The use of long-period fiber grating bending sensing rules of the fiber-optic hydrophone

A certain value, a method to analyze bending sensing rules of long-period fiber grating (LPFG) according to bending curvature, grating length and grating period was reported, based on coupled-mode theory and perturbation approximation. The transmission spectra of the bended LPFG split when the bending curvature reaches a certain value. The two resonant wavelengths of the splitting peaks change conically with the bending curvature and inversely with grating period, but are independent on grating length. The LPFG written on the B-Ge co-doping single-mode fiber proves the above conclusion. The two resonant wavelengths of the splitting peaks change conically with the bending curvature consist of fiber grating strain sensor. It has following characteristic. Firstly, resonant wavelength transforms into the frequency shift through the electro-optical heterodyne, may cause the ordinary optics wavelength survey to transform as the electricity frequency survey, improve the detection and resolution gain five or more orders of magnitude. Secondly, the two split resonant peak wavelength difference frequency is possible to obtain the higher examination sensitivity. Thirdly, use the differential motion structure to eliminate the environment disturbance. Fourthly, transmission peak sensor is suitable to consist of the small array sensor. After a resonant wavelength heterodyne photoelectric conversion for the frequency change, can make an ordinary optical wavelength into electricity to detect the frequency of detection, so that detection and resolution gain through more than five orders of magnitude. 2 the two split resonant peak wavelength difference frequency of changes in detection sensitivity can be higher. 3 Differential structure suitable for the elimination of environmental interference. 4 transmission peaks constitute a sensor suitable for small size of the sensor array. The above characteristic is suitable to constitute the new optical fiber array hydrophone. Many long peri- od fiber grating through elastic carrier curving in appropriate fixed angle, after the seal, uses as the optical fiber hydrophone array. Array works not only in space division multiplexing way, but also work in the wavelength division multiplying and the multiplying active status. The different bending strain may cause the fiber grating bias at the different wavelength, can use the optical fiber phase-shift diffraction grating separate the wavelength. Detector´s connection optical fiber length to different sensor is different, therefore the signal arrives at different time, is advantageous to the multiplying time sampling. The use of photoelectric heterodyne method of long-period gratings on the transmission spectrum of detection not only has high sensitivity but also eliminates polarization effects. Two split peaks due to resonant wavelength changes with the bending curvature of the relationship was quadratic curve relationship, the use of photoelectric heterodyne detection frequency, the frequency range up to 1012 Hz, the frequency resolution of 10 Hz. By LPFG right, photovoltaic devices and microwave heterodyne frequency meter combination can solve the interferometric fiber-optic hydrophone polarization effects and the problem of inadequate resolution, consisting of long-period grating fiber-optic hydrophone size is very small, very convenient to install. In addition the use of long-period fiber grating (LPFG), can constitute an earthquake, bridge monitoring, fatigue testing of aircraft and other important sensor, with wide applications.