Frequency doubling of acousto-optic Q-switched Nd:YVO<sub>4</sub> cascaded Raman laser for narrow pulse-width 657 nm laser

Abstract

Frequency doubling of second-Stokes in an acousto-optic Q-switched Nd:YVO<sub>4</sub> cascaded self-Raman cavity is demonstrated to achieve a narrow pulse-width red laser. A three-stage bonded YVO<sub>4</sub>/Nd:YVO<sub>4</sub>/YVO<sub>4</sub> crystal is designed by comprehensively considering the improvement of thermal effect, the performance of fundamental frequency laser and Raman conversion, to improve the Raman efficiency and output power. An LBO crystal cut for critical phase matching at room temperature is selected and used as a nonlinear optical crystal for realizing the frequency doubling of second- Stokes wave. Its phase matching angle (<i>θ</i> = 86.0°, <i>φ</i> = 0°) is very close to the non-critical phase matching angle and has a small walk-off angle, which is beneficial to the realizing of the high conversion efficiency of frequency doubling. In the experiment, the beam waist position of the pump light and the repetition frequency of the acousto-optic Q-switcher are optimized. Under an incident pump power of 14.2 W and a repetition frequency of 60 kHz, the highest average output power of 1.63 W and conversion efficiency of 11.5% are obtained for the 657 nm red laser emission. The pulse width of 657 nm red light is 11.5 ns at the maximum output power, which is much narrower than that generated by frequency doubling of ordinary neodymium-doped laser at a waveband of 1.3 μm. The result shows that the frequency doubling of the acousto-optic Q-switched Nd:YVO<sub>4</sub> cascaded self-Ramanlaser can take advantage of the pulse-width compression characteristics of Raman process to achieve a narrower pulse-width red light laser output.