65 Hz-linewidth single-frequency fiber laser utilizing a ring compound resonator incorporating double add-drop fiber rings.

We design an optical filter composed of double add-drop fiber rings (D-ADFR). Except for the free spectrum range (FSR), the single longitudinal mode (SLM) selectivity of a D-ADFR is related to the sideband suppression ratio (SBSR), defined by the intensity of the side resonant peaks to that of the main resonant peaks. A spectrum analysis method of the D-ADFR is demonstrated, considering the SBSR, for a single-frequency narrow-linewidth fiber laser with a long cavity. This analysis method can guide the optimization of SBSRs by controlling the coupling coefficients and cavity length in a D-ADFR, and it is validated by both simulations and experiments. The design principles of D-ADFR are summarized, considering a compromise between a wider FSR and superior SBSMs. According to the principles, we design the ring-cavity and linear-cavity fiber lasers with an optimized D-ADFR, and their SLM stability and linewidth are measured. The results show that the best one exhibits a narrow linewidth of ∼ 65 Hz, while keeping SLM mostly during the observation of 60 minutes. The research provides an optimization method and design principles of D-ADFR for narrow-linewidth single-frequency fiber lasers.