Bi-directional non-reciprocal optical filter and optical devices incorporating it

摘要

A bi-directional, non-reciprocal optical filter 10, comprising a transversal filter 12, two polarising beam splitters 14, 16, a Faraday rotator 18, and a photodetector 20. The transversal filter 12 comprises two two polarisation, high-birefringence (Hi-Bi) optical fibres 22, 24, and three polarisation state controllers 26, 28, 30 for rotating the polarisation state of a received optical signal by an angle ofθ1,θ2 andθ3 respectively. An optical signal input into port 1 is polarised in one polarisation state on leaving the first polarising beam splitter 14. Transmission through the Faraday rotator 18 and the first Hi-Bi fibre 22 causes the optical signal to degenerate into two orthogonally polarised modes (x, y). These modes (x, y) are coupled into the second Hi-Bi fibre 24, where each mode (x, y) degenerates into two orthogonally polarised modes, resulting in four degenerated polarised modes (xs, xp, ys, yp). These four modes (xs, xp, ys, yp) are coupled into each axis of the polarising beam splitter 16, forming four signal taps in each axis. The taps in the slow axis are coherently combined by the photodetector 20 to form the output of the filter 10 at port 2.