Quantum optics experiments frequently require the separation of single-photon-level signals from strong classical fields. In circumstances in which the signals are spectrally close, and one cannot make use of relatively simple separation methods based around differences in polarization state, optical mode, or beam direction, it is necessary to exploit the frequency difference itself as the means for signal separation. We have constructed and characterized an efficient and robust fiber-based filter, consisting of an all-fiber Fabry-Perot resonator, to achieve this goal. Our filter shows 31dB of suppression of unwanted signals and 76% transmission of the desired signal. The transmission of the filter was stabilized to within 2% of its maximum for over 35hours through simple temperature stabilization.