This article introduces the bonding crystal and gives some examples of using compound gain media.

Composite laser crystals are laser crystals obtained from different materials. Binderless diffusion bonding is usually used, and the surface is treated, for example, to bond neodymium-doped yttrium-aluminum garnet and ytterbium-doped yttrium-aluminum garnet crystals to undoped yttrium-aluminum garnet crystals.

The same can be used for Nd:YVO4. Another crystal that can be compounded is the Cr:YAG crystal (saturable absorber material for passive Q-switching), compounded with Nd:YAG. In other cases, a nonlinear crystal material for nonlinear frequency conversion needs to be bonded to the laser crystal. Composite gain media can also be made of glass or ceramic.

The optical quality of the bonding surface is very important. Several processes are required to obtain a high quality bond surface. Some processes need to work at high temperatures, others can work at room temperature. One process, for example, requires the use of a high-energy ion beam in a vacuum to remove surface disturbances prior to bonding.

The use of undoped end caps on short laser rods can reduce thermal effects and can absorb some of the heat from the doped part. This reduces the peak temperature, the probability of thermal cracking, and the strength of the thermal lens. This crystal can improve the performance of quasi-three-level neodymium-doped lasers, such as Nd:YAG lasers operating at 946nm, which generate much more heat than ordinary 1064nm wavelengths.

If sheets of different doping concentrations are glued together, the density distribution of the absorbed pump power is relatively flat. In this way, the temperature distribution of the end-pumped laser is more uniform. With proper laser design, this feature can improve overall laser performance, especially higher output power, power efficiency, and beam quality.

Others use core-doped laser rods, in which only the doped regions absorb the pump light. This also applies to edge-pumped lasers, which avoids some pumping regions that cannot be reached by the lasing mode. So this improves efficiency and enables higher beam quality. The doped part can also be seen as a waveguide, because doping usually increases the refractive index.

Very high powers can be obtained if the thin-film laser has undoped end caps. This is because if the undoped endcap has a similar refractive index to the flake, it can suppress amplified spontaneous emission. At the same time, the undoped end caps also provide additional mechanical stability, avoid stress-induced cracking, and are easy to handle.

In composite crystals with different dopings, some doped parts can act as gain media and the other can act as passive Q-switched saturable absorbers. Such crystals are commonly used in passive Q-switched microchip lasers.

In other cases, undoped end caps can suppress parasitic laser oscillations and, if cut to a suitable shape, can be used as conduits for pump radiation. In some single-frequency ring lasers, the undoped area of the beam reflection point can eliminate the spatial hole burning effect.

Composite gain media can also be ceramic. Preparation techniques for ceramics can provide a great deal of freedom for composite structures, including doping gradients. It is also possible to use both crystals and ceramics, such as growing undoped ceramics on doped single crystals.