Laser pump

The classical three-level system for pumping a working medium is used, for example, in a ruby ​​laser. Ruby is an Al ₂ O ₃ corundum crystal doped with a small amount of Cr ³⁺ chromium ions , which are the source of laser radiation. Due to the influence of the electric field of the corundum crystal lattice, the external energy level of chromium E _{2 is} split (see the Stark effect ). This is what makes it possible to use non-monochromatic radiation as a pump. ^[1] In this case, the atom transfers from the ground state with energy E ₀ to the excited state with energy near E ₂ . An atom can be in this state for a relatively short time (of the order of 10 ⁻⁸ s), a nonradiative transition to the level E ₁ occurs almost immediately, at which the atom can be much longer (up to 10 ⁻³ s), this is the so-called metastable level . There is the possibility of induced radiation under the influence of other random photons. As soon as there are more atoms in a metastable state than in the main, the generation process begins ^{[2] [3]} .

It should be noted that it is impossible to create an inversion of the populations of Cr chromium atoms by pumping directly from the level E ₀ to the level E ₁ . This is due to the fact that if absorption and stimulated emission occur between two levels, then both of these processes proceed at the same rate. Therefore, in this case, pumping can only equalize populations of two levels, which is not enough for generation to occur ^[1] .

In some lasers, for example, in neodymium, in which radiation is generated on Nd ³⁺ neodymium ions, a four-level pump scheme is used. Here between the metastable E ₂ and the main level E ₀ there is an intermediate - working level E ₁ . Stimulated emission occurs during the transition of an atom between the levels of E ₂ and E ₁ . The advantage of this scheme is that in this case it is easy to fulfill the inverse population condition, since the lifetime of the upper working level ( E ₂ ) is several orders of magnitude longer than the lifetime of the lower level ( E ₁ ). This greatly reduces the pump source requirements. ^[2] In addition, this scheme allows you to create powerful lasers operating in continuous mode, which is very important for some applications. ^[4] However, such lasers have a significant drawback in the form of low quantum efficiency, which is defined as the ratio of the energy of the emitted photon to the energy of the absorbed pump photon (η _quantum = hν _radiation / hν _pump )

Optical laser pumping implies the presence of a light source, an optical system for the concentration of this light on the working fluid of the laser and the actual working fluid of the laser. The type of lamp and the working fluid of the laser should be suitable for each other according to the emission and absorption spectra, respectively. As a light source, usually used:

• high-efficiency electric lamps ( arc , gas-discharge (including excilamps ));
• semiconductor light sources ( LEDs or other lasers );
• sunlight

Optical laser pumping is usually performed on the side of the laser’s working medium. Lasers are most often solid-state (represented as a rod made of a crystal or glass activated by impurities) or dye lasers (as a liquid dye solution in a glass tube or a jet of dye solution (“transverse pumping”)). For the most efficient use of radiation energy, the lamp and the active medium are located in a cavity with a mirror surface that directs most of the lamp light to the working medium. For high-power laser-pumped lasers, liquid cooling is provided. Semiconductor light-emitting devices are mounted on a heat sink .

Laser pumping by radiation from another laser is used when the spectrum or radiation power of the desired laser does not match the available lasers. In this case, a pair is selected from an accessible laser and a working fluid. The laser illuminates the working fluid in its emission spectrum, and the working fluid emits in the required spectrum. The radiation power is increased by irradiating the working fluid with several low-power lasers. A variety of such lasers ( solid-state laser with diode pumping , Eng. DPSS ) is widely used in the form of laser pointers of various colors. Pumping with a laser (rather than a conventional LED) simplifies the focusing system of pump radiation on the working fluid, reducing the size and increasing the design efficiency. Powerful fiber lasers on a similar principle are common in industry.

The direct pumping of lasers by electric current has been developed for two types of lasers: gas (electric discharge in the working body of the laser) and semiconductor.

In gas lasers

Gas lasers are usually a glass tube filled with a special gas or mixture of gases. Under the impact of electrons, gas molecules go into an excited state, releasing the resulting energy in the form of photon radiation. To excite the working medium of such lasers, the same methods are used as for igniting conventional gas-discharge lamps :

• Creating an electrical discharge between the electrodes inserted into the tube.
• Excitation of a discharge in a gas by high-frequency currents: induction and capacitive method.
• Excitation of a discharge in a gas by microwave irradiation with an electromagnetic field.

In semiconductor lasers

A semiconductor laser is a semiconductor device, directly in the structure of which laser radiation occurs under the influence of electric current. For this class of lasers, electric current pumping is the main method.

A gas-dynamic laser consists of a nozzle through which superheated gas reaches up to one and a half thousand degrees at a supersonic speed (up to 4 max ). The instant expansion and adiabatic cooling of the gas leaves a significant number of molecules in the excited state in the gas. Next, the working fluid enters a structure similar to gas lasers, where the excited molecules pass into the ground state, participating in stimulated emission. Often the design of such a laser is based on aircraft turbojet engines or rocket engines. The gas-dynamic pump principle, despite its low efficiency, can produce ultrahigh-energy laser radiation (up to megawatts) both in pulsed and continuous modes. ^{[5] [6] [7] [8] [9]}

Chemical reaction energy lasers are a type of gas lasers through which gaseous reactants are continuously pumped through the working zone. During a chemical reaction between the reactants, molecules are formed in an excited state, which transform into the ground state with the emission of a photon. Gas lasers can produce large radiation powers at relatively compact sizes. One of the problems of gas lasers is poor environmental friendliness due to abundant toxic exhaust.

The energy of a nuclear explosion is the most exotic way to pump lasers. Any substance in the epicenter of the explosion turns into a plasma, which, when cooled, again forms atoms, but already excited. If a long rod is preliminarily made from the starting material, then conditions can be formed in it along the axis along the axis for the generation of stimulated radiation generated as a result of the transition of atoms to the ground state. Obviously, such a laser is pulsed and disposable. Huge energy determines the x-ray range of radiation.

• A free-electron laser is a type of laser in which radiation is generated by a monoenergetic electron beam propagating in an undulator - a periodic system of deflecting ( electric or magnetic ) fields.

• Laser device
• Solid State Diode Pumped Laser
• Fiber laser
• Helium neon laser