Frequency stabilization of an injection seeded Nd:YAG ring oscillator by ramp-fire technique with a RbTiOPO₄ modulator

We report on an injection seeded Q-switched Nd:YAG zigzag slab ring oscillator with pulse energy of 67 mJ and pulse duration of 19 ns at repetition rate of 20 Hz. With a RbTiOPO₄ (RTP) electro-optic crystal as the intra-cavity phase modulator, stable single frequency operation is achieved using the ramp-fire technique with a 10 µs linear voltage ramp sweeping 2.5 free spectral ranges. Frequency jitter of output pulse at 1064 nm is measured to be 2.0 MHz over 7 min. The beam quality factor is $M_x^2=1.38\pm 0.03,M_y^2=1.29\pm 0.02$ in seeded operation, which is improved from that of $M_x^2=1.67\pm 0.01,M_y^2=1.35\pm 0.01$ in free oscillation. The measured linewidth of the output is 27 MHz, which is 1.2 times Fourier-transformed-limited. Based on the RTP crystal as the modulator, the ramp-fire technique with a high ramp speed is capable of achieving high frequency and timing stability.