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 LiNbO₃ (PPLN) because of its high nonlinear coefficient (d₃₃=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 KTiOPO₄ (PPKTP) which has reduced nonlinear coefficient compared with PPLN (d₃₃=17 pm/V), but the thickness up to 2 mm and higher damage threshold (>900 MW/cm² 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°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 × 10^-4 /nm°C) was higher than PPKTP (7.63 × 10^-5 /nm°C). It is expected that PPKTP was temperature insensitive than that of PPLN. Although PPKTP within theoretical grating period of 12.65-μm can be operated at room temperature, the optimum temperature for maximum 593 nm output was tuned to 51°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 μm, the optimum temperature could shift about 6°C in PPKTP, and roughly 3°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°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%.