What is a semiconductor laser

Since the invention of the first semiconductor laser in the world in 1962, the semiconductor laser has undergone great changes and greatly promoted the development of other science and technology. It is considered to be one of the greatest inventions of mankind in the 20th century. In recent decades, semiconductor lasers have developed more rapidly and become the fastest growing laser technology in the world. The application of semiconductor laser covers the whole field of optoelectronics, and has become the core technology of Optoelectronic Science. Due to the advantages of small volume, simple structure, low input energy, long service life, easy modulation and low price, semiconductor lasers are widely used in the field of Optoelectronics and have been highly valued by countries all over the world.

1、 Semiconductor laser
Semiconductor laser is a kind of miniaturized laser using PN junction or PIN junction composed of direct band gap semiconductor materials as its working material. There are dozens of working materials for semiconductor lasers. At present, 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 arsenic, indium phosphorus arsenic, etc. There are three main ways to excite semiconductor lasers: electric injection, optical pump and high-energy electron beam.

Most semiconductor lasers are excited by electric injection, that is, adding a forward voltage to the PN junction to generate stimulated emission in the junction plane region, that is, it is a forward biased diode. Therefore, semiconductor lasers are also called semiconductor laser diodes. For semiconductors, the transition energy is not a certain value because electrons transition between energy bands rather than between discrete energy levels, which makes the output wavelength of semiconductor lasers spread over a wide range. They emit wavelengths ranging from 0.3 to 34 μ M. Its wavelength range depends on the energy band gap of the material used. The most common is AlGaAs double heterojunction laser, whose output wavelength is 750 ~ 890nm.

Schematic diagram of laser structure

Semiconductor laser manufacturing technology has gone through a variety of processes from diffusion to liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), MOCVD (vapor deposition of metal organic compounds), chemical beam epitaxy (CBE) and their various combinations. The biggest disadvantage of semiconductor lasers is that the laser performance is greatly affected by temperature and the beam divergence angle is large (generally between several degrees and 20 degrees), so the directivity, monochromaticity and coherence are poor. However, with the rapid development of science and technology, the research of semiconductor lasers is advancing in 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 in the 21st century.

Working principle of semiconductor laser

Semiconductor laser is a coherent radiation light source. In order to generate laser, three basic conditions must be met:

1. gain conditions: the inversion distribution of carriers in the lasing medium (active region) is established. The energy band representing the electron energy in the semiconductor is composed of a series of energy levels close to continuous. Therefore, to achieve particle number inversion in the semiconductor, the number of electrons at the bottom of the conduction band in the high-energy state must be much larger than the number of holes at the top of the valence band in the low-energy state between the two energy band regions. This depends on applying a positive bias voltage to the homojunction or heterojunction, By injecting necessary carriers into the active layer, electrons are excited from the lower valence band to the higher conduction band. Stimulated emission occurs when a large number of electrons in the state of particle number inversion recombine with holes.

2. in order to obtain coherent stimulated radiation, the stimulated radiation must be fed back many times in the optical resonator to form laser oscillation. The resonator of the laser is formed by the natural cleavage surface of the semiconductor crystal as a mirror. Usually, the non light emitting end is coated with high reflection multi-layer dielectric film, and the light emitting surface is coated with antireflection film. For F-P cavity (Fabry Perot cavity) semiconductor lasers, the natural cleavage plane perpendicular to the plane of p-n junction can be easily used to form F-P cavity.

3. in order to form stable oscillation, the laser medium must be able to provide enough gain to make up for the optical loss caused by the cavity and the loss caused by the laser output from the cavity surface, and continuously increase the optical field in the cavity. This requires a strong enough current injection, i.e. sufficient particle number inversion. The higher the particle number inversion, the greater the gain, i.e. certain current threshold conditions 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 the dipole transition of electrons and holes is the basic process of light emission and light amplification in semiconductor lasers. For new semiconductor lasers, it is generally accepted that quantum wells are the fundamental driving force for the development of semiconductor lasers. Whether quantum wires and quantum dots can make full use of quantum effects has been extended to this century. Scientists have tried to make quantum dots in various materials with self-organizing structures, and GaInN quantum dots have been used in semiconductor lasers.

Development history of semiconductor lasers

Semiconductor lasers in the early 1960s were homogeneous junction lasers, which were PN junction diodes made of a material. Under high forward current injection, electrons are continuously injected into the p region and holes are continuously injected into the N region. Therefore, the carrier distribution is reversed in the original PN junction depletion region. As the electron migration speed is faster than the hole migration speed, radiation and recombination occur in the active region, emitting fluorescence, and laser occurs under certain conditions. This is a semiconductor laser that can only work in the form of pulse. 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. The single heterostructure laser first appeared (1969). The threshold current density of a single heterojunction injection laser (shld) in the p region of gaasp-n junction is reduced by one order of magnitude compared with that of a homogeneous junction laser, but the single heterojunction laser still can not work continuously at room temperature.

Since the late 1970s, semiconductor lasers have developed in two directions. One is information lasers for the purpose of transmitting information, and the other is power lasers for the purpose of improving 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).

Application of semiconductor laser

Semiconductor laser is a kind of laser with early maturity and rapid progress. Due to its wide wavelength range, simple fabrication, low cost, easy mass production, small volume, light weight and long service life, it has developed rapidly and has a wide range of applications. At present, it has more than 300 kinds.

Application in industry and technology

1) Optical fiber communication. Semiconductor laser is the only practical light source in optical fiber communication system. Optical fiber communication has become the mainstream of modern communication technology.2) CD access. Semiconductor laser has been used in optical disk memory. Its greatest advantage is that it can store a large amount of sound, text and image information. Using blue and green lasers can greatly improve the storage density of optical disks.3) Spectral analysis. Far infrared tunable semiconductor lasers have been used in environmental gas analysis, monitoring air pollution, automobile exhaust and so on. It can be used to monitor the process of vapor deposition in industry.4) Optical information processing. Semiconductor lasers have been used in optical information processing systems. The two-dimensional array of surface emitting semiconductor lasers is an ideal light source for optical parallel processing system, which will be used in computer and optical neural network.
5) Laser micromachining. With the help of high-energy ultrashort laser impulse generated by Q-switched semiconductor laser, integrated circuits can be cut and punched.
6) Laser alarm. Semiconductor laser alarm is widely used, including anti-theft alarm, water level alarm, vehicle distance alarm, etc.
7) Laser printer. High power semiconductor lasers have been used in laser printers. Using blue and green lasers can greatly improve the printing speed and resolution.
8) Laser bar code scanner. Semiconductor laser bar code scanner has been widely used in the sales of commodities, as well as the management of books and archives.
9) Pumped solid state laser. This is an important application of high-power semiconductor lasers. Using it to replace the original atmosphere lamp can constitute an all solid-state laser system.
10) High definition laser TV. In the near future, semiconductor laser TV sets without cathode ray tubes can be put on the market. It uses red, blue and green lasers, and its power consumption is estimated to be 20% lower than that of existing TV sets.

Applications in medical and life science research

1) Laser surgery. Semiconductor lasers have been used in soft tissue resection, tissue bonding, coagulation and vaporization. This technology is widely used in general surgery, plastic surgery, dermatology, Urology, gynecology and obstetrics.
2) Laser dynamic therapy. The photosensitive substances that have affinity for tumor are selectively gathered in the cancer tissue, and the cancer tissue is irradiated by semiconductor laser to produce active oxygen, so as to make it necrosis without damage to the healthy tissue.
3) Life science research. The "optical tweezers" using semiconductor lasers can capture living cells or chromosomes and move them to any position. They have been used to promote cell synthesis, cell interaction and other research, and can also be used as a diagnostic technology for forensic forensics.

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