Lyot-type flat-top fibre multiwavelength filter based on polarisation-diversity loop structure

Proposed is a Lyot-type flat-top fibre multiwavelength filter based on a polarisation-diversity loop structure, which has flat-top and lossy flat-top passbands and multiwavelength switching capability. The proposed filter is composed of a polarisation beam splitter, two half-wave plates (HWPs) and two polarisation-maintaining fibre (PMF) loops concatenated with a 60° offset between their principal axes. One of the two PMF loops is two times longer than the other. It is able to operate in a flat-top or a lossy flat-top band mode at the selected combination of the azimuthal orientation angles of two HWPs. In particular, through the adjustment of the orientation angles of the two HWPs, its multiwavelength channels can be interleaved in each mode, which cannot be realised in a previously reported Lyot-Sagnac comb filter based on a fibre coupler. In addition to the channel interleaving capability, it provides narrower passbands compared with previous fibre multiwavelength filters with the same order. To make a quantitative comparison between the passband bandwidths of the proposed filter and other filters, both -1 and -3 dB passband bandwidths of a conventional zeroth-order, a Solc-type first-order and the proposed multiwavelength filters were considered and compared with each other.