| With the developments of Fiber-Optics hydrophone technologies, the hydrophone application is now moving towards the large-scale and remote interrogation direction, which requires a lot of hydrophones to be located several hundreds or thousands kilometers from the receiving station. In this case, noises due to the remote transmission and cascaded analogue amplifiers increase significantly. They are continuously accumulated and amplified in the transmission links, which ultimately limit the scalability of the approach. Therefore it is the purpose of this paper to develop, for the first time to our knowledge, a full and systematic noise analysis for the remotely interrogated hydrophone arrays, aiming to demonstrate an analogue transmission link with acceptable noise performance. In the paper, we provided a general analytical approach for all the potential noise sources, presented a phase noise model applicable to evaluate, characterize, and optimize the performance of the remote link. Based on a synthetically noise suppression process for the Rayleigh scatter noise, cascaded amplifier noise, Stimulated Brillouin scattering, Four-Wave Mixing and so on, we designed and constructed a 400 km interrogated hydrophone array with a 8T×2W multiplexing structure, and the measured system phase noise floor is low to-97 d B re 1rad/sqrt(Hz) at 1 k Hz. The main contents and innovations of this paper are as follows:(1) Based on the use of high coherence laser sources to interrogate the hydrophones, we systematically analyzed the coherent double Rayleigh scattering(DRS) induced intensity and equivalent hydrophone phase noise. We described the operational principle of the phase-generated carrier(PGC) scheme for the suppression of the DRS-induced noise, and have got up to 20 d B of noise suppression in the experiments. Then we presented that the DRS-induced phase noise spectrum is mainly determined by the Voigt line-shape of the light source. Therefore if the laser line-width is excessively narrow, the corresponding phase noise will greatly increase at low frequencies and thus degrade the system performance. This result is helpful for us to choose suitable laser sources for long-haul hydrophone applications. In the experiments, a laser diode with Voigt line-width of 12.5 k Hz was applied to interrogate a 100 km transmission link and the PGC technique was used to demodulation the hydrophone phase. The measured result shows that the phase noise due to DRS has been significantly reduced.(2) Based on the PGC scheme, we analyzed the conversion of the amplifier-induced intensity fluctuation to the demodulated phase noise, and presented that the PGC scheme can increase the phase noise level by ~3.5d B when compared with the original intensity noise. Then we defined two novel terms: “Demodulation conversion coefficient” and “Noise aliasing coefficient” to develop a model to characterize the amplifiers-induced hydrophone phase noise. This model is also a fundamental criterion for optimizing the performances of the cascaded amplifiers.(3) With this model, a hybrid Raman amplifier(FRA) and erbium doped fiber amplifier(EDFA) scheme is constructed, which is used as a pre-amplifier in a 100 km unrepeated transmission system. The measured results show that the FRA/EDFA hybrid has significantly suppressed the amplified spontaneous emission(ASE) noise, and the noise due to the pre-amplifier is reduced by ~8d B when compared with the sole EDFA scheme. Thus, the total phase noise level of the unrepeated system is only-104.2d B.(4) For a 400 km interrogated time division multiplexing(TDM) array, we considered all the aspects which can potentially impact the phase noise of the remote array, including the optical transmitter and receiver, the TDM array and the remote transmission link, and designed a cascade of 9 EDFAs with a perfectly acceptable phase resolution. The final phase noise due to the EDFAs is only-106.2d B; the total system noise is-97.2d B~-99.0d B(@1k Hz~14k Hz). These experimental results are helpful for us to design an ultra-remotely interrogated hydrophone array.(5) We introduced adaptive noise cancellation method into remote hydrophone systems to suppress the transmission links induced highly correlated noises. These sources include polarization and phase modulation noises resulting from the disturbances of the lead fibers. Experimental results show that by the use of Normalized Least-mean-square(NLMS) and Recursive-Least-Squares(RLS) algorithms, respectively, the narrowband phase modulation noise and broadband polarization noise can all be suppressed by greater than 20 d B. And thus the lead fiber induced noises can almost been eliminated. In addition, we show that the adaptive noise cancellation results are much better than that of the conventional noise subtract method.(6) After a systematic suppression of all the potentially noises, we designed a signal processing system with improved stability for the demodulation of remote signals, and finally accomplished a 400 km interrogated 8T×2W array. The measurement results show that the averaged noise level of the array is about-97 d B@1k Hz, the averaged TDM crosstalk level is between-61 and-75 d B, the wavelength division multiplexing(WDM) crosstalk level is better than-73.2d B and the acoustical coherences between two TDM sensors and two WDM sensors are all better than 0.98. Besides a slightly increase of the phase noise, the system performances of the 400 km interrogated system are comparable with that of a short-haul system. So we consider that the integral performance of the remote interrogated hydrophone array has achieved to an acceptable high level, which potentially can be used in a practical bottom mounted application. |
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