Investigation On Heterodyne Based High Precision Optical Fiber Sensor Network Technologies And Their Applications

With the development of the networking technology and signal demodulation method for fiber optical sensor network,the sensing resolution and networking capacity of the fiber sensor network have been improved greatly.The optical fiber sensor network have attracted lots of research interesting in several fields of high precision detection requirements,such as geophysical research,seismic observation and marine exploration.Particularly,for geoscience data acquisition,it is required to sense the earth's crustal deformation with a strain resolution less than 10^-9?with dense spatial coverage.The high-precision single-point fiber laser sensing system has demonstrated extremely high measurement resolution,which represents the detection performance limit of fiber sensing technology.However,the sensing capacity of the high-precision fiber laser sensor network is limited by the fiber laser multiplexing technology.Therefore,the sensing precision and sensing capacity of fiber optic micro-vibration sensor network should be further improved to meet the requirements of the geophysical research.This dissertation focuses on the demodulation method and networking technique of the high precision heterodyne based fiber optical sensor network.The main contents include:(1)For the practical requirements of the high precision and large capacity micro acceleration sensor network,an acceleration sensor network based on distributed Bragg reflector(DBR)fiber laser has been demonstrated.The low frequency noise DBR fiber laser is designed and fabricated by structure design and wavelength selection.The T shaped cantilever transducer is introduced to realize high sensitivity and high nature frequency acceleration sensing.for the large sensing capacity,a hybrid wavelength/time division multiplexing sensing network is established.Moreover,the high resolution DBR fiber laser sensor network interrogator is developed based on super heterodyne receiver and digital quadrature frequency demodulation method.The DBR fiber laser sensor network possesses a large sensing capacity of 128,and presents a high acceleration sensing resolution of1.79×10^-8 g/?from 20Hz to 250Hz.The crosstalk between the sensors in the networks is lower than-70 d B.(2)For the amplitude fading noise in the phase sensitive optical fiber distributed strain sensor,a fading suppression method based on the multiple wavelengths multiplexing and differential-vector sum method is introduced.The fading factor and probability in the heterodyne based distributed strain sensor are simulated analyzed,which prove the fading noise can be suppressed by average a lot of Rayleigh scattered signal with independent pulse response.The multiple wavelengths based heterodyne receiver are introduced in the distributed sensor to generate multiple beat frequency signals with independent amplitude fading distribute.Moreover,the differential-vector sum method is used to synthesize the multiple beat frequency signals with different wavelength.In the experiment,a dual laser scheme distributed strain sensor is established,and the fading probability in the 5km single mode fiber is suppressed from 9.4%to 1%based on dual laser scheme.Further,with the help of the scattering enhancement micro-struct optical fiber,dual laser scheme achieve a fading free distributed sensing ability.(3)For reducing the common mode phase noise in the phase sensitive distributed strain sensor,a phase noise suppression method assisted with least square support vector machine(LS-SVM)based hysteresis model is introduced.The system phase noise factors in the distributed strain sensor are analyzed.For compensating the ultra-low frequency noise in the?-OTDR system induced by laser source frequency shift and environment temperature change,an extra identical fiber with SEPs array which is free of strain is used as the reference fiber.Moreover,a hysteresis operator based LS-SVM model is introduced to reduce the thermal hysteresis nonlinearity between the sensing fiber and reference fiber.In the experiment,a distributed strain sensor based on scattering enhancement micro-struct optical fiber is established.The distributed strain sensor present a dynamic strain resolution of 10.5p?/?above 10Hz,and ultra-low frequency sensing resolution of 166 p?at 0.001Hz.It is the first reported a sub-n?resolution in m Hz frequency range along with ultra-large sensing capacity for strain sensor network,to the best of our knowledge.(4)For the application requirement of the high precision data acquisition in geoscience research,the research of distributed micro-vibration field tracking technology for near surface geological structure imaging is carried out.Based on the multiple wavelengths multiplexing fading noise suppression method and the LS-SVM assisted common mode phase noise suppression method,a weak artificial earthquake with 0.7 on the Richter scale is detected by the distributed strain sensor.In addition,the lightweight distributed optical fiber underwater acoustic towing cable for Marine seismic exploration are designed based on the proposed distributed strain sensor and scattering enhanced microstructure optical fiber.The distributed optical fiber underwater acoustic towing cable demonstrate a high sensitivity of-136 d B re(1rad/?Pa).