Introduction to the instrument
Semiconductor laser is a device that generates laser light by using a certain semiconductor material as a working substance. The working principle is to achieve non-equilibrium loading between the energy band of semiconductor material (conduction band and valence band) or between the energy band of semiconductor material and the energy level of impurities (acceptor or donor) through a certain excitation method. The number of current particles is reversed. When a large number of electrons and holes in the state of number inversion recombine, stimulated emission occurs. There are three main excitation methods for semiconductor lasers, namely, electrical injection, optical pumping and high-energy electron beam excitation. Electrical injection semiconductor lasers are generally semiconductor surface junction diodes made of gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), zinc sulfide (ZnS) and other materials, which are forward biased The current is injected for excitation, and stimulated emission occurs in the junction plane area. Optically pumped semiconductor lasers generally use N-type or P-type semiconductor single crystals (such as GaAS, InAs, InSb, etc.) as the working material, and use lasers from other lasers as optical pump excitation. High-energy electron beam-excited semiconductor lasers generally use N-type or P-type semiconductor single crystals (such as PbS, CdS, ZhO, etc.) as working materials, and are excited by external injection of high-energy electron beams. Among semiconductor laser devices, the performance is better and the most widely used is the electric injection GaAs diode laser with double heterostructure.
Laser
Semiconductor laser was successfully excited in 1962, and achieved continuous output at room temperature in 1970. Later, after improvement, a double heterojunction type laser and a laser diode (Laserdiode) with a stripe structure were developed, which are widely used in optical fiber communications, optical disks, laser printers, laser scanners, and laser pointers (laser pointers). It is currently produced The laser with the largest volume.
The advantages of laser diodes are: high efficiency, small size, light weight and low price. Especially the efficiency of the multiple quantum well type is 20-40%, and the P-N type also reaches 15%-25%. In short, high energy efficiency is its biggest feature. In addition, its continuous output wavelength covers the range of infrared to visible light, and products with an optical pulse output of 50W (pulse width 100ns) have also been commercialized. As a laser radar or excitation light source, it can be said that it is an example of a very easy-to-use laser.
Working principle
According to the energy band theory of solids, the energy levels of electrons in semiconductor materials form energy bands. The high energy is the conduction band, and the low energy is the valence band. The two bands are separated by the forbidden band. When the unbalanced electron-hole pairs introduced into the semiconductor recombine, the released energy is radiated in the form of luminescence, which is the recombination of carrier luminescence.
Generally used semiconductor materials are divided into two categories, direct band gap materials and indirect band gap materials. Among them, direct band gap semiconductor materials such as GaAs (gallium arsenide) are higher than indirect band gap semiconductor materials such as Si. The probability of radiation transition is much higher, and the luminous efficiency is much higher.
The necessary conditions for semiconductor compound luminescence to achieve stimulated emission (ie, laser generation) are: ①The carrier density of the population inversion distribution injected into the active region from the p-type side and the n-type side is very high. When it is high, the number of electrons occupying the conduction band electronic state exceeds the number of electrons occupying the valence band electronic state, and the population inversion distribution is formed. ②Optical resonant cavity In a semiconductor laser, the resonant cavity is composed of mirrors at both ends, which is called a Fabry-Perot cavity. ③High gain is used to compensate light loss. The optical loss of the resonant cavity is mainly the loss emitted from the reflecting surface and the light absorption of the medium.
Semiconductor lasers work by injecting carriers. Laser emission must meet three basic conditions:
(1) To produce sufficient population inversion distribution, that is, high-energy state The number of particles is sufficiently larger than the number of particles in the low-energy state;
(2) There is a suitable resonant cavity that can play a feedback function, so that the stimulated emission photons are proliferated, thereby generating laser oscillations;
(3) Certain threshold conditions must be met to make the photon gain equal to or greater than the photon loss.
The working principle of semiconductor lasers is the excitation method, which uses semiconductor materials (that is, electrons) to transition between energy bands to emit light, and the cleavage surfaces of semiconductor crystals form two parallel mirrors as mirrors to form a resonant cavity. , The light is oscillated and fed back, the radiation of the generated light is amplified, and the laser is output.
The advantages of semiconductor lasers: small size, light weight, reliable operation, low power consumption, high efficiency, etc.
Packaging technology
5.1 Technology introduction
Semiconductor laser packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but there are many Great particularity. Generally, the die of the discrete device is sealed in the package, and the function of the package is mainly to protect the die and complete electrical interconnection. The semiconductor laser package is to complete the output of electrical signals, protect the normal operation of the die, and output: the function of visible light, which has both electrical and optical parameters design and technical requirements. It is impossible to simply use discrete device packaging for semiconductor lasers. .
5.2 Light-emitting part
The core light-emitting part of a semiconductor laser is a pn junction die composed of p-type and n-type semiconductors. When the minority and majority carriers are injected into the pn junction When the currents recombine, they emit visible light, ultraviolet light or near-infrared light. However, the photons emitted by the pn junction region are non-directional, that is, they have the same probability of being emitted in all directions. Therefore, not all the light generated by the die can be released, which mainly depends on the quality of the semiconductor material, the structure of the die, and the geometry. , Packaging internal structure and packaging materials, application requirements to improve the internal and external quantum efficiency of semiconductor lasers. The conventional Φ5mm semiconductor laser package is to bond or sinter a square die with a side length of 0.25mm on the lead frame. The positive electrode of the die is connected to the gold wire through a spherical contact point, and the inner lead is connected to a pin, and the negative electrode passes through The reflector cup is connected to the other pin of the lead frame, and then its top is encapsulated with epoxy resin. The function of the reflector cup is to collect the light emitted from the side and interface of the die and emit it in the desired direction and angle. The epoxy resin encapsulated on the top is made into a certain shape, which has several functions: protect the die from external corrosion; use different shapes and material properties (doped or not doped with dispersing agent) to act as a lens or a diffuse lens Function to control the divergence angle of light; the core refractive index is too much related to the air refractive index, resulting in a very small critical angle of total reflection inside the core. Only a small part of the light generated by the active layer is taken out, and most of it is easy to be in the tube. The inside of the core is absorbed through multiple reflections, and total reflection is prone to cause excessive light loss. The epoxy resin with the corresponding refractive index is selected as a transition to improve the light emission efficiency of the die. The epoxy resin used to form the tube shell must have moisture resistance, insulation, mechanical strength, high refractive index and transmittance to the light emitted by the tube core. When packaging materials with different refractive indices are selected, the packaging geometry has different effects on the photon escape efficiency. The angular distribution of luminous intensity is also related to the die structure, light output mode, and the material and shape of the packaging lens. If a sharp resin lens is used, the light can be concentrated to the axis of the semiconductor laser, and the corresponding viewing angle will be smaller; if the top resin lens is round or flat, the corresponding viewing angle will increase.
5.3 Drive Current
Under normal circumstances, the emission wavelength of a semiconductor laser varies with temperature from 0.2-0.3nm/℃, and the spectral width increases accordingly, which affects the vividness of colors. In addition, when the forward current flows through the pn junction, the heat loss will cause the junction temperature to rise. At around room temperature, for every 1°C increase in temperature, the luminous intensity of the semiconductor laser will correspondingly decrease by about 1%, and the package will dissipate heat. Color purity and luminous intensity are very important. In the past, the method of reducing the driving current was used to lower the junction temperature. The driving current of most semiconductor lasers is limited to about 20mA. However, the optical output of semiconductor lasers will increase with the increase of current. The drive current of many power semiconductor lasers can reach 70mA, 100mA or even 1A. It is necessary to improve the packaging structure, new semiconductor laser packaging design concepts and low thermal resistance packaging structures. And technology to improve thermal characteristics. For example, a large-area chip flip-chip structure is adopted, silver glue with good thermal conductivity is selected, the surface area of the metal bracket is increased, and the silicon carrier of the solder bump is directly mounted on the heat sink. In addition, in the application design, the thermal design and thermal conductivity of PCB circuit boards are also very important.
After entering the 21st century, semiconductor lasers have continued to develop and innovate with high efficiency, ultra-high brightness, and panchromatic. The luminous efficiency of red and orange semiconductor lasers has reached 100Im/W, and the green semiconductor laser is 50lm/W. , The luminous flux of a single semiconductor laser also reaches tens of Im. Semiconductor laser chips and packaging no longer follow Gong’s traditional design concepts and manufacturing production models. In terms of increasing the light output of the chip, R&D is not limited to changing the amount of impurities in the material, lattice defects and dislocations to improve internal efficiency. At the same time, how to Improve the internal structure of the die and package, enhance the probability of photons emitted inside the semiconductor laser, improve the light efficiency, solve the heat dissipation, optimize the design of light extraction and heat sink, improve the optical performance, and accelerate the surface mount SMD process. Mainstream direction.
Determining factors
The operating wavelength of semiconductor optoelectronic devices is related to the type of semiconductor materials used to make the device. There are conduction and valence bands in semiconductor materials. The upper part of the conduction band allows electrons to move freely, and the lower part of the valence band allows holes to move freely. There is a forbidden band between the conduction band and the valence band. When the electrons absorb the energy of light When jumping from the valence band to the conduction band, it turns the energy of light into electricity, and the electrons with electrical energy jump from the conduction band back to the valence band, which can turn the energy of electricity into light. At this time, the material forbidden band. The width determines the operating wavelength of the optoelectronic device. The development of material science allows us to use energy band engineering to perform various exquisite tailoring of the energy band of semiconductor materials, so that it can meet our various needs and do more for us, and also enable the work of semiconductor optoelectronic devices. The wavelength breaks through the limitation of the material forbidden band width and extends to a wider range.
Loss relationship
The cavity of the laser can be divided into a resonant cavity and an external cavity. In the resonant cavity, there are many types of laser losses, such as deflection loss. Fabry-Perot resonators have larger deflection losses, while confocal resonators have smaller deflection losses, which are suitable for low-power continuous output lasers. , Such as the non-radiative transition loss of inverted particles (this type of loss can be classified as white noise), etc., are all with large cavity length loss. The laser threshold current is just the current that can make the laser oscillate. The difference in the length of the resonator can make the threshold current different. In semiconductor lasers, the image-side emitting laser has a longer cavity and the threshold current is relatively large, while the vertical cavity surface emission The laser cavity length is extremely short, and the threshold current is very low. These are not clear in one or two sentences, and their respective rate equations are also different, which cannot be explained by one or two formulas. In addition, the different resonant cavity length can also achieve the mode selection effect, that is, the frequency of the output laser is different.
Development overview
Introduction
Semiconductor lasers are also called laser diodes (LD). In the 1980s, people absorbed the latest achievements in the development of semiconductor physics, adopted novel structures such as quantum wells (QW) and strained quantum wells (SL-QW), introduced refractive index modulation Bragg emitters and enhanced modulation Bragg emitters The latest technology, and the development of new crystal growth technologies such as MBE, MOCVD, and CBE, enables the new epitaxial growth process to precisely control crystal growth, achieve the accuracy of atomic layer thickness, and grow high-quality quantum well and strained quantum well materials. As a result, the threshold current of the manufactured LD is significantly reduced, the conversion efficiency is greatly improved, the output power is doubled, and the service life is significantly longer.
Low power
The development of low power LD used in the field of information technology is extremely fast. For example, distributed feedback (DFB) and dynamic single-mode LD used in optical fiber communication and optical switching systems, narrow linewidth tunable DFB-LD, visible light wavelengths (such as wavelengths of 670nm, 650nm, 630nm) in information processing technology fields such as optical disks Red light to blue-green light) LD, quantum well surface emitting laser and ultra-short pulse LD have all been substantively developed. The development characteristics of these devices are: single-frequency narrow linewidth, high-speed, tunable, short-wavelength and monolithic integration of optoelectronics.
High power
In 1983, the output power of a single LD with a wavelength of 800nm exceeded 100mW. By 1989, the LD with a strip width of 0.1mm reached a continuous output of 3.7W, while 1cm The linear array LD has reached 76W output, and the conversion efficiency has reached 39%. In 1992, the Americans raised the index to a new level: 1cm linear array LD continuous wave output power reached 121W, and the conversion efficiency was 45%. Many high-power LDs with output powers of 120W, 1500W, 3kW, etc. have been launched. The rapid development of high-efficiency, high-power LDs and their arrays has also provided strong conditions for the rapid development of solid-state lasers, that is, semiconductor laser pumped (LDP) solid-state lasers.
In order to meet the needs of EDFA and EDFL, high-power LD with a wavelength of 980nm has also been greatly developed. With fiber Bragg grating for frequency selective filtering, the output stability is greatly improved, and the pumping efficiency is also effectively improved.
Product classification
(1) Heterostructure laser
(2) Strip structure laser
(3) GaAIAs/GaAs Laser
(4) InGaAsP/InP laser
(5) Visible laser
(6) Far-infrared laser
(7) Dynamic single-mode laser
(8) Distributed feedback laser
(9) Quantum well laser
(10) Surface emitting laser
(11) Microcavity lasers
9.1 Application Introduction
Semiconductor lasers are a class of lasers that have matured and developed rapidly. Because of their wide wavelength range, simple manufacture and low cost , Easy to mass production, and because of its small size, light weight and long life, the variety has developed rapidly, and the application range has exceeded 300. The main application field of semiconductor lasers is Gb local area network. Semiconductor lasers with 850nm wavelength are suitable for) 1Gh/. Local area network, semiconductor lasers with wavelengths of 1300nm-1550nm are suitable for 10Gb LAN systems. The application range of semiconductor lasers covers the entire field of optoelectronics and has become the core technology of today’s optoelectronics. Semiconductor lasers are used in laser ranging, lidar, laser communications, and lasers. Simulated weapons, laser warning, laser guided tracking, ignition and detonation, automatic control, detection equipment, etc. have been widely used, forming a broad market. 1In 978, semiconductor lasers began to be used in optical fiber communication systems. Semiconductor lasers can be used as light sources and indicators for optical fiber communication and form optoelectronic subsystems through large-scale integrated circuit planar technology. Because semiconductor lasers have excellent characteristics of ultra-small, high efficiency and high-speed operation Therefore, the development of this type of device has been closely integrated with optical communication technology from the very beginning. It is used in optical communication, optical conversion, optical interconnection, parallel light wave system, optical information processing and optical storage, and optical computer peripheral equipment. It has important uses. The advent of semiconductor lasers has greatly promoted the development of information optoelectronics technology. Up to now, it is the fastest growing and most important important light source for laser fiber communications in the field of optical communications. Semiconductor lasers plus low Loss of optical fiber has had a significant impact on optical fiber communication and accelerated its development. Therefore, it can be said that without the emergence of semiconductor lasers, there would be no today's optical communication. GaAs/GaAlA. Double heterojunction lasers are important light sources for optical fiber communication and atmospheric communication. Nowadays, all long-distance, large-capacity optical information transmission systems all use distributed feedback semiconductor lasers (DFB-LD). Semiconductor lasers are also widely used in optical disks. In technology, optical disk technology is a comprehensive technology integrating computing technology, laser technology and digital communication technology. It is a large-capacity, high-density, fast, effective and low-cost information storage method. It requires the light beam generated by a semiconductor laser to write information Human and Reading.
9.2 Commonly used devices
Let’s take a look at the applications of several commonly used semiconductor lasers in detail:
Quantum well semiconductor high-power lasers It has important applications in the laser processing of precision mechanical parts, and it has also become the most ideal and high-efficiency pump light source for solid-state lasers. Due to its high efficiency, high reliability and miniaturization advantages, it has led to continuous updates of solid-state lasers.
In the printing industry and medical fields, high-power semiconductor lasers are also used. In addition, long-wavelength lasers (in 1976, people used Ga[nAsP/InP to achieve long-wavelength lasers) for optical communications, short Wavelength lasers are used for disc reading. Since the realization of GaInN/GaN blue lasers by NaKamuxa, visible light semiconductor lasers have been widely used in optical disc systems, such as CD players, DVD systems and high-density optical storage. Visible light surface emitting lasers are used in optical discs and printers. , Displays have very important applications, especially red, green and blue surface-emitting lasers are more widely used. Blue-green semiconductor lasers are used for underwater communications, laser printing, high-density information reading and writing, deep-water detection and Used in large-screen color displays and high-definition color televisions. In short, visible light semiconductor lasers are used as color display light sources, readout and writing of optical storage, laser printing, laser printing, high-density optical disc storage systems, and bar codes. Readers and pump sources of solid-state lasers have a wide range of uses. The new lasers of quantum cascade lasers are used in environmental detection and medical examination fields. In addition, because semiconductor lasers can achieve wavelength tuning by changing the magnetic field or adjusting the current, and Laser output with a very narrow linewidth can be obtained, so high-resolution spectroscopy can be carried out by using semiconductor lasers. Tunable lasers are an important tool for in-depth study of material structure and rapid development of laser spectroscopy. High-power mid-infrared (3.5lm) LD is used in There are a wide range of applications in infrared countermeasures, infrared lighting, lidar, atmospheric windows, free space communications, atmospheric surveillance, and chemical spectroscopy.
Green to ultraviolet vertical cavity surface emitters are used in optoelectronics It has been widely used in ultra-high density, optical storage. Near-field optical solutions are considered to be an important means to achieve high-density optical storage. Vertical cavity surface emitting lasers can also be used in full-color flat panel displays, large-area emission, illumination, Optical signal, optical decoration, ultraviolet lithography, laser processing and medical treatment, etc. I2). As mentioned above, semiconductor lasers have been used since the early 1980s. In the past, due to the successful development and practical application of DFB dynamic single longitudinal mode lasers, the emergence of quantum well lasers and strained quantum well lasers, the development of high-power lasers and their arrays, the successful development of visible lasers, and the realization of surface emitting lasers. , The development of unipolar injection semiconductor lasers and other major breakthroughs. Semiconductor lasers have become more and more widely used. Semiconductor lasers have become a major part of the laser industry and have become the development of information, communications, home appliance industries and military affairs in various countries. Equipment is an indispensable important basic device.
The application of semiconductor laser in semiconductor laser marking machine:
Due to its long service life, high laser utilization efficiency, and thermal energy ratio, semiconductor laser YAG laser has a series of advantages such as small size, small size, high cost performance, low power consumption and other advantages. It has become a hot product in 2010. The emergence of domestic semiconductor lasers produced by e-net laser has accelerated the replacement of YAG lasers by semiconductor lasers with semiconductor lasers as main consumables. The pace of marking machine market share.
Development process
10.1 Overview
The rapid development of semiconductor physics and the subsequent invention of transistors made scientists imagine as early as the 1950s Invented the semiconductor laser. In the early 1960s, many groups competed to conduct research in this area. In terms of theoretical analysis, the work of Nikolai Basov from the Lebedev Institute of Physics in Moscow is the most outstanding.
10.2 Early research
At the International Conference on Solid-State Device Research held in July 1962, two scholars from the Lincoln Laboratory of the Massachusetts Institute of Technology, Keyes and Quist reported the light emission phenomenon of gallium arsenide material, which aroused great interest of General Electric Research Laboratory engineer Hall (Hall), he wrote down the relevant data on the train home after the meeting. After returning home, Hal immediately formulated a plan to develop semiconductor lasers, and worked with other researchers for several weeks, and their plan was successful.
Like crystal diodes, semiconductor lasers are also based on the p-n junction characteristics of materials, and their appearance is similar to the former. Therefore, semiconductor lasers are often called diode lasers or laser diodes.
10.3 Manufacturing devices
Early laser diodes have many practical limitations. For example, they can only work with microsecond pulses at a low temperature of 77K. After more than 8 years, Bell The laboratory and the Ioffe Institute of Physics in Leningrad (St. Petersburg) have produced continuous devices that can work at room temperature. A sufficiently reliable semiconductor laser did not appear until the mid-1970s.
Semiconductor lasers are very small, and the smallest is only as big as a grain of rice. The working wavelength depends on the laser material, generally 0.6 to 1.55 microns. Due to the needs of a variety of applications, devices with shorter wavelengths are under development. According to reports, lasers with compounds with valences II to IV, such as ZnSe as the working material, have achieved output of 0.46 microns at low temperatures, while the output power of room temperature continuous devices with wavelengths of 0.50 to 0.51 microns has reached more than 10 milliwatts. But so far it has not been commercialized.
Optical fiber communication is the most important foreseeable application field of semiconductor lasers. On the one hand, it is the world-wide long-distance submarine optical fiber communication, and on the other hand, it is various regional networks. The latter includes high-speed computer networks, avionics systems, health communication networks, and high-definition closed-circuit television networks. But as far as it goes, CD players are the largest market for such devices. Other applications include high-speed printing, free-space optical communications, solid-state laser pump sources, laser pointers, and various medical applications.
Semiconductor lasers in the early 1960s were homojunction lasers. They were pn junction diodes made on a material. Under the forward large current injection, electrons were continuously injected into the p region. People, holes are constantly injected into the n region. Therefore, the carrier distribution is reversed in the original pn junction depletion region. Since the migration speed of electrons is faster than the migration speed of holes, in the active region Radiation, recombination, and fluorescence are emitted, and laser light occurs under certain conditions. This is a semiconductor laser that can only work in the form of pulses.
10.4 Second stage
The second stage of semiconductor laser development is heterostructure semiconductor laser, which is composed of two thin layers of semiconductor materials with different band gaps, such as GaAs and GaAlAs. The first appeared in the single heterostructure laser (1969). The single heterojunction implanted laser (SHLD) uses the barrier provided by the heterojunction to limit the injected electrons to the P region of the GaAsP-N junction Within this, to reduce the threshold current density, its value is an order of magnitude lower than that of a homojunction laser, but a single heterojunction laser still cannot work continuously at room temperature.
In 1970, a double heterojunction GaAs-GaAlAs (gallium arsenide-gallium aluminum arsenide) laser with a laser wavelength of 9000 Å: continuous working at room temperature was realized. The birth of the Double Heterojunction Laser (DHL) has continuously expanded the usable band, and gradually improved the line width and tuning performance. Its structure is characterized by growing a thin layer of only 0.2Eam thick, undoped, and narrow energy gap material between P-type and n-type materials, so the injected carriers are restricted in this area. Inner (active area), so less current can be injected to achieve the inversion of the number of carriers. Among semiconductor laser devices, the electro-implanted GaAs diode laser with double heterostructure is more mature, better in performance, and widely used.
With the research and development of heterojunction lasers, people have thought that if the ultra-thin film (<20nm) semiconductor layer is used as the excitation layer of the laser, it can produce quantum effects. What will the result be? ? In addition, due to the achievements of MBE and MOCVD technology. Therefore, in 1978, the world's first semiconductor quantum well laser (QWL) appeared, which greatly improved the performance of semiconductor lasers. Later, due to the maturity of MOCVD and MBE growth technology, it can grow high-quality Ultra-fine thin-layer materials, afterwards, successfully developed quantum well lasers with better performance. Compared with double heterojunction (DH) lasers, quantum well semiconductor lasers have lower threshold current, higher output power, and frequency response. Good, many advantages such as narrow spectral line, good temperature stability and high electro-optical conversion efficiency.
The structural feature of QWL is that its active area is composed of multiple or single wells with a width of about 100 people. Because the width of the potential well is smaller than the de Broglie wave of the electrons in the material The wavelength produces a quantum effect, and the continuous energy band is split into sub-levels. Therefore, it is particularly conducive to the effective filling of carriers, and the required laser threshold current is particularly low. The semiconductor laser structure is mainly used for single and multiple Quantum well, the structure of single quantum well (SQW) laser is basically a kind of laser whose active layer thickness of ordinary double heterojunction (DH) laser is made below tens of nanometers. Usually, the barrier is so thick that it is relatively thick. The periodic structure in which the electron wave functions in the adjacent potential wells do not overlap is called multiple quantum wells (MQW). The single output power of quantum well lasers is now greater than 1W, and the power density has reached more than 10MW/cm3). The output power can usually be combined with many single semiconductor lasers to form a semiconductor laser array. Therefore, when the quantum well laser adopts an array-type integrated structure, the output power can reach more than 100w. High-power semiconductor lasers (especially array devices) are developing rapidly, and products with continuous output powers of 5W, 10W, 20W and 30W have been launched. Laser arrays. Pulsed semiconductor lasers with peak output powers of 50W, 120W and 1500W have also been commercialized. A 4.5cmx9cm two-dimensional array, its peak output power has exceeded 45kW. Two-dimensional arrays with a peak output power of 350 kW have also disappeared.
10.5 Development direction
Since the end of the 1970s, semiconductor lasers have clearly developed in two directions, one type is information-type lasers for the purpose of transmitting information. The other type It is a power laser for the purpose of increasing the optical power. Driven by applications such as pumping solid-state lasers, high-power semiconductor lasers (continuous output power above 100W, pulse output power above 5W, can be called high-power semiconductor Lasers) made a breakthrough in the 1990s, marked by a significant increase in the output power of semiconductor lasers. Foreign kilowatt-level high-power semiconductor lasers have been commercialized, and the output of domestic sample devices has reached 600W [61. From an expanded perspective, first infrared semiconductor lasers, followed by a large number of 670nm red semiconductor lasers entered the application, and then, the advent of wavelengths of 650nm, 635nm, blue-green and blue semiconductor lasers have also been successfully developed, 10mw order of magnitude Violet and even ultraviolet semiconductor lasers are also being developed rapidly. [a} Semiconductor lasers developed to adapt to various applications include tunable semiconductor lasers, electron beam-excited semiconductor lasers, and the distribution of the best light source for "integrated optical circuits". Feedback lasers (DFB-LD), distributed Bragg reflection lasers (DBR-LD) and integrated dual-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, etc. Among them, tunable semiconductor lasers change the wavelength of the laser through external electric field, magnetic field, temperature, pressure, doping basin, etc. The output beam can be easily modulated. Distributed feedback (DF) semiconductor lasers emerged with the development of fiber communication and integrated optical circuits. It was successfully developed in 1991. Distributed feedback semiconductor lasers fully realized single longitudinal mode Operation, it has opened up huge application prospects in the field of coherent technology. It is a cavityless traveling wave laser. The laser oscillation is provided by the optical coupling formed by the periodic structure (or diffraction grating), and is no longer a resonance composed of cleavage planes. The cavity provides feedback. The advantage is that it is easy to obtain a single-mode single-frequency output, and is easy to couple with fiber optic cables, modulators, etc., and is particularly suitable as a light source for an integrated optical path.
Semiconductor lasers with unipolar injection use thermionic optical transitions between sub-levels in the conduction band (or in the valence band) to achieve laser emission. In sub-levels or sub-bands, it is necessary to adopt a quantum well structure. Unipolar injection lasers can obtain large optical power output. It is a high-efficiency and ultra-high-speed response semiconductor laser. It is important for the development of silicon-based lasers and Shortwave lasers are very advantageous. The invention of the quantum cascade laser greatly simplifies the way to generate lasers with specific wavelengths in a wide wavelength range from mid-infrared to far-infrared. It only uses the same material and can obtain lasers of various wavelengths in the above-mentioned wavelength range according to the thickness of the layer. Compared with traditional semiconductor lasers, this laser does not require a cooling system and can operate stably at room temperature. Low-dimensional (Quantum wire and quantum dot) laserResearch has also developed rapidly. The GaInAsP/Inp long-wavelength quantum wire (Qw+) laser from Okayama, Japan has achieved Im=6.A, l=37A/cm2 and has high quantum efficiency under 90kCW working conditions. Many scientific research institutions are working Developed a self-assembled quantum dot (QD) laser. The QDLD has high density, high uniformity and high emission power. Due to actual needs, the development of semiconductor lasers is mainly centered on reducing the threshold current density, extending the working life, and achieving room temperature Continuous work, as well as obtaining single-mode, single-frequency, narrow linewidth, and the development of various devices with different laser wavelengths.
10.6 Surface Emitter
Surface-emitting lasers (SEL) appeared in the 1990s and are particularly worth mentioning. As early as 1977, people proposed the so-called surface emission Laser, and made the first device in 1979, made a 780nm surface-emitting laser pumped by light in 1987. In 1998, GaInAIP/GaA. The surface emitting laser reaches the sub-milliamp grid current at room temperature, the output power of 8mW and the conversion efficiency of 11% [2) The semiconductor laser mentioned above, from the perspective of the cavity structure, whether it is F-P (Fabry) A Perot cavity or DBR (distributed Bragg reflection) cavity, the laser output is in the horizontal direction, collectively referred to as the horizontal cavity structure. They all emit light along the parallel direction of the substrate. However, the surface emitting laser is on the chip The upper and lower surfaces are plated with reflective films to form a vertical F-P cavity. The light output is emitted along the direction perpendicular to the substrate. The vertical cavity surface emitting semiconductor laser (VCSELS) is a new type of quantum well laser. The threshold current is low, the output light has good directivity, and the coupling efficiency is high. Through the array distribution, a relatively strong optical power output can be obtained. The vertical cavity surface emitting laser has achieved an operating temperature of up to 71°C. In addition, the vertical cavity surface emitting laser also has two unstable mutually perpendicular polarization transverse mode outputs, namely the x mode and the y mode. The research on polarization switching and polarization bistability has also entered a new stage. People can change Optical feedback, photoelectric feedback, light injection, injection current and other factors realize the control of the polarization state, and new progress has been made in the field of optical switches and optical logic devices. In the late 1990s, surface emitting lasers and vertical cavity surface emitting lasers have been rapidly developed, and multiple applications in ultra-parallel optoelectronics have been considered. 980nm, 850nm and 780nm devices have been put into practical use in optical systems. Vertical cavity surface emitting lasers have been used in high-speed Gigabit Ethernet networks. In order to meet the needs of broadband information transmission, high-speed information processing, large-capacity information storage, and miniaturization and high-precision military equipment in the 21st century, the development trend of semiconductor lasers is mainly in high-speed broadband LD, high-power ID, short-wavelength LD, basin line And quantum dot lasers, mid-infrared LD and other aspects. A series of major achievements have been made in these aspects.
Other information
——Scientists at Bell Laboratories, a research and development institution of Lucent Technologies, have successfully developed the world's first capable of emitting light continuously and reliably in the infrared wavelength spectrum. The new type of semiconductor laser. The new equipment overcomes the defects in the original broadband laser emission process, and has broad potential applications in the fields of advanced optical fiber communications and photosensitive chemical detectors. Related manufacturing technology is expected to become the basis of high-performance semiconductor lasers for optical fibers in the future.
——The paper on the properties of the new laser was published in the journal Nature on February 21, 2002. The lead author of the article and Bell Labs physicist Claire Gmachl asserted: "Ultra-wideband semiconductor lasers can be used to make highly sensitive universal detectors to detect minute traces of pollution in the atmosphere, and can also be used to make new medical devices such as breath analyzers. Diagnostic tool."
——Semiconductor laser is a very convenient light source, which is compact, durable, portable and powerful. However, typical semiconductor lasers are usually narrow-band devices and can only emit monochromatic light at a unique wavelength. In contrast, ultra-wideband lasers have significant advantages in that wavelengths can be selected in a wider spectral range at the same time. It is a goal that scientists have long pursued to produce ultra-wideband lasers that can operate in a wide range of operating environments.
——In order to develop a new type of laser, Bell Labs scientists used more than 650 standard semiconductor materials used in photonics and stacked them together to form a "multi-layer sandwich." These layers are divided into 36 groups, of which different layer groups have subtle differences in photosensitive properties, and generate light in a unique short-wavelength range, while maintaining transparency between the other groups. All these layer groups are combined, A broadband laser can be emitted.
——The new laser belongs to a high-performance semiconductor laser called a quantum cascade (QC) laser. QC laser was invented by FedericoCapasso and AlfredCho and their colleagues at Bell Labs in 1994, and its operation process is very similar to an electronic waterfall. When the current passes through the laser, the waterfall of electrons will rush down the energy ladder; whenever it hits the first-level ladder, it will emit infrared photons. These infrared photons are reflected back and forth in a semiconductor resonator containing an electronic waterfall, thereby exciting other photons. This amplification process will produce a very high output energy.
——Ultra-wideband lasers can produce 1.3 watts of peak energy in the infrared wavelength range of 6 to 8 microns. Gmachl pointed out: "Theoretically, the wavelength range can be wider or narrower. The purpose of choosing a wavelength in the range of 6 to 8 microns to emit lasers is to demonstrate our ideas more convincingly. In the future, we can use specific applications such as optical fiber The laser is tailored to the specific needs of the application."
10.8 Common parameters
The common parameters of semiconductor lasers can be divided into: wavelength, threshold current Ith, working current Iop, vertical divergence angle θ ⊥, horizontal divergence angle θ∥, monitoring current Im.
(1) Wavelength: the working wavelength of the laser tube. The wavelength of the laser tube that can be used as a photoelectric switch is 635nm, 650nm, 670nm, laser diode 690nm, 780nm, 810nm, 860nm, 980nm, etc.
(2) Threshold current Ith: the current at which the laser tube starts to generate laser oscillation. For general low-power laser tubes, its value is about tens of milliamps. It is a laser with a strained multiple quantum well structure. The tube threshold current can be as low as 10mA or less.
(3) Working current Iop: the drive current when the laser tube reaches the rated output power. This value is more important for the design and debugging of the laser drive circuit.
(4) Vertical divergence angle θ⊥: The angle at which the light-emitting band of the laser diode is opened in the direction perpendicular to the PN junction, generally around 15˚~40˚.
(5) Horizontal divergence angle θ∥: The angle at which the light-emitting band of the laser diode is opened in the direction parallel to the PN junction, generally about 6˚~10˚.
(6) Monitoring current Im: the current flowing on the PIN tube when the laser tube is at the rated output power.
Laser diodes have been widely used in low-power optoelectronic devices such as CD-ROM drives on computers, print heads in laser printers, barcode scanners, laser distance measurement, laser medical treatment, optical communications, laser pointers, etc. It has also been used in high-power equipment such as stage lighting, laser surgery, laser welding and laser weapons.
Industrial
The semiconductor laser used in industrial laser equipment is generally 1064nm, 532nm, 355nm, and the power ranges from several watts to several kilowatts. Generally used in SMT template cutting, automotive sheet metal cutting, laser marking machine is 1064nm, 532nm is suitable for ceramic processing, glass processing and other fields, 355nm ultraviolet laser is suitable for cover film opening, FPC cutting, silicon wafer cutting and scribing Wire, high-frequency microwave circuit board processing and other fields.
Military
Since semiconductor lasers have the advantages of simple structure, small size, long life, easy modulation and low price, they are widely used in military fields, such as laser guidance and tracking, laser radar , Laser fuze, optical ranging, laser communication power supply, laser simulation weapon, laser aiming warning, laser communication and laser gyroscope, etc. Developed countries in the world attach great importance to the development of high-power semiconductor lasers and their military applications.
Semiconductor laser fuze is a kind of optical fuze, which belongs to the technical category of active proximity fuze. The laser fuze detects the target through the laser, processes and calculates the laser echo information, determines the target, calculates the explosion point, and detonates at the best position in time. Once the bomb fails to capture or lose its target and the fuze fails, the self-explosion mechanism can detonate the projectile to self-destruct. Semiconductor laser fuze is the most successful application of laser detection technology in weapon systems.
Laser guidance: It makes the missile fly in the laser beam until it destroys the target.
Semiconductor laser guidance has been used in surface-to-air missiles, air-to-air missiles, surface-to-surface missiles, etc. Laser guided tracking has a very wide range of applications in the military. One of the methods of laser guidance is beam-riding guidance, also known as laser beam guidance. The laser emission system of the guidance station emits a coded and modulated laser beam into space according to a certain rule, and the center line of the beam is aligned with the target; the missile flying in the beam, when its position deviates from the center of the beam, the laser received at the tail of the missile The laser signal is detected by the device, and after information processing, the on-board solver calculates the size and direction of the projectile deviation from the center line to form a control signal; then the corresponding mechanism of the missile is manipulated by the autopilot to make it fly along the center of the beam. , Until the target is destroyed. Another laser guidance method is fiber-optic guidance. The sensor's information is transmitted to the missile controller through a released optical fiber, the displayed image is observed and the control command is sent back through the same optical fiber to achieve the purpose of controlling and manipulating the missile.
Laser ranging: mainly used in anti-tank weapons, aviation, aerospace and other fields. The rangefinder uses a semiconductor laser as the light source, which is concealed and slightly improved. It can also measure the distance between vehicles and perform digital display, and give an alarm when the safety factor is lower than the required safety factor. Semiconductor laser night vision devices and laser night vision monitors have also been important applications. The light source of the active night vision device using the semiconductor laser array is concealed and the array power is high, which can increase the monitoring distance to 1km. If equipped with a scanning and image display device, it can become a laser night vision monitor. When it is used to monitor the target, the target's activity can be transmitted to the command post through the optical cable at the right time. Choosing a longer suitable wavelength can become an all-weather monitor.
Lidar: Compared with CO2 lidar, the lidar of the semiconductor laser array is small in size, simple in structure, short in wavelength, high in precision, has a variety of imaging functions and real-time image processing functions, including various Integration of imaging, image tracking and automatic target recognition, etc. It can be used to monitor targets, measure atmospheric moisture, clouds, and air pollution; it can also be used as aircraft collision avoidance radar, airborne shear wind detection coherent light radar, accurate positioning of incoming targets, and terrain of helicopters and cruise missiles Tracking etc. Semiconductor laser radars are mainly LDs and arrays with wavelengths of 820 to 850 nm.
Laser simulation: Laser simulation is mainly a new type of military training and exercise technology developed based on semiconductor lasers. By adjusting the laser beam, period and range to achieve the purpose of simulating the characteristics of any weapon. Weapon simulation mainly uses 904nm semiconductor lasers, and eye-safe lasers are used as the basis of the tactical training system, originally called the Laser Engagement System (LES). The development of the system began in 1973, and its feasibility has been confirmed. In 1974, microprocessor technology was introduced, and LES developed into a multifunctional laser engagement system (MILES). In the same year, Syrox Electro-Optic Systems Corporation accepted a full set of MILES project development contracts and provided more than 80,000 sets of equipment to the Army for ground combat simulation. In addition, the company has also developed air-to-ground combat systems and MILES air defense prototypes. There are three countries in the world selling MILESII/SAWE systems in the United States, Britain, and Switzerland (Code); NATO countries, Israel, Argentina, Russia, and China are all developing such systems.
Deep-sea optical communications: Semiconductor lasers are an ideal light source with the advantages of anti-interference and good confidentiality. The blue-green light of the laser-to-submarine communication light source is the communication window (460～540nm) of seawater, with a penetration depth of about 300ft. The submarine can communicate with satellites or aircraft carriers using blue-green light. A frequency-doubled semiconductor high-power laser array (wavelength between 920 and 1080 nm) is one such light source.
Semiconductor laser communication: In satellite communication technology, semiconductor lasers only need smaller telescopes and lower emission power to realize free-space transmission of light and obtain extremely high data rate transmission. Laser communication technology can be used for mutual communication between orbiting satellites and between satellites and ground stations.
Characteristics
Laserdiode is a type of laser device with semiconductor materials as the working material. In addition to the common characteristics of lasers, it also has the following advantages:
(1)Small size and light weight;
(2) Low driving power and current;
(3)High efficiency and long working life;
(4) Direct electrical modulation;
(5) Easy to realize optoelectronic integration with various optoelectronic devices;
(4) p>
(6) Compatible with semiconductor manufacturing technology; mass production is possible. Because of these characteristics, semiconductor lasers have received extensive attention and research from all over the world since their inception. It has become the fastest growing, most widely used laser in the world, the first class of lasers to achieve commercialization and the largest output value in the laboratory. After more than 40 years of development, semiconductor lasers have evolved from the initial low-temperature 77K, pulsed operation to continuous operation at room temperature, and the operating wavelength has expanded from infrared and red light to blue-violet light; the threshold current has been reduced from the order of 10^5A/cm2 10^2A/cm2 magnitude; working current is as small as sub-mA; output power ranges from a few mW to array device output power up to several kW; structure develops from homojunction to single heterojunction, double heterojunction, quantum well , Quantum well array, distributed feedback type, DFB, distributed Bragg reflection type, DBR and more than 270 forms. The production method has developed from diffusion method to liquid phase epitaxy, LPE, vapor phase epitaxy, VPE, metal organic compound deposition, MOCVD, molecular beam epitaxy, MBE, chemical beam epitaxy, CBE and many other preparation processes.