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A simple stable single-longitudinal-mode (SLM) single-and dual-wavelength erbium-doped fiber ring laser using an arrayed waveguide grating as the wavelength filter is proposed and demonstrated. SLM operation is guaranteed by a saturable-absorber-based auto-tracking filter. A 40/60 optical coupler is used to form two imbalanced loss cavities in the fiber laser initially, and single-and dual-wavelength operation can be changed by properly modifying the variable optical attenuator. The side mode suppression ratios of the laser outputs are higher than 60 dB. Through switching the two fiber optical switches, the dual-wavelength spacing is tuned from the narrowest 0.82 nm to the widest 12.05 nm, which offers potential uses in the generation of terahertz (THz) radiation.