Ultra-narrow linewidth single-frequency fiber laser based on inverted-T-shaped optical fiber ring and Sagnac ring

The invention discloses an ultra-narrow linewidth single-frequency fiber laser based on an inverted-T-shaped optical fiber ring and a Sagnac ring, and belongs to the field of fiber lasers. The experimental structure consists of a pumping source (1), a wavelength division multiplexer (2), a gain optical fiber (3), a first optical isolator (4), an inverted-T-shaped optical fiber ring (5) and a first2*2 optical isolator (6), a polarization beam splitter (7), an optical circulator (8), a Bragg fiber grating (9), a first 1*2 optical coupler (10), a non-pumped erbium-doped fiber (11), a second optical isolator (12), a second 2*2 optical coupler (13) and a third 2*2 optical coupler (14). The inverted-T-shaped optical fiber ring serves as a high-performance mode filter to restrain multimode oscillation in the resonant cavity. The unpumped erbium-doped optical fiber in the Sagnca ring can be used as a saturable absorber and can ensure that the optical fiber laser can finally achieve single longitudinal mode output. Hi