In‐Fiber Switchable Polarization Filter Based on Liquid Crystal Filled Hollow‐Filament Bragg Gratings

Abstract Femtosecond laser filaments are formed by surface aberration to open nano‐hole arrays, passing through the silica cladding and guiding core cross‐section of a standard telecommunication fiber. The hollow filament grating presents a strong capillarity effect to draw nematic liquid crystal (NLC) into homogeneous alignment with the cylindrical walls and presents a high birefringent response in the second‐order Bragg stopband. The NLC provides a strong extinction ratio of up to 20 dB over a ≈5 nm band with only a moderate insertion loss of <1 dB arising between the two polarization states of the shifted stopbands. The Bragg grating follows the thermo‐optic response of the NLC to further offer dynamic tuning and switching of the polarization extinction response without an increase in the insertion loss. The flexible laser writing enables tailoring of the Bragg spectral and polarization responses in the telecommunication C‐band.