Pump Absorption And Coherence Collapse Characteristics Of Fiber Laser Hydrophone Array

Fiber laser hydrophone has a number of bene?ts, such as high sensitivity, miniature size, as well as the inherent high multiplexing capability to integrate a wavelength-division multiplexing array into a single fiber, which makes fiber laser hydrophone especially attractive for ultra thin hydrophone array. However, the pump absorption and coherence collapse makes it hard to develop a large scale fiber laser hydrophone array, and limits the application of fiber laser hydrophone. In this thesis, we mainly concern DFB (Distributed-feedback) fiber laser hydrophone and investigate the pump absorption characteristics of DFB fiber laser hydrophone, as well as the coherence collapse characteristics.The main results of this thesis are as follows:1. The factors which influence the pump threshold and the pump depletion coefficient of DFB fiber laser are investigated on the basis of the rate equation theory. The pump absorption characteristics of two-element DFB fiber laser hydrophone array is tested, and the pump power distribution in the hydrophone array is analyzed through the measurement.2. The complex cavity model of DFB fiber laser hydrophone with optical feedback is introduced. The frequency shift and coherence collapse characteristics of DFB fiber laser hydrophone are analyzed numerically.3. The coherence collapse characteristics of DFB fiber laser hydrophone and hydrophone array is tested through the measurement of longitudinal mode operation and phase noise. The total length of the lead fiber in the two-element DFB fiber laser hydrophone array can be enlarged to 140m, and that of the lead fiber in the three-element DFB fiber laser hydrophone array can be enlarged to 250m. The threshold length of the lead fiber between the adjacent elements in the array is decided by the coherence collapse characteristics of the former element, and Rayleigh backscattering of the lead fiber after the latter element does not make the former element burst into coherence collapse.4. A novel kind of DFB fiber laser sensor based on the coherence collapse characteristics is proposed. It makes the lead fiber act as sensing head, and utilizes the reflection facet and Rayleigh backscattering of the lead fiber. Experimental results indicate that the signals acting on the lead fiber can be reconstructed easily, and the feasibility of the DFB fiber laser sensor is proved.