Self-modulation oscillations and Q-switched mode-locking of Vortex Laser Formed by Coherent Superposition of Off-axis Traveling Waves*

We generated diode pumped optical vortex laser with self-modulation oscillation and Q-switched mode-locked vortex lasers in the proposed azimuthal symmetry breaking ring resonator. In the laser resonator are placed an a-cut Nd:YVO₄ crystal, a focusing lens of 10-cm-focal-length and a spiral phase plate (SPP) carrying a topological charge of positive one. Because the azimuthal symmetry is broken by the intra-cavity SPP, lights can only travel off-axis along a ring trajectory in either clockwise or counterclockwise manner. With sufficient pump, linearly polarized laser with a doughnut-shaped intensity profile and a positive unit topological charge was emitted. Without any intracavity modulating elements, vortex laser power spectrum contains a few harmonics of 82.5 kHz. The oscillations were identified as self-modulation oscillation. We also study the laser mode-locking in this vortex laser by inserting an acousto-optic modulator into the laser resonator and signature of laser mode-locking was observed within the Q-switched envelope. Notably the self-modulation oscillation and mode-locked vortex pulses were synchronized among the off-axis traveling-wave modes.