Definition: laser crystals consisting of several parts of different materials

Composite laser crystals (sometimes called hybrid laser crystals) are laser crystals which have been fabricated by combining at least two different parts. Typically, diffusion bonding of well prepared crystal surfaces is used e.g. to combine a Nd:YAG or Yb:YAG crystal with an undoped YAG crystal. The same can be done e.g. with Nd:YVO₄, or Cr:YAG (a saturable absorber material for passive Q switching) can be bonded 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.

In the following, some examples for 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 significantly stronger than for the usual 1064-nm wavelength.

• By bonding plates with different doping concentrations (→ multi-segmented rods), one can smoothen the density of absorbed pump power (which normally decays rapidly in the pumping direction) and thus the temperature rise in an end-pumped laser. 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. One may thus obtain an 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.

• By applying an undoped cap to a thin disk laser, one can preserve power scalability to very high power levels, because the undoped cap helps to suppress amplified spontaneous emission (ASE), if it has a simple refractive index. At the same time, the undoped cap can provide additional mechanical stabilization, avoiding stress fracture and also making the handling easier.

• 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 e.g. in passively Q-switched microchip lasers.

In other situations, undoped end caps can help to suppress parasitic laser oscillation, and when they are properly shaped (e.g. conically) then 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.

Note that composite gain media can also be made of ceramics. The fabrication techniques for ceramic actually 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.