S-band single-longitudinal-mode erbium-doped fiber ring laser with ultra-narrow linewidth, ultra-high OSNR, high stability and low RIN

A high-performance S-band single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring cavity laser based on a depressed cladding EDF is investigated and experimentally demonstrated. We combine a double-ring passive resonator (DR-PR) and a length of unpumped polarization maintaining (PM) EDF in the laser cavity to achieve the SLM lasing without mode hopping. The DR-PR, composed of two efficient dual-coupler fiber rings, is utilized to expand the free spectral range of the EDF ring cavity laser and to eliminate the dense longitudinal modes greatly. The PM EDF, insusceptible to random change induced by environmental perturbations, is used as a saturable absorber filter to guarantee and to stabilize the SLM operation of the EDF ring cavity laser. At the pump power of 400 mW, we obtain an SLM EDF ring laser with a linewidth as narrow as 568 Hz, an optical signal-to-noise ratio as high as 77 dB, and a relative intensity noise as low as 140 dB/Hz at the frequency over 5 MHz. Meanwhile, the stability performance of both the wavelength lasing and the output power, the dependence of the OSNR and the output power on pump power for the S-band fiber laser are also investigated in detail.