  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.
