Research On Characteristics And Application Of Few Mode Fiber

With the continuous development and improvement of fiber manufacturing technology,various kinds of optical fibers which can realize special functions emerge endlessly.The development of optical fiber technology creates basic conditions for many industrie,such as communication and sensor systems.This paper focuses on the characteristics and applications of the few-mode fiber(FMF),including the design of weakly-coupled FMF in the mode division multiplexing(MDM)system and the mode manipulation based on FMF.As we know,single-mode fiber can not meet the demand of communication capacity in the fiber communication.MDM system using FMF has been widely concerned since it can be compatible with the previous multiplexing schemes.In addition,FMF can not only be used as the transmission waveguide in the fiber comminication to expand the transmission capacity,but also be used to make various new devices because of the controllability of mode and the limited mode numbers.The mode manipulation devices based on the FMF have the advantages of simple structure,small volume,high conversion efficiency and good compatibility with optical fiber systems,which lead to it has been widely used in particle manipulation,optical imaging,material processing and MDM systems.This paper,supported by the national "973" project and the National Natural Science Fund,has the following research results:(1)A new type of weakly-coupled fiber is proposed,which solves the mismatch between the mode fields of the elliptical-core fiber and the one of commercial circular-core mode converter(MC)in the current weakly-coupled MDM systems.The effective refractive index differences between the six spatial modes in the proposed fiber are larger than 3.87×10-4 at the wavelength of 1310nm?1550nm.Compared with elliptical-core fiber,the insertion losses are reduced by 2dB and the crosstalk is reduced by 12dB,when the proposed fiber is connected with the circular-core MC.What is more,the parameters(mode intensity distributions,phase distributions,different mode group delays and multi-path interference values)of two-mode fiber and elliptical-core FMF are measured by S2 method.(2)By analogy with the two-dimensional Jones vector,a four-dimensional Jones vector is constructed to represent the higher-order asymmetric modes of the FMF.The mode bases of the Jones vector can be either linearly-polarized mode(LP)or circular-polarization orbital angular momentum(OAM)mode.In addition,we derive the conversion matrix between the two mode bases.Then,the transmission principle of high-order asymmetric modes in the FMF under stress is analyzed.Finally,a four-dimensional Jones matrix based on LP modes is used to describe the processing mathematically.(3)By means of Jones matrix analysis,a scheme,the input polarization state controls the mode orientations of higher order-asymmetric LP modes,is proposed and verified.The scheme consists of a MC,a FMF under stress and a polarizer.In theory,when the phase differences between egienmodes of the FMF satisfy certain condition,there will be a linear relationship between the angle of input linear polarization and the mode orientation of the output LP mode.In the experiment,the mode orientation with tunable 360 degrees for the first-order LP mode and 180 degrees for the second-order LP mode are generated by adjusting the input linear polarization.Based on the above scheme,the all-fiber conversion from Polarization Division Multiplexing to MDM in hybrid optical network is also realized.(4)A scheme is proposed to generate all the modes corresponding to the first-order scalar mode Poincare sphere(PS).Polarization PS and the first-order scalar mode PS are used as auxiliary tools to represent the input polarization states and the output field distributions of a segment FMF under stress.Based on the four-dimensional Jones matrix,we analyze theoretically that how the phase differences between the eigenmodes affect the mapping relationship between the input PS and the output PS.Based on the above analysis,a phase difference equation which makes the input and output PS have a linear mapping relationship is obtained.It is proved that,for a set of given phase differences,the first-order scalar mode PS can be obtained by rotating the polarization PS 90 degrees counterclockwise around S2 axis,S1 axis and S3 axis in turn.In the experiment,the mode on the equator and a meridian of the first-order scalar mode PS is generated by adjusting the input polarization.The conversion efficiencies of all the modes are more than 80%.Both the evolution of the modes and the positions on the first-order scalar mode PS are consistent with the simulation results.(5)Similarly,based on the four-dimensional Jones matrix analysis method,a scheme,generating all the cylindrical vector(CV)modes on the first-order vector mode PS by cascading two segment FMFs under different stresses,is proposed.In theoretical analysis,it is easier to find the appropriate phase differences of two segment FMFs by converting the mode bases of Jones vector from LP mode to circular-polarized OAM mode.In the experiment,eight CV modes are respectively generated on both the equator and one longitude of the first-order vector mode PS.