Introduction to laser principle, classification and use

1. Principle:

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 also 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.

2. 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.

3. Incentive pumping system

It refers to a mechanism or device that provides an energy source for the realization and maintenance of the particle number 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 concentrator. This excitation method is also called Lamp pump. 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.

4. Optical resonant cavity

It is usually composed of two mirrors with a certain geometric shape 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.

There are many types of lasers. In the following, it will be classified and introduced from the aspects of laser working material, excitation mode, operation mode, and output wavelength range.

5. Working Substances

According to the different states of the working material, all lasers can be divided into the following categories: ①Solid-state lasers (crystal and glass), the working material used in this type of laser is made by doping metal ions that can generate stimulated radiation. It is made by inserting into a crystal or glass matrix to form a luminescent center; ② gas lasers, the working substance they use is gas, and according to the different properties of the working particles that actually produce stimulated emission in the gas, they are further distinguished as atomic gases Lasers, ion gas lasers, molecular gas lasers, excimer gas lasers, etc.; ③ liquid lasers, the working substances used in such lasers mainly include two types, one is organic fluorescent dye solutions, and the other is containing rare earth metal ions. Inorganic compound solution, in which metal ions (such as Nd) play the role of working particles, and inorganic compound liquid (such as SeOCl2) plays the role of matrix; ④ Semiconductor laser, this type of laser is excited by a certain semiconductor material as a working substance The principle of emission is achieved by exciting non-equilibrium carriers between the energy bands of the semiconductor material or between the energy bands and the impurity energy level through a certain excitation method (electric injection, optical pump or high-energy electron beam injection). The number of particles is reversed, resulting in the stimulated emission of light; ⑤ Free electron laser, which is a special type of new type of laser, the working material is a directional free electron beam moving at high speed in a spatially periodically changing magnetic field, as long as the free electrons are changed. The speed of the beam can generate tunable coherent electromagnetic radiation, whose coherent radiation spectrum can, in principle, transition from the X-ray band to the microwave region, which is an attractive prospect.

6. Incentive methods

①Optical pump laser. Refers to lasers excited by optical pump, including almost all solid-state lasers and liquid lasers, as well as a few gas lasers and semiconductor lasers. ②Electrically excited laser. Most gas lasers are excited by gas discharge (DC discharge, AC discharge, pulse discharge, electron beam injection), while common semiconductor lasers are mostly excited by junction current injection, and some semiconductor lasers can also be excited by means of junction current injection. Excited by high-energy electron beam injection. ③Chemical laser. This specifically refers to a laser that uses the energy released by chemical reactions to excite the working substance, and the chemical reactions that are expected to be generated can be triggered by light, discharge, and chemical respectively. ④ nuclear pumped laser. Refers to a special type of laser that uses the energy released by small nuclear fission reactions to excite working substances, such as nuclear-pumped helium-argon lasers.

7. Operation mode

Due to the different working substances, excitation methods and application purposes used by lasers, their operation modes and working states are also different, so they can be divided into the following main types.

①Continuous laser, its working characteristics are the excitation of the working substance and the corresponding laser output, which can be continuously carried out in a long time range in a continuous manner. And semiconductor lasers belong to this category. Since the overheating effect of the device is often inevitable during continuous operation, most of them need to take appropriate cooling measures.

②Single-pulse laser, for this type of laser, the excitation of the working material and the corresponding laser emission are both a single-pulse process in terms of time. General solid-state lasers, liquid lasers and some special gas lasers, All operate in this way, and the thermal effect of the device can be ignored at this time, so no special cooling measures can be taken.

③ Repetitive pulse laser. The characteristic of this type of device is that its output is a series of repetitive laser pulses. For this reason, the device can be excited in the form of repeated pulses, or stimulated in a continuous way but modulate the laser oscillation process in a certain way. Obtaining repetitive pulsed laser output usually also requires effective cooling of the device.

④Dimming laser, which refers to a pulsed laser that adopts certain switching technology to obtain higher output power. Its working principle is that after the particle number inversion state of the working substance is formed, it does not cause laser oscillation (the switch is in the off state). ), after the particle number has accumulated to a high enough level, the switch is suddenly turned on, so that a very strong laser oscillation and high-power pulsed laser output can be formed in a short period of time (for example, 10-10 seconds) (see "Technology'" class=link>laser tuning technology).

⑤ Mode-locked laser, which is a special type of laser using mode-locking technology. Its working characteristic is that there is a definite phase relationship between different longitudinal modes in the resonant cavity, so a series of equidistant lasers can be obtained in time. For laser ultra-short pulse (pulse width 10-10 seconds) sequence, if special fast optical switching technology is further adopted, a single ultra-short laser pulse can also be selected from the above pulse sequence (see laser mode locking technology).

⑥Single-mode and frequency-stabilized lasers. Single-mode lasers refer to lasers that operate in a single transverse mode or single longitudinal mode after using certain mode-limiting techniques. Frequency-stabilized lasers refer to certain automatic control measures to make the laser output wavelength or Special laser devices whose frequency is stable within a certain precision range can also be made into special laser devices that operate in single mode and have the ability to automatically stabilize and control frequency in some cases (see Laser Frequency Stabilization Technology).

⑦ Tunable laser, in general, the output wavelength of the laser is fixed, but after using special tuning technology, the output laser wavelength of some lasers can be continuously and controllably changed within a certain range. This type of laser is called a tunable laser (see Laser Tuning Technology).

8. Band range

According to the different wavelength ranges of the output laser, various types of lasers can be divided into the following types.

① Far-infrared laser, the output wavelength range is between 25 and 1000 microns, and the laser output of some molecular gas lasers and free electron lasers falls into this region.

②Mid-infrared lasers refer to laser devices whose output laser wavelengths are in the mid-infrared region (2.5-25 microns), represented by CO molecular gas lasers (10.6 microns) and CO molecular gas lasers (5-6 microns).

③Near-infrared lasers refer to laser devices whose output laser wavelengths are in the near-infrared region (0.75-2.5 microns), represented by neodymium-doped solid-state lasers (1.06 microns), CaAs semiconductor diode lasers (about 0.8 microns) and some gas lasers, etc. .

④Visible lasers refer to a class of laser devices whose output laser wavelengths are in the visible spectral region (4000-7000 angstroms or 0.4-0.7 microns), represented by ruby lasers (6943 angstroms), helium-neon lasers (6328 angstroms), and argon-ion lasers (4880 angstroms, 5145 angstroms), krypton ion lasers (4762 angstroms, 5208 angstroms, 5682 angstroms, 6471 angstroms) and some tunable dye lasers.

⑤Near-ultraviolet laser, whose output laser wavelength range is in the near-ultraviolet spectral region (2000-4000 angstroms), represented by nitrogen molecular lasers (3371 angstroms) xenon fluoride (XeF) excimer lasers (3511 angstroms, 3531 angstroms), fluorine Krypton (KrF) excimer laser (2490 angstroms) and some tunable dye lasers, etc.

⑥ Vacuum ultraviolet lasers, whose output laser wavelength range is in the vacuum ultraviolet spectral region (50-2000 angstroms) are represented by (H) molecular lasers (1644-1098 angstroms), xenon (Xe) excimer lasers (1730 angstroms), etc.

⑦ X-ray laser refers to a laser system whose output wavelength is in the X-ray spectral region (0.01-50 angstroms). Soft X-rays have been successfully developed, but are still in the exploratory stage.

Nine, the main purpose

Lasers are one of the essential core components in modern laser processing systems. With the development of laser processing technology, lasers are also constantly advancing, and many new types of lasers have appeared. The lasers used in early laser processing were mainly high-power CO2 gas lasers and lamp-pumped solid-state YAG lasers. From the perspective of the development history of laser processing technology, the first laser to appear was the sealed-off CO2 laser tube in the mid-1970s. Since its development, the fifth generation of CO2 lasers, diffusion-cooled CO2 lasers, have appeared. From the development, it can be seen that the early CO2 lasers tended to increase the laser power. However, when the laser power reached a certain requirement, the beam quality of the laser was paid attention to, and the development of the laser shifted to increasing the beam quality. The diffusion-cooled slat-type CO2 laser that appears close to the diffraction limit has better beam quality, and has been widely used since it was launched, especially in the field of laser cutting, which is favored by many companies.

At the beginning of the 21st century, another new type of laser, the semiconductor laser, appeared. Compared with traditional high-power CO2 and YAG solid-state lasers, semiconductor lasers have obvious technical advantages, such as small size, light weight, high efficiency, low energy consumption, long life, and high metal-to-semiconductor laser absorption. With the continuous development of semiconductor laser technology, other solid-state lasers based on semiconductor lasers, such as fiber lasers, semiconductor-pumped solid-state lasers, and chip lasers, have also developed rapidly. Among them, fiber lasers have developed rapidly, especially rare earth-doped fiber lasers, which should be widely used in the fields of fiber communication, fiber sensing, and laser material processing.

Due to the various outstanding characteristics of lasers, they are quickly used in industry, agriculture, precision measurement and detection, communication and information processing, medical treatment, military and other aspects, and have caused revolutionary breakthroughs in many fields. In addition to the use of lasers in military communication, night vision, early warning, ranging, etc., a variety of laser weapons and laser-guided weapons have also been put into practical use.

1. The laser is used as a heat source. The laser beam is small and with huge power, such as focusing with a lens, it can concentrate the energy on a tiny area and generate huge heat. For example, people can use the concentrated and extremely high energy of laser to process various materials, and can drill 200 holes on a needle; laser as a kind of effect on biological organisms such as stimulation, mutation, burning, vaporization, etc. It has achieved good results in the practical application of medical treatment and agriculture.

2. Laser ranging. As a distance measuring light source, laser can measure long distances with high precision due to its good directionality and high power.

3. Laser communication. In the field of communication, a light-conducting cable that transmits signals with a laser column can carry the amount of information equivalent to that carried by 20,000 telephone copper wires.

4. The application of controlled nuclear gathering in the air. The laser is injected into the mixture of deuterium and tritium, and the laser brings them huge energy, which produces high pressure and high temperature, which causes the two nuclei to aggregate into helium and neutron, and at the same time emit huge radiation energy. Since the laser energy can be controlled, the process is called controlled nuclear fusion.

In the future, with the further research and development of laser technology, the performance of the laser will be further improved and the cost will be further reduced, but its application scope will continue to expand and will play an increasingly important role.