Adaptive optical signal-to-noise ratio recovery for long-distance optical fiber transmission

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.