Tapered depressed-cladding waveguide lasers modulated by Ag nanoparticles embedded in SiO2

We report on fabrication of tapered circular depressed-cladding waveguides in Nd:YAG crystal by femtosecond laser direct writing (FsLDW). Based on the fabricated waveguides, for the first time to the best of our knowledge, efficient Q-switched mode-locked (QML) lasers are realized by employing silver nanoparticles (NPs) embedded in fused silica as saturable absorber (SA) element. Laser pulses as short as 25 ps with a repetition rate of 7.7 GHz are generated at a wavelength of 1064 nm. We systematically explore the influence of incident-output facet and the reduction factors on guiding properties and laser performance by conducting a comparative study of different tapers, and it has been found that the waveguides with tapered structures can suppress multi-mode guiding while ensure fundamental-mode output for waveguide lasers. Our work paves a new way toward developing three-dimensional (3D) waveguide for applications in on-chip ultrafast laser sources.