What is the semiconductor laser?
Publish：Box Optronics Time：2022-03-30 Views:1391
semiconductor laser
A semiconductor laser is a miniaturized laser that uses a Pn junction or Pin junction composed of a direct band gap semiconductor material as the working substance. There are dozens of semiconductor laser working materials. The semiconductor materials that have been made into lasers include gallium arsenide, indium arsenide, indium antimonide, cadmium sulfide, cadmium telluride, lead selenide, lead telluride, aluminum gallium arsenide, indium Phosphorus, Arsenic, etc. There are three main excitation methods of semiconductor lasers, namely electric injection type, optical pump type and high-energy electron beam excitation type. The excitation method of most semiconductor lasers is electrical injection, that is, a forward voltage is applied to the Pn junction to generate stimulated emission in the junction plane region, that is, a forward-biased diode. Therefore, semiconductor lasers are also called semiconductor laser diodes. For semiconductors, since electrons transition between energy bands rather than discrete energy levels, the transition energy is not a definite value, which makes the output wavelength of semiconductor lasers spread over a wide range. on the range. The wavelengths they emit are between 0.3 and 34 μm. The wavelength range is determined by the energy band gap of the material used. The most common is the AlGaAs double heterojunction laser, which has an output wavelength of 750-890 nm.
The semiconductor laser fabrication technology has experienced from the diffusion method to liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), MOCVD method (metal organic compound vapor deposition), chemical beam epitaxy (CBE) ) and various combinations of them. The biggest disadvantage of semiconductor lasers is that the laser performance is greatly affected by temperature, and the divergence angle of the beam is large (generally between a few degrees and 20 degrees), so it is poor in directivity, monochromaticity and coherence. However, with the rapid development of science and technology, the research of semiconductor lasers is advancing in the direction of depth, and the performance of semiconductor lasers is constantly improving. Semiconductor optoelectronic technology with semiconductor laser as the core will make greater progress and play a greater role in the information society of the 21st century.
How semiconductor lasers work?
A semiconductor laser is a coherent radiation source. To make it generate laser light, three basic conditions must be met:
1. Gain condition: The inversion distribution of carriers in the lasing medium (active region) is established. In the semiconductor, the energy band that represents the electron energy is composed of a series of energy levels that are close to continuous. Therefore, in the semiconductor In order to achieve population inversion, the number of electrons at the bottom of the conduction band of the high-energy state must be much larger than the number of holes at the top of the valence band of the low-energy state between the two energy band regions. The heterojunction is forward biased to inject necessary carriers into the active layer to excite electrons from the valence band with lower energy to the conduction band with higher energy. Stimulated emission occurs when a large number of electrons in a state of population inversion recombine with holes.
2. To actually obtain coherent stimulated radiation, the stimulated radiation must be fed back multiple times in the optical resonator to form laser oscillation. The laser resonator is formed by the natural cleavage surface of the semiconductor crystal as a mirror, usually in The end that does not emit light is coated with a high-reflection multilayer dielectric film, and the light-emitting surface is coated with an anti-reflection film. For the F-p cavity (Fabry-Perot cavity) semiconductor laser, the F-p cavity can be easily formed by using the natural cleavage plane of the crystal perpendicular to the p-n junction plane.
3. In order to form a stable oscillation, the laser medium must be able to provide a sufficiently large gain to compensate for the optical loss caused by the resonator and the loss caused by the laser output from the cavity surface, etc., and continuously increase the optical field in the cavity. This requires a strong enough current injection, that is, there is enough population inversion, the higher the degree of population inversion, the greater the gain obtained, that is, a certain current threshold condition must be met. When the laser reaches the threshold, the light with a specific wavelength can resonate in the cavity and be amplified, and finally form a laser and output continuously. It can be seen that in semiconductor lasers, the dipole transition of electrons and holes is the basic process of light emission and light amplification. For new semiconductor lasers, it is currently recognized that quantum wells are the fundamental driving force for the development of semiconductor lasers. Whether quantum wires and quantum dots can take full advantage of quantum effects has been extended to this century. Scientists have tried to use self-organized structures to make quantum dots in various materials, and GaInN quantum dots have been used in semiconductor lasers.
Development History of Semiconductor Lasers
The semiconductor lasers of the early 1960s were homojunction lasers, which were pn junction diodes fabricated on one material. Under the forward large current injection, electrons are continuously injected into the p region, and holes are continuously injected into the n region. Therefore, the inversion of the carrier distribution is realized in the original pn junction depletion region. Since the migration speed of electrons is faster than that of holes, radiation and recombination occur in the active region, and fluorescence is emitted. lasing, a semiconductor laser that can only work in pulses. The second stage of the development of semiconductor lasers is the heterostructure semiconductor laser, which is composed of two thin layers of semiconductor materials with different band gaps, such as GaAs and GaAlAs, and the single heterostructure laser first appeared (1969). The single heterojunction injection laser (SHLD) is within the p region of the GaAsP-N junction to reduce the threshold current density, which is an order of magnitude lower than that of the homojunction laser, but the single heterojunction laser still cannot Continuous work at room temperature.
Since the late 1970s, semiconductor lasers have obviously developed in two directions, one is an information-based laser for the purpose of transmitting information, and the other is a power-based laser for the purpose of increasing the optical power. Driven by applications such as pumped solid-state lasers, high-power semiconductor lasers (continuous output power of more than 100mw and pulse output power of more than 5W can be called high-power semiconductor lasers).
In the 1990s, a breakthrough was made, which was marked by a significant increase in the output power of semiconductor lasers, the commercialization of high-power semiconductor lasers at the kilowatt level abroad, and the output of domestic sample devices reaching 600W. From the perspective of the expansion of the laser band, first infrared semiconductor lasers, followed by 670nm red semiconductor lasers, were widely used. Then, with the advent of wavelengths of 650nm and 635nm, blue-green and blue-light semiconductor lasers were also successfully developed one after another. Violet and even ultraviolet semiconductor lasers of the order of 10mW are also being developed. Surface-emitting lasers and vertical-cavity surface-emitting lasers have developed rapidly in the late 1990s, and a variety of applications in super-parallel optoelectronics have been considered. 980nm, 850nm and 780nm devices are already practical in optical systems. At present, vertical cavity surface emitting lasers have been used in high-speed networks of Gigabit Ethernet.
Applications of semiconductor lasers
Semiconductor lasers are a class of lasers that mature earlier and progress faster. Because of their wide wavelength range, simple production, low cost, and easy mass production, and because of their small size, light weight, and long life, they have rapid development in varieties and applications. A wide range, currently more than 300 species.
1. Application in industry and technology
1) Optical fiber communication. Semiconductor laser is the only practical light source for optical fiber communication system, and optical fiber communication has become the mainstream of contemporary communication technology.
2) Disc access. Semiconductor lasers have been used in optical disk memory, and its greatest advantage is that it stores a large amount of sound, text and image information. The use of blue and green lasers can greatly improve the storage density of optical discs.
3) Spectral analysis. Far-infrared tunable semiconductor lasers have been used in ambient gas analysis, monitoring air pollution, automobile exhaust, etc. It can be used in industry to monitor the process of vapor deposition.
4) Optical information processing. Semiconductor lasers have been used in optical information systems. Two-dimensional arrays of surface-emitting semiconductor lasers are ideal light sources for optical parallel processing systems, which will be used in computers and optical neural networks.
5) Laser microfabrication. With the help of high-energy ultra-short light pulses generated by Q-switched semiconductor lasers, integrated circuits can be cut, punched, etc.
6) Laser alarm. Semiconductor laser alarms are widely used, including burglar alarms, water level alarms, vehicle distance alarms, etc.
7) Laser printers. High-power semiconductor lasers have been used in laser printers. Using blue and green lasers can greatly improve printing speed and resolution.
8) Laser barcode scanner. Semiconductor laser bar code scanners have been widely used in the sales of goods, and the management of books and archives.
9) Pump solid-state lasers. This is an important application of high-power semiconductor lasers. Using it to replace the original atmosphere lamp can form an all-solid-state laser system.
10) High Definition Laser TV. In the near future, semiconductor laser TVs without cathode ray tubes, which utilize red, blue, and green lasers, are estimated to consume 20 percent less power than existing TVs.
2. Applications in medical and life science research
1) Laser surgery. Semiconductor lasers have been used for soft tissue ablation, tissue bonding, coagulation and vaporization. This technique is widely used in general surgery, plastic surgery, dermatology, urology, obstetrics and gynecology, etc.
2) Laser dynamic therapy. The photosensitive substances that have an affinity for the tumor are selectively accumulated in the cancer tissue, and the cancer tissue is irradiated with a semiconductor laser to generate reactive oxygen species, aiming to make it necrotic without damaging the healthy tissue.
3) Life science research. Using the "optical tweezers" of semiconductor lasers, it is possible to capture live cells or chromosomes and move them to any position. It has been used to promote cell synthesis and cell interaction studies, and can also be used as a diagnostic technology for forensic evidence collection.
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