Stress-induced waveguides written by femtosecond laser in phosphate glass

A 10-mm-long cylindrical shell was written in neodymium doped phosphate glass by a femtosecond laser delivering pulses with a pulse repetition of 1 kHz, energies of 90 uJ and duration of 120 fs. The pulses were focused below the glass surface by an objective producing ablation filaments about 200 um in length. During processing, the sample was placed on a three-dimensional (3-D) translation stage, which moved along an enclosed pattern in the horizontal plane followed by a minor descent less than the filament length in the vertical direction. As this procedure continued, a cylinder, which demonstrated optical waveguiding, was fabricated with a rarified periphery and densified center region due to plasmonic expansion and outward shockwave upon laser ablation. The refractive-index contrast, propagation loss, near- and farfield mode distribution, and microscopic fluorescence micrograph of the waveguides were measured. 1-to-N splitters with adjustable splitting ratio were also fabricated using current approach indicating its 3-D processing flexibility. Compared with previous femtosecond laser fabrication methods, waveguides prepared in this approach exploit both depressed cladding and stress-induced refractive index increase in core region and show controllable mode conduction, strong field confinement, large numerical aperture, low propagation loss, acceptable thermal stability and intact core region.