Research On The Dispersion Control,Characterization And Compensation Of Few Mode Fibers

Mode-division multiplexing（MDM）promises orders of magnitude increase in single-fiber transmission capacity.However,the commercialization of MDM systems has a significant challenge,namely,how to reduce the computational complexity and realize real-time transmission.To achieve this goal,it is necessary to study the dispersion control,characterization and compensation schemes in few-mode fiber（FMF）links deeply.Finally,people can make breakthroughs through fiber design,fiber characterization and improvement of equalization algorithms.Based on this,the main work of the paper includes:（1）In order to reduce the differential mode group delay（DMGD）in FMFs,the global optimization and design of the structure parameters is studied.Based on the trench-assisted graded-index profile,the influence of the relevant geometric parameters on the DMGD characteristics is explored by simulation.Besides,the fiber parameters corresponding to the minimum DMGD have been found.Finally,the DMGD of the six-mode fiber at 1310 nm is less than 7 ps/km and the overall DMGD of another six-mode fiber is less than 52 ps/km in the whole C-band.（2）In order to make iterative feedback on the fiber design and to provide reference for the receiver algorithm,the integrated scheme on the chromatic dispersion and inter-modal dispersion measurement in FMFs based on microwave photonic methods is studied.Firstly,a scheme based on the offset coupling model is proposed,with a 36.43 km long two-mode fiber measured.The high-precision inter-mode dispersion and chromatic dispersion value of the fundamental mode are obtained simultaneously.Then,the scheme based on a free-space photodetector is proposed,with the theory modified according to the orthogonality between different modes.For the first time,high-precision DMGD value and the chromatic dispersion values of each mode are obtained simultaneously.The DMGD accuracy between the two modes is 0.25 ps/km,while the accuracy of chromatic dispersion is 0.0044ps/km/nm and 0.0034 ps/km/nm for LP₀₁ mode and LP₁₁ mode,respectively.（3）In order to further reduce the computational load of MDM system,a frequency domain equalization recursive least squares（RLS-FDE）algorithm is proposed and experimentally demonstrated to accelerate the convergence speed of the receiver equalization.Based on the orthogonality between frequency bins,RLS-FDE with overlap-save method is proposed and applied to the six-spatial-mode experimental datasets from 1000 km FMF transmission,achieving nearly 17.5 times convergence speed increase compared with the frequency domain least mean square algorithm（LMS-FDE）.Then,RLS-FDE is modified by adding a special envelope to the variable forgetting factor.The convergence speed achieves nearly 18.7 times increase than that of LMS-FDE algorithm,while its final convergence performance is also better than LMS-FDE.