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Definition of the gain medium

gain medium,Nd: YAG,titanium sapphire . 

In laser physics, the laser gain medium is the medium (generally in the form of a beam of light) that magnifies the power of light. In a laser, the medium requires to make up for the loss of the resonator as well as is commonly described as the active laser medium. The gain medium can additionally use for optical fiber amplifiers. Gain refers to the degree of boosting.

Considering that the gain medium enhances the power of the intensified beam, the medium itself requires to receive the energy, that is, via a pumping process, usually developed to either existing (electrical pumping) or input light wave (optical pumping), and also the pump wavelength is smaller sized than that of the signal light.

Kinds of laser gain media

There are many kinds of gain media. The common ones are the following:

- Some straight bandgap semiconductors, such as GaAs, AlGaAs, and InGaAs, are usually pumped by an electric present in the form of quantum Wells (see semiconductor lasers).

- Laser crystals or glasses, such as Nd: YAG( neodymium-doped yttrium aluminum garnet, see yttrium aluminum garnet laser), Yb: YAG( Ytterbium aluminum garnet laser), Yb: glass, Er: YAG (Erbium doped YAG), or titanium sapphire, in solid sheet form (see volume laser) or optical glass fiber (fiber laser, fiber amplifier). These crystals or glasses are doped with laser-active ions (mostly trivalent rare-earth ions, in some cases, shift metal ions) and also pumped with light waves. Lasers utilizing these media are commonly referred to as drugged insulator lasers.

- Ceramic gain media are normally also doped with rare earth aspect ions.

- A laser dye, generally a liquid solution, is used in color lasers.

- Gas lasers utilize several gases or a blend of gases, normally pumped by a discharge gadget (such as CO2 and excimer lasers).

- Unique gain moderators consist of chemical gain arbitrators (which transform chemical power into light), nuclear pumping mediators, and oscillators in complimentary electron lasers (which transfer power from a rapid electron beam right into a beam).

Crucial physical impacts

Most of the time, the physical basis of the amplification process is stimulated radiation, in which the incident photon creates more photon radiation, and the ecstatic laser-active ion initially shifts to a somewhat lower energy fired-up state. There is a difference between the four-level gain medium and also the three-level gain medium.

An amplification procedure that happens much less often is promoted Raman scattering, which includes altering several of the higher energy pumped photons right into reduced power photons and also phonons (related to latticework vibrations). If the case light power is very high, the gain will certainly reduce after the gain medium gets to gain saturation. The amplifier can not add a randomly big quantity of power to the event beam at a restricted pump power.

In laser amplifiers, the variety of ions in the upper degree decreases at saturation as a result of promoted radiation.

The gain medium has a thermal impact because part of the pump light power is exchanged for warmth. The resulting temperature level gradient as well as mechanical anxiety will create the prism result and distort the enhanced beam of light. These results can damage the beam of light top quality of the laser, decrease its performance, and also damage the gain medium (thermal fracturing).
Associated physical residential or commercial properties of laser gain medium.

In laser applications, the physical buildings of lots of gain media are very important.

It mostly includes:

- In the laser shift procedure needing wavelength area, the best optimal gain happens in this region.

- The substratum has a high level of transparency in the working wavelength region.

- Good pump light, and reliable pump absorption.

- Suitable upper-level lifetime: enough time for Q-switched applications and also brief enough for rapidly modulated power.

- High quantum efficiency is obtained from typical quenching effects, thrilled state absorption, and also similar processes or helpful results such as multiphoton transitions or energy transfers.
- Perfect four-level habits due to the fact that quasi-three-level behavior presents some other additional restraints.

- High strength as well as long life, chemical security.

- For solid-state gain media: Base media need to be of great optical quality, can be reduced or brightened of very excellent quality (ideal firmness), allow the high focus of laser-active ions to be doped without forming clusters, have good chemical stability, have a great thermal conductivity and also reduced thermo-optical coefficient (weak thermal prism effect at high power operation), resistance to mechanical anxiety, optical isotropy is usually required, But in some cases birefringence (lowering the result of thermal depolarization) and also gain connected with polarization is needed (see the polarization of laser radiation).

- Low pump power threshold at a high gain: The product of radiation cross-section and also high-ranking lifetime is bigger.

- The beam top quality of the pump light source is low: high pump absorption is needed.

- Wavelength tuning: Needs big gain data transfer

- Ultrashort pulse generation: the gain range is large as well as flat; Ideal diffusion and nonlinearity.

- Passive mode-locked lasers without Q-switching security: sufficiently big laser cross-sections.

- High power pulse amplification (favorable feedback amplifier): Impact of high optical damages threshold and also not expensive saturation on gain.

Note that there are circumstances where conflicting needs are needed. For instance, very low quantum flaws are incompatible with a four-level system. A big gain data transfer represents a smaller sized laser cross-section than the optimal instance, and also the quantum problem is not so little. The condition in the solid-state gain medium raises the gain data transfer and also lowers thermal conductivity.

A brief pump absorption length is beneficial however worsens the thermal impact.

The needs for the gain medium vary from situation to instance. For that reason, lots of gain media are really important for applications, as well as it is needed to choose the right gain media when enhancing the style of the laser.

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