Stable Sub-KHz Linewidth Fiber Laser Using Saturable Absorber Integrated Four-Subring Resonators

This study proposed a high-quality fiber ring laser combining four-subring resonators and a saturable absorber within a ring cavity. An un-pumped erbium-doped fiber is introduced as the saturable absorber (SA) within the ring structure. By employing the vernier effect, we arrange four subrings with different lengths to form the four-subring resonators, achieving a single-longitudinal-mode (SLM) operation. At a pump power of 100 mW, the laser output power and wavelength deviations during a one-hour test were measured to be 0.01 dB and 0.059 nm, respectively. Additionally, using the delayed self-heterodyne measurement system and Lorentzian fit analysis, we measured an ultra-narrow linewidth of 623.1 Hz.