Fiber Bragg gratings made with a phase mask and 800-nm femtosecond radiation

High-quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm. Induced index modulations of $1.9\times {10}^{-3}$ were achieved with peak power intensities of $1.2\times {10}^{13} \mathrm{W}/{\mathrm{cm}}^2$ without any fiber sensitization. The fiber gratings are stable and did not erase after 2 weeks at 300 °C. The primary mechanism of induced index change results from a structural modification to the fiber core.