Hydrothermally cladded crystalline fibers for laser applications [Invited]

Various rare earth doped single crystal YAG and sesquioxide fibers have been drawn using the laser heated pedestal growth (LHPG) method. Crystalline core/clad fibers have been successfully fabricated using a two-step hydrothermal growth method applied to core fibers. The development of a two-step process is essential to the growth of reasonable cladding layers. Various shapes of the cladding layer were observed, i.e., octagon, hexagon, square and intermediate shapes. EDX study shows a stoichiometry of YAG composition of the clad and there is no significant diffusion of the rare earth ion across the core/clad interface. No birefringence was observed under cross-polarized light indicating that a negligible stress between core/clad crystals was formed during hydrothermal process. Acid etching is an effective method to obtain faceted YAG fibers with a reduced diameter in a controlled manner. An etch rate of ∼0.15 µm/min was measured using 50:50 mixtures of phosphoric acid and sulfuric acid at elevated temperature. Scattering loss of 0.05 dB/cm and a net peak gain of 19 dB was measured from 10% Yb:YAG core/YAG clad fiber. The hydrothermal technique is a versatile epitaxial method to growth undoped YAG cladding of various thicknesses onto doped YAG core fibers and has a great potential to pave the way forward for improving laser performance.