Distributed Optical Fiber Sensors Based on Optical Frequency Domain Reflectometry: A review
Distributed optical fiber sensors (DOFS) offer unprecedented features, the most unique one of which is the ability of monitoring variations of the physical and chemical parameters with spatial continuity along the fiber. Among all these distributed sensing techniques, optical frequency domain reflectometry (OFDR) has been given tremendous attention because of its high spatial resolution and large dynamic range. In addition, DOFS based on OFDR have been used to sense many parameters. In this
... ew, we will survey the key technologies for improving sensing range, spatial resolution and sensing performance in DOFS based on OFDR. We also introduce the sensing mechanisms and the applications of DOFS based on OFDR including strain, stress, vibration, temperature, 3D shape, flow, refractive index, magnetic field, radiation, gas and so on. of the photo-detectors need to be increased. The sensing range will be deteriorated due to the energy weaken by a short pulse width of the input light. The SNR will be also decreased by increasing bandwidth of the photo-detectors. In addition, the peak pulse power of input light in OTDR cannot be increased greatly to restrain the nonlinear effects in FUT . Among all the distributed sensing techniques, OFDR has been given tremendous attention because of its high spatial resolution and large dynamic range. Eickhoff et al. firstly presented the OFDR method by using Rayleigh backscattering of an optical fiber in 1981, which is similar to the technology of frequency modulated continuous wave (FMCW) radar  . A basic OFDR configuration consists of a tunable laser source (TLS) which optical frequencies can be tuned linearly in time without any mode hops and an interferometer that comprises a test path and a reference path. The reference path is considered as a local (LO) oscillator whereas the FUT is connected to the test path. Interferences are generated between the LO signals and back-reflected light in FUT coming from the test path that contains Rayleigh backscattering and Fresnel reflection. The beat frequencies are obtained by a Fourier transform applying to the interferences signals. If the tuning rate of the TLS is a constant, the beat frequencies are proportional to the length of FUT . OFDR is firstly applied to test the components and assemblies in the optical fiber networks at a short range as an order of tens to hundreds of meters, but the spatial resolution of OFDR can range from up to ten micrometers to several millimeters     . Along with the technological progress of the narrow linewidth of TLS and compensation methods of nonlinear phase noise in OFDR, the sensing range can be up to an order of hundred kilometers     . In the distributed sensing methods based on OFDR, in 1998, Froggatt et al. present a distributed high sensitivity strain and temperature sensing method with a spatial resolution at an order of millimeters to centimeters using Rayleigh backscattering spectra (RBS) shifts in OFDR [19, 20] . Rayleigh scattering is generated by random refractive index variations along a FUT, and it can be considered as a distributed, weak fiber Bragg grating (FBG) with random periods. The strain or temperature variations on FUT can result in a local RBS spectral shifts and they are obtained from the cross-correlation between two RBS measurements. One is as a measured RBS and the other is as a referential RBS. Now the sensing parameters in OFDR-based distributed sensing methods have been extended to strain, stress, vibration, temperature, 3D shape, flow, refractive index, magnetic field, gas and so on. In this review, we survey the compensation of nonlinear phase noise in OFDR as a key technology for improving the sensing range, the spatial resolution and sensing performances in DOFS based on OFDR. We also introduce the sensing mechanisms and the applications of DOFS based on OFDR including strain, stress, vibration, temperature, 3D shape, flow, refractive index, magnetic field, radiation and gas.