An in-line quasi-Sagnac interferometer based comb filter used for tunable multi-wavelength fiber laser

•A quasi-Sagnac interferometer comb filter used for multi-wavelength fiber laser was reported.•The filtering function was theoretically analyzed and simulated by Jones matrix.•A laser ring cavity was constructed by the filter, EDF, SOA, WDM and pump source.•14 unique laser lines were achieved with the SNR of ~40 dB.•This filter exhibited tunable ability, switchable ability and good stability.

An in-line quasi-Sagnac interferometer comb filter based on high birefringence (Hi-Bi) fiber is proposed and experimentally demonstrated. This quasi-Sagnac interferometer is constructed by integrating a high-birefringence (Hi-Bi) fiber in between two fiber-coupled polarization controllers (PCs) followed by a fiber mirror. The filtering function is theoretically analyzed and simulated by the Jones matrix method. The extinction ratio and wavelength interval of the output spectrum can be flexibly adjusted by rotating the PCs. By only using a pump light source and gain medium, stable multi-wavelength lasing operation can be realized at room temperature. These advantages of tunable ability, switchable ability and good stability make the proposed filter to have broad application prospects in the field of tunable multi-wavelength fiber laser.