Growth, Cladding, and Characterization of Single Crystal Optical Fiber: The Next Evolution in Directed Energy Systems

High-power fiber lasers are steadily increasing in maximum power and rapidly approaching theoretical maximum power output from a single fiber aperture. The low thermal conductivity of silica fibers is an inherent limitation to the dissipation of heat from the fiber core, and the buildup of heat within the laser gain media is the principal driver of transverse mode instability within fiber systems. This limitation necessitates the exploration of alternate fiber host materials. Due to their high thermal conductivity, excellent transmission range, ability to host most of the optically active rare earth ions, and success as bulk laser materials, single-crystalline YAG and Lu­2O3 are excellent candidates as high-power fiber lasers. Lu2O3 is especially promising due to its ability to maintain high thermal conductivity upon Yb-doping. Here, we grow Yb:YAG single-crystal fibers to demonstrate the effective commissioning of the LHPG system at Clemson University, and the system is used to grow the first reported sub-200 mm, Yb:Lu2O3 single-crystal fibers via the LHPG technique. Moreover, the Yb:Lu2O3 fiber cores are clad with single crystalline Lu2O3 for the first time via the hydrothermal synthesis technique. The crystallinity of the clad fiber system is confirmed through single-crystal X-ray diffraction, SEM imaging, and Raman spectroscopy, and the core-cladding interface properties are specifically addressed. Spectroscopic properties of the unclad Yb:Lu2O3 fibers are measured for various dopant concentrations, and amplification is demonstrated within the unclad system. Parallel thermal conductivity measurements along the fiber axis of a YAG-clad, Yb:YAG fiber core are modeled to probe the phonon properties across the core-cladding interface. The results reported in this study suggest that single crystal fibers show great potential for high-power laser systems, although further development is needed before high power levels can be achieved.