Research On The Mechanism And Monitoring Of Nonlinear Effect In Ultra High Speed Optical Fiber Transmission Systems

The demand of data rate from users is growing since the 21st century.The fast developing of cloud computing,big data and so on also needs expansion and update for the communication networks.Therefore it needs higher transmission rate of the optical fiber transmission networks to fulfill the ever-growing demand for capacity.In traditional intensity modulation/direct detect(IM/DD)based optical fiber communication systems,the number of bits in each symbol is limited.The symbol rate is also limited because of the limitation of instrument bandwidth and sample rate.The system robustness against chromatic dispersion,polarization mode dispersion and other impairments drops sharply if the system has higher capacity.Therefore the IM/DD systems cannot fulfill the demand of communication capacity from users.Compared to IM/DD based optical fiber communication systems,the coherent optical fiber communication systems have better receiver sensitivity.It can also improve the spectrum efficiency by using higher modulation format.The signal can also be processed by digital signal processing(DSP)technology in electrical domain.Because of these advantages,the coherent optical fiber communication systems become the first choice of the optical fiber transmission systems with high speed and long distance.In coherent optical fiber communication systems,the DSP technology can compensate linear distortions well such as chromatic dispersion and polarization mode dispersion.Therefore the fiber nonlinearity becomes the bottleneck of the development coherent optical fiber communication systems.This paper thoroughly investigates the model mechanism,suppression and monitoring solutions of nonlinear effect of optical fiber in ultra-high speed optical fiber transmission systems.This paper first begins with the mechanism and verifies the usability of the Gaussian Noise model.This paper also investigates the optimum symbol rate of sub-channels which makes the fiber nonlinear effect minimum in wavelength division multiplexing systems,which suppresses the nonlinear effect effectively.This paper then utilizes Fractional Fourier Transformation(FrFT)of Linear Frequency Modulation(LFM)pilots,frequency domain extraction of differential pilot(DP)and some other methods,which monitor the chromatic dispersion,Optical Signal to Noise Ratio(OSNR)and nonlinear effect in the optical fiber links effectively.The research contents and innovations of this paper contain two aspects:(1)The mechanism of nonlinear effect:Research content:The nonlinear noise model is the basic of suppression,monitoring and compensation of nonlinear noise.The Gaussian Noise model is the key point and hot topic of the nonlinear noise model research.To verify the usability of the Gaussian Noise model,this paper first verifies the Gaussian Noise model in wavelength division multiplexing systems with three Nyquist channels.The research reveals that using the Gaussian Noise model will makes the estimation more accurate when the system contains pre-dispersion.In wavelength division multiplexing systems,the power of nonlinear noise will change according to different symbol rate of sub-channels and number of sub-channels when the total capacity,total bandwidth and modulation format are fixed.If the symbol rate of sub-channels is chosen to make the power of nonlinear noise minimum,the nonlinear noise can be effectively suppressed and then the system performance can be improved.This paper investigates the symbol rate which makes the nonlinearity minimum for wavelength division multiplexing systems.This paper discusses several parameters which change the power of nonlinearity and these parameters includes phase-matching effect,spectrum gap between sub-channels,number of spans,modulation format and so on.After theoretical analysis and system verification for each of these parameters,this paper finds out the optimum symbol rate of sub-channels,which makes the nonlinearity minimum.The nonlinearity is effectively suppressed and the performance of the system is improved.Innovation:This paper makes comprehensive and detailed mechanism analysis of phase-matching effect,spectrum gap between sub-channels,number of spans,modulation formats and some other issues which affect the optimum symbol rate of sub-channels.The results provide powerful theoretical evidence for the suppression of nonlinear effect in the system and improvement of transmission performance.(2)The monitoring of nonlinear effect:Research content:The monitoring of chromatic dispersion,OSNR and inter-channel nonlinear effect are the important issues to be solved in the maintenance and optimization of optical fiber communication systems.Chromatic dispersion is one of the most important influencing parameter in the model,suppression,monitoring and compensation of fiber nonlinearity.The monitoring of noise in optical transmission system is an important aspect of system maintenance and optimization.In optical transmission systems,the noise in the received signal mainly includes the Amplified Spontaneous Emission(ASE)noise and nonlinear noise of the link and the monitoring of ASE noise corresponds to the monitoring of OSNR.Among all kinds of nonlinear effect,the intra-channel nonlinear effect can be compensated by algorithms.Therefore the monitoring of inter-channel nonlinear effect becomes the key point and difficulty of the monitoring of nonlinear noise.In former inter-channel nonlinear effect monitoring solutions,the solution by angular squeezing of DP is the most typical method.However,its complexity is too high and it underestimates the power of inter-channel nonlinear noise.This paper first proposes a chromatic dispersion monitoring solution based on FrFT of frequency domain LFM pilots.This solution first inserts two LFM pilots with different center frequencies in frequency domain.After conducting FrFT of the LFM pilots at the receiver,the time delay is measured by measuring the peak position difference of two LFM pilots at the fractional domain.Then the chromatic dispersion can be monitored.The results reveal that this chromatic dispersion monitoring solution has good robustness against noise.This solution can also be achieved by instruments without DSP.The monitoring resolution and monitoring range can be changed flexibly.It can be used in dynamic reconfigurable optical networks to estimate chromatic dispersion as well.This paper then proposes a joint multiple system parameters monitoring solution of chromatic dispersion,OSNR and inter-channel nonlinearity based on FrFT of time domain LFM pilots.This solution inserts two LFM pilots in the front of the frame of the channel under test and the center frequencies of the two LFM pilots are different at two polarizations.The LFM pilots from two polarizations at the receiver are processed by FrFT respectively.The value of chromatic dispersion can be obtained by the measuring the peak position difference in fractional domain.After compensation of the chromatic dispersion,the power of ASE noise is measured in frequency domain and the OSNR can be calculated.Then the LFM pilots are transformed into fractional domain by FrFT.The power of the residual signal after removing the peak in fractional domain can be treated as the combination of power of inter-channel nonlinearity and power of ASE noise.By subtracting the power of ASE noise measured in frequency domain from this combination of power,the power of inter-channel nonlinearity can be obtained.By the above steps,this solution realizes anti-noise chromatic dispersion monitoring,anti-nonlinearity OSNR monitoring and highly accurate inter-channel nonlinearity monitoring.At last,this paper proposes a monitoring solution of inter-channel nonlinearity based on frequency domain extraction of DP.First the channel under test is inserted by DP in the front of the frame.The ASE noise is measured at the frequency zone which is far away from the received DP in frequency domain.Therefore the total power of ASE noise in the channel can be calculated.After removing the peak in frequency domain,the combination of the power of inter-channel nonlinearity and the power of ASE noise can be obtained.Then the power of inter-channel nonlinearity can be obtained after subtracting the total power of ASE noise in the channel.Compared to the inter-channel nonlinear effect monitoring solution by angular squeezing of DP,this solution estimates the inter-channel nonlinear effect more accurately and has lower computational complexity.Innovation:1.This paper innovatively proposes chromatic dispersion monitoring solution by FrFT of frequency domain LFM pilots.2.This paper innovatively proposes chromatic dispersion,OSNR and inter-channel nonlinear effect monitoring solution by FrFT of time domain LFM pilots.3.This paper innovatively proposes an inter-channel nonlinear effect monitoring solution by frequency domain extraction of DP.This paper begins with the mechanism of fiber nonlinearity and investigates the usability of Gaussian Noise model.The symbol rate of sub-channels which makes the nonlinearity minimum is analyzed,which suppresses fiber nonlinearity and improves the performance of the system.At last,this paper proposes several chromatic dispersion,OSNR and inter-channel nonlinearity monitoring solutions,which evaluate the parameters of the system effectively.These solutions also provide important support for the maintenance and optimization of the ultra high speed optical fiber transmission system.