Research On Events Analysis And Algorithms In Fiber Distributed Measurement

The principle of fiber distributed measurement system is to detect the stresses change along the fiber, in which fiber is considered to be both the detection element and transmission media. For years, fiber distributed measurement system is discussed widely, for it posses many advantages, such as anti-electromagnetic interference, portable and high accuracy. Two aspects will be studied here, which are events detection and analysis algorithm in distributed measurement system based on Optical Time Domain Reflection and fiber tiny deformation detection and correction in Millimeter wave local oscillator distribution.In Optical Time-Domain Reflection technique, the echo along the fiber (such as Rayleigh scattering, Fresnel reflection, Raman scattering, Brillouin scattering) which is the reflected signal of injected light is measured, so that the physical elements distributed characteristics along the fiber is acheved. For the echo reflects the change of physical elements along the fiber link, it is called distributed measurement.For example, according to backward Rayleigh scattering, the transition loss of fiber can be measured. According to Raman scattering, distributed temperature measurement can be realized. And according to Brillouin scattering, the stress distributed along the fiber can be abtained. Therefore, in this paper, the Optical Time-Domain Reflectmetry (OTDR) is taken for example, and an OTDR event analysis algorithm is discussed by improving OTDR data modeling, which is based on single-edge exponential window of Short Fourier Transform—which is also called Gabor transform and binary signals detection theory. It is verified that it can increase detection rates of the fiber faults in far away with no increase of computation.Traditional OTDR events detection algorithms are based on exponential function modeling, which is weak on long distance events detection. For this, linear modeling is proposed according to the characteristic of the OTDR data, to improve the remote events detection, in which the generation of noise in linear modeling is carefully discussed. What's more the computation is decreased and the events resolution is optimized by carefully picking up the parameters when OTDR data is processed by Gabor transform. By processing the Gabor representation of OTDR data using binary signals detection theory, both location and type of the connection events along the fiber can be detected. Therefore, the algorithm proposed in this paper can increase the speed and efficiency of OTDR events analysis with no loss of accuracy, which means that it is much more applicable.Fiber tiny deformation detection and correction in Millimeter wave local oscillator distribution is another application in fiber measurement. The phase of the light traveling in fiber will drift caused by physical stress outside the fiber. Therefore, for high stability local signal distributed to far away in fiber, the phase of it has to be corrected. Michelson interferometer is widely used in this occasion, according to its high sensitivity, large dynamic measurement range and strong anti-electromagnetic interference. In this paper, a system of phase stabilized distribution of millimeter wave local oscillator based on Michelson interferometer is proposed. According to the theory model and experimental system, it is verified that the proposed system is feasible.For avoiding polarization fading in Michelson interferometer, two anti-polarization fading techniques are introduced in the system, including Faraday rotate mirror added in the end of the fiber and polarization insensitive receiver. The two techniques play an important role in avoiding polarization fading. In this scheme, the millimeter wave is modulated to the carrier light, and then it transmits along a long arm interferometer system. And the shifted phase of light traveling in the fiber will be detected in the receiver. By controlling the fiber stretcher, the phase of local oscillator is corrected. What's more, the experimental system of this scheme is studied carefully. The relationship of phase drift of local oscillator, the length drift of optical fiber and coherent heterodyne output signal are tested. The experimental results are in almost perfect agreement with the theory model, which indicates that the proposed system is feasible. It is a basis for the correction of optical fiber length drift and the transmission of phase stabilized local oscillator by feedback controlling the delay of optical fiber using coherent heterodyne signal.