Semiconductor lasers are quantum generators based on a semiconductor active medium in which optical amplification is generated by stimulated emission during a quantum transition between energy levels at a high concentration of charge carriers in the free zone.
Semiconductor laser: principle of operation
In the normal state, most electrons are located at the valency level. When photons supply energy exceeding the energy of the discontinuity zone, the semiconductor electrons enter the state of excitation and, having overcome the forbidden zone, pass into the free zone, concentrating at its lower edge. At the same time, 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 enhanced 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 zone - the place where semiconductors come in contact with p- and n-type conductivity. In addition, there are semiconductor lasers with an optical energy supply, in which a beam is formed when light photons are absorbed, as well as quantum cascade lasers, the operation of which is based on transitions inside the bands.
Composition
The standard compounds used both in semiconductor lasers and in other optoelectronic devices are as follows:
- gallium arsenide;
- gallium phosphide;
- gallium nitride;
- indium phosphide;
- indium gallium arsenide;
- aluminum gallium arsenide;
- gallium indium arsenide nitride;
- gallium indium phosphide.
Wavelength
These compounds are direct-gap semiconductors. Indirect (silicon) light does not emit with sufficient strength and efficiency. 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 3- and 4-component semiconductor compounds, the energy of the discontinuity zone can continuously vary over a wide range. For 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 rupture zone.
While the most common semiconductor lasers operate in the near infrared, some emit red (gallium-indium phosphide), blue or violet (gallium nitride). Semiconductor lasers (lead selenide) and quantum cascade lasers create medium infrared radiation.
Organic Semiconductors
In addition to the aforementioned 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, organic lasers with optical energy supply have only been developed, and high-efficiency electric pumping has not yet been achieved.
Varieties
Many semiconductor lasers have been created that differ in parameters and applied value.
Small laser diodes produce a high-quality beam of end radiation, the power of which ranges from several to five hundred milliwatts. The crystal of the laser diode is a thin plate of a rectangular shape, which serves as a waveguide, since the radiation is limited to a small space. The crystal is doped on both sides to create a pn junction of a large area. Polished ends create a Fabry - Perot optical resonator. A photon passing through the resonator will cause recombination, the radiation will increase, and the 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 resonator for generating short pulses can produce mode locking.
Semiconductor lasers with an external resonator consist of a laser diode, which plays the role of an amplifying medium as part of a larger laser cavity. Able to change wavelengths and have a narrow emission band.
Injection semiconductor lasers have a wide-band emission region and 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-layer, forming a double heterojunction. There is no lateral light retention mechanism, which results in high beam ellipticity and unacceptably high threshold currents.
Powerful diode arrays, consisting of an array of broadband diodes, are capable of producing a beam of mediocre quality with a power of tens of watts.
Powerful two-dimensional arrays of diodes can generate power of hundreds and thousands of watts.
Surface-emitting lasers (VCSEL) emit a high-quality light beam with a power of several milliwatts perpendicular to the plate. On the radiation surface, resonator mirrors are applied in the form of layers in the wavelengths with different refractive indices. Several hundreds of lasers can be made on a single crystal, which opens up the possibility of their mass production.
VECSEL lasers with optical energy input and an external resonator are capable of generating a good-quality beam with a power of several watts during mode synchronization.
The operation of a quantum-cascade semiconductor laser is based on transitions within the bands (in contrast to interband ones). These devices emit in the middle region of the infrared part of the spectrum, sometimes in the terahertz range. They are used, for example, as gas analyzers.
Semiconductor lasers: application and basic aspects
Powerful diode lasers with high-performance electric pumping at moderate voltages are used as a 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 mid-infrared. Created devices that also allow you to change the frequency of fading.
Laser diodes can quickly switch and modulate optical power, which is used in transmitters of fiber-optic communication lines.
Such characteristics have made semiconductor lasers technologically the most important type of quantum generators. They apply:
- in telemetry sensors, pyrometers, optical altimeters, rangefinders, sights, holography;
- in fiber optic systems for optical data transmission and storage, coherent communication systems;
- in laser printers, video projectors, pointers, barcode 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 of solid-state lasers, control of chemical reactions, in industrial sorting, industrial engineering, ignition systems, air defense systems.
Pulse output
Most semiconductor lasers generate a cw beam. Due to the short residence time of electrons at the conduction level, they are not very suitable for generating pulses with Q-switching, but the quasi-continuous mode of operation can significantly increase the power of a quantum generator. In addition, semiconductor lasers can be used to generate ultrashort pulses with mode locking or switching gain. The average power of short pulses, as a rule, is limited to a few milliwatts, with the exception of optical-pumped VECSEL lasers, the output of which is measured by multi-watt picosecond pulses with a frequency of tens of gigahertz.
Modulation and stabilization
The advantage of a short-term electron stay in the conduction band is the ability of semiconductor lasers to high-frequency modulation, which for VCSEL lasers exceeds 10 GHz. This has found application in optical data transmission, spectroscopy, laser stabilization.