Femtosecond laser writing low-loss waveguides in silica glass: highly symmetrical mode field and mechanism of refractive index change

Abstract

Here, single-mode waveguides are written in silica glass by femtosecond (fs) laser. The waveguides exhibit a low propagation loss of 0.15 dB/cm and a symmetrical mode field. The loss performance and the mode field symmetry are demonstrated to strongly depend on the pulse repetition and polarization of fs laser, respectively. Raman and photoluminescence properties indicate that an increase of refractive index originates from an increment in the total density of glass networks consisting of small and large rings, but only increasing the number of small rings does not necessarily lead to an increase in refractive index. The long working distance of the objective lens promises to fabricate low-loss waveguides in a large depth of glass, which would inspire manufacturing integrated photonic devices by fs laser writing. Clarifying the mechanism of refractive index change offers a new perspective to understand the fs laser-matter interaction.