Theoretical And Experimental Research On Sensing Characteristics Of Birefringent Fiber Gratings

Supported by National Natural Science Foundation of China, theoretical and experimental investigations have been carried out systematically on the sensing characteristics of birefringent fiber gratings in this dissertation, with a background of water-depth measurement for fiber-optic hydrophone towed array. The birefringence distributions of both PANDA and side-hole fibers are studied, together with the influence rules of geometry configuration on birefringence. A wavelength scanning method is adopted to measure the fiber birefringence and the results are identical with that of theoretical calculation. Coupled-mode theory is applied to analyze the mode coupling phenomena between polarization-modes in birefringent fiber gratings, and the spectral characteristic of birefringent fiber gratings is identified. Since the conventional wavelength scanning method can't effectively measure the spectra of birefringent fiber gratings, a novel spectrum detection method for birefringent fiber grating, based on polarization detection, is presented. Modeling of birefringent fiber grating as temperature and pressure sensor is completed, together with the analysis of geometry configuration influence on temperature and pressure sensitivities, which is identified by experiments. Two package technologies, axisymmetric package and side-hole package, are applied to single-mode, PANDA and side-hole fiber gratings, which is to study their influences on temperature and pressure sensitivities. The experimental results show that, with side-hole package technology, the pressure sensitivities can be enhanced much greater than with the other. At last, the experimental results of fiber grating as accelerometer, hydrophone and bathometer are presented.The main results and creative points in the dissertation were as follows:1. A systemic research on the birefringence of side-hole fiber is presented. The research results show that the birefringence of circular-core side-hole fiber relies mainly on stress birefringence, its fast axis is parallel to the link line of the two holes, and its intrinsic birefringence increases exponentially with the angle and ellipticity of side holes.2. Side-hole fiber and grating are fabricated for the first time in China. Their birefringence characteristic, temperature, pressure and load sensitivities are studied experimentally. The peak separation of bared side-hole fiber grating has a temperature sensitivity coefficient of -0.05pm/℃, which is only 1/200 that of single-mode fiber grating. The pressure sensing with bared side-hole fiber grating between 0～3.5MPa is realized insensitive to temperature.3. Circular Side-hole package structure is applied to single-mode, PANDA and side-hole fiber gratings. The enhancements of pressure sensitivity are 1750 times, 1702 times and 2688 times respectively. Such experiment results of pressure sensitivity enhancement on PANDA and side-hole birefringent grating are reported for the first time.4. A novel fiber grating hydrophone with side-hole package structure is designed. At the frequency of 500Hz, its sensitivity can reach - 153dB (0dB=1pm/uPa) and the minimal sonar pressure that can be detected is 61 dB (0dB = 1uPa/Hz^1/2 ) .5. Water-depth measurement experiments with two birefringent fiber gratings of wavelength division multiplex were carried out, and the water-depth measurement accuracy achieves better than 0.5m, which satisfies the accuracy requirements of water-depth measurement for fiber-optic hydrophone towed array.