THERMAL EFFECTS IN THE EYE-SAFE RING OPTICAL PARAMETRIC OSCILLATOR BASED ON KTiOPO4 CRYSTALS

For an eye-safe optical parametric oscillator (OPO) built on the basis of a three-mirror ring cavity, each section of which (the space between adjacent plane mirrors) contains a x-cut KTiOPO4 (KТР) crystal having a size of 15(х) × 7(y) × 7(z) mm3, the thermal effects due to idler wave absorption in KTP crystals were investigated. These thermal effects were evaluated by means of experimental measurement of the change in the OPO performance (divergence of the output beam and pulse energy) when transferring the OPO from the mode of generation of occasional single pulses to the mo de of generation of periodically repetitive pulses. It was found when the eye-safe OPO pumped by multimode YAG: Nd laser radiation generates 8-ns pulses with a repetition rate of 10 Hz and an energy of 30–35 mJ, thermal distortions of KTP crystals placed in metal holders at their natural air-cooling, are moderate. The total effect of positive thermolenses induced in nonlinear crystals leads to an increase in the divergence of the beam of the eye-safe OPO by 10 % and to a decrease in the efficiency of the OPO by 0.76 %, by virtue of fact that the induced thermal lenses are not ideal and thereby introduce additional aberration losses into the OPO cavity. The theoretical simulation of the OPO operation in the plane-wave approximation with the use of a system of three coupled first-order abridged differential equations showed that among three KTP crystals the KTP crystal placed first in the path of pump radiation in the OPO is the largest thermal load and the action of the most intense beams.