Localized High Birefringence Induced in SMF-28 Fiber by Femtosecond IR Laser Exposure of the Cladding

A process for creating a local high fiber birefringence and large variation in refractive-index modulation with a polarization in fiber Bragg gratings using post writing exposures with femtosecond pulses is presented. A birefringence as high as ~8 times 10^-4 was obtained in the core of SMF-28 fiber by exposing the cladding of the fiber in close proximity to the core/cladding interface with a modulated multibeam interference field created by a phase mask. The birefringence is likely due to the induced stress rather than the direct asymmetric refractive index modification.