Research On Optical Fiber Link Damage Monitoring Technology Based On Fractional Fourier Transform

With the development of optical fiber communication system in the direction of ultra-long distance,ultra-large capacity and ultra-high speed,the performance of the optical fiber communication system will be affected by the optical signal distortion including optical fiber loss,dispersion and nonlinearity.The maximum transmission distance will be limited by the optical fiber loss.Dispersion will lead to pulse broadening and exacerbate intersymbol interference.Nonlinear will cause signal distortion.With the increasing transmission rate of optical fiber communication system,transmission distance being longer and longer and the signal spectrum being more intensive,optical fiber link damage will be more serious,which will restrict the upgrading of optical fiber communication rate.Fortunately,optical amplifier technology has solved the problem of optical fiber loss.Therefore,the main problem at present are dispersion and nonlinearity in the optical fiber,which brings great challenges to signal quality analysis and measurement.Dispersion and nonlinearity can be regarded as chirp signal in the optical fiber communication system.The chirp signal has obviously energy convergence effect in fractional domain.Therefore,the fractional Fourier transform is an effective tool to study dispersion and nonlinear noise.This dissertation focuses on the monitoring of optical fiber communication system damage with fractional Fourier transform.The main contents and conclusions are as follows:(1)The basic theory and characteristics of fractional Fourier transform are studied.Firstly,the relationship between fractional Fourier transform and traditional Fourier transform is discussed,the basic definition of fractional Fourier transform and the definition of time-frequency plane rotation are introduced and the application of fractional Fourier transform in optical fiber communication are also introduced.It has been found that the chirp signal has a significant energy convergence characteristic in the fractional domain through fractional Fourier transform.Therefore,the fractional Fourier transform is suitable for processing chirp signal.(2)The method to measure third-order dispersion of optical fiber communication system based on fractional Fourier transform was developed.The 112Gbit/s DP-QPSK and 224Gbit/s DP-16QAM high-order modulation optical transmission simulation systems are established.The simulation results show that the method is robust to nonlinear noise and ASE noise,which is of great significance for the monitoring of third-order-dispersion.(3)The method of using fractional Fourier transform to improve the performance of optical fiber communication system has been developed.The fractional Fourier transform has some suppression effects on dispersion and nonlinearity in the optical fiber communication system.Based on the theoretical analysis,the simulation verification is given and the 40Gbit/s and 100Gbit/s OOK and QPSK simulation systems are set up.The signal is fractional Fourier transformed before the receiver.Residual dispersion values and fractional Fourier transform order can be found to get optimal system performance.This dissertation focuses on the monitoring of optical fiber communication system damage with fractional Fourier transform.The method to measure the third-order dispersion based on fractional Fourier transform and the mutual restraint between residual dispersion and nonlinear noise in fractional domain are discussed in the dissertation.The above studies for the optical fiber link damage monitoring is of great significance.