All polarization maintaining erbium-doped fiber dual-ring laser with ultra-low threshold and ultra-narrow linewidth based on artificial random feedback structure

An ultra-low threshold and ultra-narrow linewidth erbium-doped fiber (EDF) hybrid laser based on all polarization maintaining dual-ring structure combined with artificial random scattering feedback is proposed and demonstrated. In this work, a 7-cm-long artificial random scattering fiber (ARF) inscribed by femtosecond laser is employed as a feedback structure for multi longitudinal mode suppression and a model is established to theoretically analyze the formation progress of achieving single longitudinal mode (SLM) laser output by the ARF feedback. A sub-ring, consisting of 2 m unpumped EDF-based saturable absorber is employed to produce an auto-tracking narrowband filter to ensure the SLM laser output. We achieve a SLM laser with an ultra-low threshold of 7 mW, an ultra-narrow linewidth of 315 Hz and an OSNR of 70 dB. In addition, the output power and center wavelength fluctuation of the proposed laser are 0.285 mW and 3.3 pm, respectively, over one hour.