Polarization-independent dual-peak filter based on a cylindrically symmetric helical small-period fiber grating

A polarization-independent dual-peak narrow-band filter is proposed and demonstrated theoretically and experimentally, which is realized by using a helical long-period fiber grating (HLPG) but with a period small down to tens of micrometers. Unlike those excessively tilted fiber gratings (Ex-TFGs) or the conventional long-period fiber gratings (LPGs) but with a small period down to tens of micrometers where the generated dual-peak pairs (DPPs) are all of the strong polarization-dependence, the DPPs obtained in this study are of the polarization-independent, which is the first time, to the best of our knowledge, that the underlying mechanism for generation of the polarization-independent DPPs in transmission spectrum of the helical small-period fiber grating (HSPFG) has been revealed both theoretically and experimentally.