Widely Wavelength-Tunable Parity-Time Symmetric Single-Longitudinal-Mode Fiber Ring Laser With a Single Physical Loop

A wavelength-tunable single-longitudinal-mode fiber ring laser based on parity-time (PT) symmetry for mode selection is proposed and experimentally demonstrated. The PT symmetry is implemented using a single physical fiber-optic loop, which functions equivalently to two mutually coupled optical loops with an identical geometry but balanced gain and loss. In the proposed system, a linearly polarized light wave is split into two, which are traveling in a Sagnac loop incorporating a polarization beam splitter (PBS) and two polarization controllers (PCs). By controlling the polarization states of the light waves injected into the PBS via tuning the PCs, the gain and loss of the two equivalent loops corresponding to the light waves traveling in the clockwise (CW) and counter-clockwise (CCW) directions are controlled. When the gain and loss are balanced and exceed the coupling coefficient, PT-symmetry breaking condition is met, and single-longitudinal-mode lasing is achieved. The proposed PT-symmetric fiber ring laser is experimentally demonstrated. Single-longitudinal-mode lasing with a tunable wavelength from 1530 to 1565 nm and a narrow linewidth as small as 390 Hz is achieved. The sidemode suppression ratio is 41.9 dB confirming the effectiveness of using PT symmetry for mode selection.