DFB-FL Hydrophone

With the development of the noise elimination techniques of submarine, optical fiber hydrophone has got a rapid progress, in order to meet the passive sonar and some important civil application. The distributed feedback fiber laser (DFB-FL) sensor based on the techniques of fiber Bragg Gratings can have a steady single model operational factor, very narrow band-with, long coherent length, stable result. It also can detect very small strain, be easy to be used for quasi-distributed sensing. For this, DFB-FL is popular in towing array sonar.The DFB-FL hydrophone can get enough detection sensitivity with a 5 centimeter long optical fiber, while other optical fiber hydrophone uses a 100 meter long conventional optical fiber to get the same sensitivity. In contrast with piezoelectricity hydrophone, the DFB-FL hydrophone has a small sensor head because of omitting electronic installation and copper wire. The DFB-FL can be multiplexed along a single fiber using wavelength division multiplexing (WDM) techniques. So with this technique, a smaller array of DFB-FL hydrophones can be developed. The demodulation method contributes in how the hydrophone works, the range of the hydrophone and its performance, so wavelength demodulation is one of the most important techniques to achieve a hydrophone system.The purpose of this paper is to discuss the demodulation system design of one hydrophone.Chapter 1:On the basis of knowing the principle and characteristic of conventional hydrophone, the superiority of optical fiber hydrophone is presented. Then this thesis introduces each kind of different optical fiber hydrophone, and proposes the plan of DFB-FL hydrophone in this paper. A mass of material indicates that the DFB-FL hydrophone has a good prospect.Chapter 2: The principle of DFB-FL gets an analyse in detail after introducing the basic structure and output theory of traditional fiber laser. The DFB-FL consists of λ/4-shifted Er-doped fiber grating, 980nm pump, WDM, coupler and isolator. Each part of DFB-FL has been explained. At last, how the DFB-FL works as sensor is bright to us.Chapter 3: The demodulation system of DFB-FL hydrophone introduces a wavelength demodulation method of unbalancing M-Z interferometer. Through principle analysis, the demodulation process is clear and finally the system gets an intensity signal output containing phase information changing unceasingly. For the inherent noise characteristic of the interferometer, a PZT modulator is used to compensate the phase drifting. It's proved that this anti-phase declining method is effective theoretically and experimentally.Chapter 4: On the foundation of above analysis, this thesis designs a demodulation system based on unbalancing M-Z interferometer. The design and debug of circuit part about this system are analyzed. This part consists of the design of photoelectricity transformation, amplifying circuit, filtering circuit and feedback controlling circuit.Chapter 5: A series of experiments has been done using a sine-wave exciter. The experimental results are gathered by data acquisition card and oscillograph separately. Through Fourier transforms, experimental results show the consistent frequency spectrum characteristic with acoustic source signal. Lastly, the DFB-FL hydrophone can get exact acoustic signals and work well.Chapter 6: On the basis of analyzing experiment results, the advantages and shortcomings of this hydrophone plan are commentated, the packing structure of DFB-FL and use of this hydrophone on array sonar research have been prospected.