Sub-100 Hz linewidth, highly stable single frequency fiber laser based on the randomly distributed feedback structure

Narrow-linewidth fiber lasers are crucial for applications such as remote distributed sensing and coherent detection LiDAR, which demand exceptional spectral purity and phase stability. Herein, we report a high-performance single-longitudinal-mode (SLM) erbium-doped fiber (EDF) laser that achieves a sub-100 Hz linewidth. The laser's architecture is designed based on a theoretical model developed to elucidate the quasi-continuous cascaded feedback mechanism of the randomly distributed feedback structure (RDFS). We leverage numerical simulations via the proposed model to investigate how the laser linewidth is affected by key parameters, such as RDFS length, feedback coefficient, and initial gain bandwidth. The analysis provides a clear rationale for balancing output power with long-term SLM stability. Accordingly, we construct a dual-component EDF laser employing a self-injection locking mechanism. It combines a saturable absorber-based interferometric ring with an optimized RDFS to simultaneously achieve robust SLM operation and significant linewidth narrowing. Experimentally, the laser demonstrates an ultra-narrow linewidth of approximately 95 Hz, an output power exceeding 4 mW, and a relative intensity noise below -140 dB/Hz at frequencies above 0.5 MHz, while maintaining stable SLM operation for over two hours.