. Electrons in the free zone recombine with holes, emitting energy equal to the energy of the discontinuity zone, in the form of photons. Recombination can be amplified by photons with a sufficient level of energy. The numerical description corresponds to the Fermi distribution function.

Device

The semiconductor laser device is a laser diode pumped by the energy of electrons and holes in the pn-junction region-the point of contact of semiconductors with p- and n-type conductivity. In addition, there are semiconductor lasers with an optical energy supply, in which the beam is formed by the absorption of photons of light, as well as quantum cascade lasers whose operation is based on transitions within the bands.

Composition

Standard connections used in both semiconductor lasers and other optoelectronic devices are as follows:

• Gallium arsenide;
• Gallium phosphide;
• Gallium nitride;
• Indium phosphide;
• Indium gallium arsenide;
• Arsenide of aluminum-gallium;
• Arsenide-nitride gallium-indium;
• Gallium-indium phosphide.

Wavelength

These compounds are direct-gap semiconductors. Indirect (silicon) light with sufficient strength and efficiency does not radiate. The radiation wavelength of a diode laser depends on the degree of approximation of the photon energy to the energy of the rupture zone of a particular compound. In the 3- and 4-component compounds of semiconductors, the energy of the discontinuity zone can vary continuously over a wide range. In AlGaAs = Al _x Ga _1-x As, for example, an increase in the aluminum content (an increase in x) results in an increase in the energy of the discontinuity zone.

While the most common semiconductor lasers operate in the near-IR part of the spectrum, some emit red (gallium-indium phosphide), blue or violet (gallium nitride) in color. Average infrared radiation is produced by semiconductor lasers (lead selenide) and quantum cascade lasers.

Organic Semiconductors

In addition to the above inorganic compounds, organic can also be used. The corresponding technology is still under development, but its development promises to significantly reduce the cost of production of quantum generators. So far only organic lasers with optical energy supply have been developed, and highly effective electric pumping has not yet been achieved.

Varieties

A lot of semiconductor lasers are created, differing in parameters and application value.

Small laser diodes produce a high-quality beam of face radiation, whose power ranges from several to five hundred milliwatts. The laser diode crystal is a thin rectangular plate that serves as a waveguide, since the radiation is limited to a small space. The crystal is doped on both sides to create a large-area pn junction. The polished ends create an optical Fabry-Perot resonator. The photon, passing through the resonator, will cause recombination, the radiation will increase, and generation will begin. They are used in laser pointers, CD- and DVD-players, as well as in fiber-optic communication.

Low-power monolithic lasers and quantum generators with an external cavity to form short pulses can produce mode synchronization.

Semiconductor lasers with an external resonator consist of a laser-diode, which plays the role of an amplifying medium in the composition of a larger laser cavity. They are able to change the wavelengths and have a narrow band of radiation.

Injection semiconductor lasers have a region of radiation in the form of a wide band, they can generate a low-quality beam with a power of several watts. They consist of a thin active layer located between the p- and n-layers, forming a double heterojunction. The mechanism of light confinement in the lateral direction is absent, which results in a high beam ellipticity and unacceptably high threshold currents.

Powerful diode arrays, consisting of an array of broadband diodes, are capable of producing a mediocre beam with a power of tens of watts.

Powerful two-dimensional arrays of diodes can generate power in hundreds and thousands of watts.

Surface-emitting lasers (VCSEL) emit a qualitative beam of light several milliwatt perpendicular to the plate. On the surface of the radiation, the resonator mirrors are placed in the form of layers in one-fourth of a wave of a wave with different refractive indices. A single crystal can produce several hundred lasers, which opens the possibility of their mass production.

VECSEL lasers with an optical power supply and an external resonator are able to generate a good quality beam with a power of several watts during mode synchronization.

The work of a quantum-cascade type semiconductor laser is based on transitions inside the zones (in contrast to interband ones). These devices emit in the middle of the infrared part of the spectrum, sometimes in the terahertz range. They are used, for example, as gas analyzers.

Semiconductor lasers: application and main aspects

Powerful diode lasers with high-efficiency electric pumping at moderate voltages are used as means of supplying energy to high - performance solid-state lasers.

Semiconductor lasers can operate in a wide frequency range, which includes the visible, near infrared and middle infrared part of the spectrum. Devices have been created that also allow changing the frequency of the outburst.

Laser diodes can quickly switch and modulate optical power, which finds application in transmitters of fiber-optic communication lines.

Such characteristics have made semiconductor lasers the most technologically important type of quantum generators. They are applied:

• In telemetry sensors, pyrometers, optical altimeters, range finders, sights, holography;
• In optical fiber systems of optical transmission and storage of data, systems of coherent communication;
• In laser printers, video projectors, pointers, bar code scanners, image scanners, CD players (DVD, CD, Blu-Ray);
• In security systems, quantum cryptography, automation, indicators;
• In optical metrology and spectroscopy;
• In surgery, dentistry, cosmetology, therapy;
• For water treatment, material processing, pumping solid-state lasers, chemical reaction control, industrial sorting, industrial engineering, ignition systems, air defense systems.

Pulse output

Most semiconductor lasers generate a continuous beam. Because of the short duration of the electrons stay at the level of conductivity, they are not very suitable for generating pulses with Q-switching, but the quasi-continuous mode of operation makes it possible to significantly increase the power of the quantum generator. In addition, semiconductor lasers can be used to generate ultrashort pulses with mode synchronization or gain-switching. The average power of short pulses is usually limited to several milliwatts, with the exception of VECSEL lasers with optical pumping, whose output is measured by multi-watt picosecond pulses of a frequency of tens of gigahertz.

Modulation and stabilization

The advantage of short-term electron stay in the conduction band is the ability of semiconductor lasers to high-frequency modulation, which in VCSEL lasers exceeds 10 GHz. This has found application in optical data transmission, spectroscopy, and stabilization of lasers.