Analysis of laser-diode end-pumped intracavity frequency-doubled, passively Q-switched and mode-locked Nd:YVO_4 laser

摘要

Simultaneous Q-switching and mode locking in a laser-diode end-pumped intracavity frequency-doubled Nd:YVO_4/KTP green laser using a Cr~(4+):YAG saturable absorber with 81% initial transmission is experimentally demonstrated. At an incident pump power of 6 W, 300 mW of average green power was obtained, which is around four times higher than the cw green power obtained without the Cr~(4+):YAG crystal. The repetition rate of the mode-locked green pulses was 400 MHz and the individual mode-locked pulse width was measured to be 500 ps. The repetition rate of the Q-switched envelope of the mode-locked pulses was 15 kHz at 6 W of incident pump power. The energy of the mode-locked pulse at the peak of the Q-switched envelope was estimated to be 1 μJ and the peak power was estimated to be 2.4 kW. The measured width and the total energy of the Q-switched envelope of the mode-locked pulses was 47 ns and 21 μJ, respectively, at the maximum incident pump power. An analysis of the system by incorporating a nonlinear loss term due to the intracavity second-harmonic generation to the general recurrence relation for the mode-locked pulses under the Q-switched envelope at the fundamental wavelength has been presented. Using a hyperbolic secant square function to model the mode-locked pulse, the temporal shape of a single Q-switched pulse at the second-harmonic wavelength has been reconstructed. The theoretical calculations of the pulse parameters like pulse energy, peak power, pulse width and pulse-symmetry factor have shown fairly good agreement with the experimental results.