Introduction to the types, uses and principles of lasers

Type of use

The pure quality and stable spectrum of light emitted by lasers can be applied in many ways.

Ruby laser: The original laser was a ruby that was excited by a bright flash 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 diodes we use. This intense light emission, which lasts only a few nanoseconds, is ideal 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 produce only brief pulses of light.

Helium-neon laser: In 1960, scientists Ali Javan, William R.Brennet Jr. and Donald Herriot designed a helium-neon laser. It was the first gas laser, a type of equipment commonly used by holographic photographers. Two advantages: 1. Continuous laser output is generated; 2. No flash bulb is needed for light excitation, but gas is excited by electricity.

Laser diode: Laser diode is one of the most commonly used lasers at present. 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 photons generated by spontaneous emission pass through the semiconductor, once they pass near the emitted electron-hole pairs, they can be stimulated to recombine to generate new photons, which induce the recombination of excited carriers to emit new photons The phenomenon is called stimulated emission. If the injection current is large enough, the carrier distribution opposite to the thermal equilibrium state will be formed, that is, the population of the particles is reversed. When a large number of carriers in the active layer are reversed, a small amount of photons generated by spontaneous radiation will generate induced radiation due to the reciprocal reflection at both ends of the resonator, resulting in positive feedback of frequency-selective resonance, or a gain for a certain frequency. When the gain is greater than the absorption loss, coherent light with good spectral lines can be emitted from the PN junction—laser. The invention of laser diodes has made laser applications rapidly popular, and various applications such as information scanning, optical fiber communication, laser ranging, laser radar, laser discs, laser pointers, supermarket payment collection, etc., are constantly being developed and popularized.

Principle introduction

Except for the free electron laser, the basic working principle of all kinds of lasers is the same. The essential conditions for laser generation are particle number inversion and gain greater than loss, so the essential components in the device include excitation (or pumping) source and working medium with metastable energy level. Excitation is the excitation of the working medium to an excited state after absorbing external energy, creating conditions for the realization and maintenance of particle 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 dominant, thereby realizing light amplification. The common component in the laser is the resonator, but the resonator (see optical resonator) is not an essential part. The resonator can make the photons in the cavity have a consistent frequency, phase and running direction, so that the laser has Good directionality and coherence. Moreover, it can shorten the length of the working substance very well, and can also adjust the mode of the generated laser light by changing the length of the resonant cavity (that is, mode selection), so generally lasers have a resonant cavity.

Laser working substance

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

Incentive pumping system

Refers to the mechanism or device that provides the energy source for the realization and maintenance of the particle number inversion of the laser working substance. Depending on the working substance 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). It uses the light emitted by an external light source to irradiate the working substance to achieve particle number inversion. The entire 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 pumped. ②Gas discharge excitation. The gas discharge process in the gas working substance is used to realize the inversion of the number of particles, and the entire excitation device is usually composed of a discharge electrode and a discharge power supply. ③ chemical incentives. The 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 triggering measures. ④ nuclear energy incentives. It uses the fission fragments, high-energy particles or radiation produced by small nuclear fission reactions to excite the working material and realize the inversion of the number of particles.

Optical cavity

It is usually composed of two mirrors with certain geometric shape and optical reflection characteristics combined in a specific way. The functions are as follows: ①Provide optical feedback capability, so that the stimulated radiation photons go back and forth in the cavity multiple times to form coherent continuous oscillation. ② Limit the direction and frequency of the reciprocating oscillating beam in the cavity to ensure that the output laser has a certain directionality and monochromaticity. The effect of the resonant cavity ① is determined by the geometric shape (radius of curvature of the reflecting surface) and the relative combination of the two mirrors that usually make up the cavity; and the effect ② is determined by the given resonant cavity type for different travel directions and Light of different frequencies has different selective loss characteristics.