Femtosecond laser inscribed phase-shifted fiber Bragg gratings

A highly-birefringent phase-shifted fiber Bragg grating (PS-FBG) was inscribed with femtosecond laser and a uniform phase mask. The PS-FBG was fabricated from an ordinary FBG in case over exposure was applied. The spectral evolution from FBG to FS-FBG was observed in accompany with a decrease in transmission loss, a `blue´ shift of dip wavelength, a decrease in cladding mode loss, and an increase in insertion loss. High birefringence was demonstrated experimentally with the existence of PS-FBG only in TM polarization. The formation of the PS-FBG may probably due to a negative index change induced by the high-intensity center of Gaussian laser beam.