Types and applications of laser generators

Laser Classification Lasers can be classified in two ways. One is to classify from the material state of the activated medium. This can be divided into gas, liquid, solid and semiconductor lasers. All types of lasers have their own characteristics. The monochromaticity of gas lasers is strong. For example, the monochromaticity of helium-neon lasers is 100 million times higher than that of ordinary light sources, and gas lasers have a wide variety of working substances, so they can generate lasers of many different frequencies. However, due to the low gas density, the output power of the laser is correspondingly small; on the contrary, the solid-state laser has high energy and high output power, but there are few types of working substances and poor monochromaticity; the biggest feature of the liquid laser is that the wavelength of the laser can be Continuous transformation within a certain range. This kind of laser is especially suitable for occasions with strict requirements on laser wavelength; semiconductor lasers are characterized by small size, light weight and simple structure, but the output power is small and the monochromaticity is poor. Another classification method is to classify according to the particle structure of the active medium, which can be divided into atoms, ions, molecules and free electron lasers. Helium-neon lasers produce lasers that are emitted by neon atoms, and ruby ​​lasers produce lasers that are emitted by chromium ions. There are also carbon dioxide molecular lasers, whose frequency can be continuously changed. And can cover a wide frequency range. The method of activating the medium in various lasers is also different. There are generally three methods: the use of high-intensity light, electrons from a charged power source, and a third, less commonly used method, nuclear radiation.

Lasers used in fiber-optic communications In fiber-optic communications, there are three types of light sources: semiconductor lasers, semiconductor light-emitting diodes, and non-semiconductor lasers. In the actual optical fiber communication system, the first two are usually selected. Instead of semiconductor lasers, such as gas lasers, solid-state lasers, etc., although they are the earliest coherent light sources, they are not suitable for use with small optical fibers due to their large size, and are only used in some special places.

Semiconductor lasers Semiconductor lasers are laser diodes, denoted as LDs. It was invented by former Soviet scientist H.Γ. Basov in 1960. The structure of a semiconductor laser is usually composed of a P layer, an N layer and an active layer forming a double heterojunction. The light emission of semiconductor lasers is based on the principle of stimulated emission of light. Most electrons in the state of population inversion distribution will emit photons synchronously when excited by external incident photons. The stimulated radiation photons and incident photons not only have the same wavelength, but also the same phase and direction. In this way, strong emission light is obtained by excitation by weak incident light, which plays a role of light amplification. However, the optical amplifying function alone cannot form optical oscillation. Just like an oscillator in an electronic circuit, only the amplification function cannot generate electrical oscillation, and a positive feedback circuit must be designed so that the power lost in the circuit can be compensated by the amplified power. Similarly, in the laser, the feedback concept of the electronic circuit is also borrowed, and a part of the amplified light is fed back to further amplify, generate oscillation, and emit laser light. Such instruments used to achieve amplified feedback of light are called optical resonators. The advantages of semiconductor lasers: small size, high coupling efficiency, fast response speed, wavelength and size adapted to the size of the fiber, direct modulation, and good coherence.

Semiconductor light-emitting diodes Similar to semiconductor lasers, semiconductor light-emitting diodes are also a PN junction, and they also use an external power supply to inject electrons into the PN junction to emit light. Semiconductor light-emitting diodes are referred to as LEDs, which are composed of a P layer formed by a P-type semiconductor, an N layer formed by an N-type semiconductor, and an active layer formed by a double heterostructure in the middle. The active layer is a light-emitting region, and its thickness is about 0.1 to 0.2 μm.

The structural tolerances of semiconductor light-emitting diodes are not as tight as those of lasers, and there are no resonators. So, the light emitted is not laser light, but fluorescence. LEDs are devices that work with an applied forward voltage. Under the action of forward bias, the electrons in the N region will diffuse in the positive direction and enter the active layer, and the holes in the P region will also diffuse in the negative direction and enter the active layer. The electrons and holes entering the active layer are trapped in the active layer due to the effect of the heterojunction barrier, forming a population inversion distribution. These electrons with population inversion distribution in the active layer will generate spontaneous emission light when they recombine with holes through transition. Semiconductor light-emitting diodes are simple in structure, small in size, small in operating current, easy to use, and low in cost, so they are widely used in optoelectronic systems.

There are many ways to classify lasers, which can be divided according to the material it cuts, according to its power, and according to the frequency band. Laser equipment can be divided into visible light, infrared, ultraviolet, X-ray, and multi-wavelength tunable according to the wavelength band. At present, industrial infrared and ultraviolet lasers, such as CO2 laser 10.64um infrared laser, krypton lamp pumped YAG laser 1.064um infrared laser, xenon lamp pumped YAG laser 1.064um infrared laser, semiconductor side pumped YAG laser 1.064um infrared laser.

There are many types of lasers, which can be divided into solid, gas, liquid, semiconductor and dye types:

(1) Solid-state lasers are generally small and sturdy, with high pulse radiation power and a wide range of applications. Such as: Nd:YAG laser. Nd (neodymium) is a rare earth element, YAG stands for yttrium aluminum garnet, and its crystal structure is similar to ruby.

(2) The semiconductor laser is small in size, light in weight, long in life and simple in structure, and is especially suitable for use in aircraft, warships, vehicles and spaceships. Semiconductor lasers can change the wavelength of laser light through external electric fields, magnetic fields, temperature, pressure, etc., and can directly convert electrical energy into laser energy, so they are developing rapidly.

( 3 ) The gas laser uses gas as the working substance, and has good monochromaticity and coherence. The laser wavelength can reach thousands of kinds, and it is widely used. The gas laser has simple structure, low cost and convenient operation. It is widely used in industry and agriculture, medicine, precision measurement, holographic technology, etc. Gas lasers have various excitation methods such as electrical energy, thermal energy, chemical energy, light energy, and nuclear energy.

(4) Dye lasers with liquid dyes as working substances came out in 1966 and are widely used in various scientific research fields. There are about 500 kinds of dyes that can generate laser light. These dyes are soluble in alcohol, benzene, acetone, water or other solutions. They can also be contained in organic plastics in solid form, or sublimated to vapor, in gaseous form. Therefore, dye lasers are also called “liquid lasers”. The outstanding feature of dye lasers is that the wavelength is continuously tunable. A wide variety of fuel lasers are available at low cost, high efficiency, and output power comparable to gas and solid-state lasers for applications in spectroscopic spectroscopy, photochemistry, medical care, and agriculture.

(5) There are many types of infrared lasers with wide application range. It is a new type of infrared radiation source, which is characterized by high radiation intensity, good monochromaticity, good coherence and strong directionality.

(6) X-ray lasers have important value in scientific research and military affairs, and have advantages in laser anti-missile weapons; biologists can use X-ray lasers to study molecular structures in living tissues or learn more about cell functions; use X-ray lasers to shoot Photographs of molecular structures, resulting in high-contrast biomolecular images.

(7) Chemical lasers Some chemical reactions produce enough high-energy atoms to release large energy, which can be used to produce laser action.

(8) Free electron lasers These types of lasers are more suitable for generating very high power radiation than other types. Its working mechanism is different. It obtains tens of millions of volts of high-energy adjustment electron beams from the accelerator, and passes through the periodic magnetic field to form energy levels of different energy states and generate stimulated radiation.

(9) Excimer lasers, fiber guided wave lasers, etc.

Laser principle overview and application

A laser is a device that emits laser light. The first microwave quantum amplifier was made in 1954, and a highly coherent microwave beam was obtained. In 1958, A.L. Xiaoluo and C.H. Townes extended the principle of microwave quantum amplifier to the optical frequency range, and pointed out the method of generating laser. In 1960 T.H. Maiman and others made the first ruby ​​laser. In 1961, A. Jia Wen et al made a helium-neon laser. In 1962, R.N. Hall and others created the gallium arsenide semiconductor laser. Since then, there have been more and more types of lasers. According to the working medium, lasers can be divided into four categories: gas lasers, solid-state lasers, semiconductor lasers and dye lasers. Recently, free electron lasers have also been developed. The working medium is a high-speed electron beam moving in a periodic magnetic field. The laser wavelength can cover a broad band from microwave to X-ray. According to the working mode, there are several types such as continuous, pulsed, Q-switched and ultra-short pulsed. High-power lasers are usually pulsed output. There are thousands of laser wavelengths emitted by various types of lasers. The longest wavelength is 0.7 mm in the microwave band, and the shortest wavelength is 210 angstroms in the far ultraviolet region. Lasers in the X-ray band are also being studied.

Except for free electron lasers, the basic working principle of various lasers is the same, and the essential components of the device include excitation (or pumping), a working medium with metastable energy levels, and a resonator (see Optical Resonator) 3 parts. Excitation is the excitation of the working medium to an excited state after absorbing external energy, creating conditions for realizing and maintaining the population inversion. The excitation methods include optical excitation, electrical excitation, chemical excitation and nuclear energy excitation. The working medium has a metastable energy level so that the stimulated emission dominates, thereby realizing optical amplification. The resonator can make the photons in the cavity have the same frequency, phase and running direction, so that the laser has good directionality and coherence.

Laser working material refers to the material system used to achieve particle number inversion and generate stimulated radiation amplification of light, sometimes also called laser gain medium, which can be solid (crystal, glass), gas (atomic gas, ionic gas) , molecular gases), semiconductors, and liquids. The main requirement for the laser working material is to achieve a large degree of population inversion between the specific energy levels of its working particles as much as possible, and to keep this inversion as effectively as possible during the entire laser emission process; To this end, the working substance is required to have suitable energy level structure and transition characteristics.

Excitation (pump) system refers to a mechanism or device that provides an energy source for the realization and maintenance of the population inversion of the laser working material. Depending on the working material and the operating conditions of the laser, different excitation methods and excitation devices can be adopted, and the following four are common. ① Optical excitation (optical pump). The whole excitation device is usually composed of a gas discharge light source (such as xenon lamp, krypton lamp) and a condenser. ②Gas discharge excitation. The particle number inversion is realized by the gas discharge process that occurs in the gas working substance. The entire excitation device is usually composed of a discharge electrode and a discharge power source. ③ chemical incentives. Particle number inversion is achieved by using the chemical reaction process that occurs inside the working substance, and usually requires appropriate chemical reactants and corresponding initiation measures. ④ Nuclear energy incentives. It uses fission fragments, high-energy particles or radiation produced by small nuclear fission reactions to excite working substances and achieve population inversion.

Optical resonant cavities are usually composed of two mirrors with certain geometric shapes and optical reflection characteristics combined in a specific way. The functions are: ① Provide optical feedback capability, so that stimulated radiation photons travel back and forth in the cavity for many times to form a coherent continuous oscillation. ② The direction and frequency of the reciprocating oscillating beam in the cavity are limited to ensure that the output laser has a certain directionality and monochromaticity. The effect of the resonant cavity ① is determined by the geometry (radius of curvature of the reflecting surface) and the relative combination of the two mirrors that usually constitute the cavity; Different frequencies of light have different selective loss characteristics.

Several common lasers and their uses are described as follows:

Nd: YAG laser, 1064nm, solid-state laser, the maximum output power of continuous laser is 1000W, which can be used for laser cutting metal.

Ho: YAG, solid-state laser that produces eye-safe 2097nm and 2091nm lasers for radar and medical applications.

He-Ne laser, 632.8nm, gas laser, power of several mW, used for collimation, positioning, holography, etc.

CO2 laser, gas laser, output wavelength 10.6um, widely used in laser processing, medical, atmospheric communication and other military applications.

N2 molecular laser, gas laser, output ultraviolet light, the peak power can reach tens of megawatts, the pulse width is less than 10ns, and the repetition frequency is tens to kilohertz. It can be used as a pump source for tunable fuel lasers, and can also be used for fluorescence analysis. , detection of pollution, etc.

There are roughly three principles to achieve laser wavelength tuning. Most tunable lasers use working substances with broad fluorescence lines. The resonators that make up the laser have very low losses only in a very narrow wavelength range. Therefore, the first is to change the wavelength of the laser light by changing the wavelength corresponding to the low-loss region of the resonator by some elements (such as gratings). The second is to shift the energy level of the laser transition by changing some external parameters (such as magnetic field, temperature, etc.). The third is to use nonlinear effects to achieve wavelength conversion and tuning (see nonlinear optics, stimulated Raman scattering, optical frequency doubling, and optical parametric oscillation). Typical lasers belonging to the first tuning method include dye lasers, chrysoberyl lasers, color center lasers, tunable high-pressure gas lasers and tunable excimer lasers.

Tunable lasers are mainly divided into: current control technology, temperature control technology and mechanical control technology in terms of implementation technology.

Among them, the electronic control technology realizes wavelength tuning by changing the injection current. It has ns-level tuning speed and wide tuning bandwidth, but the output power is small. Auxiliary grating directional coupling back-sampling reflection) laser. The temperature control technology changes the laser output wavelength by changing the refractive index of the active region of the laser. The technique is simple, but slow and has a narrow tunable bandwidth of only a few nm. Based on temperature control technology, there are mainly DFB (distributed feedback) and DBR (distributed Bragg reflection) lasers. Mechanical control is mainly based on MEMS (Micro-Electro-Mechanical Systems) technology to complete wavelength selection, with large adjustable bandwidth and high output power. Based on mechanical control technology, there are mainly DFB (distributed feedback), ECL (external cavity laser) and VCSEL (vertical cavity surface emitting laser) and other structures. The principles of tunable lasers from these aspects are explained below.

Based on current control technology

The general principle based on current control technology is to change the current of the fiber grating and the phase control part at different positions in the tunable laser, so that the relative refractive index of the fiber grating will change, resulting in different spectra, which are generated by different regions of the fiber grating. The superposition of different spectrums selects a specific wavelength, thereby generating the desired specific wavelength of laser light.

A tunable laser based on current control technology adopts SGDBR (Sampled Grating Distributed Bragg Reflector) structure.

This type of laser is mainly divided into a semiconductor amplification area, a front Bragg grating area, an active area, a phase adjustment area and a rear Bragg grating area. The front Bragg grating region, the phase adjustment region and the rear Bragg grating region respectively change the molecular distribution structure of the region through different currents, thereby changing the periodic characteristics of the Bragg grating.

For the spectrum generated in the active region (Active), the spectrum with small difference in frequency distribution is formed in the front Bragg grating region and the rear Bragg grating region respectively. For the required specific wavelength of laser light, the tunable laser applies different currents to the front Bragg grating and the back Bragg grating respectively, so that only the specific wavelength overlaps and other wavelengths do not overlap the spectrum in these two regions, so that the required specific wavelengths can be output. At the same time, the laser also includes a semiconductor amplifier area, so that the output laser light power of a specific wavelength can reach 100mW or 20mW.

Based on mechanical control technology

Based on mechanical control technology, MEMS is generally used to achieve. A tunable laser based on mechanical control technology adopts the MEMs-DFB structure.

Tunable lasers mainly include DFB laser arrays, tiltable MEMs mirrors and other control and auxiliary parts.

For the DFB laser array area there are several DFB laser arrays, each of which can generate specific wavelengths spaced at 25Ghz intervals within a bandwidth of about 1.0nm. The required specific wavelength is selected by controlling the rotation angle of the MEMs lens, so as to output the required specific wavelength of light.

Another tunable laser based on VCSEL structure ML series series, its design is based on optically pumped vertical cavity surface emitting laser, using semi-symmetric cavity technology, using MEMS to achieve continuous wavelength tuning. At the same time, large output optical power and wide spectral tuning range can be obtained by this method, and the thermistor and TEC are packaged together to have stable output in a wide temperature range. A broadband wavelength controller is integrated into the same package for precise frequency control, and the front-end tapped optical power detector and optical isolator are used to provide stable output power. This tunable laser can deliver 10/20mW optical power in both C-band and L-band.

The main disadvantage of tunable lasers based on this principle is that the tuning time is relatively slow, generally requiring a tuning stabilization time of several seconds.

Based on temperature control technology

The temperature-based control technology is mainly used in the DFB structure. The principle is to adjust the temperature in the laser cavity so that it can emit different wavelengths.

The wavelength adjustment of a tunable laser based on this principle technology is realized by controlling the InGaAsP DFB laser to work at -5–50℃. The module has built-in FP etalon and optical power detection, and the laser of continuous light output can be locked on the grid of 50GHz interval specified by ITU. There are two independent TECs in the module, one is used to control the wavelength of the laser, and the other is used to ensure the constant temperature operation of the wavelength locker and power detection detector in the module. The module also has a built-in SOA to amplify the output optical power.

The disadvantage of this control technology is that the tuning width of a single module is not wide, generally only a few nm, and the tuning time is relatively long, generally requiring a tuning stabilization time of several seconds.

At present, tunable lasers basically use current control technology, temperature control technology or mechanical control technology, and some suppliers may use one or both of these technologies. Of course, as the technology develops, other new tunable laser control technologies may also emerge.
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Post time: Jul-26-2022

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