Research On Embedded Few-mode Optical Time Domain Reflectometer

With the rapid development of information technology,the demand for bandwidth in all walks of life is higher and higher.Because Shannon's limit theorem limits the capacity of optical communication network based on single-mode fiber,revolutionary new technologies are urgently needed to solve the current network capacity challenge.A mode division multiplexing technology based on few mode optical fiber emerges as the times require.The technology uses orthogonal spatial modes as independent propagation channels,which can improve the capacity of optical communication system.However,the unique damage characteristics such as mode dependent loss,mode coupling and differential mode group delay will affect the transmission capacity of the mode division multiplexing technology and reduce the transmission distance of the mode division multiplexing system;In addition,the transmission characteristics of different spatial modes are different,and the spatial modes will interact with each other due to crosstalk,which makes it difficult to detect the link failure of the few mode fiber.Therefore,it is particularly important to study the damage measurement technology and fiber fault detection technology of few mode fiber.It is very important to optimize the structure of the few mode fiber,provide data support for the damage compensation of the mode division multiplexing system,and improve the transmission capacity of the optical communication system.At present,pulse response technology,wavelength scanning interferometry and back Rayleigh scattering measurement technology are the mainstream damage detection technologies of few mode fiber.In this paper,an embedded few mode optical time domain reflectometer is developed based on the measurement technology of back Rayleigh scattering.The principle is to use the back Rayleigh scattering effect of optical pulse in the transmission process to obtain the fiber damage and fiber structure parameters.Based on the theoretical analysis of the spatial mode orthogonality and the back Rayleigh scattering characteristics of the few mode optical fiber,the hardware platform of the embedded few mode optical time domain reflectometer is built respectively,and the human-computer interaction software of the embedded few mode optical time domain reflectometer is developed.The dynamic range,the frequency and the frequency of the embedded few mode optical time domain reflectometer are analyzed The performance of the instrument in fault location and measurement mode coupling,differential mode group delay and mode dependent loss is verified.The main contents and innovations of this paper are as follows1.Aiming at the problem that the minority mode OTDR needs to measure the transmission characteristics of different spatial modes,this paper proposes an optical fiber fault detection system with photon lantern and minority mode circulator,which uses photon lantern to realize mode multiplexer and mode demultiplexer.Finally,experiments show that the system can excite multiple spatial modes in the minority mode optical fiber,The different spatial modes in the few mode fiber are separated,which lays a foundation for the subsequent damage detection and link fault diagnosis of the few mode fiber.2.Mode dependent loss,mode coupling and differential mode group delay are unique to damage detection of few mode fiber.In this paper,the theoretical model of back Rayleigh scattering based on the few mode fiber is established firstly,and then through the in-depth study of the damage mechanism of the few mode fiber,the relationship between the mode dependent loss,mode coupling and the slope of the back Rayleigh scattering curve of the few mode fiber is deduced theoretically,and the detection results of the above damage parameters are obtained,while the differential mode group delay is obtained by the time-of-flight method.Finally,experiments show that the system can achieve the purpose of measurement,and realize the original design intention of multi-mode and multi parameter measurement.3.In the process of optical fiber link fault diagnosis,Kalman filter algorithm is used to remove the noise in the measurement data.The simulation and final experimental results show that the algorithm can keep the original transformation trend of the measurement curve,make the curve smooth,and improve the fault diagnosis accuracy of the few mode OTDR.4.Aiming at the problems of large volume,high power consumption and high cost of current few mode OTDR.In this paper,the embedded device is used as the data processing and human-computer interaction platform of the few mode OTDR.The specific model is zynq-7000 So C Hardware Platform of Xilinx company.The final experimental results show that the embedded device can meet the design requirements and is more portable.