All-Fiber Circular Polarization Filter Realized by Using Helical Long-Period Fiber Gratings

A simple method to produce an all-fiber circular polarization filter is proposed and experimentally demonstrated, which is realized based on the utilization of two consecutively cascaded single-helix helical long-period fiber gratings but with opposite helicities (SHLPGs-OHs). The proposed SHLPGs-OH can be used as a circular polarizer as well as an equivalent half wave-plate, which enables us to turn a left-circular (right-circular) polarization light to a right-circular (left-circular) polarization one. As a concept-proof example, a polarization-converter with an extinction ratio (between the left- and the right-circular polarization light) of ~20 dB, insertion loss of ~10 dB, and a bandwidth of ~4 nm has been successfully demonstrated.