A theoretical study of a passive fiber ring resonator with a Sagnac loop filter is reported. It is used to construct a stable multiwavelength fiber laser at room temperatures. The gain medium is an elliptical-core erbium-doped fiber. The principle of operation is based on the polarization-dependent gain phenomena in erbium-doped fiber. The effects of the different parameters of the doped fiber and the loop filter are experimentally studied.