The working principle of various lasers and the application range

How lasers work Except for free electron lasers, the basic working principles of various lasers are the same. The necessary conditions for laser generation are population inversion and gain greater than loss, so the essential components of the device have excitation (or pumping) ) source and the working medium with metastable energy levels. 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. A common component in a laser is a resonator, but the resonator (see optical resonator) is not an essential component. The resonator can make the photons in the cavity have a consistent frequency, phase and running direction, so that the laser has the same frequency, phase and direction. Good directionality and coherence. Moreover, it can shorten the length of the working material very well, and can adjust the mode of the generated laser (ie mode selection) by changing the length of the resonant cavity, so the general laser has a resonant cavity. Laser working substance 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 gas) ), 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. There are many types of lasers. It can be classified from the aspects of laser working material, excitation mode, operation mode, and output wavelength range. , this paper next introduces fiber lasers, ultrafast lasers, quantum cascade lasers, and terahertz. 1. Fiber Laser Fiber laser refers to a laser that uses rare earth element-doped glass fiber as a gain medium. Fiber laser can be developed on the basis of fiber amplifier: under the action of pump light, it is easy to form high power density in the fiber, resulting in the laser working substance of the laser. The energy level is "number inversion", and the laser oscillation output can be formed when a positive feedback loop (constituting a resonant cavity) is properly added. Fiber lasers have a wide range of applications, including laser fiber optic communications, laser space telecommunication, industrial shipbuilding, automobile manufacturing, laser engraving, laser marking, laser cutting, printing rolls, metal and non-metal drilling/cutting/welding (brazing, quenching, etc.) Water, cladding, and deep welding), military defense and security, medical equipment and equipment, large-scale infrastructure, as a pump source for other lasers, etc. According to the type of fiber material, fiber lasers can be divided into: 1. Crystal fiber laser. The working material is laser crystal fiber, mainly including ruby single crystal fiber laser and nd3+: YAG single crystal fiber laser. 2. Nonlinear optical fiber lasers. There are mainly stimulated Raman scattering fiber lasers and stimulated Brillouin scattering fiber lasers. 3. Rare earth doped fiber laser. The matrix material of the optical fiber is glass, and rare earth element ions are doped into the optical fiber to activate it to make a fiber laser. 4. Plastic fiber laser. Fiber lasers are made by doping laser dyes into the core or cladding of plastic optical fibers. Application of fiber laser Fiber lasers are very suitable for amplifying to higher power under continuous wave or quasi-continuous wave operation to meet the application requirements of microelectronics. In these applications, beam quality, precision and stability are critical. In many applications, controlling and changing the laser processing energy and power input plays a decisive role in the processing process. Application in welding: Using the excellent beam quality of the fiber laser to obtain a long working focal length, it is possible to obtain a large working range through the ordinary two-dimensional galvanometer system, which not only simplifies the design, but also reduces the cost. Laser welding reduces the impact of stress on the internal components of the material, thereby greatly improving the overall product yield. Application in laser marking: Due to the extremely short pulse width, it is easy to achieve extremely high peak laser intensity with low pulse energy. Due to the extremely high intensity and very short interaction time of the laser with the matter, thermal diffusion is restricted to a very small area, and the concentrated laser energy density results in rapid vaporization of the material. Therefore, pulsed fiber lasers can ablate high-quality, precise patterns on the surface of selected materials in laser marking applications. Since the distance between the two laser marking points along the scanning path is proportional to the scanner speed and inversely proportional to the pulse repetition rate, high repetition rate pulsed fiber when the laser scanner is controlled by a digital state space servo Lasers are an important part of designing high-quality, high-speed laser marking systems. Application in industrial drilling: The laser has achieved great flexibility through pulse waveform control, and can be used in drilling applications. Greater amplitude means greater peak power. The higher peak power and pulse energy provided by the waveform WFO produces larger diameter holes. Changing the frequency, the peak power and pulse energy change, and the aperture also changes. Therefore, different apertures on the micron scale can be changed by the frequency and pulse characteristics of the laser. Applications in rock and soil material processing: Fiber lasers are significantly superior to any other type of laser in construction site applications, including mining, tunneling, cutting, and rock and concrete drilling. Fiber lasers can deliver enough energy to long-range targets through very long optical fibers. Fiber laser's ultra-high electro-optical conversion efficiency (30%), good beam quality, vehicle mobility, and equipment stability and maintenance-free characteristics make it the best choice for such applications. Today, the rapid development and increasing progress of dense wavelength division multiplexing and optical time division multiplexing technology accelerate and stimulate the progress of multi-wavelength fiber laser technology, supercontinuum fiber laser and so on. At the same time, the emergence of multi-wavelength fiber lasers and supercontinuum fiber lasers provides an ideal solution for realizing Tb/s DWDM or OTDM transmission at low cost. As far as the technical approach to its realization is concerned, technologies such as spontaneous emission amplified by EDFA, femtosecond pulse technology, and super light-emitting diodes have also appeared. With the rapid development of technology in optical communication and related fields, fiber laser technology is continuously advancing in breadth and depth; technological progress, especially new optical fiber devices based on fiber grating, filter, fiber technology, etc., are coming to market one after another , which will provide new countermeasures and ideas for the design of fiber lasers. It is foreseeable that fiber lasers will play an important role in future optical communication, military, industrial processing, medical, optical information processing, full-color display and laser printing. As the representative of the third generation of laser technology, fiber laser has unparalleled technical advantages of other lasers. In the short term, fiber lasers will mainly focus on high-end applications. With the popularity of fiber lasers, cost reductions and productivity improvements, they may eventually replace most of the world's high-power carbon dioxide lasers and most YAG lasers. 　　2. Ultrafast lasers Ultrafast lasers are lasers developed from Amberpico series picosecond lasers and Amberfemto series femtosecond lasers based on SESAM mode-locking technology. Amberpico series picosecond lasers have ultra-short pulse width (less than 15ps), high single pulse energy (maximum single pulse energy 30mJ), high repetition rate (above 1kHz) and reliable excellent output performance, Amberfemto series femtosecond laser pulse width is less than 200fs, repetition frequency 1Hz-100kHz optional, with excellent spatial mode and excellent power stability. High-efficiency double, triple, and even quadruple light output can be achieved. The wavelength range covers infrared, green and ultraviolet, and the shortest wavelength can reach 266/263nm. The two are powerful research tools in scientific research fields such as satellite ranging, laser fine micromachining, nonlinear optics, laser spectroscopy, biomedicine, strong field optics, and condensed matter physics. 3. Quantum Cascade Laser Quantum cascade lasers (QCLs) are a new type of unipolar semiconductor device based on the principle of electron transition between conduction and subbands in semiconductor quantum wells and phonon-assisted resonance tunneling. Different from the electron-hole recombination stimulated emission mechanism of traditional p-n junction semiconductor lasers, only electrons participate in the stimulated emission process of QCL, and the selection of the lasing wavelength can be realized by the energy band tailoring of the potential well and the potential barrier in the active region. QCL has led the revolution of semiconductor laser theory, mid-infrared and THz semiconductor light sources. It is an ideal light source for trace gas monitoring and free space communication. It has great application prospects in the fields of public safety, national security, environment and medical science. A quantum cascade laser (QCL) is a mid-infrared monopolar light source based on electronic transitions between subbands, and its working principle is completely different from that of ordinary semiconductor lasers. The lasing scheme uses the separated electronic states caused by the quantum confinement effect in the thin layer of semiconductor heterojunction perpendicular to the thickness of the nanometer to generate population inversion between these excited states. The active region of the laser is coupled by The multi-level cascade composition of quantum wells (usually more than 500 layers) realizes the multi-photon output of single electron injection. The emergence of quantum cascade lasers pioneered the development of mid- and far-infrared semiconductor lasers using wide-bandgap materials, setting a new milestone in the development history of mid- and far-infrared semiconductor lasers. In 1994, Federico Capasso and colleagues Zhuo Yihe and others pioneered the invention of quantum cascade lasers at Bell Labs. This is regarded as a revolution in the field of semiconductor lasers. In 2000, the research group of Chinese scientist Li Aizhen (currently an academician of the National Academy of Sciences) was the first in Asia to develop a semiconductor quantum cascade laser in the 5-8 micron band, which made China enter the ranks of countries that master the development technology of such lasers. Because the quantum cascade laser is a combination of quantum engineering and advanced molecular beam epitaxy technology, it is different from the conventional semiconductor laser in the working principle, and its characteristics are better than the ordinary laser. Due to the high technical content, the development of related products has important social and economic value. It is understood that the quantum cascade laser is a difficult quantum engineering. Many tunable mid-infrared lasers (pulsed and infrared) based on quantum cascade lasers have entered industrialization abroad, and are high-tech industries that countries are scrambling to study. The quantum cascade laser integrates quantum engineering and molecular beam epitaxy technology. It is the real embodiment of the core technology of national nanometer and quantum devices. Technological breakthroughs in this area will activate my country's civilian market. It has an urgent application prospect in infrared communication, long-distance detection, air pollution monitoring, industrial smoke analysis, chemical process monitoring, molecular spectroscopy research, non-destructive medical diagnosis and so on. 　4. Terahertz THz waves (terahertz waves) or THz rays (terahertz rays) were officially named from the mid-to-late 1980s. Before that, scientists would collectively refer to them as far-infrared rays. Terahertz waves refer to electromagnetic waves with frequencies in the range of 0.3THz to 3THz, and wavelengths in the range of 0.1mm (100um) to 1mm, between microwaves and infrared. In fact, as early as a hundred years ago, scientists have been involved in this band. In 1896 and 1897, Rubens and Nichols were involved in this band, and the infrared spectrum reached 9um (0.009mm) and 20um (0.02mm), and then there were records of reaching 50um. In the following nearly 100 years, far-infrared technology has achieved many achievements and has been industrialized. However, there are very few research results and data involving the terahertz band, which is mainly limited by effective terahertz generating sources and sensitive detectors, so this band is also called the THz gap. The unique properties of terahertz give communication (broadband communication), radar, electronic countermeasures, electromagnetic weapons, astronomy, medical imaging (label-free genetic inspection, cell-level imaging), non-destructive testing, security inspection (biochemical inspection) and other fields had a profound impact. Because of the high frequency of terahertz, its spatial resolution is also high; and because its pulse is very short (on the order of picoseconds), it has high temporal resolution. Terahertz imaging technology and terahertz spectroscopy technology thus constitute the two main key technologies for terahertz applications. At the same time, because the terahertz energy is very small, it will not cause damage to the material, so it has more advantages compared with X-rays. In addition, since the resonance frequencies of the vibration and rotation frequencies of biological macromolecules are in the terahertz band, terahertz has a good application prospect in the agricultural and food processing industries such as grain selection and selection of excellent strains. The application of terahertz is still under continuous development and research, and its broad scientific prospects are recognized by the world