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 flexible transmission control capability. With novel spectral functionality 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 photorefractive grating devices.