  In long-distance fiber optic transmission, the optic fiber link and
erbium-doped fiber amplifiers can introduce excessive noise, which reduces the
optical signal-to-noise ratio (OSNR). The narrow-band optical filters can be
used to eliminate noise and thereby improve OSNR. However, there is a relative
frequency drift between the signal and the narrow-band filter, which leads to
filtered signal instability. This paper proposes an adaptive OSNR recovery
scheme based on a Fabry-Perot (F-P) cavity with mode width of 6 MHz. Utilizing
the comb filtering of F-P cavity, the noise around the carrier and sidebands of
the signal is filtered out simultaneously. To avoid frequency mismatch, we
propose a double-servo scheme to suppress relative frequency drift between the
signal and the F-P cavity. We constructed a stable radio frequency transfer
system based on passive phase compensation and compared our scheme with other
OSNR recovery schemes based on optical filters. Compared to the schemes based
on dense wavelength division multiplexing (DWDM) and Waveshaper, our scheme
demonstrates an improvement in OSNR of carrier by at least 12 dB and sidebands
by at least 23.5 dB. The short-term transfer stability (1 s) is improved by one
order of magnitude compared to DWDM and half an order of magnitude compared to
Waveshper. This scheme can be applied to the recovery of signals with low OSNR
in long-distance fiber optic transmission, improving signal quaility and
extending the transmission distance limit.
