| With the development of the marine science, fiber optic hydrophones haveattracted much attention as sensing elements for exploring the ocean. Modernrequirements in application such as the seismic exploration are large scale sensor arrayswith large dynamic range. However, the current signal detection technology hasseverely restrict the further development and hindered the practicability of the fiberoptic hydrophones. The main problem is that the limited dynamic range cannot meet therequirements in the applications of seafloor strata exploration field, and hinders thepractical process of fiber optic hydrophone. Heterodyne detection technology has thecharacteristics of large dynamic range and low algorithm complexity, which provides asolution to solve these bottle-neck problems. In addition, the corresponding opticsystem structure is more suitable to construct the large scale fiber optic sensor network.Thus the development of fiber optic hydrophone array based on heterodyne detectionhas great significance to promote the practical of fiber optic hydrophone and theprogress of the marine science.Several key technologies are studied in this dissertation, including theimplementation of all digital heterodyne signal detection, the expansion of the upperlimit of dynamic range (ULDR), the analysis and suppression of the noise of the system,and the analysis and suppression of the crosstalk of the array system. Firstly, thecharacteristics of the array structure corresponding to the heterodyne detection areanalyzed. The quadrature signal acquisition and quadrature demodulation algorithminvolved in the heterodyne detection method are discussed. A method to implement alldigital demodulation process is designed. To maximize the ULDR of the heterodyneinterferometic system, the restriction caused by the heterodyne frequency, thequadrature demodulation algorithm and the digital sampling rate are analyzed andcompared. In the aspect of system noise analysis, three main noises are analyzed intheory, including the phase noise of the laser associated with the linewidth of the laser,the phase noise introduced by the frequency instability of the frequency shifter and thephase noise caused by the Rayleigh scattering in the leading fiber. The impacts of thenoise on the performance of the system are also discussed. Some important results areachieved, including the requirement to the linewidth of the laser can be reduced in aquasi-equal-arm interferometry, a scheme based on the sync optic reference signal tosuppress the frequency shifting noise is proposed, and a method by modulating thefrequency of the laser to suppress the Rayleigh scattering noise is designed. All thesuppression measures are verified by experiments. A fiber optic hydrophone arraydemonstration system with4sensors based on heterodyne detection is built. Thecrosstalk of the array system is theoretical analyzed and experimentally tested, and an integrated test for the performance of the heterodyne detection system is executed.The main research results and innovations are as follows:A scheme based on sync optic reference signal to suppress the frequency shiftingnoise is proposed, which can reduce the noise level of the heterodyne detection systemto lower than-100dB/√Hz in the frequency range of above300Hz. The noiseelimination of this scheme is achieved in the phase demodulation process withoutadditional signal processing. This scheme can also reduce the requirement on theperformance of the frequency shift device, which makes it possible for the heterodynedetection technology to be widely used in the fiber optic hydrophone system with highrequirement on the noise performance.The ULDR is decided by both the heterodyne frequency and quadrature heterodynedemodulation algorithm. The restrictions to the ULDR of the heterodyne frequency,arctangent algorithm and DCM algorithm are discussed in theory. An optimal parameterconfiguration scheme that sets the heterodyne frequency as a quarter of sampling rateand chooses the arctangent algorithm as the demodulation algorithm is proposed. Basedon the performance of the current system, the ULDR can reach1000rad at1kHz, whichincreases2orders of magnitude than that of current detection system. Considering theresult of the noise level after suppression, the dynamic range of the heterodyne systemcan expand to160dB.The theoretical research on the linewidth characteristics of the frequencymodulated laser is carried on based on delayed self-heterodyne method. The relationshipbetween linewidth with the modulation amplitude, frequency and the length of delayfiber is obtained, which laid a theoretical foundation to suppress the phase noise thatcaused by the coherent Rayleigh scattering in long-distance transmission by modulatingthe frequency of the laser. And the feasibility of the method is verified by experiments.This suppression method can decrease more than10dB for the noise level in one-waytransmission fiber of50km, which makes the system noise level close to-100dB/√Hz inthe frequency range above200Hz, and can decrease5~10dB for the noise level inbidirectional transmission fiber of2km.A lookup table–rotation-approximation associated fast actangent algorithm forquadrature demodulation is proposed. This algorithm can acquire an arctangentcalculation result with10-6rad precision only by a look-up table, a coordinate rotationand a small amount approximation. With this fast algorithm, we can realize the fullydigital signal phase demodulation of the heterodyne detection scheme. |
PDF Full Text Request