All-Fiber Bandpass Filter Based on Helicoidal Long-Period Grating Pair and Null Core Hollow Optical Fiber With Flexible Transmission Control

We report an all-fiber bandpass filter (BPF) based on a null core hollow optical fiber serially concatenated between a pair of helicoidal long-period fiber gratings (HLPFGs) with flex-ible transmission control capability. With novel spectral function-ality of HLPFGs that can be varied by the torsional stress to the helix, and core mode blocking in a hollow core fiber, the proposed device showed unique BPF characteristics whose transmission can be flexible and altered along with a low polarization-dependent loss and low thermal variation, which have not observed in prior pho-torefractive grating devices.