Bottom-mounted and all-optical transmitted interferometric fiber-optic hydrophone array systems have the advantages of underwater uncharged and high reliability and are widely used in subsea oil exploration, marine acoustic exploration, and other fields. However, with an increase in the remote transmission distance, the phase noise of the system increases sharply due to the nonlinear effect of the fiber and high optical losses, which limit the detection performance. In optical communication, the schemes of remote pumped optical amplification (ROPA) and fiber Raman amplification (FRA), combined with large effective area and low-loss optical fibers, are used in long-distance unrepeated transmission systems, and good noise index and low bit error rate have been achieved. However, because optical-fiber hydrophone systems are based on a coherent detection scheme with high sensitivity, phase noise is a critical factor for the performance evaluation. Thus, the actual performance cannot be determined by only the noise index. However, only a few analyses and experimental studies on the phase noise characteristics of fiber hydrophone systems with remote pumped amplification and new-type fiber transmission structures have been reported. In this study, we developed a phase noise model based on a remote all-optical transmission and amplification structure for optical-fiber hydrophone systems with a hybrid time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) array scheme. System parameters, such as the transmission link and remote gain position, were optimized through the model, which effectively reduced the system noise. The proposed noise model and optimization method can be applied to unrepeated fiber-optic hydrophone systems as they greatly improve the all-optical transmission distance and remote detection performance.

Based on the structure of a remote-transmitted and amplified hydrophone system with a dedicated pump path scheme (Fig. 1), we developed a phase noise model of the remote amplification. First, the cascaded noise index of the hybrid optical amplification is calculated by comprehensively considering system parameters, such as the loss of the fiber hydrophone array, loss of the round-trip transmission link, remote pump power, and gain coefficient of the unit pump light. Second, the noise index is correlated with the beat intensity noise induced by the cascade amplification spontaneous emission noise at the receiver of the hydrophone system. Finally, combined with the phase-demodulation conversion coefficient, the TDM sampling aliasing, and other factors, the optical intensity beat noise is converted into a demodulated phase noise, and an equivalent phase noise model of remote optical amplification is obtained. Fig. 2 shows the simulation results of the phase noise associated with each stage of the amplifiers and the total noise of the remote amplification. The ROPA and FRA are the main noise sources in the remote system, and the huge transmission link loss combined with the insufficient input optical power of the remote pumped unit (RGU) are the key factors limiting the system's performance.

Based on the noise model, transmission-link-induced noise was simulated and optimized (Fig. 3) using a hybrid transmission scheme, which uses a large-effective-area and low-loss fiber (Type G.654) for the pump and signal-light transmission in the optical-fiber hydrophone downlink and an ultra-low-loss optical fiber (Type ULL-G.652) for signal-light transmission in the uplink. Also, in an experimental system with a 100-km transmission and a 4-WDM×8-TDM array scheme, the measured loss of the pump light is reduced by 2.1 dB (Fig. 4) compared with that of the traditional single-mode optical-fiber link (Type G.652), and Raman scattering of the pump light is also effectively reduced (Fig. 5). Then, the phase noise of the short system (Fig. 7) and the complete noise of the 100-km system (Fig. 6) were measured, and the phase noises independently induced by the remote amplification were obtained (Tab. 1). The results show that, with the combination of the G.654 and ULL-G.652 transmission links, the remote amplification noise can be reduced by 4.3 dB compared with that of the conventional G.652 link, reaching a low noise level of -98.1 dB@1 kHz. This reveals the effectiveness of the phase noise model (Tab. 1). Furthermore, the model was applied to a 150-km transmission system to optimize the position of the RGU, and the simulated result (Fig. 8) shows an optimal position of 115 km with -93.7-dB noise. Based on this result, an experiment was conducted, and the result shows a remote-amplification-induced noise of -93.2 dB@1 kHz (Fig. 9), which is consistent with the simulation result.

In this study, we developed a phase noise model based on an all-optical transmitted and amplified optical-fiber hydrophone array system. By analyzing the noise sources and characteristics, we propose a hybrid transmission link using large-effective-area and low-loss optical fibers. Both theoretical and experimental results show that the remote-amplification-induced phase noise in a 100-km transmission and 4-WDM×8-TDM system can be reduced to a low noise level of about -98 dB@1 kHz, which is 4 dB-5 dB lower than that of the traditional single-mode fiber link, revealing that the proposed transmission structure can effectively improve the noise performance of hydrophone systems. The RGU position in a 150-km transmission system was also optimized using the model, and the measured noise of the remote amplification is low (-93.2 dB@1 kHz), which is consistent with the simulation results. The proposed noise model and optimization methods for all-optical transmission and amplification systems can be applied to the design, implementation, and performance evaluation of remotely interrogated optical-fiber hydrophone systems, which could provide a critical technical support for extending remote transmission distances and improving the detection performance of such systems.