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semiconductor laser[¦sem·i·kən¦dək·tər ′lā·zər]
a laser whose active material is a semiconductor crystal. In contrast to other kinds of lasers, semiconductor lasers make use of radiative quantum transitions between the allowed energy bands of a crystal rather than between discrete energy levels of atoms, molecules, and ions. The atoms forming the crystal lattice are excited and radiate collectively in a semiconductor laser. This difference determines an important characteristic of such lasers, namely, small size and compactness (the volume of the crystal ranges from 10-6 to 10–2 cm3). A coefficient of optical amplification of up to 104 cm-1 can be obtained in semiconductor lasers, although smaller values are usually sufficient for laser excitation. Other semiconductor-laser characteristics with practical importance are high efficiency of conversion of electrical energy into the energy of coherent radiation (up to 30–50 percent); fast response, which permits its direct modulation within broad-band frequencies (more than 109 gigahertz); simplicity of design; the possibility of varying the wavelength X of the radiation; and the existence of a large number of semiconductors that continuously cover the interval of wavelengths from 0.32 to 32 microns (μ).
Luminescence in semiconductors. When conduction electrons and holes recombine in semiconductors, energy is released. This energy may be emitted in the form of quanta of radiation (luminescence), or it may exhibit itself as lattice vibrations—that is, it may become heat. The fraction of radiative recombination events in such semiconductors as Ge and Si is very low, but in some semiconductors, such as GaAs and CdS, it may approach 100 percent after purification and doping.
In order to observe luminescence, some method of exciting (pumping) the crystal must be used—that is, a method of generating excess electron-hole pairs is needed. Such a method may involve the use of light, fast electrons, or an electric field. When the rate of formation of excess electron-hole pairs is low, radiative recombination is random, or spontaneous, in character and is made use of in nonlaser semiconductor light sources. To obtain laser action, that is, the generation of coherent radiation, it is necessary to create a special state in a luminescing crystal—a state with a population inversion.
The recombination of an electron-hole pair may be accompanied by the emission of a quantum of radiation close in energy to the width AS of the energy gap of the semiconductor (Figure 1, a). Here, the wavelength is λ ≈ hc/Δℰ, where h is Planck’s constant and c is the speed of light.
Note on Figure 1. Energy diagrams: (a) pumping and radiative recombination in a semiconductor, (b) optical gain in the presence of a population inversion near the band edges; (ε0) bottom of conduction band, (εr) top of valence band, (Δε) width of energy gap, and quasi Fermi levels for conduction electrons and holes
Population inversion in semiconductors. Optical quantum amplification in a semiconductor may be observed if the conduction band is filled with electrons near its bottom ℰc to a greater extent than the valence band is filled near its top ℰv. Since more electrons are at the upper levels than at the lower levels, transitions with the emission of quanta predominate over transitions with absorption, although the probabilities of forced transitions are the same in both directions. The occupancy of the bands is commonly described by means of quasi Fermi levels, which separate states with an occupation probability greater than 1/2 from states with an occupation probability less than 1/2. If ℰfe and ℰfh are the quasi Fermi levels for electrons and holes, then the condition for population inversion with respect to transitions with an energy hv, where v is the frequency of the radiation, is expressed by the formula
The maintenance of such a state requires a high rate of pumping to make up for the loss of electron-hole pairs owing to radiative transitions. Because of these forced transitions, the radiant flux increases (Figure 1, b)—that is, optical amplification occurs.
The following methods of excitation are used in semiconductor lasers: (1) injection of current carriers across a p-n junction, a heterojunction, or a metal-semiconductor contact (injection lasers), (2) pumping with a fast-electron beam, (3) optical pumping, and (4) pumping by means of breakdown in an electric field. Semiconductor lasers of the first two types have been developed most extensively.
Injection lasers. A p-n junction laser is a semiconductor diode in which two plane-parallel surfaces perpendicular to the p-n junction (Figure 2) form an optical cavity; the coefficient of reflection from the faces of the crystal is approximately 20–40 percent. A population inversion occurs when a forward current of high density passes through the diode. The current density threshold is approximately 1 kiloampere/cm2; at reduced temperature it is approximately 102 amperes/cm2 (Figure 3). Heavily doped semiconductors are used to obtain sufficiently intense injection.
Heterojunction injection lasers first appeared in 1968. An example is the double heterostructure shown in Figure 4. The active layer of GaAs is enclosed between two semiconductor heterojunctions. One heterojunction is of the p-n type and is used to inject electrons; the other is of the p-p type and reflects the injected electrons by impeding their diffusive spreading from the active layer, an effect called electron confinement. With the same pumping current, a higher concentration of electron-hole pairs
and, consequently, greater optical amplification are achieved in the active layer of the heterostructure than in p-n junction semiconductor lasers. Another advantage of the heterostructure is that optical confinement is achieved: the dielectric wave guide formed by the active layer confines the radiation propagating along the structure within the active layer. As a result, the optical amplification is used most efficiently. At T = 300°K, p-n junction lasers require a current density more than ten times as great as that required by heterojunction lasers. Heterojunction lasers thus permit of continuous operation at temperatures up to 350°K.
The power output of injection lasers can reach 100 watts for pulsed operation. In the case of continuous operation, it exceeds 10 watts (GaAs) in the near infrared (λ = 850 nanometers) and is about 10 milliwatts (PbxSn1–xTe) in the middle infrared (λ = 10 μ). A shortcoming of injection lasers is the low directivity of the radiation owing to the small dimensions of the active radiating region. In other words, the radiation exhibits high divergence as a result of diffraction. Another shortcoming is the relatively broad spectrum of the radiation in comparison with gas lasers.
Electron-beam-pumped lasers. When a semiconductor is bombarded with fast electrons having an energy W ~ 103-106 electron volts, electron-hole pairs are produced in the crystal. The number of pairs created by one electron is ~ W/3Δℰ. This method is applicable to semiconductors with an energy gap of any width. The power output of the laser reaches 106 watts because of the possibility of exciting a large volume of a semiconductor (Figure 5). An electron-beam-pumped laser contains an electron gun, a focusing system, and a semiconductor crystal in the form of an optical cavity, which are housed in a vacuum envelope. The technical advantage of such a laser is that the electron beam can be rapidly moved (scanned) over the crystal, thereby providing an additional method of controlling the emission. Since an appreciable part of the energy of the electron beam is spent on heating the crystal lattice, the efficiency is limited to approximately 1/3; an energy 3Δℰ is expended on each electron-hole pair, and a photon with an energy of approximately Δℰ is emitted.
Semiconductor laser materials. Primarily binary compounds of the type A3B5, A2B6 and A4B6 and mixtures thereof—solid solutions—are used in semiconductor lasers (see Table 1). Such materials are direct-gap semiconductors, in which interband radiative recombination can occur without the participation of phonons or other electrons and therefore has the highest probability among the recombination processes. In addition to the materials listed in Table 1, there are some other promising but still inadequately studied materials, such as other solid solutions, that are suitable for semiconductor lasers. In solid solutions the quantity Δℰ depends on the chemical composition; consequently, a semiconductor laser for any wavelength from 0.32 to 32 μ can be produced.
|Table 1. Semiconductor lasers|
|Semiconductor||Wavelength of radiation (μ)||Maximum operating temperature (°K)||Method of excitation|
|1 Electron-beam pumping 2Optical pumping 3Breakdown in an electric field ‘Injection|
|CdS.........||0.49–0.53||300||E, O2, B3|
|CdS1_xSex ........||0.49–0.68||77||E. O|
|CdSe .......||0.68–0.68||77||E. O|
|GaAs1-xPx....||0.62–0.9||300||E, O, I4|
|AlxGa1-xAS ....||0.62–0.9||300||O, I|
|GaAs ...........||0.83–0.90||450||E, O, I, B|
|InP.......||0.90–0.91||77||O, I, B|
|InP1-xAsx ....||0.90–3.1||77||O, I|
|InAs.........||3.1–3.2||77||E, O, I|
|InSb.........||5.1–5–3||100||E, O, I|
|PbTe .......||6,4–6,5||100||E, O, I|
|PbSe ...........||B.4–8.5||100||E, O, I|
|PbxSn1-xTe ........||6.4–31.8||100||E, O, I|
Applications. Semiconductor lasers are used in (1) optical communication, for example, the portable optical telephone and multichannel stationary communication lines; (2) optical detection and ranging and special automation systems, such as range-finding, aitimetry, and automatic-tracking systems; (3) optical electronics, for example, the emitter in an optron, logic circuits, address devices, and holographic memory systems; (4) special illumination technology, such as in high-speed photography and the optical pumping of other lasers; (5) the detection of contaminants and impurities in various media; and (6) laser projection television (Figure 6).
Historical survey. The first work on the possibility of using semiconductors to construct a laser was published in 1959 by N. G. Basov, B. M. Vul, and Iu. M. Popov. The use of p-n junctions for this purpose was proposed in 1961 by Basov, O. N. Krokhin, and Popov. A semiconductor laser using a GaAs crystal was first built in 1962 in the laboratories of the American scientists R. Hall, M. I. Nathan, and N. Holonyak, Jr. Earlier in 1962, Soviet scientists, including D. N. Nasledov and S. M. Ryvkin, completed an investigation of the radiative properties of p-n junctions that indicated the appearance of stimulated emission under an intense current. In the USSR, the basic research resulting in the invention of the semiconductor laser was awarded a Lenin Prize in 1964. The research was conducted by Vul, Krokhin, Nasledov, A. A. Rogachev, Ryvkin, Popov, A. P. Shotov, and B. V. Tsarenkov. The first electron-beam-pumped laser was built in 1964 by Basov, O. V. Bogdankevich, and A. G. Deviatkov. In the same year, Basov, A. Z. Grasiuk, and V. A. Katulin reported the development of an optically pumped semiconductor laser. In 1963, Zh. I. Alferov of the USSR proposed the use of heterostructures for semiconductor lasers. Such lasers were constructed in 1968 by Alferov, V. M. Andreev, D. Z. Garbuzov, V. I. Korol’kov, D. N. Trefiakov, and V. I. Shveikin; these scientists were awarded the Lenin Prize in 1972 for their research on heterojunctions and for their development of heterojunction devices.
REFERENCESBasov, N. G., O. N. Krokhin, and Iu. M. Popov. “Poluchenie sostoianii s otritsatel’noi temperaturoi v p-n-perekhodakh vyrozhdennykh poluprovodnikov.” Zhurnal eksperimental’noi i teoreticheskoi fiziki, 1961, vol. 40, issue 6.
Basov, N, G.”Poluprovodnikovye kvantovye generatory.” Uspekhi fizicheskikh nouk. 1965, vol. 85, issue 4.
Pilkun, M.”Inzhektsionnye lazery.” Uspekhi fizicheskikh nauk, 1969, vol, 98, issue 2.
Eliseev, P. G.”Inzhektsionnye lazery na geteroperekhodakh.” Kvantovaia elektronika, 1972, issue 6 (12).
Basov, N. G., V, V. Nikitin, and A. S. Semenov. “Dinamika izlucheniia inzhektsionnykh poluprovodnikovykh lazerov.” Uspekhi fizkheskikh nauk, 1969, vol. 97, issue 4.
P. G. ELISEEV and IU. M. POPOV
laser diodeA semiconductor-based laser used to generate analog signals or digital pulses for transmission through optical fibers. Both laser diodes and LEDs (light-emitting diodes) are used for this purpose, but the laser diode generates a smaller beam that is easier to couple with the smaller core of singlemode fibers. Laser diodes are designed to emit light either from their edge or their surface, the latter providing a circular beam that couples better with the round core of the fiber.
Laser diodes work on the same principle as the bigger gas lasers. They function as an optical oscillator by stimulating a chain reaction of photon emission inside a tiny chamber. In edge-emitting lasers, the semiconductor waver is cleaved, and the inherent properties of the semiconductor create reflective ends that may or may not be enhanced with additional reflective films. With vertical cavity surface emitting lasers (VCSELs), the reflectivity has to be added.
The most common semiconductors used in laser diodes are compounds based on gallium arsenide (750 to 900 nm in the infrared), indium gallium arsenide phosphide (1200 to 1700 nm in the infrared) and gallium nitride (near 400 nm in the blue). See laser and solid state laser.
|Laser diodes such as these are all fabricated using semiconductor processes just like CPU and memory chips. Note that the VCSELs (vixels) produce a circular beam that couples well with a fiber.|
|A Tunable Laser Diode|
|Laser diodes also make great sensors. This laser diode from Jet Propulsion Labs is tunable to different frequencies in order to detect various gases both on this and other planets. (Image courtesy of JPL's Microdevices Laboratory; Robert M. Brown, photographer)|