Research On Optical Signal-to-Noise Ratio In Optical Fiber Transmission Systems In High Nonlinear Regimes

With the explosive development of new technologies such as mobile Internet,Internet of Things,cloud computing and big data,the global demand for network capacity continues to grow at a high speed.Optical fiber conmmunication system is the backbone of modern communication network,and it also needs to be continuously expanded.Traditional optical transmission systems based on intensity modulation/direct detection(IM/DD)are simple in modulation and demodulation and low in cost,but they do not contain phase information of signals,which limits the modulation format and transmission rate.In contrast,coherent optical transmission systems contain phase information and can use many spectrum-efficient and higher-order modulation formats to improve spectral efficiency.The high sensitivity of coherent receivers can improve the non-relay distance and compensate the linear transmission damage in transmission optical fibers by using digital signal processing(DSP)technology in the electrical domain.In conclusion,coherent optical transmission system is the first choice for large capacity and long distance transmission system.Optical Performance Monitoring technology plays an important role in maintaining the normal operation of the system and fault management,because it can provide accurate information about the quality of transmission links and the health of optical signals.Optical signal-to-noise ratio(OSNR)is directly related to bit error rate(BER),which can directly measure the transmission quality of the system.Therefore,as one of the important parameters of optical performance monitoring,OSNR monitoring technology has been focused on.Traditional point-to-point wavelength division multiplexing(WDM)systems assume that the noise spectrum is uniform,and the out-of-band OSNR monitoring technology based on linear interpolation can be used by using the channel protection interv al between adjacent wavelengths.With the rapid development of high-speed coherent optical communication system and dynamic optical network,in order to improve the spectrum utilization,the channel protection interval of WDM system is getting smaller and smaller.The out-of-band OSNR monitoring technology based on linear interpolation is no longer applicable,which also makes OSNR monitoring change from out-of-band monitoring to in-band monitoring.In-band OSNR monitoring technology separates signal power and noise power by using different characteristics of amplified spontaneous emission(ASE)noise and signal,and then directly measures noise power to obtain OSNR monitoring value of the system.In coherent optical communication systems,with the increase of transmission distance,more channel power is needed to overcome the loss of optical fibers.However,larger channel transmission power will lead to higher nonlinear effects of the system,so the transmission system environment with high nonlinear effects is inevitable.The traditional in-band OSNR monitoring technology is no longer applicable.Therefore,it is of great significance to study effective OSNR monitoring schemes in high nonlinear regimes.High-order modulation format is used to improve the spectral efficiency of the system.At the same time,higher OSNR is needed.Because of the existence of nonlinear effect,blindly improving OSNR will leads to system capacity reduction and affects system performance.Nonlinear effect limits the capacity of optical fiber transmission system.Therefore,the study of monitoring and suppression schemes for nonlinear effects is a key issue to improve the performance of high-speed and long-distance optical fiber transmission systems.In this paper,a nonlinear effect monitoring scheme based on signal correlation function and correction factor is proposed.The scheme uses signal correlation function and correction factor to represent the nonlinear noise,so as to separate the nonlinear noise from the total noise obtained by the multi-order statistical moment OSNR monitoring scheme,and finally obtains the nonlinear effect monitoring scheme.Then,this paper proposes a nonlinear suppression scheme based on constant transformation for probability shaping of encoders and decoders with fixed length,reversibility and low complexity.Finally,this paper proposes effective OSNR monitoring schemes for single channel PM-QPSK system,single channel PM-16QAM system and PM-16QAM-WDM system in high nonlinear regimes.Details of the research are listed as follows:(1)Research on Nonlinear EffectsResearch contents:With the introduction of coherent translmission system,linear impairments can be compensated by DSP technologies.The capacity of optical fiber transmission system is limited by the nonlinear effect of the system.With the increasing demand for capacity,spectrum efficiency and transmission distance,the nonlinear effect becomes the key obstacle to be solved urgently in the next generation optical fiber transmission system.In this paper,the product of correlation function and calibration factor is used to represent the nonlinear noise in single channel PM-16QAM system,to separate nonlinear noise and the total noise measured by high-order statistical moments based OSNR monitoring scheme,so as to obtain the nonlinear effect monitoring scheme.Compared with the nonlinear effect monitoring scheme based on error vector amplitude correction(EVM-ACF),the proposed method can achieve less nonlinear monitoring errors under the same conditions.At the same time,this scheme explores the formla of calibration factor with transmission distance and transmission launch power.Then this paper proposes for the suppression of nonlinear effects,in high-order modulation systems,a constant-length,reversible and low-complexity encoder and decoder based on probability shaping technology.By using the idea of dictionary-order distribution matching,the 0/1 distribution random sequence with length fixed to k is mapped to a series of n-length symbols with specified probability distribution PA(ai),amplitude value ai.The mapped set of symbolic sequences is sequentially arranged in dictionary-order.Nonlinear noise is simulated by using Gauss noise(GN)model,and multi-channel,multi-span and long-distance optical fiber channel is simulated.The performance of probabilistic shaping and uniform distribution systems is compared,which offers a method for the next better study of the suppression scheme of nonlinear effects.Research conclusions:1.The correlation function of received signals and calibration factor of signal are used to correct the high-order statistical moment OSNR estimation method in a single-channel PM-16QAM system to monitor the nonlinear effects of the system.The formula between calibration factor and transmission distance and transmission channel power is explored and validated.2.This paper presents a constant-length,reversible and low-complexity encoder and decoder for probability shaping,which can effectively suppress the nonlinear effects of the system.(2)Research on OSNR estimationResearch contents:The capacity of optical network is growing steadily,and the optical network architecture is becoming more dynamic,complex and transparent.Optical signals in high-speed optical fiber networks are vulnerable to various transmission link impairments,which change dynamically over time.Therefore,an appropriate monitoring mechanism must be established throughout the optical network to provide real information about the quality of transmission links.OSNR,an direct indicator related to BER,is one of the most important indicators in optical performance monitoring,which can directly measure the transmission quality of the system.The traditional definition of OSNR is the ratio of signal power to ASE noise power.However,in actual optical fiber transmission systems,with the increasing demand for transmission capacity,transmission distance,transmission channel launch power is needed to be enlarged to overcome the optical fiber loss.However,larger channel launch power leads to higher nonlinear effects of the system,which can not be ignored,so that the receiived signal noise includes ASE noise and nonlinear noise in the transmission links.Therefore,it is of great significance to study an effective OSNR monitoring scheme for high-speed long-distance transmission in high nonlinear regimes.Firstly,this paper presents an OSNR estimation method for a single channel PM-QPSK systems in high nonlinear regimes,which uses the correlation function of signal amplitude noise to correct high-order statistical-moments based OSNR estimation method.The scheme uses the product of signal amplitude noise and calibration factor to represent nonlinear noise,and separates the total noise measured in high-order statistical-moments based OSNR estimation method and nonlinear noise to obtain the OSNR estimation.The relationship between the calibration factor and transmission distance and transmission power is proposed and well verified.This scheme realizes accurate OSNR monitoring in high nonlinear regimes.Secondly,in this paper,an OSNR estimation method is presented,using signal correlation function to correct multi-order statistical-moments based OSNR estimation method in a single channel PM-16QAM system in high nonlinear regimes.In this scheme,the product of correlation function and calibration factor is used to represent the nonlinear noise.The total noise measured by multi-order statistical-moments based OSNR estimation method and nonlinear noise are separated to obtain the OSNR estimation errors.The formula between the calibration factor and transmission distance and channel launch power is proposed and verified.This scheme realizes accurate OSNR monitoring in high nonlinear regimes.In addition,the proposed method is applied in PM-QPSK system and PM-16QAM system,while the simulation results are compared and analyzed.Finally,in the PM-16QAM-WDM system with high nonlinearity,the OSNR estimation method based on multi-order statistical-moments is corrected by the auto-correlation function and cross-correlation function of each polarization state and adjacent channel,and the total noise measured by the multi-order statistical-moments based OSNR estimation method and nonlinear noise are separated to obtain the OSNR estimation errors.It is for the first time to explore and prove the formula relationship of weight factor between auto-correlation function and cross-correlation function and the channel launch power,transmission distance and channel number.This scheme realizes accurate OSNR monitoring in high nonlinear regimes.Research conclusions:1.In PM-QPSK system with high nonlinearity,the OSNR monitoring method is presented using correlation function of signal amplitude noise and calibration factor to correct the high-order statistical-moments based OSNR estimation method,and the formula relationship between calibration factor and transmission distance and channel launch power is presented and is well verified.2.In PM-16QAM system with high nonlinearity,the OSNR estimation method is presented using signal correlation function(instead of correlation function of signal amplitude noise)and calibration factor to correct multi-order statistical-moments based OSNR estimation method,and the formula relationship between calibration factor and transmission distance and channel launch power is presented and is well verified.3.In PM-16QAM-WDM system with high nonlinearity,the OSNR estimation method based on multi-order statistical-moments is corrected by using the auto-correlation function and cross-correlation function of each polarization state and adjacent channels.The formula relationship of the weight factor between autocorrelation function and cross-correlation function and the channel launch power,transmission distance and channel number is explored and proved for the first time.In this paper,the influence of nonlinearity on the system is studied based on the nonlinear effects of optical fibers.A new nonlinearity monitoring method for single channel PM-16QAM system is presented,which is based on the product of correlation function and calibration factor to separate the total noise estimated by high-order statistical moments and nonlinear noise;A fixed-length,reversible and low-complexity encoder and decoder based on constant transform is presented to realize probability shaping for nonlinear suppression scheme;An OSNR estimation method based on correlation function of signal amplitude noise and high-order statistical-moments OSNR estimation method is presented in single channel PM-QPSK system with high nonlinearity;An OSNR estimation method based on signal correlation function of signal and multi-order statistical-moments OSNR estimation method is presented in PM-16QAM system with high nonlinearity;An OSNR monitoring scheme based on auto-correlation function and cross-correlation function and statistical moments OSNR estimation method is presented in PM-16QAM-WDM system with high nonlinearity.The results of this paper are valuable for the monitoring and suppression of nonlinearity and OSNR in high-speed optical fiber communication systems.