Composite 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.
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 of 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.
|||R. Zhou et a l., “Continuous-wave, 15.2 W diode-end-pumped Nd:YAG laser operating at 946 nm”, Opt. Lett. 31 (12), 1869 (2006), DOI:10.1364/OL.31.001869|
|||R. Wilhelm et al., “Power scaling of end-pumped solid-state rod lasers by longitudinal dopant concentration gradients”, IEEE J. Quantum Electron. 44 (3), 232 (2008), DOI:10.1109/JQE.2007.911702|
|||Y. T. Chang et al., “Comparison of thermal lensing effects between single-end and double-end diffusion-bonded Nd:YVO4 crystals for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions”, Opt. Express 16 (25), 21155 (2008), DOI:10.1364/OE.16.021155|
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