Introduction to laser theory, classification and application

Lasers - devices that emit lasers.

The first microwave quantum amplifier was made in 1954, and the microwave beam was highly coherent.

In 1958, A.L. scholl and C.H. tuns applied the principle of microwave quantum amplifiers to the range of light frequencies. In 1960, T.H. maiman et al. made the first ruby laser.

In 1961, a. jarvin et al. made A he-ne laser.

In 1962, R.N. hall et al. created gallium arsenide semiconductor lasers.

In the future, there will be more and more kinds of lasers.

According to the working medium, the laser can be divided into four categories: gas laser, solid laser, semiconductor laser and dye laser.

Recently, free electron lasers have been developed. High-power lasers are usually pulsed.　

I. principle:

All kinds of lasers have the same basic operating principles except the free electron laser. The essential condition for producing laser is particle number inversion and gain over loss. Therefore, the indispensable component of the device has two parts: excitation (or pumping) source and working medium with metastable energy level.

Excitation is the excitation of the working medium after absorbing foreign energy to the excited state, to achieve and maintain the particle number reversal conditions.

There are optical, electric, chemical and nuclear incentives.

The substable energy level of the working medium makes the stimulated radiation dominant, thus realizing the optical amplification.

The common components of a laser include a resonant cavity, but the cavity (see the optical cavity) is not an essential component. The cavity allows the photons in the cavity to have a consistent frequency, phase and operating direction, thus giving the laser a good directivity and coherence.

Moreover, it can shorten the length of the working material and adjust the mode of the laser produced by changing the length of the cavity (i.e. mode selection). Therefore, most lasers have resonant cavity.

Laser working substance

The term refers to the substance system used to realize particle number reversal and generate light by stimulated radiation amplification, sometimes also called laser gain medium, they can be solid (crystal, glass), gas (atomic gas, ion gas, molecular gas), semiconductor and liquid medium.

The main requirement for the laser working material is to achieve a large degree of particle number inversion between the specific energy levels of the working particles and keep the inversion as effective as possible during the whole process of laser emission.

Therefore, it is required that the working substance has proper energy level structure and transition characteristics.

3. Pumping system of excitation

Means a mechanism or device that provides energy for a laser working substance to realize and maintain a particle number reversal.

Depending on the working substance and the operating conditions of the laser, different excitation methods and devices can be adopted.

Optical actuation (light pump).

The whole excitation device is usually composed of gas discharge light sources (such as xenon lamp, krypton lamp) and concentrator. This excitation method is also called lamp pump.

, gas discharge excitation.

The entire excitation device is usually composed of the discharge electrode and the discharge power supply.

Chemistry.

The inversion of particle number is achieved by using the chemical reaction process occurring in the working substance, which usually requires appropriate chemical reactants and corresponding initiation measures.

Nuclear energy.

Fission fragments, high-energy particles or radiation produced by small nuclear fission reactions are used to excite the working material and realize particle number reversal.

　　Optical cavity

It is usually composed of two mirrors with certain geometric and optical properties.

The effect is to provide optical feedback so that excited radiation photons travel back and forth in the cavity many times to form coherent continuous oscillations.

The direction and frequency of the oscillating beams in the cavity are limited to ensure that the output laser is directional and monochromatic.

The cavity effect is determined by the geometric shape (radius of curvature of reflective surface) and relative combination of the two mirrors that usually constitute the cavity.

The forces are determined by the selective loss characteristics of different moving directions and different frequencies of light in a given cavity type.

There are many types of lasers.

In the following part, the classification of laser working substance, excitation mode, operation mode and output wavelength range will be introduced.

Working substance

All the lasers can be divided into the following categories according to the different physical states of the working substance: the rest solid laser (crystal and glass).

The gaseous laser is a gas, and it can be further divided into atomic gas laser, ionic gas laser, molecular gas laser and excimer gas laser according to the different properties of the working particles in the gas that actually generate stimulated emission.

The working substances adopted by this kind of laser include organic fluorescent dye solution and inorganic compound solution containing rare earth metal ions, in which metal ions (such as Nd) act as working particles and inorganic compound liquids (such as SeOCl2) act as substrate.

(4) semiconductor laser, the laser is a semiconductor material role as working substance produced by stimulated emission of radiation, the principle of which is through certain incentives (electric injection pump, light or high energy electron beam injection), between the band gap of semiconductor material or between band and impurity level, by stimulating the carrier and the balance of population inversion, the role of light are produced by stimulated emission of radiation;

(5) free electron laser, this is a special type of new type of laser, work material for periodic changes in the space of the high speed motion in magnetic field directional free electron beam, as long as the speed of change of free electron beam can produce tunable coherent electromagnetic radiation, in principle, the coherent radiation spectrum can transition from X-ray wavelengths to microwave area, so it is very tempting prospects.

Vi. Incentives

Light pump laser.

Refers to lasers that are pumped by light, including almost all solid and liquid lasers, as well as a few gas and semiconductor lasers.

An electrically excited laser.

Most gas lasers are excited by gas discharge (dc discharge, ac discharge, pulse discharge, electron beam injection), while most common semiconductor lasers are energized by junction current injection. Some semiconductor lasers can also be excited by high-energy electron beam injection.

Chemical lasers.

This is a laser that USES the energy released by chemical reactions to excite the working material. The chemical reactions can be triggered by light, discharged or chemically triggered respectively.

Is the nuclear pump laser.

A type of special laser, such as a nuclear pumped helium-argon laser, that USES the energy released by a small nuclear fission reaction to excite the working material.

　　Vii. Operation mode

Due to different working materials, excitation modes and application purposes, the operation mode and working state of the laser are different, which can be divided into the following main types.

The continuous laser is characterized by the excitation of the working substance and the corresponding laser output, which can be carried out continuously in a long time range. The solid laser excited by the continuous light source and the gas laser and semiconductor laser operated by the continuous electric excitation are of this kind.

Due to the inevitable overheating effect of devices in continuous operation, most of them need to take proper cooling measures.

(2) a single pulse laser, for this type of laser, material incentives and corresponding laser emission, from the time all is a process of single pulse, the general solid state laser, liquid laser, as well as some special gas laser, adopt this way, the heating effect of the device at this time can be ignored, so it can not take special cooling measures.

(3) repetitive pulse laser, such devices is characterized by its output is a series of repeated laser pulse, therefore, the device can be appropriate incentives, in the form of repetitive pulse or motivation on the basis of continuous modulation laser oscillation process but in a certain way, to get repetitive pulse laser output, usually also requires effective cooling measures are taken to the device.

(4) the laser, which is specifically refers to the adoption of a certain switch technology to achieve high power output of pulsed laser, its working principle is in the work state of population inversion matter does not make it after the formation of lasing oscillation (switch is closed), after waiting for particles accumulate to a high enough level, suddenly instantaneous switch, which can be in a relatively short period of time (10 ~ 10 seconds, for example) form a very strong laser oscillation and high power pulse laser output (see '" class = link > laser technology).

(5) mode-locked lasers, which is a kind of special type laser used mode-locked technology, whose work is characteristic by the resonance cavity has a definite phase relation between different longitudinal modes, therefore can obtain a series of view is equally spaced in time ultrashort pulse laser, pulse width 10 to 10 seconds) sequences, if further adopt special fast optical switch technology, from the selection of single pulse sequence of ultrashort laser pulses (see mode-locked laser technology).

6 single mode and the laser frequency stabilization, the single mode laser refers to the adoption of a certain limit after mold technology is in a state of single transverse mode or single longitudinal mode operation of the laser, laser frequency stabilization measures refers to the adoption of a certain automatic control the laser output wavelength or frequency stability under certain precision within the scope of the special laser devices, in some cases, also can be made into both single-mode operation and special laser capable of automatic frequency stability control devices (see the laser frequency stabilization technology).

In general, the output wavelength of a tunable laser is fixed, but the output wavelength of some lasers can be changed in a continuous and controllable range by using a special tuning technique. This type of laser is called tunable laser (see laser tuning technique).

Band range

Different types of lasers can be divided into the following types according to the wavelength range of the output laser.

The output wavelength range of the far-infrared laser is between 25 ~ 1000 microns. The laser output of some molecular gas laser and free electron laser falls into this area.

The nir laser refers to a laser device whose output laser wavelength is in the mid-infrared region (2.5 ~ 25 microns), which is represented by CO molecular gas laser (10.6 microns) and CO molecular gas laser (5 ~ 6 microns).

The passive near-infrared laser is a laser device whose output laser wavelength is in the near-infrared region (0.75 ~ 2.5 microns), represented by neodymium-doped solid laser (1.06 microns), CaAs semiconductor diode laser (about 0.8 microns) and some gas lasers.

(4) the visible laser, refers to the output laser wavelength in the visible spectral range (4000 ~ 7000 or 0.4 ~ 0.7 microns) of laser device, representatives for the ruby laser (6943), he-ne laser (6328), argon ion laser (4880, 5145), krypton ion laser (4762, 5208, 5682, 6471), and some of the tunable dye laser, etc.

The near-ultraviolet laser, whose output laser wavelength range is in the near-ultraviolet spectrum region (2000-4000 angstroms), is represented by nitrogen molecular laser (3371 angstroms) fluorinated xenon (XeF) excimer laser (3511 angstroms, 3531 angstroms), krypton fluoride (KrF) excimer laser (2490 angstroms) and some tunable dye lasers.

The wavelength range of the output laser is in the region of vacuum ultraviolet spectrum (50 ~ 2000 angstrom), represented by (H) molecular laser (1644 ~ 1098 angstrom), xenon (Xe) excimer laser (1730 angstrom), etc.

Soft x-rays have been developed, but are still in the exploratory stage.

Ix. Main purposes

Laser is one of the essential core components in modern laser processing system.

With the development of laser processing technology, the laser has been developing and many new lasers have appeared.

The early laser processing lasers are high-power CO2 gas laser and light pumped solid YAG laser.

From the development history of laser processing technology, the first laser emerged in the mid-1970s sealed CO2 laser tube. Up to now, the fifth generation CO2 laser -- diffusion cooled CO2 laser has emerged.

It can be seen from the development that the early CO2 laser is inclined to the development direction of improving the laser power. However, when the laser power reaches a certain requirement, the beam quality of the laser is taken seriously, and the development of the laser is transferred to the beam quality of high adjustment.

The diffusion cooling slat CO2 laser which appears near the diffraction limit has a good beam quality and has been widely used, especially in the field of laser cutting, which is favored by many enterprises.

In the early 21st century, another new type of laser - semiconductor laser emerged.

Compared with traditional high power CO2 and YAG solid laser, semiconductor laser has the obvious technical advantages, such as the mention of small, light weight, high efficiency, low energy consumption, long life and high metal uptake of semiconductor laser, with the continuous development of semiconductor laser technology, the semiconductor laser based other solid laser, such as fiber lasers, semiconductor pump solid state laser, such as slab laser development is also very quickly.

Among them, fiber lasers developed rapidly, especially rare earth doped fiber lasers should be widely used in the fields of fiber communication, fiber sensing and laser material processing.

Due to its outstanding characteristics, the laser has been used in many fields such as industry, agriculture, precision measurement and detection, communication and information processing, medical treatment and military, and has made revolutionary breakthroughs in many fields.

In the military, laser has been used for communications, night vision, early warning, ranging and other aspects, a variety of laser weapons and laser guidance weapons have also been put into use.

1. The laser is used as a heat source.

The laser beam is small and carries a huge amount of power. Focusing with a lens, for example, can concentrate energy on a tiny area and generate huge amounts of heat.

For example, people can use the concentrated and extremely high energy of laser to process various materials and drill 200 holes on a needle.

As a means to cause stimulation, variation, cauterization and vaporization on biological organisms, laser has achieved good results in the practical application of medicine and agriculture.

2. Laser ranging.

As a range-finding light source, the laser can measure very far distance due to its good directivity, high power and high precision.

3. Laser communication.

In communications, an optical cable that USES a laser beam to transmit signals can carry as much information as 20,000 copper wires.

4. Application of controlled nucleus gathering in air.

By shooting the laser into a mixture of deuterium and tritium, the laser gives them huge amounts of energy, producing high pressure and high temperature, causing the two nuclei to fuse into helium and neutrons, and releasing huge amounts of radiation energy at the same time.

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 laser will be further improved and the cost will be further reduced, but its application range will be further expanded, and it will play an increasingly huge role.