Tunable Hz Intrinsic Linewidth Single-Polarization Linear Cavity Fiber Laser Based on Parity-Time Symmetric Optics With a High-Q Microresonator

High-performance tunable fiber laser with ultra-narrow linewidth and single polarization is driving the progress in fundamental research and engineering applications. However, it is still challenging to achieve single polarization tunable fiber laser due to the redundant laser cavity and the lack of polarization selectivity of ordinary optical fibers. Here, we propose and demonstrate a novel scheme for the generation of an ultra-narrow linewidth and single polarization linear cavity fiber laser based on parity-time (PT) symmetry in combination with a high-Q Fabry Pérot (FP) resonator. The PT symmetry can be achieved between two subspaces in a single physical loop based on optical polarization diversity and controllability. Benefitting from the FP resonator with a high Q factor of 10⁷, the measured laser side mode suppression ratio reaches 73.3 dB and the high frequency white noise floor is ∼2.62 Hz²/Hz, corresponding to an intrinsic linewidth of about 8.2 Hz. The laser output has a high polarization extinction ratio of ∼35.46 dB and its maximum output power fluctuation is less than 1.6% over more than 60 min due to the high polarization sensitivity of the FP resonator. The wavelength tuning range is 52.15 nm (from 1523.95 to 1576.1 nm), covering the entire flat region of the erbium doped gain fiber. The proposed PT-symmetric system realizes an ultra-narrow linewidth and single-polarization tunable single-frequency fiber laser in a robust and efficient way, which provides a new insight to improve the performance of other laser sources.