LAUSR

A tunable single-longitudinal-mode (SLM) narrow linewidth Brillouin fiber laser based on parity-time (PT) symmetry is proposed and experimentally demonstrated. In the proposed scheme, the dual selecting-mode mechanism is employed to achieve the SLM operation. The first, a narrow-band gain curve induced by stimulated Brillouin scattering (SBS) can be acted as a filter for selecting mode preliminary in the fiber ring cavity. The second, a PT-symmetric structure is formed by using a Sagnac loop, in which by adjusting a single polarization controller, the PT symmetry can be broken, resulting in the SLM oscillation. The SBS effect can be further beneficial to the linewidth narrowing. Using the proposed scheme, the SLM lasing with an optical signal to noise ratio of 50.1 dB and an ultra-narrow linewidth of 300 Hz at 1547.90 nm is obtained. Compared with the original linewidth of pump laser, the linewidth-reduction ratio can be up to 680 times. The available wavelength ranges from 1528.4 to 1563.4 nm, covering 35 nm. The key advantage of the proposed laser is that by constructing a simple Sagnac loop, the SLM selection based on PT symmetry and linewidth narrowing benefiting from SBS effect are accomplished simultaneously. Moreover, it is worth noting that the wavelength tunability is virtually unrestricted, only depending on the pump wavelength.