Research On The Key Technologies Of Fiber-Optic Hydrophone Array Based On Sagnac Interferometers

The fiber-optic hydrophone array based on Sagnac interferometers presents several advantages over the acoustic sensor array based on Mach-Zehnder (MZ) interferometers. The former is insensitive to signal fading and source phase noise and can be easily desensitized to polarization fading just by inserting a Lyot depolarizer in the interferometer.This dissertation presents our efforts to develop the key technologies of Sagnac based sensor array (SSA) for acoustic sensing. The acoustic response characteristics in the Sagnac interferometer and the main factors that may limit the performances of the SSA are analyzed. The signal processing, Time-Division Multiplexing and anti-noise-pulses-overlap in the folded Sagnac sensor array (FSSA) are proposed and demonstrated. The dynamic range and crosstalk are analyzed in the double folded SSA (DFSSA), and a DFSI with 173dB dynamic range is presented. A 4-sensor SSA is developed first time in China, and the possibility of using the SSA as an acoustic/rotation sensor in the same array is demonstrated by experiments.The main innovations and creative points in the dissertation are as follows:1. It proposes a signal processing technique with alternative phase bias in the DFSSA. With this technique, the requirement of stability of optical power can be decreased, the phase drift induced by non-reciprocity can be measured and compensated, and the low-frequency noise due to the detection system and the optical source can be reduced obviously in the output signal.2. It proposes a time division-multiplexing (TDM) scheme, which is compatible with the active phase bias technique mentioned above, to avoid the overlapping of the signal and noise pulses in the FSSA. A stableπphase bias is observed and accounted in the FSSA incorporating an orthoconjugate reflector (Faraday rotator mirror), which can be used to eliminate the optical noise pulses under small signal situation.3. It proposes a double folded Sagnac hydrophone array whose dynamic range is adjustable, and analyzes the crosstalk effects. A DFSI with about 173dB dynamic range is demonstrated experimentally.4. It proposes and demonstrates experimentally the concept of using the SSA as an acoustic/rotation sensor with the same array. This technology can be used in fiber-optic towed array for the shape measurement and in the bottom-mounted array for security.