Theoretical analysis of Sagnac loop mirror with an inline high birefringence fiber ring resonator: Application in single-frequency fiber lasers

We newly propose and theoretically analyze a Sagnac loop mirror with an inline ring resonator containing a piece of high birefringence fiber (HBF) and erbium-doped fiber (EDF). With the EDF properly pumped to offset the passive loss in the inline ring resonator, narrowband transmission peaks with large effective free spectral range can be realized by the intrinsic vernier effect between orthogonally polarized lights traveling along the two primary axes in the HBF. In addition, this property is independent of polarization states of the input signal, which makes the proposed Sagnac loop mirror suitable for a narrowband filter applied to single-frequency fiber lasers.