Birefringent π-Phase-Shifted Fiber Bragg Gratings for Sensing at 1000 °C Fabricated Using an Infrared Femtosecond Laser and a Phase Mask

Birefringent π-phase-shifted Bragg gratings for multi-parameter sensing at temperatures ~1000 °C are written inside a standard single mode silica optical fiber (SMF-28) with infrared femtosecond pulses and a special phase mask one half of which is shifted with respect to the other by 5/4 of the mask period. The birefringence is caused by the presence of light-induced sub-wavelength periodic planar nanostructures in the fiber core, whose orientation is controlled by the laser polarization, and is maximized when the laser pulse polarization is aligned perpendicular to the fiber core. The birefringence can reach ~4.2 × 10
^-4
at room temperature at the 1.5 × 10
^-4
level after 100 h annealing at 1000 °C. Erasure and rewriting of the planar nanostructures inside fiber Bragg gratings by changing the laser pulse polarization is demonstrated.