Polarization-Independent Flat-Top Band-Rejection Filter Based on the Phase-Modulated HLPG

A novel method enabling to produce a flat-top band-rejection filter is firstly proposed and experimentally demonstrated, which is achieved by using a phase-modulated helical long-period fiber grating (HLPG). Unlike most of the long-period fiber grating (LPG)-based and the HLPG-based band-rejection filters reported so far, the proposed band-rejection filter is inherently polarization-independent. Moreover, the proposed HLPG does not need a complex apodization in the grating's profile, which considerably facilitates the fabrication processes and makes the designed HLPG particularly suitable for fabrication with using the CO2 laser-writing platform. As a typical example, a polarization-independent band-rejection filter with a bandwidth of & x007E;10 nm& x0040;-20dB and a rejection depth of & x007E;28 dB has been successfully achieved.