Optical Frequency Domain Reflectometry (OFDR) is a relatively new directionfor sensor developments. Comparing with traditional Optical Time DomainReflectometry (OTDR) method, OFDR has the advantages of high signal to noiseratio, high spatial resolution, and high sensitivity and so on. However, OFDR has thedeficiencies in the measurement range, the spatial resolution and the sensingperformance at long measurement distances, which limit the scope of its applications.This dissertation focuses on solving several difficult problems, such as phasenoise, laser tuning nonlinearity, and polarization effects, and finding a sensingmechanism using fiber Rayleigh scattering information. It also presents several novelmethods for improving measurement range, spatial resolution and sensingperformance at long measurement distances, as well as their theoretical analysis,experimental verifications and system implementations. The main contents are asfollows:1) By analyzing OFDR signal and laser phase noise, a method for measuringtunable laser dynamic line width is presented. In addition, another method ispresented to achieve a long range OFDR system beyond laser coherent length byincreasing the laser frequency tuning speed while measuring the phase noise of thereflected optical signal. With this method, we can measure Rayleigh scattering at adistance of120km and Fresnel reflections at a distance up to170km with areflectivity of-14.6dB and a spatial resolution of200m.2) By analyzing the mechanism of the nonlinear tuning on OFDR signal, twomethods are presented to compensate the tuning nonlinearity of a TLS in an OFDRsystem by using non-uniform FFT and the deskew filter respectively. Theexperimental results demonstrated a factor of100times enhancement to the spatialresolution of the same OFDR system without nonlinearity compensation. Usingdeskew filter, our system can detect a Fresnel reflection reflectivity down to-55dB ata distance of80km and an impressive spatial resolutions of just1.6m. This methodhas a high computational efficiency and makes real-time measurements possible.3) A phenomenon called polarization-induced sidebands (PIS) in Rayleighbackscatter spectra is described. Using a special polarization diversity detection scheme, this method successfully removes PIS and achieve accurate distributed strainsensing in the range of0.75to225in a50m standard single mode fiber.4) A distributed vibration sensing method based on a correlation analysis of OFDR’sRayleigh scattering signals is presented. Using this method, our system can achieve ameasurable range of12km and a measurable vibration frequency up to2kHz with aspatial resolution of5m. Moreover, the preliminarily measurement result of thefrequency and location of two vibration events located at different positions along thetest fiber are also presented. |