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Gain Media

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Buyer's Guide

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Definition: media for laser amplification

German: Verstärkungsmedien

Categories: optical amplifiers, lasers, optical materials

How to cite the article; suggest additional literature

Within the context of laser physics, a laser gain medium is a medium which can amplify the power of light (typically in the form of a light beam). Such a gain medium is required in a laser to compensate for the resonator losses, and is also called an active laser medium. It can also be used for application in an optical amplifier. The term gain refers to the amount of amplification.

As the gain medium adds energy to the amplified light, it must itself receive some energy through a process called pumping, which may typically involve electrical currents (electrical pumping) or some light inputs (→ optical pumping), typically at a wavelength which is shorter than the signal wavelength.

Types of Laser Gain Media

There are a variety of very different gain media; the most common of them are:

Compared with most crystalline materials, ion-doped glasses usually exhibit much broader amplification bandwidths, allowing for large wavelength tuning ranges and the generation of ultrashort pulses. Drawbacks are inferior thermal properties (limiting the achievable output powers) and lower laser cross sections, leading to a higher threshold pump power and (for passively mode-locked lasers) to a stronger tendency for Q-switching instabilities. See the article on laser crystals versus glasses for more details.

The doping concentration of crystals, ceramics and glasses often has to be carefully optimized. A high doping density may be desirable for good pump absorption in a short length, but may lead to energy losses related to quenching processes, e.g. caused by upconversion via clustering of laser-active ions and energy transport to defects.

Important Physical Effects

In most cases, the physical origin of the amplification process is stimulated emission, where photons of the incoming beam trigger the emission of additional photons in a process where e.g. initially excited laser ions enter a state with lower energy. Here, there is a distinction between four-level and three-level gain media.

A less frequently used amplification process is stimulated Raman scattering, involving the conversion of some higher-energy pump photons into lower-energy laser photons and phonons (related to vibrations e.g. of the crystal lattice).

For high levels of input light powers, the gain of a gain medium saturates, i.e., is reduced. This naturally follows from the fact that for a finite pump power an amplifier cannot add arbitrary amounts of power to an input beam. In laser amplifiers, saturation is related to a decrease in population in the upper laser level, caused by stimulated emission.

Thermal effects can occur in gain media, because part of the pump power is converted into heat. The resulting temperature gradients and also subsequent mechanical stress can cause lensing effects, distorting the amplified beam. Such effects can spoil the beam quality of a laser, reduce its efficiency, and sometimes even destroy the gain medium (thermal fracture).

Relevant Physical Properties of Laser Gain Media

A great variety of physical properties of a gain medium can be relevant for use in a laser. The desirable properties include:

Note that in many situations there are partially conflicting requirements. For example, a very low quantum defect is not compatible with four-level behavior. A large gain bandwidth typically means that laser cross sections are smaller than ideal, and that the quantum defect cannot be very small. Disorder in solid-state gain media increases the gain bandwidth, but also reduces the thermal conductivity. A short pump absorption length can be advantageous, but also tends to exacerbate thermal effects.

It is apparent that different situations lead to very different requirements on gain media. For this reason, a very broad range of gain media will continue to remain important for applications, and making the right choice is essential for constructing lasers with optimum performance.

See also: gain, gain bandwidth, gain saturation, laser transitions, quantum defect, laser crystals, rare-earth-doped gain media, transition-metal-doped gain media, ceramic gain media, laser crystals versus glasses, solid-state lasers, four-level and three-level gain media, amplifiers, transition cross sections, doping concentration

In the RP Photonics Buyer's Guide, 64 suppliers for laser crystals and glasses are listed.

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Color-coded field penetration in a Bragg mirror as a function of wavelength.

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