Research On Mode Coupling Measurement Technology In Few-Mode Fibers

With the advancement of the society informatization process,bandwidth consumption business,leading to data traffic,emerges constantly such as the Internet of things,large data centers,IPTV and so on.In the past twenty years,data traffic is growing at a rate estimated to 60% annually,and in the foreseeable a few years,the growth trend will not change.It brings huge pressure on backbone which chooses single-mode fibers as transmission medium.With the rapid development of WDM technology,the communication capacity based on single-mode optical fibers has been fully exploring,seeking a new way of expansion is imminent.In order to solve this problem,the new transmission technology is becoming a hot spot to study and compete for in academic and technical fields.Mode division multiplexing(MDM)technology has attracted much attention in recent years because it can break through the traditional single-mode optical fiber communication capacity limits and multiply the capacity of optical fiber communication system.However,with the increase of transmission capacity and transmission rate,the impairment of the few-mode fibers,such as mode coupling and differential group delay,seriously affects the transmission performance of the system.The existence of mode coupling which occurs between the original independent transmission channels leads to channel crosstalk and energy exchange.Mode coupling is a fundamental obstacle for MDM to affect transmission distance and the quality of the signals.Therefore,in this paper,aiming at this problem,mode coupling measurement is studied from the two aspects of theory and experiment.Rayleigh scattering in the optical fiber is firstly introduced,and then,a brief introduction to the transmission properties of the few-mode fibers is described,including its intensity profile distribution,the orthogonality between different modes,and the influence of the mode coupling.Then,according to the characteristic of few-mode fibers,a Rayleigh scattering model of few-mode fibers is established,which theoretically proves the effectiveness of mode coupling measurement by back scattering light.Finally,an experimental system is set up to implement the measurement of mode coupling distribution.Firstly,the Rayleigh scattering in the optical fiber is introduced and the Rayleigh scattering model of single-mode fibers is given,and then,the Optical Time Domain Reflectometer(OTDR)is introduced,mainly including the measuring principle andthe characteristic parameters,which is helpful for the establishment of OTDR in few-mode fibers.Then the mode transmission characteristics of few-mode fibers are introduced.Starting from maxwell's equations,the field structure of the fiber and the orthogonality between modes are deduced,in addition,the mode coupling of degenerate modes and nondegenerate modes is analyzed,finally,according to the characteristic of few-mode fibers,a Rayleigh scattering model of few-mode fibers is established.Secondly,an experimental system is established to implement the measurement of mode coupling distribution.The measuring principle is introduced firstly,on this basis,the mode coupling distribution of 9.8 km long three-mode fibers is measured,through calculating mode coupling coefficient under circumstance of different exciting modes,the symmetry of mode coupling is verified.From the two ends of the optical fibers to measure the distribution of mode coupling,the results are compared with those obtained using a conventional transmission method,the two results agreed well.In addition,the influence of incident light pulse peak power,pulse repetition frequency,pulse width and Fresnel reflex on the results of measurement is analyzed.Finally,the mode coupling between LP01-LP11 modes at splice point is measured,the relation between mode coupling along the fibers and at splice point is deduced by analyzing the mode coupling matrix in different circumstance.We take the measure of some specific location,and then,using the Newton iterative method to compute the value of mode coupling at splice point,agreed well with measurements using the transmission method.