Polarimetric parity time symmetry in a photonic system

Parity-time symmetry: Achieving single-mode lasing in long cavities Parity-time symmetry can be implemented between two sub-spaces in a single long cavity to achieve stable single-mode lasing without needing expensive optical filters. Jiejun Zhang and colleagues at China's Jinan University demonstrated this by manipulating linearly and orthogonally polarized light waves in a single-loop fiber-ring laser. They adjusted the gain, loss and coupling between the polarized light waves, to ensure that the system operates with parity-time symmetry breaking. This generated a single-mode light wave that inherently covers a narrow part of the electromagnetic spectrum (2.4 kHz), but has a larger measured 'linewidth' of 129 kHz, due to the long cavity's susceptibility to environmental disturbances. This linewidth could be reduced by actively stabilizing or isolating the cavity. The single physical loop design provides simplicity and highly improved stability to the system. Parity-time (PT) symmetry has attracted intensive research interest in recent years. PT symmetry is conventionally implemented between two spatially distributed subspaces with identical localized eigenfrequencies and complementary gain and loss coefficients. The implementation is complicated. In this paper, we propose and demonstrate that PT symmetry can be implemented between two subspaces in a single spatial unit based on optical polarimetric diversity. By controlling the polarization states of light in the single spatial unit, the localized eigenfrequencies, gain, loss, and coupling coefficients of two polarimetric loops can be tuned, leading to PT symmetry breaking. As a demonstration, a fiber ring laser based on this concept supporting stable and single-mode lasing without using an ultranarrow bandpass filter is implemented.