Effect of chirp on the index contrast of waveguides written in BK7 glass with ultrashort laser pulses

We report results of a systematic study of laser-writing of waveguides within a block of BK7 glass using chirped laser pulses of sub-100 fs duration. Use of positively-chirped laser pulses enhances the refractive index contrast that is achieved. Measurements using the z-scan technique are utilized to shed light on the mechanism responsible for chirp-dependent index change. Purity of the sample is found to be a crucially important parameter for optimizing index contrast and achieving low propagation loss; extremely low loss values (as low as 0.04 dB/cm) are achieved by us for 633 nm light.