Research On High-performance Distributed Optical-fiber Vibration Sensing Technology

Distributed optical fiber sensors(DOFSs)are sensors that employ the optical fiber to detect measurands and transmit light.With the merit of simultaneously doing measurements over the whole sensing fiber,DOFSs can be used in many industrial areas.For example,DOFSs can be used in health monitoring in civil engineering where vibration events need to be detected.Vibration applied to the fiber can modulate the refractive index and can further modulate the phase of light propagating in the fiber.By finding the additional phase term in the optical field,the detecting of vibration event can be made.There are 2 main methods to make distributed vibration sensing,the time-domain method and the frequency-domain method and they are both studied in this thesis.The frequency-domain method here is based on optical frequency domain reflectometry(OFDR).In the OFDR system,the frequency-tuned light was produced by external modulation.Firstly,the light emitting by laser passed a phase modulator and then a side band was filtered by a optical band pass filer.With the frequency modulated continuous wave technique,linewidth of the laser was measured.With the OFDR,a reflection at a distance of 4.1km could be detected with spatial resolution of 0.7m.A vibration event with frequency of 30kHz occurring at a position of 4.1km was detected and the detection worked for different amplitude.The influence of the size of frequency windows in the frequency recognition procedure is discussed.The other method,time-domain method is based on phasesensitive optical time domain reflectometry(?OTDR).The heterodyne detection and phase demodulation was used to realize high-performance vibration detection.A vibration event with frequency of 300Hz was detected at a distance of 23km.The ability of quantitative measurement of amplitude and distance-independent detection was demonstrated.To reduce the performance degrades causing by interference fading,a dual-wavelength source was used in ?OTDR and an experiment was done to demonstrate it.For vibration measurement,the frequency-domain method mentioned in this thesis has a wide response band while the time-domain method has the ability of quantitative measurement of am-plitude.The 2 methods have their applicative occasions,respectively.