Research On Distributed Optical Fiber Multi-parameters Sensing Based On Optical Frequency Domain Reflectometry

Optical Frequency Domain Reflectometry(OFDR)is a new method for distributed optical fiber measurement and sensing.When comparing with traditional optical time domain reflectometry(OTDR),OFDR has a lot advantages such as high dynamic range,sensitivity and spatial resolution.However,this OFDR method has its own limits in much more sensing parameters,measurement distance,higher spatial resolution and so on,which make it difficult to extend its application.In this thesis,we focus on some important points in OFDR areas,including phase noise,nonlinear tuning of laser source,system sampling errors and Rayleigh scattering sensing mechanism.We present sensing methods for more parameters based on OFDR.We present methods for improving spatial resolution in long range OFDR and we also present methods for disturbance sensing in long range optical fiber.Finally,we analyze our methods theoretically and verify with experiments.My main work in this thesis are as follows: 1.We present a method for improving the spatial resolution and amplitude by an optimized deskew filter in long range optical frequency-domain reflectometry(OFDR).As the nonlinear phase estimated from an auxiliary interferometer is used to compensate the nonlinearity effect in the beating signals generated from a main OFDR interferometer,the spatial resolution and amplitude of reflection peak in long range(i.e.80 km)are deteriorated by a residual nonlinearity effect due to the estimation inaccuracy of the nonlinear phase.The estimation accuracy of the nonlinear phase is improved by the higher orders of Taylor expansion and high accuracy of estimation of the time delay in the auxiliary interferometer using cepstrum.We experimentally demonstrate that the amplitude of a reflection peak at 80 km increases 20.5 dB and the spatial resolution is up to 80 cm that is about 187 times enhancement when compared with that of the same OFDR trace without nonlinearity compensation.2.We present a 40 km distributed disturbance optical fiber sensor(DDOFS)based on optical frequency domain reflectometry(OFDR).Using the local Rayleigh backscattering(RB)in the optical frequency domain after deskew filter processing,we can extract the position and level information of the disturbance by the cross-correlation analysis of the signals between the disturbed and non-disturbed states.Our measurement range can be up to 40 km long,the location error for disturbance is about 11.6 m to 23.2 m.We also realize the location of two simultaneous disturbances in the test fiber.3.We present a method to realize temperature variation measurement in cryogenic environment(e.g.at 76K)using Rayleigh backscattering spectra(RBS)shift in standard single mode optical fiber by optical frequency-domain reflectometry(OFDR).Our experiments show that at a relatively high temperature(e.g.above 195K),minimal measurable temperature variation is 0.21 K with effective sensing segment size of 8cm.When the temperature is very low(e.g.at 76K),minimal measurable temperature variation can still maintain 0.34 K by simply increasing effective sensing segment size of fiber to 48 cm.4.We present a distributed optical fiber magnetic field and current sensor based on magnetostriction using Rayleigh backscattering spectra(RBS)shift in optical frequency-domain reflectometry(OFDR).The magnetostrictive Fe-Co-V alloy thin films are attached to single mode fiber(SMF).We detect the strain coupled to SMF caused by magnetic field using RBS shift.We measure the range of the magnetic field is from 0 mT to 143.3 mT.The minimal measurable magnetic intensity variation is 12.9 mT when the spatial resolution is 4 cm and it can be improved to 5.3 mT by deteriorating the spatial resolution to 14 cm.We measure the current range from 0 A to 11.8 A and 0 A to 12.14 A at two locations using proposed method and obtain distributed current trace.