Application Of Fractional Fourier Transform In Chromatic Dispersion Monitoring

With the development of Internet of things,big data and cloud computing,the demands of data for users is growing.Therefore,optical fiber communication systems are developing towards ultra-large capacity and ultra-high symbol rate.The coherent optical fiber communication system has strong receiver sensitivity,utilizing high-order modulation format to improve the spectrum efficiency,and processing signal by digital signal processing in the electrical domain,thereby being widely used.In coherent optical fiber communication systems,linear damage such as dispersion can be compensated by digital signal processing,and nonlinear effects become the main factors to constrain system performance.In the study of nonlinearity monitoring and compensation,chromatic dispersion is not only an important parameter of nonlinear estimation,but also affects the implementation of nonlinear compensation.Therefore,researching the chromatic dispersion in fiber link is a prerequisite for nonlinearity monitoring and compensation.At the same time,chromatic dispersion is an important indicator of optical performance monitoring,and many compensation methods for chromatic dispersion are based on chromatic dispersion estimation.Thus,chromatic dispersion monitoring is of great significance in the research of optical fiber communication systems.Fractional Fourier transform is the generalization of the traditional Fourier transform and has good energy concentration characteristics for the chirp signal.The chromatic dispersion and nonlinearity in the fiber link can be regarded as chirp in the frequency domain and time domain,respectively.Therefore,the fractional Fourier transform is used for chromatic dispersion monitoring.The research contents of this paper is as follows:1.The definition,properties and numerical calculation of fractional Fourier transform are researched.Compared with traditional Fourier transform,the advantages of fractional Fourier transform are analyzed.The applications of fractional Fourier transform are introduced in the field of optical communication,radar and sonar.2.A optimal order searching method in fractional domain with low computational complexity is proposed.The method divides the searching process into coarse search with fewer sampling points and fine search with more sampling points.Compared with one-step searching method,muti-step sear-ching method can significantly reduce the computational complexity with the same measurement accuracy.The feasibility of the method is verified by measuring the chirp coefficient of the chirp signal.At the same time,the proposed method is applied to analyze residual chromatic dispersion and self-phase modulation optimal compensation,chromatic dispersion estimation in the optical fiber communication systems.3.The rotation characteristics of chromatic dispersion and self-phase modulation on the time-frequency plane are researched.The theoretical formula of chromaric dispersion and self-phase modulation optimal compensation is derived.The correctness of the formula is verified by split-step Fourier method.The 28 GBaud RZ-DQPSK is built to determine the optimal residual chromatic dispersion at the lowest bit error rate with different launch powers,fits the relationship between launch power and optimal residual chromatic dispersion.The fitting relationship is consistent with the theoretical formula.Thus,the theoretical formula is applied to search the optimal residual chromatic dispersion in optical fiber communication systems.4.The limitation of the fractional Fourier transform based blind chromatic dispersion monitoring method in the sampling rate is researched.The fractional Fourier transform based chromatic dispersion monitoring method with ultra-low sampling rate is proposed.The simulation of a 28 GBaud QPSK optical fiber communication system is built.The simulation results show the feasibility and robustness to ASE noise and nonlinear noise.The 20.5 GBaud QPSK system is established in experiments.The chromatic dispersion measurement error is less than 100 ps/nm and 150 ps/nm when transmission distance is 100 km and 200 km,respectively.The feasibility of the chromatic dispersion monitoring method with ultra-low sampling rate has been experimentally verified.