Research On The Static Sensitivity And Dynamic Characteristic Of Bar-type And Sine Fiber Optic Hydrophone Unit

The fiber optic hydrophone was developed to satisfy the urgent demands of underwater acoustics applications, especially for anti-submarine purposes. This new underwater acoustic sensor based on optical fiber and optoelectronics technology is now well developed and widely used in military and civil applications. The optical fiber hydrophone system includes the wet end whose signal sensing and transferring are all fiber based, and thus has the merits of light weight, low cost and immunity to electromagnetic interference.Fiber optic hydrophone unit is the basis of fiber optic hydrophone system. In this paper, bar-type and sine fiber optic hydrophone units, which are both based on planar circular thin plate structure, are put forward for the first time. Their static sensitivities are formulated, in which the static sensitivity of bar-type fiber optic hydrophone system considers two cases of the elastic diaphragm with and without a small rigid thin plate. Influences of the initial tensile loading of fiber and corresponding parameters of the unit systems on their static sensitivity are investigated theoretically and analytically, analytical results are compared with those of ANSYS numerical simulation. Using Rayleigh's method, the approximate fundamental frequency formula of bar-type fiber optic hydrophone unit is derived, influences of the diaphragm plate and fiber on which are analyzed comprehensively. In addition, ANSYS is employed to simulate the lower order natural frequencies and modes of the fiber optic hydrophone unit, whose characteristics are discussed. Finally, the static sensitivity testing of bar-type and sine fiber optic hydrophone units, based on vibration liquid column method and electromagnetism force generator, are carried out to verify the correction of theoretical formulas. The analytical and experimental results are compared each other, the deviation between them is analyzed. Furthermore, the resonant frequencies of bar-type fiber optic hydrophone unit are measured.Based on these studies, the following conclusions can be obtained:1. Analytical results of the static sensitivity of bar-type fiber optic hydrophone unit, which is based on the small deflection bending theory of thin plate, are found to agree well with those of ANSYS numerical simulation, revealing the significant theoretical situation and enough practical value of this expression. The static sensitivity of bar-type fiber optic hydrophone unit decreases with the thin plate thickness and the fiber length increasing. With the fiber length determined, its static sensitivity increases nearly linearly with the diaphragm plate radius increasing.2. The static sensitivity of the sine fiber optic hydrophone increases with the thin plate thickness decreasing, and with the thin plate radius increasing. When the material properties and geometrical parameters of the thin plate, as well as the fiber's effective length are given, its static sensitivity possesses an optimum design value with respect to the characteristic parameter of sine force amplifying mechanism.3. By means of Rayleigh's method, the fundamental frequency of the bar-type fiber optic hydrophone unit is derived. In addition, ANSYS is employed to simulate its natural frequencies, and numerical results are seen to be in good agreement with the analytical results, meaning that this expression can be used to optimize the hydrophone unit design. The fundamental frequency of the hydrophone unit increases with the fiber length decreasing, and with the diaphragm plate thickness increasing. It is affected considerably by the fiber stiffness, and weakly by the fiber mass.4. Experimental results are found to be very close to theoretical results, indicating the practical application value of the bar-type and sine fiber optic hydrophone units, both based upon planar circular thin plate structure. Furthermore, dynamic characteristic experiment of the bar-type fiber optic hydrophone shows that in the frequency range of 0.1-6.5kHz its amplitude-frequency response is plain, with the undulation less than 3dB. It reveals that this hydrophone has excellent frequency response characteristic, and is well applicable to dynamic pressure measurement.