Experimental determination and the theoretical model of an equivalent temperature of nonlinear optical crystals interacting with high-power laser radiation

We present the results of the use of resonant ultrasound spectroscopy to study changes in temperature of crystals during their interaction with high-power laser radiation. To measure the nonuniform temperature of a crystal, use is made of the frequency (calibrated at uniform temperature θ) of the nth mode of the piezoelectric resonance Rf_n(θ) and its change ΔRf_n(P_in) upon crystal irradiation by laser light with an average power Pin. A spatially inhomogeneous distribution of thermodynamic temperature in the crystal at a fixed laser power Pin is set in correspondence with equivalent temperature θ_eq(P_in), defined in this paper. A mathematical model is proposed, justifying the correctness of its introduction. This parameter can be used to study any piezoelectric crystals interacting with laser radiation.