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Laser Gain Media

Definition: media for laser amplification

Alternative terms: active media, active laser media

More specific terms: ceramic laser gain media, rare-earth-doped laser gain media, transition-metal-doped laser gain media, chromium-doped laser gain media, erbium-doped laser gain media, neodymium-doped laser gain media, praseodymium-doped laser gain media, thulium-doped laser gain media, ytterbium-doped laser gain media, four-level and three-level laser gain media, quasi-three-level laser gain media, laser crystals, rare-earth-doped fibers

German: Verstärkungsmedien

Categories: optical materialsoptical materials, laser devices and laser physicslaser devices and laser physics, optical amplifiersoptical amplifiers


Cite the article using its DOI: https://doi.org/10.61835/z5z

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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, in contrast to passive optical elements, not providing amplification. 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 electric 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 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 laser gain media, and others are quasi-three-level laser 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 thermal lensing effects, distorting the amplified beam, and there can also be depolarization loss. Thermal 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 laser 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. For that purpose, quantitative laser modeling and simulation is often helpful.

More to Learn

Encyclopedia articles:


The RP Photonics Buyer's Guide contains 53 suppliers for laser gain media. Among them:

Questions and Comments from Users


How can manufacturers determine concentrations of rare-earth dopants in active media?

The author's answer:

The doping concentration is indeed an important parameter of a gain medium such as a laser crystal or a rare-earth-doped fiber. In some cases, it can be determined simply by measuring the amount of dopants added during fabrication. In other cases, it may be necessary to measure it, e.g. with spectroscopic measurements, where however additional parameters need to be known (e.g. absorption cross-sections).

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