Polarization-Diversity-Loop-Based Optical Fiber Comb Filter with Polarization-Controlled Dual Transmission Channel Spacings

In this paper, we propose an optical comb filter based on a polarization-diversity loop structure, whose two transmission channel spacings (TCS’s) can be alternately switched between two principal axes of the filter by controlling waveplates contained in the filter. The proposed filter consists of a polarization beam splitter (PBS), three half-wave plates (HWPs), one Faraday rotator (FR), and two polarization-maintaining fiber (PMF) segments with different lengths (L and 2L) and equal birefringence (B). As the PMF segments are utilized as birefringence elements, the TCS of the filter is determined by the product of their effective length L_e and birefringence B. For an input signal introduced into each principal axis of the filter, L_e can be the sum of or difference between the two lengths (L and 2L) of the two PMF segments, that is, L or 3L, resulting in a TCS of S ₁ or S ₂, respectively, according to angular combination of the orientation angles of the three HWPs. At a specific set of the three HWP angles, the proposed filter can create two sinusoidal transmittance functions with different TCS's (S ₁ and S ₂) at the two principal axes, respectively. Also, the two TCS's (S ₁ and S ₂) are switchable with each other depending on three HWP angles for each principal axis. With the fabricated filter, the flexible switching between S ₁ and S ₂ could be successfully implemented for two orthogonally polarized input signals aligned along the two principal axes of the filter by properly controlling the three HWPs, without any modification of the filter configuration.