We describe the experimental construction of a single-longitudinal-mode erbium-doped fiber ring laser equipped with a ring resonator filter. The system is designed to avoid mode hopping and can be frequency scanned for spectroscopic sensor applications. We modified a conventional ring resonator filter to have the capability to reduce transmittance of unwanted modes, which is essential for frequency scanning without mode hopping. (C) 1999 Optical Society of America. OCIS codes: 060.2320, 060.2410, 140.3500, 140.3510, 140.3560, 140.3570, 140.3600.