We have performed high repetition rate femtosecond laser bulk modification of TiO2-SiO2 based glass (Schott N-SF8 glass), leading to a decrease in the refractive index near the focal volume. From Raman and X-ray microanalysis we have associated the decrease in the refractive index to a volume expansion due to glass network modifications induced by the laser irradiation. By writing two lines close together we have been able to confine the optical mode and obtain propagation losses of 0.7 dB/cm in the near infrared.