Widely-wavelength-tunable brillouin fiber laser with improved optical signal-to-noise ratio based on parity-time symmetric and saturable absorption effect

A widely-wavelength-tunable Brillouin fiber laser (BFL) with improved optical signal-to-noise ratio (OSNR) based on parity-time (PT) symmetric and saturable absorption (SA) effect is present. This novel BFL realizes PT symmetry and SA effect through polarization-maintaining erbium-doped fiber (PM-EDF) Sagnac loop, which is composed of a PM-EDF, a coupler and two polarization controllers (PCs). By using the inherent birefringence characteristic of PM-EDF, two feedback loops in orthogonal polarization state are formed when the Strokes signal in injected. One of these loops provides gain in the clockwise direction with in the Sagnac loop, while the other loop generates loss in the counterclockwise direction. By adjusting the PCs to control the polarization state of the PM-EDF, a single-longitudinal-mode (SLM) BFL can be achieved, as the PT symmetry is broken when the SA participating stimulated Brillouin scattering (SBS) gain and loss are well-matched and the gain surpasses the coupling coefficient. Compared to previous BFLs, the proposed BFL has a more streamlined structure and a wider wavelength tunable range, at the same time, it is not being limited by the bandwidth of the erbium-doped fiber amplifier while still maintaining narrow linewidth SLM output. Additionally, thanks to SA effect of the PM-EDF, the PT symmetric SBS gain contract is enhanced, resulting in a higher optical signal-to-noise (OSNR). The experimental results show that the laser has a wide tunable range of 1 526.088 nm to 1 565.498 nm, an improved OSNR of 77 dB, and a fine linewidth as small as 140.5 Hz.