Low-loss waveguides in borosilicate glass fabricated by high-repetition-rate femtosecond chirp-pulsed oscillator

We describe fabrication and characterisation of smooth low-loss waveguides in BK7 optical glass by means of direct femtosecond inscription with chirp-pulse oscillator, operating at 800 nm and 11 MHz repetition rate. The waveguides were characterized in terms of refractive index, dimensions, mode profiles and insertion losses at 633 nm and 1539 nm, when coupled with a standard optical fibre. This work allowed us to optimise a set of experimental parameters and provide guidelines on their choice. High pulse repetition rate of our laser system allowed us to investigate the regimes with higher sample scanning speed, than commonly used. In this paper we demonstrated the refractive index contrast as high as 0.02 in smooth low-loss structures.