A compact single-mode fiber ring depolarizer

In this paper, a degree of polarization (DOP) tunable single-mode fiber ring depolarizer is proposed and demonstrated. The fiber ring is made of a 2×2 coupler and a piece of single-mode fiber delay line, which is wound into a coil performing as a half-wave plate. For a random input state of polarization, the full depolarization can be realized by rotating the whole set of fiber ring through a proper angle, and for a linearly-polarized input light, the output DOP can change from one to zero. In our experiment for a fiber ring with a delay line of 10 meters, a Fabry-Perot (FP) laser with 8-meter coherent length is depolarized with a DOP less than 1%.