Energy-transfer upconversion and thermal lensing in high-powerend-pumped Nd:YLF laser crystals

摘要

Thermal lensing in an end-pumped Nd:LiYF4 rod, undernlasing and nonlasing conditions, has been investigated. Under lasingnconditions, a weak thermal lens, with dioptric power varying linearlynwith pump power, was observed. Under nonlasing conditions, where higherninversion densities were involved, hence relevant to Q-switchednoperation or operation as an amplifier, a much stronger thermal lens wasnmeasured, whose power increased nonlinearly with pump power. Thisndifference has been attributed to the increased heat deposition due tonthe subsequent multiphonon decay following various interionicnupconversion processes, which increase strongly under nonlasingnconditions, and is further exacerbated by the unfavorable temperaturendependencies of heat conductivity and the rate of change of thenrefractive index with temperature. A strategy for reducing upconversionnand its associated thermal loading, without degrading laser performance,nis discussed