Average power effects in periodically poled crystals

The introduction of periodically poled crystals with high non-linear coefficients has lowered significantly the threshold for parametric processes. This progress enables pumping frequency conversion devices with low pulse energy, Q-switched, diode-pumped, solid-state lasers. New non-linear optical ferroelectric materials, such as KTP and Stoichiometric Lithium Tantalate (SLT) were proven to exhibit adequate deff, higher optical damage resistance and lower photo-refractivity in comparison to well-known periodically poled Lithium Niobate. Advances in poling technology have enabled the production of relatively thick periodically poled crystals from those materials. Thus, in principal much higher average power levels can be converted. We have investigated the effects that limit frequency conversion efficiency as power levels are increased. Average power induced thermal lensing and thermal phase mismatching were considered. The resulting power limitations are discussed, and under some assumptions quantitative expressions for these limits were formulated. Thermal lensing imposes a limit on the local power density. Thermal phase mismatching imposes a limit on the overall power.