Accurate measurements of longitudinal temperature distribution of PPLN crystal in the process of second-harmonic generation.

Phase-matching temperature tuning curves and longitudinal temperature distribution of periodically poled lithium niobate (PPLN) crystal were measured in the process of second-harmonic generation (SHG) of 1070 nm pump wavelength using tiny transparent piezoelectric crystals as the temperature sensors. The temperatures of the crystal sensors placed along the PPLN length were determined directly by measuring the induced frequency shifts of their piezoelectric resonances, which were excited in a noncontact manner by a probe radio-frequency electric field. Such temperature sensors do not suffer additional heating conditioned by absorption of scattered radiation and also do not have any contact wires. Due to the high factors of piezoelectric resonances, the measurement accuracy of 0.05°C was achieved for a local determination of PPLN surface temperature during SHG. The self-consistent theoretical model that describes the SHG process in the presence of induced nonuniform temperature distribution of the nonlinear-optical crystal is introduced.