Research On Strain Demodulation Method Of Optical Frequency-domain Reflectometry

Nowadays,due to the fast development of sensing technology towards intelligent and networking applications,distributed optical fiber sensors have attracted intensive research attention because of their superior advantages,e.g.,immunity to electromagnetic interference,stability and short response time,and continuous monitoring capability.As a branch of distributed optical fiber sensing technology,optical frequency-domain reflectometry(OFDR)has been widely used in many fields,such as optical fiber communications,petrochemical industry,electrical power industry and biomedicine,due to its high signal-to-noise ratio,high sensitivity and spatial resolution.For example,OFDR distributed system can be used for faulty detection over optical fiber link,and realize distributed measurement of temperature,strain or other physical quantities.It has very high spatial resolution,which can reach millimeter level.The main contents of this thesis are as follows:Firstly,study the sensing principle based on Rayleigh backscattering in optical fibers,and set up an OFDR system.In order to compensate measurement errors caused by nonlinear wavelength tuning from the laser source,we adopt an auxiliary interferometer whose output is used as trigger signals for data acquisition to ensure equal frequency sampling;the polarization diversity scheme is also utilized in the system to eliminate the polarization fading effect.Then,complete programming to control the tunable laser source and data acquisition card,and realize the function of optical link quality monitoring in the OFDR system.The sensitivity of strain and temperature of the sensing system are also calibrated.In addition,we have investigated the system's measurement accuracy variations with respect to different spatial resolution.We conclude that the spatial resolution of our OFDR system reaches2.7mm with a strain accuracy of 1.19??and temperature accuracy of 0.14~oC.The GPU is used to speed up the signal processing of the OFDR system.The running time of data processing is reduced from a few minutes to less than one second,which enables the real-time strain measurement.Finally,we investigate the OFDR measurement accuracy under external vibrations,as this situation is very common for practical applications.We find out that the measurement errors are significantly enlarged when the leading fiber is under vibration disturbance.We also study the course of this problem in detail.