Basic Components and Classification of Lasers

Basic components of a laser

Lasers, though diverse, produce laser light through excitation and stimulated radiation, so the basic composition of a laser is fixed, by consisting of three parts: the working substance, the excitation source, and the optical resonant cavity.

1.1 Laser working substances

A laser working substance is a system of substances, sometimes called a laser gain medium, that is used to invert the number of particles and produce an amplification of the excited radiation of light. The main requirement for a laser working substance is to realize a large degree of particle number inversion between specific energy levels of its working particles as far as possible, and to maintain this inversion as effectively as possible throughout the laser emission process, for which the working substance is required to have a suitable energy level structure and hopping characteristics. The existence of substable energy levels is very favorable for the realization of particle number inversion.

Laser working substances can be solid (crystals, glass), gaseous (atomic, ionic, molecular gases), semiconductor and liquid media. In different lasers, the activating particles may be atoms, molecules, and ions, and the basic principles of laser generation are similar for all kinds of substances. We collectively refer to substances that have realized the inversion of the set number as activation media or gain media, which have the ability to amplify optical signals.

The laser working substance determines the wavelength of laser light that can be radiated by the laser, which is determined by the jump between the two energy levels in the substance that form the laser radiation. Currently, there are thousands of substances capable of generating lasers under laboratory conditions, and the wavelengths of lasers that can be generated range from the vacuum ultraviolet to the far infrared, with lasers in the X-ray band also being studied.

1.2 Pump Source

The role of the pump source is to excite the laser-working matter, pumping activated particles from the ground state to higher energy levels for particle number inversion. According to the different working substances and laser operating conditions, different excitation methods and excitation devices can be adopted, and the following four kinds are common:

(1) Optical excitation. Optical pumping is the use of an external light source issued by the light to irradiate the laser working material to realize the particle number inversion, the whole excitation device, usually by the gas discharge light source and concentrator composition. Solid-state lasers generally use ordinary light sources or semiconductor lasers as the pumping source to irradiate the laser working material.

(2) Gas discharge excitation. For the gas laser working substance, usually the gas is sealed in a fine glass tube, the voltage is added at both ends of it, and the excitation is carried out by the method of gas discharge, and the whole excitation device usually consists of the discharge electrode and the discharge power supply.

(3) Chemical motivation.Chemical excitation utilizes a chemical reaction process occurring within the laser working substance to achieve particle number inversion and usually requires appropriate chemical reactants and corresponding initiation measures.

(4) Nuclear energy incentives. Nuclear excitation is the use of fission fragments, energetic particles or radiation produced by small nuclear fission reactions to excite the laser working matter and to invert the particle number.

Although semiconductor lasers are a type of solid-state laser, they use the method of injecting current, relying on the flow of current through the medium to produce the compounding process of electrons and holes to form optical radiation, and therefore do not require an external pumping source.

From the energy point of view, the pumping process is the outside world to provide energy to the particle system process. The source of laser energy in the laser is converted by the excitation device from other forms of energy (such as light, electricity, chemical, thermal energy, etc.). In order to obtain a continuous laser output, it is necessary to pump continuously to maintain the number of particles in the upper energy level more than the lower energy level.

1.3 Optical Resonant Cavity

Optical resonant cavities serve two main purposes：

-Generation and maintenance of laser oscillations

The role of the optical resonant cavity is first of all to increase the effective length of the laser working medium, making it possible for the excited radiation process to exceed the spontaneous radiation and become dominant; at the same time to provide optical positive feedback, so that the activation of the radiation generated in the medium can be many times through the medium, and so that the beam in the cavity back and forth once during the process by the excited radiation provided by the gain exceeds the loss suffered by the beam, which will enable the beam to be amplified in the cavity and to maintain the self-oscillation oscillation.

- Controlling the quality of the output laser beam

The characteristics of the laser beam are inextricably linked to the structure of the resonant cavity, which can limit the direction and frequency of the oscillating beam inside the cavity to ensure high monochromaticity and high directionality of the output laser. By adjusting the geometrical parameters of the optical resonant cavity, it is also possible to directly control the lateral distribution characteristics of the beam, spot size, oscillation frequency and beam divergence angle.

Classification of lasers

Depending on the substance in which the laser works, lasers can be divided into the following categories:

1. Gas lasers

Gas lasers use gases and metal vapors as working substances.

Depending on the nature of the working particles in the gas that produce the effect of excited radiation, gas lasers can be further classified into atomic gas lasers, ion gas lasers, molecular gas lasers and so on.

2. Solid-state lasers

Solid-state lasers use a solid activation medium as the working material.

The solid laser working material is usually a matrix material, such as a crystal or glass, doped with a small amount of metal ions (called activation ions), and the particle jumps occur between the different working energy levels of the activation ions.

The wavelength coverage of solid-state lasers is mainly located in the visible to near-infrared wavelength band, with thousands of excitation spectral lines, characterized by high output energy and various operation modes. The devices are compact, robust and durable.