Mode-couplings in two cascaded helical long-period fibre gratings and their application to polarization-insensitive band-rejection filter

Mode-couplings occurring in cascaded single-helix helical long-period fibre gratings (SHLPGs) have been analysed. Using this type of helical long-period fibre grating combined with a cladding-mode stripper, a simple and efficient method of production of a polarization-insensitive flat-top band-rejection filter was proposed and experimentally verified. Unlike the previous SHLPG-based methods in which obtained spectra are strongly dependent on the polarization status of the incident light, the proposed method enables production of a flat-top filter with neither polarization-dependence nor reduction of the rejection depth. As a typical example, a flat-top filter with a rejection depth of ∼17 dB, a bandwidth of ∼14 nm@1 dB, and the maximum polarization-dependent loss (PDL) of ∼1.5 dB was successfully demonstrated.