Parity-time symmetric forward Brillouin fiber laser

Parity-time (PT) symmetry breaking in non-Hermitian systems provides a transformative approach to surpass conventional laser mode-selection limitations. We demonstrate a PT-engineered forward Brillouin fiber laser that enables precise opto-acoustic mode control within an all-fiber architecture. By constructing dual coupled resonators with controlled gain-loss interplay, implemented through an erbium-doped fiber amplifier and polarization controllers, we experimentally achieve non-Hermitian PT-symmetry breaking. The proposed control architecture enables selective excitation of forward Brillouin scattering (FBS) oscillation modes, achieving single-longitudinal-mode lasing with ultra-narrow linewidth at the sub-hertz level. The polarization-dependent FBS effect further permits dynamic mode tuning through polarization manipulation. Our system establishes a high-coherence opto-acoustic platform for ultra-precision FBS sensors and optomechanical devices while advancing nonreciprocal photon–phonon engineering.