Research On High-Speed Time-Division Multiplexing System Based On Fiber-Optic Hydrophone Array

Time-division multiplexing (TDM) is considered as one of the most basic and effective schemes for signal transmission in the large fiber-optic hydrophone array. The rapid attenuation of optical power and the crosstalk of underwater acoustic signal are the key problems of application systems.To satisfy the need of fiber-optic hydrophone detection, the power of laser reaches several hundreds mill watt. But this will produce a serious nonlinear phenomenon in fiber-optic, which is called stimulated Brillouin scattering (SBS). This will increase the optical loss in long distance communication using high coherent laser, and destroy the spectral character of the laser. This phenomenon has been analyzed in theory and some experiments have been carried out. And the efficient conclusion has been given to improve the threshold of SBS and to eliminate the input power limitation set by SBS in the fiber-optic hydrophone TDM system.Both high coherent and low coherent laser are used in fiber-optic hydrophone array respectively. Theoretical analyses have indicated that these two lasers have different effect on the TDM system. Recently, with the development of the PGC digital demodulation scheme, high coherent laser has been mainly applied. The coherent length of laser is much longer than the length of total delay coils in TDM system. This will produce serious interference between multiplexing signals in TDM system. The system crosstalk model has been set up theoretically and the relationship between optical pulse modulator and system crosstalk has been analyzed and emulated. And the conclusion has been drawn that the channel crosstalk between 8 sensors is greater than -35dB when the extinction ratio of acousto-optic modulator (AOM) is 40dB.Based on theoretical and experimental analyses, detailed qualification has been given about the TDM system. A narrowband and tunable ring laser emits high coherent light, an AOM with high extinction ratio produces optical pulse, fiber-optic coils with different lengths implements delay among multiplexing channels, a 1 MSPS, 16-bit A/D samples the TDM signal and two DSP realizes synchronization and process of the signal. Finally, a high-speed TDM polarization-maintaining hydrophone system at 1550nm has been designed and set up. The crosstalk, measure in laboratory, reaches -35dB and the noiselevel achieves 10-5 rad Hz . The experiment result is in good agreement with the theoretical and emulation value.