Gain Medium


In laser physics, the laser gain medium is the medium (usually in the form of a beam) that amplifies the power of light. In a laser, the medium requires to make up for the loss of the resonator and is typically described as the active laser medium. The gain medium can additionally apply it to optical fibre amplifiers. Gain refers to the level of boosting.

Since the gain medium increases the power of the enhanced light beam, the medium itself requires to obtain the power, that is, through a pumping process, generally created to either present (electrical pumping) or input light wave (optical pumping), and also the pump wavelength is smaller than that of the signal light.

Kinds of laser gain media

There are lots of kinds of gain media. The typical ones are the following:

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

– Laser crystals or glasses, such as Nd: YAG( neodymium-doped yttrium aluminium garnet, see yttrium aluminium garnet laser), Yb: YAG( Ytterbium aluminium garnet laser), , Yb: glass, Er: YAG (Erbium doped YAG), or ti sapphire, in strong sheet form (see volume laser) or optical glass fibre (fibre laser, fibre amplifier). These crystals or glasses are doped with laser-active ions (primarily trivalent rare-earth ions, in some cases, transition steel ions) as well as pumped with light waves. Lasers using these media are usually referred to as doped insulator lasers.

Er,Yb glass

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

– A laser dye, normally a liquid service, is used in colour lasers.

– Gas lasers utilize several gases or a mix of gases, typically pumped by a discharge tool (such as carbon dioxide and also excimer lasers).

– Unique gain conciliators include chemical gain mediators (which transform chemical energy right into light), nuclear pumping mediators, and also oscillators in free electron lasers (which transfer energy from a quick electron beam right into a beam).

Essential physical effects

Most of the time, the physical basis of the amplification process is boosted radiation, in which the event photon triggers more photon radiation and also, the thrilled laser-active ion very first shifts to a somewhat reduced power ecstatic state. There is a difference between the four-level gain medium as well as the three-level gain medium

A boosting procedure that takes place much less frequently is stimulated Raman spreading, which involves transforming several of the higher-power pumped photons right into lower-power photons and phonons (related to latticework resonances). If the event light power is very high, the gain will certainly decrease after the gain medium gets to gain saturation. The amplifier can not add a randomly huge quantity of power to the case beam at a minimal pump power. In laser amplifiers, the variety of ions in the top level lowers at saturation as a result of promoted radiation.

The gain medium has a thermal result due to the fact that part of the pump light power is exchanged for warmth. The resulting temperature level slope and also mechanical stress will create the prism effect and also misshape the intensified light beam. These impacts can ruin the beam of light high quality of the laser, decrease its effectiveness, and also damage the gain medium (thermal breaking).

Associated physical properties of laser gain medium.

In laser applications, the physical residential or commercial properties of several gain media are very important. It generally includes:

– In the laser transition procedure calling for wavelength region, the most effective height gain occurs in this region.

– The substratum has a high degree of transparency in the functioning wavelength region.

– Good pump light and efficient pump absorption.

– Ideal upper-level lifetime: long enough for Q-switched applications and also short enough for rapidly modulated power.

– High quantum efficiency is obtained from common quenching results, excited state absorption, as well as similar procedures or beneficial effects such as multiphoton transitions or power transfers.

– Suitable four-level habits due to the fact that quasi-three-level habits introduce a few other extra restraints.

– High toughness as well as long life, chemical security.

– Reduced pump power limit at a high gain: The product of radiation cross-section as well as high-ranking lifetime is bigger.

– The beam of light high quality of the pump light source is reduced: high pump absorption is needed.

– Wavelength tuning: Requires huge gain transmission capacity

– Ultrashort pulse generation: gain range is vast as well as flat; Proper diffusion as well as nonlinearity.

– Passive mode-locked lasers without Q-switching stability: adequately huge laser cross-sections.

– High power pulse amplification (positive responses amplifier): Result of high optical damage threshold and not too high saturation on gain.

For solid-state gain media:

Base media required to be of good optical top quality, can be cut or brightened of extremely top quality (ideal firmness), enable the high focus of laser-active ions to be doped without forming clusters, have great chemical security, have good thermal conductivity and also low thermo-optical coefficient (weak thermal prism effect at high power procedure), resistance to mechanical anxiety, optical isotropy is normally required, But in some cases birefringence (decreasing the result of thermal depolarization) and gain associated with polarization is required (see the polarization of laser radiation).

Note that there are situations where contradictory demands are called for. For instance, very reduced quantum problems are inappropriate with a four-level system. A large gain transmission capacity corresponds to a smaller laser cross-section than the perfect situation, and also, the quantum problem is not so little. The problem in the solid-state gain medium increases the gain data transfer and reduces thermal conductivity.

A short pump absorption length is beneficial however aggravates the thermal result.

The needs for the gain medium differ from case to situation. Consequently, numerous gain media are essential for applications, and also it is necessary to choose the best gain media when enhancing the style of the laser.

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