Few Mode Time Domain Reflectometer Based On VC

With the continuous development of new technologies such as big data,cloud computing and 5G,data traffic has shown explosive growth,and the demand for transmission bandwidth has been rising.The capacity of the traditional single-mode fiber communications channel is expected to reach the Shannon-Hartley in the next 20 years.So it is extremely urgent to study new fiber to solve the bandwidth capacity problem.To this end,researchers have proposed a mode division multiplexing technology,which uses the orthogonality between modes to make signals transmit independently through different modes,so as to achieve expansion and solve the problem of insufficient bandwidth capacity in single mode fiber communication system.However,few-mode fibers have their own characteristic parameters,such as mode coupling,mode-dependent loss,differential mode group delay,which is very important to design and optimize the structure of FMF and evaluate the quality of FMF.Therefore,it is necessary to measure these these parameters in practical applications.Optical time domain reflectometer(OTDR)is an important instrument for measuring fiber parameters and evaluating fiber quality.The commercial optical time domain reflectometer available,which can not measure multiple modes simultaneously,is realized based on the principle of backscattering of single-mode optical fibers.If you want to study few-mode fibers,you need to measure different modes of few-mode fiber simultaneously.So,it is necessary to develop a few-mode time-domain reflectometer that can measure the characteristics of each mode in a few-mode fiber.In this paper,the structure of few mode time domain reflectometer is designed,and functions of the hardware and software modules of the few mode time domain reflectometer are elaborated in detail.This software is developed under VC platform.The parameter setting module,data processing module and control display module in software are focused on.At last,a prototype of the few-mode time-domain reflectometer is built to measure 6-mode optical fibers.In this paper,firstly,the mode characteristics of few-mode fibers are analyzed by Maxwell's equations,and the mode orthogonality of few-mode fibers is proved by Lorentz reciprocity theorem.According to the back-trapping theory,the back-direction Rayleigh scattering model of the few-mode fiber is established,which provides a theoretical basis for implementation of few-mode time domain reflectometer.Then,the system structure of the few-mode time-domain reflectometer is introduced,and working process of the few-mode time-domain reflectometer is analyzed.The various components of the hardware system of the few-mode time domain reflectometer are described in detail.The realization of software system of the few-mode time domain reflectometer is highlighted.The software is developed under VC platform.The software implention adopts modular design approach and is divided into initialization module,parameter setting module,data processing module,control display module and data saving module.The parametersetting module,data processing module and control display module are mainly explained.The parameter setting module is focused on the data transmission methods of laser,FPGA,high-speed data acquisition card with PC.The data processing module is focused on the data processing algorithms of the software system,including data average algorithm,Kalman filtering algorithm,and fault location algorithm.The control display module is focused on the method of drawing waveforms,such as coordinate transformation,double buffering,and so on.Next,develop a ptototype of few-mode time domain reflectometer for joint debugging of hardware and software.Measurements were made on 6-mode fibers that are 4-km and 12 km.The performance indicators of the few-mode time domain reflectometer were analyzed.Then change the measurement parameters and analyze the influence of different parameters on the measurement results.By simulating the reflection event and the non-reflection event,the fault point of the few-mode fiber was successfully located.The mode-dependent loss and mode coupling of the fundamental mode and high-order mode of the few-mode fiber are measured.As a result,the function of the few-mode time domain reflectometer is experimentally verified.Finally,the main work of the full text is summarized.The objective analysis of the few-mode time domain reflectometer in this paper is carried out,and looking forward to the optimization direction of the few-mode time-domain reflectometer in the future.