  An all-polarization maintaining (PM) single-longitudinal-mode (SLM)
erbium-doped fiber laser (EDFL) with ultra-high optical signal-to-noise ratio
(OSNR), ultra-narrow linewidth and extremely high stability is proposed and
experimentally demonstrated. A double-ring passive subring resonator (DR-PSR)
composed of two single-coupler fiber rings and a length of unpumped EDF-based
saturable absorber filter is designed and employed in the EDFL to serve as the
efficient SLM selecting element to guarantee SLM lasing with excellent output
performance. The all-PM structure enables the proposed EDFL to present strong
ability to resist the environment disturbance. At the pump power of 100 mW, we
obtain an SLM EDFL with an ultra-high OSNR of 83 dB and an ultra-narrow
linewidth of 459 Hz. For the SLM operation, the all-PM EDFL processes
outstanding stability performance of both the wavelength lasing and the output
power. The maximum fluctuations of the center wavelength and output power are
0.012 nm and 0.01 dB.
