Diode-pumped Q-switched yellow laser with single-pass sum-frequency mixing: comparison between PPLN and PPKTP

The use of periodically poled materials as nonlinear crystal for quasi-phase-matching (QPM) recently has attracted more attention in solid-state laser for second-harmonic-generation (SHG) and optical-parametric-oscillator (OPO). The conversion efficiency of quasi-phase-matching can be highest while the periodic reversal of the sign of nonlinear coefficient of material occurs in the phase matching period. Most of research have focused on LiNbO/sub 3/ (PPLN) because of its high nonlinear coefficient (d/sub 33/=27 pm/V). However, the disadvantages of PPLN in thickness of 0.5 mm and lower damage threshold are needed to be concerned. An emerging material KTiOPO/sub 4/ (PPKTP) which has reduced nonlinear coefficient compared with PPLN (d/sub 33/=17 pm/V), but the thickness up to 2 mm and higher damage threshold (>900 MW/cm/sup 2/ for 5 ns pulse) can be applied. We report a comparative study of the periodically poled material, PPLN and PPKTP, as nonlinear crystal in external cavity sum frequency mixing for 593 nm yellow-orange laser generation in dual wavelength all solid-state laser system. The measured temperature bandwidth of PPKTP was about 6/spl deg/C that was about twice of it in PPLN. From phase mis-matching analysis, it can be found that the slope of phase mis-matching with temperature of PPLN (1.136 /spl times/ 10/sup -4/ /nm/spl deg/C) was higher than PPKTP (7.63 /spl times/ 10/sup -5/ /nm/spl deg/C). It is expected that PPKTP was temperature insensitive than that of PPLN. Although PPKTP within theoretical grating period of 12.65-/spl mu/m can be operated at room temperature, the optimum temperature for maximum 593 nm output was tuned to 51/spl deg/C. The possible reason could be caused by the grating poling tolerance. Because the optimum phase matching temperature was sensitive to the grating period value if this value shifts 0.01 /spl mu/m, the optimum temperature could shift about 6/spl deg/C in PPKTP, and roughly 3/spl deg/C in PPLN. The highest output power of single pass sum frequency generation (593 nm) was 70.2-mW at 30 kHz by PPKTP setting temperature at 51/spl deg/C and the single pass conversion efficiency was about 5.57%; the maximum output power of 593-nm was 64.9-mW at 20 kHz with PPLN as nonlinear crystal and the efficiency was about 6.03%.