Research On Performance Monitoring Technology Of Flexible Coherent Optical Transmission System Based On PAM4 Optical Label

With the rapid growth of various high-rate and high-bandwidth services,the data traffic carried by underlying coherent optical transmission system is increasing significantly.Optical network is developing towards more efficient resource utilization,more flexible resource allocation,more complex networking,and more complex network structure.Therefore,the low cost,low complexity and high reliability optical performance monitoring schemes are urgently needed to ensure the efficient and stable operation of flexible coherent optical transmission systems.However,most of the existing optical performance monitoring schemes have many problems,such as high cost,low reliability,single monitoring function and difficulty in large-scale deployment.The monitoring schemes based on optical label "bundling" transmission with service signals naturally has the advantages of high reliability and online monitoring.But there are still some problems such as less carrying information and expensive devices,which are unqualified for the requirements of multi-parameter online monitoring of large capacity flexible coherent optical transmission system.Therefore,this paper focuses on the above problems of optical performance monitoring of flexible coherent optical transmission system,and carries out research on optical performance monitoring technology of flexible coherent optical transmission system based on PAM4 optical labels.The main research work and innovations are as follows.1.In view of the problems of the existing optical performance monitoring schemes based on optical labels,such as simple modulation format,less information carrying capacity,high implementation cost and difficulty in large-scale application,an optical performance monitoring schemes of flexible coherent optical transmission system based on PAM4 optical labels is proposed,which loads PAM4 optical labels through the digital domain,and uses low-bandwidth photodetector to directly detect and receive PAM4 optical labels at the receiver.Based on the research,a simulation platform of 16GBaud PM-mQAM DWDM system was established,and the influence of 4 different modulation formats,different transmission rate of service signals and optical labels with different modulation depths was simulated and analyzed.In addition,a single carrier 1G/2G/4GBaud PM-QPSK/16QAM offline experimental platform was built to verify the influence between flexible modulated service signal and PAM4 optical label,and optimize the key parameters of the optical label monitoring system with flexible modulated service signal.2.In view of the fact that PAM4 optical label has more abundant highorder harmonic components in the spectrum,as well as the frequency and the amplitude of the high-order harmonic components are related to the mean value and sequence characteristics of PAM4 digital optical labels,the traditional optical label power monitoring scheme based on spectrum integration is hard to applicable.An optical power monitoring scheme based on PAM4 optical label is proposed,and an OSNR monitoring scheme is further designed.The simulation results of 16GBaud PM-QPSK/16QAM DWDM show that when the modulation depth is 0.1,four wavelength channel conditions,and the service signals take corresponding the OSNR capacity limit of BER=2E-2,the optical power monitoring error is less than 0.5 dB.When the corresponding noise factor of EDFA is between 4 and 6dB,the optical power monitoring error is less than 0.6dB and the OSNR monitoring error is less than 0.6dB in the case of 25 cross-segment fiber transmission in 4 wavelength channels.The off-line experiment results of 1G/2G/4GBaud QPSK/16QAM for single carrier show that the power monitoring errors are all below 0.3dB under single channel condition.3.In the process of using low-bandwidth PD to directly detect optical labels,PD responds to channel frequency components of all wavelengths in its bandwidth,resulting in background interference from low-frequency components of service signals to optical label demodulation,and then SNR of optical labels deteriorates seriously with the increase of channel transmission number.According to the above reasons,digital label signal is difficult to recover correctly and demodulate reliably.Therefore,an optical label coding scheme based on QC-LDPC is proposed.The proposed scheme takes into account the characteristics of short code length of optical labels and the requirements of low complexity of encoding and decoding,and adopts(3,9,14)QC-LDPC code with the code length of 126 bits to obtain higher encoding gain.Simulation results of 16GBaud Pm-QPSK/16QAM DWDM service transmission and optical label monitoring system shows that when the service signal is set to the OSNR limit corresponding to BER=2E-2,at the modulation depth of 0.15 optical label,QC-LDPC coding can increase the number of monitoring channels supported by optical labels from 10 to 32.