定义：
激光放大的介质。

在激光物理中，激光增益介质就是能放大光功率的介质（一般以光束的形式）。在激光中介质需要补偿掉谐振腔的损耗，也通常被称为激光活性介质。它也可以被应用到光纤放大器中。增益是指被放大的程度。 

由于增益介质使被放大的光束的能量增加，介质本身也需要接收能量，也就是通过泵浦过程，一般是设计到电流（电泵浦）或者输入光波（光泵浦），并且泵浦的波长要小于信号光的波长。 

激光增益介质的种类 
有许多种的增益介质，常见的有以下几种： 

1. 一些直接带隙半导体，例如GaAs, AlGaAs, InGaAs，它们通常是由电流泵浦，以量子阱的形式（参阅半导体激光器） 
2. 激光晶体或者玻璃，例如Nd:YAG(掺钕钇铝石榴石，参阅钇铝石榴石激光器)，Yb:YAG(掺镱YAG)，Yb:玻璃，Er:YAG（掺铒YAG），或者钛蓝宝石，以固体片状形式（参阅体激光器）或者光学玻璃光纤（光纤激光器，光纤放大器）。这些晶体或者玻璃掺杂了一些激光活性离子（大多数情况下为三价稀土元素离子，有时为过渡金属离子），并且用光波泵浦。采用这些介质的激光器通常被称为掺杂绝缘体激光器。 
3. 陶瓷增益介质，通常也掺杂稀土元素离子。 
4. 在染料激光器中采用激光染料，通常是液体溶液。 
5. 气体激光器是采用一些气体或者气体混合物，通常采用放电器泵浦（如CO2激光器和受激准分子激光器）。 
6. 一些特殊的增益介质，如化学增益介质（将化学能转化成光能），核能泵浦介质，还有自由电子激光器中的波荡器（将快电子束中的能量转移到光束中）。 

相比于大多数的晶体材料，离子掺杂的玻璃具有更大的放大带宽，允许大的波长调谐以及产生超短脉冲。缺点则是稍差的温度特性（限制得到的输出功率）和小的激光截面，导致大的泵浦功率阈值（对于无源锁模激光器）以及调Q不稳定性。可以参阅词条激光晶体与玻璃的对比来获得更多的信息。

晶体、陶瓷和玻璃的掺杂浓度通常需要小心的进行优化。在短波长处存在强泵浦吸收的情况下需要掺杂浓度比较高，但是这也会引起与淬灭过程相关的能量损耗，例如由于激光活性离子团簇引起的上转换过程和能量转移到缺陷。 

重要的物理效应 
大多数情况下，放大过程的物理基础为受激辐射，也就是入射的光子引发更多的光子辐射，这是激发的激光活性离子先跃迁一个稍低能量的激发态。四能级增益介质和三能级增益介质的过程是有差别的。 

不经常发生的放大过程是受激拉曼散射，涉及到将一些高能量的泵浦光子变为低能级的光子和声子（与晶格振动相关）。 如果入射光功率很高，增益介质达到增益饱和后，增益会降低。也就是说，在有限的泵浦功率时，放大器不能将任意多的功率加到入射光束中。在激光放大器中，饱和情况下上能级离子数降低，由于受激辐射的原因。 

增益介质中存在热效应，由于一部分泵浦光功率被转化成热量。产生的温度梯度和随之而来的机械应力会引起棱镜效应，使放大的光束发生畸变。这些效应会毁掉激光器的光束质量，降低效率，优势甚至摧毁增益介质（热破裂）。 

激光增益介质的相关物理性质 
在激光应用中，许多增益介质的物理性质都很重要。主要包括： 

1. 在需要波长区域的激光跃迁过程，最好峰值增益发生在此区域 
2. 在工作波长区域基底介质具有高度的透明度 
3. 好的泵浦光源，高效的泵浦吸收 
4. 合适的上能级寿命：在调Q开关应用时要足够长，需要对功率进行很快调制时需要足够短 
5. 高的量子效率，由低淬灭效应，激发态吸收和类似过程得到，或者从有利效应，如多光子跃迁或能量转移得到 
6. 理想的四能级行为，因为准三能级行为引入一些其它额外的限制 
7. 高强度和长寿命，化学稳定性 
8. 对于固态增益介质：基地介质需要具有好的光学质量，可以切割或者抛光的很高质量（合适的硬度），允许掺杂高浓度的激光活性离子而不会形成团簇，化学稳定性好，好的热传导性和低热光系数（高功率工作时弱的热棱镜效应），抗机械应力，光学各向同性通常需要，但有时双折射（减小热去极化效应）和与偏振相关的增益也会需要（参阅激光辐射的偏振） 
9. 高增益时的低泵浦功率阈值：辐射截面与上能级寿命的乘积比较大 
10. 对泵浦光源的光束质量要求低：高的泵浦吸收是需要的 
11. 波长调谐：需要大的增益带宽 
12. 超短脉冲产生：增益谱宽并且平坦；合适的色散和非线性 
13. 无需调Q稳定性的无源锁模激光器：足够大的激光截面 
14. 高能脉冲放大（正反馈放大器）：高的光学损伤阈值和不太高的饱和对增益的影响 

注意在有些情况下需要一些相互冲突的要求。例如，非常低的量子缺陷是与四能级系统不符合的。大的增益带宽对应的激光截面与理想状况下相比较小，并且这样量子缺陷不会很小。固态增益介质中的无序性提高了增益带宽，但是同时也降低了热传导性。短的泵浦吸收长度是有利的，但是这会加剧热效应。

不同的情况下对增益介质的要求不同。因此，很多增益介质对于应用还是非常重要，在优化激光器的设计时选择合适的增益介质是非常必要的。

Definition: media for laser amplification

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 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:

• Certain direct band gap semiconductors such as gallium arsenide, indium gallium arsenide or gallium nitride are typically pumped with electrical currents, often in the form of quantum wells (→ semiconductor lasers). A substantially different amplification mechanism is utilized in quantum cascade lasers, another type of semiconductor lasers.
• Certain laser crystals and glasses such as Nd:YAG (neodymium-doped yttrium aluminum garnet → YAG lasers), Yb:YAG (ytterbium-doped YAG), Yb:glass, Er:YAG (erbium-doped YAG), or Ti:sapphire are used in the form of solid pieces (→ bulk lasers) or optical glass fibers (→ fiber lasers, fiber amplifiers). These crystals or glasses are doped with some laser-active ions (in most cases trivalent rare earth ions, sometimes transition metal ions) and optically pumped. Lasers based on such media are sometimes called doped insulator lasers.
• There are ceramic laser gain media, which are also normally doped with rare earth ions.
• Laser dyes are used in dye lasers, typically in the form of liquid solutions.
• Gas lasers are based on certain gases or gas mixtures, typically pumped with electrical discharges (e.g. in CO2 lasers and excimer lasers).
• More exotic gain media are chemical gain media (converting chemical energy to optical energy), nuclear pumped media, and undulators in free electron lasers (transferring energy from a fast electron beam to a light beam).

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:

• a laser transition in the desired wavelength region, preferably with the maximum gain occurring in this region
• a high transparency of the host medium in this wavelength region
• a pump wavelength for which a good pump source is available (in case of an optically pumped laser); efficient pump absorption
• a suitable upper-state lifetime: long enough for Q-switching applications, short enough if fast modulation of the power is required
• a high quantum efficiency, obtained via a low prevalence for quenching effects, excited-state absorption and the like, but also possibly by strong enough beneficial effects such as certain multi-phonon transitions or energy transfers
• ideally, four-level behavior, because quasi-three-level behavior introduces various additional constraints
• robustness and a long lifetime, chemical stability
• for solid-state laser gain media: a host medium which is available with good optical quality in the required size, can be cut and polished with good quality (appropriate hardness), allows for high doping with laser-active ions without clustering, is chemically stable (e.g., not hygroscopic), and has a good thermal conductivity and low thermo-optic coefficients (for weak thermal lensing in high-power operation) and high resistance to mechanical stress; optical isotropy can be desirable, but in other cases birefringence (reducing thermal depolarization) and possibly polarization-dependent gain is preferable (see also: polarization of light)
• for high gain, low threshold pump power: a high product of emission cross section and upper-state lifetime (σ−τ product)
• for low beam quality requirements on the pump source: high pump absorption may be helpful
• for wavelength tuning: a large gain bandwidth
• for ultrashort pulse generation: a broad and smoothly shaped gain spectrum; suitable chromatic dispersion and nonlinearity are also sometimes important
• for passive mode locking without Q-switching instabilities: high enough laser cross sections
• for high energy pulse amplification (e.g. in regenerative amplifiers): a high optical damage threshold and not too high saturation fluence of the gain

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 is often helpful.