| As the development of the under-water acoustic counterwork and the increasing of the detection need of the submarine, the large-scale and high reliability optical hydrophone array has been an important means of the high performance under-water acoustic detection. The fiber Bragg grating(FBG) hydrophone array based on path-match interferometry not only has the advantages of the FBG, such as small size, high reliability and low cost, but also can achieve high sensitivity as the interference system. It has been a main technology scheme of the large-scale optical fiber hydrophone array. The breakthrough of this technology will establish the base to improve the scale and the reliability of many kinds of under-water acoustic equipments, such as thin-line towed array, large-scale shore-based array and hull array. It will provide more high performance under-water detection equipments to detect the submarine.The key technology of the FBG hydrophone array based on path-match interferometry is to fabricate tens or hundreds FBG pairs with different wavelengths in one fiber, which forms many optical interference cells. It will have very high requirement and difficulty in the fabrication of the FBG, process monitoring and signal processing. In order to satisfy the application requirements of the FBG hydrophone array, this paper began with the basic theory of the FBG. Then the structure of the FBG hydrophone array was designed. The methods to eliminate polarization-induced fading, crosstalk analysis and reduction, technologies to reduce the noise were analysed. Finally, four miniature FBG hydrophones and a four-element FBG hydrophone array were made. The mounting structure of the sensing elements and the basic performance of the array were studied.The main results of this thesis are as follows:1. For the wavelength-mismatch problem of the FBGs, a pre-pull method was used to control the wavelength of the FBG. The principle of this method was theoretically analysed. The relationship of the wavelength change of the FBG fabricated in Corning single mode fiber and the pre-pull was-1.35nm/N. Then the fabrication of the wavelength match FBGs was realizaed taking account of the hydrogen load and annealling, which provided technology support for FBG hydrophone array and guaranteed the normal operation of the optical system. Meanwhile, for the wavelength mismatch between the narrow linewidth laser source and the wavelength of all FBGs in the FBG hydrophone array, a polarization-maintainning FBG(PM-FBG) was used in an optical fiber laser source to choose the output wavelength. The optical spectra of the PM-FBG incidenced by different polarized light were tested and the Gauss distribution of the reflectivity of the PM-FBG and the amuzith of the input polarized light was obtainned. Then the wavelength match, low phase noise and high interference single-longitudinal-mode fiber ring laser was made. Its relative intensity noise(RIN) and phase noise were-120 d B Hz and-115 d B Hz@1k Hz respectively, which could satisfy the practical application.2. Polarization-induced fading(PIF) is a serious problem in the FBG hydrophone array. Traditionally, the Faraday rotation mirrors(FRMs) are used to eliminate the PIF in the interference type optical fiber hydrophone. This method could notbe used in the FBG hydrophone array because of its pecial optical structure. Therefore, finding a suitful and simple method to eliminate the PIF in the FBG hydrophone array is a problem must be solved when it comes to the pratical applications. In this paper, the polarization property at different points of the FBG Fabry-Perot(FBG-FP) system was analyzed. Then the relationships of the visibility and the refraction of the leading fiber, compensated interferometer and FBG-FP cavity were obtained. Finally, methods to eliminate the PIF could be used in this system were theoretically analyzed and experimentally demonstrated. Method to switch the polarizations of the interference pulses can be used to eliminate the PIF, which combined with layer-peeling algorithm can also be used to reduce crosstalk. It is very suitful to the time division multiplexing(TDM) FBG hydrophone array.3. The serious crosstalk induced by multiple reflections in the TDM FBG hydrophone array restricts the multiplexing scale of the array. The equations of the interference intensity of the FBG sensor systems with different multiplexing number were attained. The relationships between the size of the crosstalk and the reflectivity and multiplexing number were theoritically deduced, simulated and experimetally tested. The instability of the crosstalk induced by random phase relationships of the sensors is discovered, theoretically analyzed and experimentally demonstrated using statistical method. It established experimental groundwork for the large-scale FBG hydrophone array.4. For the pratical application of the FBG hydrophone array, the sensing mechanism and encapsulation structure of the air-core mandrel FBG hydrophone were analysed. The acoustic sensitivities of the FBG hydrophones with 18 mm and 12 mm diameters were-149.0d B and-145.7d B respectively and a flattened frequency response was achieved with a fluctuation of less than ±1d B within 2.5 k Hz. Then the smallest FBG hydrophone with 7.4mm inner diameter and 10 mm outer diameter was made. Its acoustic sensitivity is-149.4d B, the fluctuation was less than ±0.6d B within 2.5 k Hz and the level of the equivalent noise spectrum was 47.4d B.5. Finally, a four-element FBG hydrophone array was made and its acoustic sensitivity and crosstalk were tested. The acoustic sensitivity of the elements didn’t change in the array. The crosstalks of the second, third and fourth hydrophone were-39.4d B,-26.7d B and-22.2d B respectively. The design and facture of the low noise, high sensitivity, high reliablility and low cost path-match interferometry FBG hydrophone array were realized. It established the groundwork for the making and pratical application of the large-scale FBG hydrophone array. |
PDF Full Text Request