Femtosecond laser fabricated semi-tapered waveguide for Q-switched vortex laser generation.

Dielectric waveguides are widely recognized as excellent and versatile components for integrated multifunctional photonic chips, thanks to their strong optical confinement capabilities. In this study, we present a novel semi-tapered depressed-cladding waveguide structure, designed and fabricated using femtosecond laser direct writing technology. The optical guiding performance of this semi-tapered waveguide is systematically analyzed by characterizing its loss characteristics. Leveraging the high gain and superior laser performance of the semi-tapered waveguide platform, we successfully demonstrate an efficient vortex waveguide laser. Moreover, we achieve a stable Q-switched vortex laser with short pulse duration and high repetition rate by utilizing an SbTe thin film as a saturable absorber. This work exemplifies the potential for developing three-dimensional waveguides for on-chip integrated pulsed lasers.