Inscription of long-period gratings in pure silica and Germano-silicate fiber cores by femtosecond laser irradiation

Long-period gratings (LPGs) have been inscribed by femtosecond laser radiation into fiber optics with Ge-doped and pure silica cores. Grating parameters of 16-dB rejection and 0.3-dB insertion loss are obtained for a Ge-doped fiber core. Numerical calculation of the transmission characteristics yielded a good correlation between the measured and synthetic data showing that an induced refractive index change up to 3×10^-4 can be achieved. Inscription of an LPG in pure silica fiber core is demonstrated and thought to be first achieved in this fiber type. Greater laser intensity is required in pure silica compared with Ge-doped cores to produce a refractive index change of similar magnitude.