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Principle composition and application of laser

Laser is a device that can emit laser. According to the working medium, lasers can be divided into four categories: gas lasers, solid lasers, semiconductor lasers and dye lasers. Recently, free electron lasers have been developed. High-power lasers are usually pulsed. Output.

The working principle of laser:
Except for free electron lasers, the basic working principles of various lasers are the same. The indispensable conditions for laser generation are population inversion and gain greater than loss, so the indispensable components in the device are excitation (or pumping) source and working medium with metastable energy level. Excitation means that the working medium is excited 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 metastable energy level of the working medium makes the stimulated radiation dominate, thereby realizing optical amplification. Common components in lasers include resonant cavity, but resonant cavity (see optical resonant cavity) is not an indispensable component. The resonant cavity can make the photons in the cavity have the same frequency, phase and running direction, so that the laser has Good directionality and coherence. Moreover, it can shorten the length of the working material well, and can also adjust the mode of the generated laser by changing the length of the resonant cavity (ie mode selection), so generally lasers have resonant cavities.

The laser is generally composed of three parts:
1. Working substance: At the core of the laser, only the substance that can achieve energy level transition can be used as the working substance of the laser.
2. Encouraging energy: its function is to give energy to the working matter, and to excite atoms from low-energy level to high-energy level of external energy. Usually there can be light energy, thermal energy, electric energy, chemical energy, etc.
3. Optical resonant cavity: The first function is to make the stimulated radiation of the working substance go on continuously; the second is to continuously accelerate the photons; the third is to limit the direction of the laser output. The simplest optical resonant cavity is composed of two parallel mirrors placed at both ends of a helium-neon laser. When some neon atoms transition between the two energy levels that have achieved population inversion, and radiate photons parallel to the direction of the laser, these photons will be reflected back and forth between the two mirrors, thus continuously causing stimulated radiation. Very strong laser light is produced very quickly.

The quality of the light emitted by the laser is pure and the spectrum is stable, which can be used in many ways:
Ruby laser: The original laser was that ruby was excited by a bright flashing bulb, and the laser produced was a "pulse laser" rather than a continuous and stable beam. The quality of the speed of light produced by this laser is fundamentally different from the laser produced by the laser diode we are using now. This intense light emission that lasts only a few nanoseconds is very suitable for capturing easily moving objects, such as holographic portraits of people. The first laser portrait was born in 1967. Ruby lasers require expensive rubies and can only produce short pulses of light.

He-Ne laser: In 1960, scientists Ali Javan, William R. Brennet Jr. and Donald Herriot designed a He-Ne laser. This is the first gas laser. This type of laser is commonly used by holographic photographers. Two advantages: 1. Produce continuous laser output; 2. Do not need flash bulb for light excitation, but use electric excitation gas.

Laser diode: The laser diode is one of the most commonly used lasers. The phenomenon of spontaneous recombination of electrons and holes on both sides of the PN junction of the diode to emit light is called spontaneous emission. When the photon generated by spontaneous radiation passes through the semiconductor, once it passes the vicinity of the emitted electron-hole pair, it can excite the two to recombine and produce new photons. This photon induces the excited carriers to recombine and emit new photons. The phenomenon is called stimulated emission.

If the injected current is large enough, the carrier distribution opposite to the thermal equilibrium state will be formed, that is, the population inversion. When the carriers in the active layer are in a large number of inversions, a small amount of spontaneous radiation produces induced radiation due to the reciprocating reflection of the two ends of the resonant cavity, resulting in frequency-selective resonant positive feedback, or gaining a certain frequency. When the gain is greater than the absorption loss, a coherent light with good spectral lines-laser light can be emitted from the PN junction. The invention of the laser diode allows laser applications to be rapidly popularized. Various types of information scanning, optical fiber communications, laser ranging, lidar, laser discs, laser pointers, supermarket collections, etc., are constantly being developed and popularized.