Type-II Bragg gratings with a 300 nm period fabricated using tightly focused femtosecond pulses and the phase mask technique

1000°C-resistant Bragg gratings (i.e., Type-II Bragg gratings) with a 300 nm period are fabricated in non-photosensitized silica-based optical fibers using tightly focused ultraviolet/visible (400 nm) femtosecond pulses and a phase mask. Five millimeter-long Bragg gratings with a -10 dB transmission dip and -0.1 dB off-resonance insertion loss are demonstrated. Inscription of such gratings is also possible through protective acrylate coatings on the fiber. The Bragg wavelength drift and changes in the transmission of the fiber Bragg gratings are monitored in the course of isothermal annealing performed at 1000°C for 120 hours.