Title: Temperature noncritical processes in nonlinear and electrooptical crystals of mm2 symmetry group

Abstract

Temperature insensitivity of optical processes used in high average power laser systems is rather important problem that needs to be solved for a number of applications. Use of external thermal stabilization systems is not always effective and acceptable due to dimensions, weight or transit-time limitations. Besides, it is practically difficult or impossible to compensate powerful beam propagation channel local overheating caused by linear or multiphoton absorption in the crystal volume. In general temperature-noncritical (TNC) interactions in crystals are realized in the direction where the first-order temperature derivative of the refractive index is equal to zero. When this condition is satisfied simultaneously with the phase-matching condition for chosen frequency converter the corresponding TNC harmonic generation process might be realized. This work is focused on theoretical analysis and experimental study of the temperature noncritical frequency conversion in the nonlinear crystals of mm2 group and temperature not sensitive electro-optic modulation in E-O crystals of the same group. The possibilities of broadband TNC operation for the most promising representatives of mm2 crystal group have been demonstrated both numerically and experimentally. In our experiments with frequency converters the positive wing of temperature range for second harmonic generation (1064 to 532 nm) in KTP (KTiOPO4) crystal increased from 25 °C/cm for the conventional cut to 210°C/cm for TNC cut. The corresponding temperature bandwidth for the third harmonic conversion in LBO(LaBaO3) crystal (1064 and 532 nm to 355 nm) changed from 3°C for the conventional cut to the 130 °C for TNC cut. The reached conversion efficiencies under saturation conditions (two-pass schemes) were measured to be close to the case of the conventional cut utilization. Widely used E-O Q-switches based on the KTP, RTP and RKTP crystals usually have dual-crystal design to compensate the dependence of a crystal birefringence on ambient temperature. Introducing of temperature gradients in the crystals affects dramatically on the Pockels Cells contrast. In some important applications the TNC cut for the crystals allows use of a single-crystal Pockels Cell design with stable performance in a wide range of the E-O crystals internal and ambient temperatures instead of using duel-crystal design.