In July 1962, held in the solid-state device research international conference, the United States Massachusetts Institute of Technology Lincoln Laboratory, two scholars Keyes (Keyes) and Quist (Quist) reported gallium arsenide material light emission phenomena, which caused the General Electric Research Laboratory engineers Hal (Hall) is very interested in the meeting after the return home on the train, he wrote down the relevant data. After returning home, Hall immediately developed a plan to develop semiconductor lasers, and together with other researchers, after several weeks of struggle, their plans were successful.
Like crystal diodes, semiconductor lasers are based on the p-n junction properties of the material and are similar in appearance to their predecessors, so they are often referred to as diode lasers or laser diodes. Early laser diodes had many practical limitations, for example, they could only be operated at a low temperature of 77 K with microsecond pulses, and it took more than eight years before continuous devices capable of operating at room temperature could be fabricated by Bell Laboratories and the Ioffe Institute of Physics in Leningrad (St. Petersburg). Sufficiently reliable semiconductor lasers, on the other hand, did not appear until the mid-1970s.
Semiconductor lasers are very small, the smallest being the size of a grain of rice. The operating wavelength depends on the laser material and is generally 0.6 to 1.55 micrometers, with shorter wavelength devices in development for a variety of applications. It has been reported that the lasers with Ⅱ ~ Ⅳ valence element compounds, such as ZnSe as the working material, have been obtained 0.46 micron output at low temperature, and the output power of the room temperature continuous device with a wavelength of 0.50 ~ 0.51 micron has been up to more than 10 milliwatts. However, it has not been commercialized so far.
Fiber-optic communication is the most important foreseeable application field of semiconductor laser, on the one hand, the world-wide long-distance submarine fiber-optic communication, and on the other hand, various area networks. The latter includes high-speed computer networks, avionics systems, health communication networks, high-definition closed-circuit television networks and so on. However, for all intents and purposes, laser jukeboxes are the largest market for such devices. Other applications include high-speed printing, free-space optical communications, solid-state laser pumping sources, laser indication, and various medical applications.
The semiconductor laser in the early 1960s was a homogeneous junction laser, which was made on a material pn junction diode in the forward high current injection, electrons are constantly injected into the p region, holes are constantly injected into the n region. Thus, in the original pn junction depletion zone to achieve the carrier distribution reversal, due to the migration of electrons faster than the migration of holes, in the active region of radiation, composite, emitting fluorescence, under certain conditions occur laser, which is a form of semiconductor lasers can only work in the form of pulses. The second stage of the development of semiconductor lasers is the heterostructure semiconductor lasers, which is composed of two different bandgap semiconductor material thin layer, such as GaAs, GaAlAs, the first single heterostructure lasers (1969). The single heterojunction injected laser (SHLD) utilizes the potential barrier provided by the heterojunction to confine the injected electrons within the P-region of the GaAsP one-N junction as a means of reducing the threshold current density, the value of which is reduced by an order of magnitude compared to that of the homojunction lasers, but the single heterojunction lasers are still not capable of continuous operation at room temperature.
In 1970, the realization of the laser wavelength of 9000Å: room temperature continuous operation of the double heterojunction GaAs-GaAlAs (gallium arsenide a gallium aluminum arsenic) laser. The birth of the double heterojunction laser (DHL) has led to a continuous broadening of the available wavelength bands and a gradual improvement in linewidth and tuning performance. Its structure is characterized by the growth of a thin layer of only 0. 2 Eam thick, undoped, material with a narrow energy gap between the P-type and n-type materials, so that the injected carriers are confined to this region (active region), and thus less current injection can be achieved to invert the number of carriers. Among the semiconductor laser devices, the more mature, better-performing and more widely used are electrically injected GaAs diode lasers with a double heterostructure.
With the development of research on heterojunction lasers, it has occurred to people that if an ultra-thin film (< 20nm) semiconductor layer is used as an excitation bracket layer of the laser such that quantum effects can be produced, what would be the result? Plus, due to the achievement of MBE,MOCVD technology. As a result, the world's first semiconductor quantum well laser (QWL) appeared in 1978, which dramatically improved the performance of semiconductor lasers. Later, due to the maturity of MOCVD, MBE growth technology, can grow high-quality ultra-fine thin-layer materials, after that, it has successfully developed a better performance of the quantum well laser, quantum well semiconductor lasers and double heterojunction (DH) lasers compared with the appendicular value of the current is low, the output power is high, the frequency response is good, the spectral line is narrow and the temperature stability of the good and high efficiency of electro-optical conversion and so on. There are many advantages.
QWL in the structure is characterized by its active region is composed of more than one or a single well width of about 100 people of the potential well, due to the width of the potential well is less than the wavelength of the electron's DeBlois wave in the material, resulting in a quantum effect, the continuous energy band splitting into sub-energy levels. As a result, the efficient filling of carriers is particularly favored and the required excitation read-across current is particularly low. Semiconductor laser structure is mainly used in single and multiple quantum well, single quantum well (SQW) laser structure is basically the ordinary double heterojunction (DH) laser active layer thickness into the following tens of nm of a laser, the potential barrier is usually thicker than the neighboring potential wells in the electronic wave function does not occur in the overlap of the periodic structure known as the multi-quantum well (MQW ). Quantum well laser single output power is now greater than 1 w, bear the power density has reached l OMW/cm3 above) and in order to get a larger output power, usually can be many individual semiconductor lasers combined together to form a semiconductor laser array. Therefore, the quantum well laser when the array-type integrated structure, the output power can reach more than l00w. High-power semiconductor lasers (especially the array device) has developed rapidly, and has introduced products with continuous output power of 5 W, 10 W, 20 W and 30 W laser arrays. Arrays of pulsed semiconductor lasers with peak output powers of 50 W. 120 W and 1500 W have also been commercialized. A 4.5 cm x 9 cm two-dimensional array with a peak output power of more than 45 kW has been commercialized. Two-dimensional arrays with a peak output power of 350 kW have also been commercialized. Since the late 1970s, semiconductor lasers have clearly developed in two directions, one being information-based lasers for the purpose of transmitting information. The other is to increase the optical power for the purpose of power-type lasers. Pumped solid-state lasers and other applications, driven by high-power semiconductor lasers (continuous output power of more than 100W, pulsed output power of more than 5W, can be called high-power semiconductor lasers) made a breakthrough in the 1990s, which is marked by a significant increase in the output power of semiconductor lasers, foreign kilowatts of high-power semiconductor lasers have been commercialized, and the domestic sample Device output has reached 600W [61. If from the perspective of the laser band is extended, first infrared semiconductor lasers, followed by 670nm red semiconductor lasers into a large number of applications, then, the wavelength of 650nm, 635nm came out, blue-green light, blue light semiconductor lasers have also been successfully developed, the violet and even ultraviolet semiconductor lasers of the order of 10mw, but also in the In order to adapt to a variety of applications and the development of semiconductor lasers and tunable semiconductor lasers, electron beam excitation semiconductor lasers, as well as the best light source as an "integrated light path" Distributed Feedback Laser (DFB a LD), Distributed Bragg Reflective Laser (DBR a LD) and integrated dual-waveguide lasers. waveguide lasers. In addition, there are high-power aluminum-free lasers (aluminum is removed from semiconductor lasers to obtain higher output power, longer life and lower cost tubes), mid-infrared semiconductor lasers and quantum cascade lasers. Among them, tunable semiconductor lasers can easily modulate the output beam by changing the wavelength of the laser light through the applied electric field, magnetic field, temperature, pressure, doping basin, etc.. Distributed feedback (DF) type semiconductor laser is accompanied by fiber optic communications and the development of integrated optical circuits and appeared, it was successfully developed in 1991, distributed feedback type semiconductor laser completely realize the single longitudinal mode operation, in the field of coherent technology and opened up a huge application prospects it is a cavity-free traveling wave laser, laser oscillation is provided by the formation of a periodic structure (or diffraction grating) coupled to provide light, and no longer By the solution of the surface of the resonant cavity to provide feedback, the advantage is easy to obtain a single mode, single frequency output, easy to couple with fiber optic cables, modulators, etc., especially suitable for the integrated optical path of the light source.
Unipolar injection of semiconductor lasers is the use of the conduction band (or valence band) between the sub energy levels of hot electron optical jump to realize the laser emission, naturally, to make the conduction band and the valence band of the existence of the sub energy levels or sub energy bands, which must be used in the quantum well structure. Unipolar injection lasers are capable of obtaining large optical power outputs, are semiconductor lasers with commercial efficiency and super-commercial speed response, and are favorable for the development of silicon-based lasers and short-wave lasers. The invention of quantum cascade lasers has greatly simplified the way of generating specific wavelength lasers in such a wide wavelength range as the mid-infrared to the far-infrared. It is possible to obtain lasers of various wavelengths in the above wavelength range by using only one material and depending on the thickness of the layers. Compared to conventional semiconductor lasers, these lasers do not require a cooling system and can be operated stably at room temperature. Low-dimensional (quantum line and quantum dot) laser research is also developing rapidly, Japan okayama's GaInAsP/Inp long-wavelength quantum line (Qw +) laser has been done 9OkCW operating conditions Im = 6.A, l = 37A/cm2 and has a high quantum efficiency. Numerous research organizations are developing self-assembled quantum dot (QD) lasers, which have been characterized by high density, high uniformity, and high emission power. Due to practical needs, the development of semiconductor lasers has centered around the reduction of broadband current densities, the extension of operating life, the realization of continuous operation at room temperature, as well as the obtaining of single-mode, single-frequency, narrow-linewidth and the development of a variety of devices with different excitation wavelengths. The 1990s saw the emergence and special mention of surface emitting lasers (SELs), as early as 1977, the so-called surface emitting lasers were proposed, and the first device was made in 1979, and a 780 nm surface emitting laser pumped with light was made in 1987.In 1998 GaInAIP/GaA. surface emitting lasers achieved sub-milliamperes of room temperature network current, 8mW output power and 11% conversion efficiency[2] The semiconductor lasers mentioned earlier, in terms of the cavity structure, whether it is a F a P (Fabry-Perot) cavity or DBR (Distributed Bragg Reflective) cavity, the laser output is in the horizontal direction, collectively referred to as the horizontal cavity structure. They all emit light in the parallel direction of the substrate chip. While the surface emitting laser is coated with reflective film on the upper and lower surfaces of the chip to form a perpendicular direction of the F a P cavity, the light output is sent in the direction perpendicular to the substrate, vertical cavity surface emitting semiconductor laser (VCSELS) is a new type of quantum well laser, it is a low value of the broad value of the excitation current, the directionality of the output light is good, the coupled efficiency is high, through the arrayed distribution of the light can be obtained by the output of a fairly strong optical power, the VCSELS has been realized. Vertical-cavity surface-emitting lasers have realized operating temperatures up to 71°C. In addition, vertical-cavity surface-emitting lasers also have two unstable perpendicularly polarized transverse mode outputs, i.e., x-mode and y-mode, and the research on polarization switching and polarization bi-stability has entered a new stage, where people can control the polarization state by changing the factors of optical feedback, opto-electronic feedback, optical injection, injection current and so on and make new progress in the field of optical switching and optical logic devices. At the end of the 1990s, surface-emitting lasers and vertical-cavity surface-emitting lasers were rapidly developed and have been considered for a variety of applications in ultra-parallel optoelectronics. 980mn, 850nm, and 780nm devices have been utilized in optical systems. Vertical-cavity surface-emitting lasers have been used in high-speed networks for gigabit Ethernet. In order to meet the needs of the 21st century for broadband information transmission, high-speed information processing, large-capacity information storage, and small-sized, high-precision military equipment, the development trend of semiconductor lasers is mainly in the high-speed broadband LDs, high-power IDs, short-wavelength LDs, potted-line and quantum-dot lasers, mid-infrared LDs, and so on. A series of significant results have been achieved in these areas.