High Performance Fast Distributed Optical Fiber Sensing Based On Optical Frequency-Agile Technology

Distributed optical fiber sensing technology has been used in the large infrastructure,geological disasters,Internet of things system,geophysical exploration and other fields.This research owns great economic and scientific value,which has become a key technology for countries to develop vigorously.Among these,the fast distributed optical fiber sensing technology has become one of the main research hotspots,and has made a series of important research progress.However,there are still many problems that need to be solved in the research of fast distributed fiber sensing,such as the low sampling rate and large data storage space,and need to find effective solutions in the fields of long-range fast,single-end fast and high-sensitivity fast measurements.In this thesis,high performance and fast distributed optical fiber sensing are carried out based on the optical frequency-agile technology,and the main research contents are as follows:Aiming at the problems of low sampling rate and large storage space in the Brillouin optical time-domain analysis(BOTDA)system,this thesis proposes a sensing scheme based on compressed sensing technology,which is able to realize the compressed sampling of the Brillouin gain spectrum.The principal component analysis algorithm is used to construct the transform domain dictionary,so that the Brillouin gain spectrum owns sparsity in the transform domain.Compared with the traditional uniform spectrum sampling with a 4MHz frequency step,the Brilouin gain spectrum can be recovered with 30% data that used in the traditional scheme.It increases the sampling rate by 3.3 times and reduces the storage space by 70%,which greatly relieves the hardware pressure of the system.Aiming at the problem of long measurement time in the field of long-range BOTDA measurements,this thesis proposes a sensor scheme based on the optical chirp chain(OCC)modulation and Brillouin loss scheme.The OCC modulated probe wave is able to offer a wide spectrum amplification on the pump pulse,which can effectively improve the signal-to-noise ratio by compensating the pump pulse transmission loss and increasing the probe wave Brillouin threshold.The Brillouin loss spectrum is measured in each optical chirp duration,so that the measurement time is only limited by the fiber length and the averaging times.Hundreds of kilometers distributed sensing has been completed within several seconds,and the experiment demonstrates that the 150 km distributed measurement time is only 3.2s.In order to improve the sensing performance,the differential pulse-width pair technology is introduced to reduce the measurement error introduced by the optical chirp modulation noise and the distortion of the pump pulse.Meanwhile,the pattern recognition algorithm can be used to achieve the extraction of intrinsic Brillouin frequency shift with a high accuracy.Aiming at the slow demodulation speed of the frequency-scanning scheme in the field of single-end Brillouin optical time-domain reflectometry(BOTDR),this thesis proposes a sensor scheme based on the optical frequency-agile technology,which allows the fast frequency-scanning of the reference wave.After filtering and detecting,the spontaneous Brillouin gain spectra can be fast reconstructed.The specific frequency modulation schemes include the optical frequency scanning and optical chirp chain modulation.Using the optical frequency scanning shceme,a single-ended fast distributed BOTDR system has been experimentally studied with a 172 m sensing fiber and its sampling rate is 62.5Hz limited by the fiber length,frequency-scanning number and the averaging times.The other single-ended fast BOTDR has been studied based on the optical chirp chain modulation and its sampling rate can be increased to 500 Hz on a 400 m sensing fiber,only limited by the fiber length and the averaging times.Aiming at the tradeoff between strain resolution and dynamic strain range in the field of fast distributed fiber measurements,this thesis proposes a dual-mechanism distributed optical fiber sensing by combining the Rayleigh scattering and Brillouin scattering,where the phase-OTDR and BOTDA sensors are enabled by using the same set of frequency-scanning optical pulses.A 6.8n? high-sensitivity relative vibration measurement is realized by the Rayleigh signals,while the Brillouin signal provides an absolute strain information.The measurement time of the dual-mechanism distributed sensing system is limited by the fiber length,frequency-scanning number and the averaging times.A high-sensitivity distributed measurement is experimentally demonstrated with a k Hz sampling rate on a 50 m sensing fiber.