Self-mode-locked Laguerre-Gaussian beam with staged topological charge by thermal-optical field coupling.

A light beam with a helical phase is associated with an optical vortex and carries optical orbital angular momentum. Mode-locked optical vortex pulses impart orbital angular momentum to photons in short pulses and have attractive applications. However, due to the conflict between mature mode-locking and the generation of optical vortices, directly generated mode-locked optical vortex short pulses seem to be unavailable, thus constraining the development and applications of optical vortex short pulses. Laguerre-Gaussian (LG) modes are eigenfunctions for a laser cavity. Besides carrying optical orbital angular momentum, LG beams also have self-healing and quasi-nondiffracting properties. Here, we report the realization of a self-mode-locked LG lasers with tunable orbital angular momentum. By coupling between the thermal and optical fields, the orbital angular momentum was found to be staged. These results verify the possibility of direct mode-locking of optical vortices, and may open the way for several applications of short pulses. Moreover, mode-locked pulses with high-repetition rates also have particularly attractive applications such as optical frequency comb spectroscopy, high capacity optical networks, spectroscopy of metallic nanoparticles, arbitrary waveform generation, etc..