High-Coherence, Physically Separable Dual-Frequency Fiber Laser Based on Bidirectional Dual-Path Ring Cavity

Dual-frequency lasers with narrow linewidth and high coherence serve as essential light sources for systems such as heterodyne detection, LiDAR, and precision interferometry. However, existing technologies cannot directly separate the two frequency components at MHz-scale differences, which remains a persistent bottleneck in this field. In this paper, we present a dual-frequency fiber laser based on a bidirectional dual-path ring cavity. The proposed laser supports flexible switching between single-frequency and dual-frequency operation while allowing straightforward physical separation of the two outputs via intrinsic beam routing. In single-frequency mode, the two beams exhibit Lorentzian linewidths of 1.1 kHz and 1.16 kHz, respectively. In dual-frequency operation, the laser produces a beat signal at 470 MHz with a 3-dB linewidth of 340.2 Hz and a signal-to-noise ratio (SNR) exceeding 70 dB. This dual-frequency fiber laser provides a novel and practical source for heterodyne detection and LiDAR-based measurement systems.