LASER MATERIALS: Composite rods improve thermal management

Aug. 1, 1996
Buildup of excess heat in the laser rods of high-energy solid-state lasers can often be detrimental to laser operation by causing such effects as thermal lensing.

Buildup of excess heat in the laser rods of high-energy solid-state lasers can often be detrimental to laser operation by causing such effects as thermal lensing. Thermal management is, therefore, a critical design issue for laser designers. One approach to addressing the problem involves using composite materialslaser rods consisting of undoped segments of crystal bonded to doped segments. The design separates the rod endfaces from the heat-producing active region, and the undoped segments act as heat sinks.

Rod segments are connected by diffusion bonding in which the segment endfaces are precision polished, optically contacted, then heat-treated at temperatures below the crystal melting point to enhance bond strength. No inorganic or organic bonding aid or adhesive is used. When the process is complete, users can grind, polish, and optically coat the finished composite without damage to the bond.

Tests have shown that the bonded segments exhibit flexural strength comparable to nonbonded control samples, and the bonded interfaces do not measurably degrade laser performance through scattering, reflection, or beam-quality degradation.

Composite rods are made by Onyx Optics Inc. (Dublin, CA), which produces them from a variety of hosts, including yttrium aluminum garnet (YAG), yttrium lithium fluoride (YLF), yttrium vanadate (YVO4), lithium aluminum strontium fluoride (LiSAF), and sapphire. Designs include rod, slab, and waveguide configurations.

High-power ytterbium-doped YAG

The composite approach significantly enhanced performance of a continuous-wave (CW) ytterbium-doped YAG (Yb:YAG) laser. The device produced an average power of 131 W at 1030 nm. In this design undoped YAG endcaps were bonded to a 50-mm-long YAG rod doped with 0.5% ytterbium. A multilayer dichroic coating on the input face provided high transmission at the 941-nm pump laser wavelength. The coating was highly reflective at 1030 nm, causing the input end of the rod to act as a cavity reflector; a conjugate coating was applied to the output end of the rod. Lawrence Livermore National Laboratory (LLNL, Livermore, CA) researcher Camille Bibeau discussed the system at CLEO '96 (Anaheim, CA, paper #CPD23-2).

The composite approach prevented damage to the rod endfaces by reducing thermal loading and stress. "Endcaps are absolutely essential for thermal management," says Bibeau's collaborator Raymond Beach. "They allow you to remove the optical coating from the part of rod that produces heat and also result in a uniform thermal profile." Without endcaps, Beach continues, the rod center gets hot and endfaces tend to bulge out, an effect that can actually fracture the rod at the endface. "With composites, the caps hold the endface in compression, which eliminates the problem."

Thulium laser design

A second LLNL group has applied the composite approach and coaxed 115 W of CW power from a diode-pumped thulium-doped YAG laser. By contrast, a noncomposite rod produced only 6 W of output. The group minimized the heat load at the pump input end of the rod by pumping at 805 nm, off the main absorption peak at 785 nm. A 5-mm undoped endcap applied to each end of the 2%-doped rod isolated the endface from heat generated by the gain medium; the completed rod was 5.5 cm long and 3 mm in diameter. Eric Honea of LLNL reported the work at CLEO '96 (paper #CWN1).

Onyx and LLNL teams are also studying composite hosts made of dissimilar materials, such as erbium-doped YAG bonded to sapphire. "It allows us another degree of freedom in the way we design resonators," explains Beach, citing controlled mode suppression as another benefit. According to Onyx president Helmuth Meissner, other investigations include using composites of nonlinear crystals for quasi-phase-matching stacks or to simplify the coating process. "The challenge is to be able to polish the material properly," says Meissner, "especially crystals such as BBO [beta barium borate]. If we can apply a coating to a material of matching refractive index that can be coated readily, then bond it to a crystal, that would be a useful composite."

About the Author

Kristin Lewotsky | Associate Editor (1994-1997)

Kristin Lewotsky was an associate editor for Laser Focus World from December 1994 through November 1997.

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