Optical parametric generation at extremely low pump irradiance in a long periodically poled Lithium Niobate

Optical parametric generator (OPG) is a very attractive optical down-conversion configuration since it is a single pass process and no cavity mirror's alignment is required. Thus the system configuration is much more simple and robust. Traditionally, OPG processes were demonstrated using a pump source with a pulse length of the order of picoseconds or less. This is because GW/cm² order of magnitude pump irradiance was required to excite an OPG process, and such irradiance in nanosecond long pulses commonly damages the non-linear crystal. The introduction of periodically poled crystals with high non-linear coefficients has significantly lowered the threshold for parametric processes. This progress in non-linear crystals enables exciting OPG processes at less than 100MW/cm² irradiance, using nanoseconds long pulses from Q-switched lasers. We present an OPG with a threshold of less than 10 MW/cm² using an 80 mm long Periodically Poled Lithium Niobate (PPLN) non-linear crystal. High signal conversion efficiency and high power were obtained at 25 nanosecond pulse length, 10 kHz repetition rate pumping without damaging the crystal. Theoretical approaches for explaining this OPG regime are discussed.