Semiconductor lasers are quantum generators based on a semiconductor active medium in which optical amplification is created 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 electrons of the semiconductor come into a state of excitation and, having overcome the forbidden zone, pass into the free zone, concentrating at its lower edge. Simultaneously, the holes formed at the valence level rise to its upper boundary. Electrons in the free zone recombine with holes, radiating an energy equal to the energy of the discontinuity zone in the form of photons. Recombination can be enhanced by photons with sufficient energy levels. The numerical description corresponds to the Fermi distribution function.
Device
Semiconductor laser deviceis a laser diode pumped with the energy of electrons and holes in the p-n-junction zone - the point of contact of semiconductors with p- and n-type conductivity. In addition, there are semiconductor lasers with optical energy supply, in which the beam is formed by absorbing photons of light, as well as quantum cascade lasers, whose operation is based on transitions within 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;
- gallium aluminum arsenide;
- gallium-indium arsenide nitride;
- gallium-indium phosphide.
Wavelength
These compounds are direct-gap semiconductors. Indirect-gap (silicon) light does not emit with sufficient strength and efficiency. The wavelength of the diode laser radiation depends on the degree of approximation of the photon energy to the energy of the discontinuity zone of a particular compound. In 3- and 4-component semiconductor compounds, the discontinuity zone energy can continuously vary over a wide range. For AlGaAs=AlxGa1-xAs, 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 infrared, some emit red (indium gallium phosphide), blue or violet (gallium nitride) colors. Mid-infrared radiation is produced by semiconductor lasers (lead selenide) and quantum cascade lasers.
Organic semiconductors
In addition to the above-mentioned inorganic compounds, organic ones 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 efficient electric pumping has not yet been achieved.
Varieties
Many semiconductor lasers have been created, differing in parameters and applied value.
Small laser diodes produce a high-quality beam of edge radiation, the power of which 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 p-n junction of a large area. The polished ends create an optical Fabry-Perot resonator. A photon passing through the resonator will cause recombination, the radiation will increase, and generation will begin. Used in laser pointers, CD and DVD players, and fiber optic communications.
Low-power monolithic lasers and quantum generators with an external resonator to form short pulses can produce mode locking.
Laserssemiconductor with an external resonator consist of a laser diode, which plays the role of an amplifying medium in the composition of a larger laser resonator. They are capable of changing wavelengths and have a narrow emission band.
Injection semiconductor lasers have an emission region in the form of a wide band, 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 mechanism for keeping light in the lateral direction, which results in high beam ellipticity and unacceptably high threshold currents.
Powerful diode bars, 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 in the hundreds and thousands of watts.
Surface emitting lasers (VCSELs) emit a high-quality beam of light with a power of several milliwatts perpendicular to the plate. Resonator mirrors are applied on the radiation surface in the form of layers of ¼ wave length with different refractive indices. Several hundred lasers can be made on a single chip, which opens up the possibility of mass production.
VECSEL lasers with optical power supply and an external resonator are able to generate a beam of good quality with a power of several watts in mode locking.
The operation of a semiconductor laser quantum-cascade type is based on transitions within the zones (as opposed to interzones). These devices emit in the mid-infrared region, 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 electrical pumping at moderate voltages are used as a means of powering high-efficiency solid-state lasers.
Semiconductor lasers can operate over a wide frequency range, which includes the visible, near-infrared, and mid-infrared portions of the spectrum. Devices have been created that also allow you to change the frequency of the emission.
Laser diodes can rapidly switch and modulate optical power, which finds application in fiber optic transmitters.
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 optical transmission and data storage systems, 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,materials processing, solid-state laser pumping, chemical reaction control, industrial sorting, industrial engineering, ignition systems, air defense systems.
Pulse output
Most semiconductor lasers generate a continuous beam. Due to the short residence time of electrons at the conduction level, they are not very suitable for generating Q-switched pulses, but the quasi-continuous mode of operation allows a significant increase in the power of the quantum generator. In addition, semiconductor lasers can be used to generate ultrashort pulses with mode locking or gain switching. The average power of short pulses is usually limited to a few milliwatts, with the exception of optically pumped VECSEL lasers, whose output is measured by multi-watt picosecond pulses with a frequency of tens of gigahertz.
Modulation and stabilization
The advantage of the short stay of an electron in the conduction band is the ability of semiconductor lasers to high-frequency modulation, which for VCSEL lasers exceeds 10 GHz. It has found application in optical data transmission, spectroscopy, laser stabilization.
