定义：
对泵浦光的吸收，例如在激光器或者激光放大器中

在光泵浦激光器或放大器中，采用泵浦光可以为增益介质提供能量。只有被吸收的泵浦光能被这一装置采用，例如，光学参量放大器为小信号提供增益而不会吸收大部分的能量。因此，泵浦光是否被有效吸收对激光器或者放大器的功率效率影响很大。

目录

• 1 计算泵浦吸收
• 2 泵浦吸收饱和
• 3 泵浦吸收测量
• 4 优化泵浦吸收

计算泵浦吸收
对于非常小的泵浦强度，或者装置中激光活性原子或者离子被激发的程度可以忽略，吸收成都可以简单的采用掺杂浓度Ndop，增益介质长度L和泵浦光波长时吸收截面 σabs 来得到：泵浦吸收程度：
 

由于吸收截面与波长相关，因此泵浦吸收程度也与波长有关。有些增益介质的吸收带宽较小，这样吸收效率就非常小，例如如果采用的泵浦光源具有有限的带宽，或者辐射波长依赖于装置温度的变化。
各向异性激光晶体中，吸收截面也会对偏振态敏感。例如在钒酸盐激光器和钨酸盐激光器中，会产生很显著的偏振效应：在某一偏振态泵浦吸收效率很高，而另一正交方向则相对吸收不完全。这是最好采用偏振的泵浦光源，并且调节好偏振方向。当采用光纤耦合的二极管激光器时，这时的问题是，泵浦光的偏振态可能不是线性的，会随着温度或者光纤移动而改变，因此激光器的输出功率是不稳定的。
以上的方程也不适宜于饱和吸收情况，下面会具体讨论。

泵浦吸收饱和
一些激光器或者放大器装置中激光活性离子的激发程度很高，这通常是基于准三能级增益介质时存在的情况。这是吸收在某种程度上达到饱和，原因很简单，就是因为要吸收光的处于基态的原子或者离子密度太小，被耗尽了。
泵浦光饱和程度历来与信号光强。在激光器中，达到激光器阈值时，激光过程会使上能态粒子数达到相对较低的水平，因此泵浦饱和程度也被钳制住（稳态工作时）；大于阈值时，就不依赖于泵浦功率了，如果激发态密度在横向非常依赖于功率的情况除外。
在增益介质的某个部分很强的激发程度不会产生严重的泵浦吸收效应。原因在于某一部分泵浦吸收变小会提高在增益介质其它部分的泵浦强度。对泵浦吸收有影响的是整体吸收的泵浦功率。

泵浦吸收测量
理论上，计算泵浦吸收很简单：需要测量入射泵浦功率和剩余泵浦功率。然而，许多情况下这种测量不易进行：
测量剩余的泵浦光比较困难，例如，如果泵浦光碰到一些谐振腔镜子，尤其是当泵浦光束发散比较严重的情况，非常难获取所有剩余的泵浦光。在端泵浦激光器中，谐振腔镜通常为了测量而被移除，但是如果没有激光作用那么就不存在降低泵浦功率的机制了。而在边泵浦激光器或者多通激光器中存在的问题是泵浦光在增益介质中的辐射。
入射泵浦功率有时也不容易测量。如果泵浦光来自于激光二极管（参阅二极管泵浦激光器），泵浦功率和辐射波长也会受泵浦光背向反射到二极管的光的影响，如果采取双向泵浦，则两泵浦光会相互影响。
由于以上原因，最好将激光器设计在低强度吸收的情况（未饱和），与理论上的吸收饱和相结合。在许多情况下（例如大多数的Nd:YAG激光器），泵浦饱和现象不是很强，可以被忽略。

优化泵浦吸收
在激光器设计时，通过采用很长的增益介质，并且掺杂浓度很高，可以理论上得到非常高效的泵浦吸收。然后，也存在一些限制因素：
在端泵浦的激光器中，长的晶体提高了对泵浦光束质量的需求，因此得到高光束质量的激光器更加困难。
对于非常高的掺杂浓度，淬灭效应会降低增益介质的效率。
在准三能级增益介质中，只有当泵浦强度大于某一个特定的透明强度时才能得到正增益。因此，当晶体长度满足一些泵浦光未被吸收（如果它不会被反射回来）的条件时，端泵浦的准三能级激光器可以达到最好的功率效率。
在许多情况下，泵浦吸收效率在90%是可以接受的，因为在测量吸收效率时的一些负的边效应会占主导地位。
 

Definition: the absorption of pump light, e.g. in a laser or a laser amplifier

More general term: absorption

In an optically pumped laser or amplifier, the gain medium is powered by supplying it with pump light. Only absorbed pump light can be useful in such a kind of device, whereas e.g. an optical parametric amplifier provides amplification (gain) for a small signal without absorbing significant amounts of power. Therefore, it is essential for the power efficiency of a laser or laser amplifier that the pump light is efficiently absorbed.

Calculating Pump Absorption

For very small pump intensities, or for devices operating with a negligible degree of excitation of the laser-active atoms or ions, the degree of absorption can be simply calculated from the doping concentration N_dop, the length L of the gain medium, and the absorption cross section σ_abs at the pump wavelength: the degree of power absorption is

Due to the wavelength dependence of the absorption cross sections, the degree of pump absorption also depends on the wavelength. For some gain media with a small absorption bandwidth, the absorption efficiency can be significantly compromised, e.g. when using a pump source with finite optical bandwidth, or when the emission wavelength depends on a varying device temperature.

For anisotropic laser crystals, the absorption cross sections can also be polarization-dependent. Pronounced polarization effects can occur e.g. in various types of vanadate lasers and tungstate lasers: pump absorption can be fairly efficient for one polarization, and fairly incomplete for the other polarization direction. It is then best to use a polarized pump source and properly adjust the polarization direction. Problems can arise e.g. when using a fiber-coupled diode laser as the pump source, where the polarization state of pump light may not only not be linear, but also change with temperature or when the fiber is moved; the laser's output power is then not stable.

The above equation is not valid in cases with absorption saturation, as discussed below.

Saturation of Pump Absorption

Some laser or amplifier devices exhibit a significant degree of excitation of the laser-active ions; this is particularly the case for those based on a quasi-three-level laser gain medium. The absorption is then saturated to some extent, simply because the density of atoms or ions in the ground state, from where they can absorb light, is depleted.

The degree of pump saturation will in general depend on the intensity of signal (laser) light. In a laser, for example, the lasing process may clamp the upper-state population to some relatively low level, so that the degree of pump saturation is also clamped (in steady-state operation) as soon as the laser threshold is reached: above threshold, there is no further dependence on pump power, except if the transverse profile of the excitation density is significantly power-dependent.

Note that a high level of excitation in some portion of the gain medium does not inevitably lead to significant pump saturation effects. The point is that a reduced pump absorption at some point can increase the pump intensity in other parts of the gain medium. What matters is essentially the overall absorbed pump power.

Measurement of Pump Absorption

In principle, the measurement of pump absorption is trivial: the incident pump power as well as the residual pump power have to be measured. However, there are many situations where this is not easy to do:

• The access to residual pump light can be difficult, e.g. when the pump light hits some resonator mirror – particularly when there is significant divergence of the pump beam, so that it is difficult to capture all of the residual pump light. In an end-pumped laser, a resonator mirror is sometimes removed for that measurement, but without laser action there is then no effect of reduced pump saturation. Other problems apply to side-pumped lasers, or to lasers with multiple passes of pump radiation through the gain medium.
• The incident pump power is also not always easy to measure. If the pump light comes from a laser diode (→ diode-pumped lasers), both the power and emission wavelengths may even be somewhat modified by back-reflection of pump light into the diode, or in cases with bidirectional pumping the two pump sources may influence each other.

For such reasons, it is often better to work out a laser design based on the knowledge of (unsaturated) low-intensity absorption, combined with theoretical knowledge on absorption saturation. In many cases (e.g. for most Nd:YAG lasers), pump saturation effects are not very strong and can be ignored altogether.

Optimizing Pump Absorption

Very efficient pump absorption could in principle be obtained with a laser design based on a gain medium with a large length and/or a high doping concentration. However, there are various limitations:

• In an end-pumped laser, a long crystal length increases the demands on the beam quality of the pump source, and may make it more difficult to obtain a high beam quality of the laser.
• For too high doping concentrations, quenching effects may reduce the efficiency of the gain medium.
• In a quasi-three-level laser gain medium, a positive gain is achieved only for pump intensities above a certain transparency intensity. Therefore, the best power efficiency of an end-pumped quasi-three-level laser is achieved when the crystal length is chosen such that some portion of the pump light remains unabsorbed (if it cannot be reflected back).

In many cases, a pump absorption efficiency of e.g. 90% is considered to be satisfactory, as the negative side effects of measures for a further increased absorption efficiency would dominate.

Bibliography