定义：
增益介质需要低温工作的激光器。

低温工作的激光器的概念并不新颖：历史上第二个激光器就是低温的。最开始采用这一概念就是很难实现室温工作，对于低温工作研究的热情是从1990s高功率激光器和放大器发展开始的。 
在高功率激光光源中，热效应，例如去极化损耗，热透镜或者激光晶体的弯曲都会影响光源的性能。通过低温冷却，可以有效抑制很多有害的热效应，即需要将增益介质冷却到77K甚至4K。冷却效应主要包括： 

1. 增益介质的特传导性极大的被抑制，主要是因为绳子的平均自由程增加了。因此，温度梯度急剧降低。例如，当温度从300K降低到77K时，YAG晶体的热导率增加了7倍。 
2. 热扩散系数也急剧减小。这与温度梯度的减少一起会导致热透镜效应减小，因此应力破裂的可能性降低。 
3. 热光系数也减小，进一步减小热透镜效应。 
4. 稀土离子的吸收截面增大，主要是由于热效应引起的展宽降低。因此，饱和功率降低，激光增益增加。因此，阈值泵浦功率降低，在Q开关工作时鞥能得到更短的脉冲。通过提高输出耦合器的透射率，斜率效率可以提高，因此寄生谐振腔损耗影响变得不太重要。 
5. 准三能级增益介质总低能级的粒子数降低，因此降低阈值泵浦功率，功率效率提高。例如，产生1030nm光的Yb:YAG可以看做是室温下的准三能级系统，但是在77K为四能级系统。Er：YAG也是相同的情况。 
6. 根据增益介质的不同，有些淬灭过程的强度会降低。 

结合以上因素，低温工作可以极大的提高激光器的性能。特别是低温冷却激光器可以得到非常高的输出功率，而不会产生热效应，即可以得到很好的光束质量。 
需要考虑的一个问题是，在低温冷却激光晶体中，辐射光和吸收光的带宽会减小，因此波长调谐范围变窄，对泵浦激光器的线宽和波长稳定性要求更加严格。但是，这一效应通常很少发生。 
低温冷却通常采用的是冷却剂，例如液氮或者液氦，理想情况下，制冷剂在贴着激光晶体的管道中循环。冷却剂会及时的补充或者在闭环中循环利用。为了避免凝固，通常需要将激光晶体放在真空室中。 
激光晶体在低温工作的概念也可以应用于放大器。可以采用钛蓝宝石制作正反馈放大器，平均输出功率在几十瓦。 
尽管低温冷却装置会使激光系统变得复杂，更加常见的冷却系统通常也不太简单，并且低温冷却的效率允许降低一定的复杂性。 

Definition: lasers where the gain medium is operated at cryogenic temperatures

Alternative term: cryogenically cooled lasers

The idea of operating lasers at very low temperatures is not exactly new: the second laser in history already was a cryogenic one [1]. While this concept was originally used just because room-temperature operation was hard to achieve, a renewed interest in cryogenic operation for high-power lasers and amplifiers developed in the 1990s.

In high-power solid-state laser sources, thermal effects such as depolarization loss, thermal lensing or even fracture of the laser crystal can be a real problem limiting the performance. A number of the detrimental thermal effects can be effectively suppressed by cryogenic cooling, meaning cooling of the gain medium to low temperatures such as 77 K (the temperature of liquid nitrogen) or even 4 K (liquid helium). The main effects of such cooling are:

• The thermal conductivity of the gain medium is strongly increased, mainly because the mean free path length of phonons is increased. Therefore, temperature gradients are strongly decreased. As an example, the thermal conductivity of YAG increases by a factor of 7 when the temperature is reduced from 300 K to 77 K.
• The thermal expansion coefficient is also strongly reduced. This together with the reduced temperature gradients reduces thermal lensing from bulging and stress, and of course the tendency for stress fracture.
• The thermooptic coefficient (dn / dT) is also reduced, further reducing thermal lensing.
• The laser and absorption cross sections of rare earth ions are increased, essentially because thermally induced broadening is reduced. As a result, saturation powers are reduced, and the laser gain is increased. Consequently, the threshold pump power is reduced, and shorter pulses can be obtained in Q-switched operation. The slope efficiency can be increased by increasing the output coupler transmission, so that parasitic resonator losses become relatively less important.
• The thermal population of the lower laser level in quasi-three-level laser gain media is reduced, which again reduces the threshold pump power and leads to laser designs with increased power efficiency [5]. For example, Yb:YAG for 1030-nm emission behaves as a quasi-three-level system at room temperature, but as a four-level system at 77 K. The same is true for Er:YAG lasers emitting at 1.6 μm [8].
• Depending on the gain medium, the strength of certain quenching processes may be reduced.

The combination of these factors allows for strong improvements in laser performance. In particular, cryogenically cooled lasers have the potential for generating much higher output powers without excessive thermal effects, i.e. with good beam quality.

A possible concern is that the bandwidth of both the emission and absorption of the cryo-cooled laser crystal may be reduced, which may lead to a narrower range for wavelength tuning and to more stringent requirements on the linewidth and wavelength stability of the pump laser. However, this effect does not necessarily occur.

Cryogenic cooling may be achieved with a cryogen such as liquid nitrogen or helium, ideally circulating through channels in a cooling finger which is attached to the laser crystal. The cryogen may be taken from some supply, which is refilled from time to time, or recycled in a closed loop, containing e.g. a Stirling engine. To avoid condensation, one usually has to operate the laser crystal in a vacuum chamber with optical windows.

Of course, the concept of operating the laser crystal at a very low temperature can also be applied to amplifiers. It is used e.g. to build regenerative amplifiers based on Ti:sapphire with average output powers of tens of watts.

Although cryogenic cooling arrangements certainly add to the complexity of such a laser system, more conventional cooling systems are also not always very simple, and the great effectiveness of cryogenic cooling may allow for a reduction in complexity at other places.

Some kinds of semiconductor lasers, for example quantum cascade lasers, can also benefit from operation at cryogenic temperatures. In some cases, continuous-wave operation can be achieved only under such conditions, or at least the achieved performance becomes much better.

Bibliography