Coilable Crystalline Fiber (CCF) Lasers and their Scalability

Report No. ARL-TR-6850
Authors: Mark Dubinskiy, Larry D. Merkle, and Helmuth Meissner
Date/Pages: March 2014; 58 pages
Abstract: Conventional fiber lasers offer excellent beam quality and efficiency for high power lasers, but their composition (glass) poses significant disadvantages in pump absorption, gain, and thermal conductivity. All-crystalline fiber lasers, particularly if double-clad, would improve all these properties, greatly facilitating power scaling. Yet, progress in fabricating all-crystalline fibers has been limited thus far. The U.S. Army Research Laboratory (ARL) in partnership with Onyx Optics studied fully crystalline waveguides fabricated using current bonding techniques, with geometry similar to conventional double-clad fibers. We measured refractive indices to determine the parameters for erbium- or thulium-doped YAG (Y3Al5O12)/YAG/spinel (MgAl2O4) double-clad waveguides with a single-mode core. We also investigated YAG doped with ytterbium as an inner cladding material, as its index can be adjusted to facilitate design of a single-mode core with large area. We characterized the thermal conductivity of nominally polycrystalline samples of each material. We performed laser experiments on both single- and double-clad Er:YAG cores of different sizes, and have obtained world's-first and world-record laser performance. This includes 25-W continuous wave (CW) output due to cladding-pumping of what we believe is the first truly double-clad all-crystalline YAG waveguide laser, and a record room-temperature slope efficiency (with respect to absorbed pump) of nearly 93 % with excellent beam quality from a core-pumped waveguide.
Distribution: Approved for public release
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Last Update / Reviewed: March 1, 2014