定义： 
包含不同材料或者不同化学组分的激光晶体。

复合激光晶体（也称为混合激光晶体）是采用不同的材料得到的激光晶体。通常采用的是无粘合剂的扩散粘合，表面经过处理，例如可以将掺钕钇铝石榴石和掺镱钇铝石榴石晶体与无掺杂钇铝石榴石晶体粘接起来。 同样的也可以用于 Nd:YVO4。另一个可以复合的晶体是Cr:YAG晶体（用于被动Q开关的饱和吸收器材料），与Nd:YAG复合。还有的情况，需要把用来进行非线性频率转换的非线性晶体材料粘接到激光晶体上。复合增益介质也可以由玻璃或者陶瓷制作。 

 粘接面的光学质量非常重要。需要采用一些过程来得到高质量粘接面。有些过程需要工作在高温下，其它的可以工作在室温。有一个过程，例如，需要在粘接前采用真空中的高能离子束消除表面的干扰。  

下面给出了几个采用复合增益介质的例子： 

1. 在短激光棒采用无掺杂端帽可以减少热效应，可以从掺杂部分吸收一部分热。这会降低峰值温度，热破裂的概率以及热透镜的强度。这种晶体可以提高准三能级掺钕激光器的性能，例如工作在946nm处的Nd:YAG激光器，其产热比普通的1064nm波长时强很多。 
2. 如果将不同掺杂浓度的晶体片粘在一起，那么吸收泵浦功率的密度分布比较平坦。这样端泵浦激光器的温度分布更加均匀。采用合适的激光器设计，这一特点就会提高整体的激光器性能，尤其是输出功率、功率效率和光束质量更高。 
3. 还有的采用芯掺杂的激光棒，其中只有掺杂区域吸收泵浦光。这也适用于边泵浦激光器，这避免了激光模式无法达到有些泵浦区域。因此这可以提高效率，并且能够得到更高的光束质量。掺杂部分也可以看做一个波导，因为掺杂通常会提高折射率。 
4. 若薄片激光器具有无掺杂端帽，可以得到很高的功率。这时因为如果无掺杂端帽具有与薄片相似的折射率，那么它可以抑制放大的自发辐射。同时，无掺杂端帽还能提供额外的机械稳定，避免应力引起的破裂，且易于操作。 
5. 在具有不同掺杂的复合晶体中，有些掺杂部分可以作为增益介质，另一个可以做为被动Q开关的饱和吸收器。这种晶体通常用在被动Q开关微片激光器。

其它情况下，无掺杂端帽可以抑制寄生激光振荡，如果切割成合适的形状可以用作泵浦辐射的管道。在一些单频环形激光器中，光束反射点的未掺杂区域可以消除空间烧孔效应。 

复合增益介质也可以采用陶瓷。陶瓷的制备技术可以为复合结构提供很大的自由度，包括掺杂梯度。也可以同时采用晶体和陶瓷，例如在掺杂单晶上生长无掺杂陶瓷。

Definition: laser crystals consisting of several parts of different materials or with different chemical compositions (e.g. doping concentrations)

Alternative terms: hybrid laser crystals, core-doped rods

More general terms: laser crystals

Composite laser crystals (sometimes called hybrid laser crystals) are laser crystals which have been fabricated by combining different parts. Often, one combines parts with and without a laser-active dopant, or with different dopant concentrations, in order to achieve certain advantages in a laser design.

Typically, adhesive-free diffusion bonding of carefully prepared crystal surfaces is used, e.g., to combine an Nd:YAG or Yb:YAG crystal with an undoped YAG crystal. The same can be done e.g. with Nd:YVO4. Another possibility is to bond a Cr:YAG crystal (a saturable absorber material for passive Q switching) to Nd:YAG.

In other cases, a nonlinear crystal material for nonlinear frequency conversion is bonded to a laser crystal. Composite gain media can also be made of glasses and from ceramics.

Composite gain media can also be made of ceramics. The fabrication techniques for ceramics introduce a lot of freedom for composite structures, including doping gradients. It is also possible to combine single crystals and ceramics, e.g. to grow undoped ceramic around a doped single crystal.

The optical quality of bonded interfaces is essential. Different processes have been developed for obtaining high-quality bonds. Some of these operate at high temperatures, while others can be performed also at room temperature. One may, for example, use irradiation with high-energy ions in a vacuum to remove any disturbing surface layers before bonding. In any case, preparing very flat surfaces is essential.

Examples for Using Composite Laser Crystals

In the following, some examples of the use of composite gain media are given:

• Undoped end caps on a short laser rod can reduce thermal effects by extracting some of the heat through the end faces of the doped part. This reduces the peak temperature, the tendency for thermal fracture, and (for several reasons) the strength of thermal lensing. Such crystals improve e.g. the performance of quasi-three-level neodymium lasers, such as Nd:YAG operated at 946 nm, where the heat generation is substantially stronger than for the usual 1064-nm wavelength. (While the quantum defect is smaller, high pump intensities need to be applied, and the high excitation level leads to increased losses by quenching effects.)

• With an undoped cap on the disk of a thin-disk laser, power scalability can be maintained up to very high power levels. This is because the undoped cap helps to suppress amplified spontaneous emission (ASE) and parasitic lasing, if it has a similar refractive index. At the same time, the undoped cap can provide additional mechanical stabilization, avoiding stress fracture and also making the handling easier.

• When plates with different doping concentrations are bonded together (multi-segmented rods), the density of absorbed pump power (which normally decays rapidly in the pumping direction) can be smoothed. This leads to a more uniform temperature rise in an end-pumped laser, particularly when the crystal is pumped from one side only. With suitable laser designs, this feature can be converted into improved overall performance, in particular for a higher output power, power efficiency, and beam quality.

• Another approach is to use a core-doped rod, where pump light is absorbed only in the region covered by the laser beam. This is suitable also for side pumping of lasers, as it avoids pumping regions of the crystal which cannot be accessed by the lasing modes. It may therefore be possible to obtain enhanced efficiency and possibly a better beam quality. The doped part may even act as a waveguide, as the doping often somewhat increases the refractive index.
• In a composite crystal with different dopants, one part can act as the gain medium and the other one as a saturable absorber for passive Q switching. Such parts are used in, e.g., passively Q-switched microchip lasers.
• In other situations, undoped end caps which are properly shaped (e.g. conically) can act as ducts for pump radiation.
• In some single-frequency ring lasers, an undoped section at a point of beam reflection can eliminate spatial hole burning effects.

Bibliography