Strengthening of very large crystalline and polycrystalline Nd:YAG rods for high-power laser applications

A multistep thermochemical etching procedure was applied to very large Nd³⁺:YAG rods to increase their fracture strength. The strengthening procedure combined selection of high-quality material, fine centerless grinding, thermochemical etching, and (after completion of the lapping, polishing and AR coating) an additional hot thermochemical etching, with rod ends protected with poly-tetra-fluoro-ethylene (Teflon) caps. The final cleaning step, not previously reported, is essential in removing fracture causing contaminations on the rod surface. A unique thermal load-to-fracture technique was applied on test rods to measure their fracture strength. The rods were thermally loaded up to fracture by means of optical pumping in a specially designed laser pump chamber. The results thus obtained were analyzed by Weibull distribution statistics appropriate to these tests. The strengthened laser rods of this study sustained a maximum pump power density of \( I_{{\ell_{\hbox{max} } }} \) = 500 W cm⁻¹. This value is higher by a factor of four over untreated rods and also higher than any previously published data for such large rods. High-power diode-pumped laser heads were operated with the strengthened crystalline and polycrystalline Nd:YAG rods, yielded output power of ~ 3 kW, when pumped with 7 kW. Such performance was routinely achieved without any instance of rod fracture. Reliability of the strengthening procedure was further demonstrated by the failure-free operation of an azimuthally polarized high-power master-oscillator power-amplifier system (composed of oscillator, preamplifier, and six power amplifiers), emitting an output power in excess of 10 kW.