Research On Optical Performance Monitoring Methods For Complex Modulation Formats

Fiber communications have seen tremendous growth over the last decade fueled mainly by the Internet traffic growth both in terms of number of users and the bandwidth consumed by each user.To comply with the enormous bandwidth requirements posed by the growth in data traffic,fiber-optic communication networks have evolved drastically.For example,advanced optical modulation formats offering high spectral efficiencies have been successfully employed.Furthermore,dynamic network architectures utilizing reconfigurable optical add-drop multiplexers(ROADMs)also have been employed.In dynamic optical networks,flexible transmitted power,modulation format,bit rate,bandwidth and path have been incorporated to promote better utilization of available transmission capacity.To reduce the operating costs,ensure optimum resources utilization and guarantee adequate operation and management of dynamic optical networks,it is essential to be able to continuously monitor various optical performance parameters.Among various optical performance parameters,optical signal to noise ratio is very important as it can directly reflect the quality of optical link.Therefore,in this paper,we focus on optical signal to noise ratio monitoring.The main research efforts are summarized as follow.1.An OSNR monitoring method based on normalized autocorrelation function is proposed.The method is based on the difference of normalized autocorrelation function between signal and noise.The relation between OSNR and normalized autocorrelation function of noisy signal is analyzed theoretically and evaluated experimentally.The experimental results show that the ultra-large OSNR monitoring range from-10 dB to 25 dB with error less than±0.5 dB is achieved for 32 Gbaud PDM-QPSK signal.To the best of our knowledge,the monitoring range is the largest among all reported methods,which can cover almost all communication scenarios.The experimental results also show that the method is insensitive to environment with good repeatability.In addition,we realize OSNR monitoring without the prior information of the signal by polynomial fitting of normalized autocorrelation function of signal.Experimental results show that the upper limit of monitoring range is extended to 27dB,which is compatible with more complex modulation formats.2.Two optical performance monitoring methods based on neural networks are proposed.First,we propose the combination of normalized autocorrelation functions and neural networks to realize modulation format recognition and OSNR monitoring.Experimental results show that for 28 Gbaud QPSK,16QAM and PAM4 signals,the accuracy of modulation format recognition is 100%and the OSNR monitoring range is from 0 to 30dB with error less than±0.5 dB.In addition,we put forward the method based on the combination of spectra of low-speed sampling signals with neural networks to realize multi-parameter optical performance monitoring,including modulation format recognition,bit rate recognition and OSNR monitoring.The spectra are obtained by Fourier transform of the output waveforms of low-speed photodetector,which can effectively reduce the cost of optical performance monitors.Simulation results show that the accuracy of modulation format recognition and bit rate recognition is 100%for QPSK and 16QAM signals with bit rates of 25 Gbaud,28 Gbaud and 32 Gbaud.The OSNR monitoring range for aforementioned signals is from 9.5 dB to 27.5 dB with error less than±0.5 dB.3.Three OSNR monitoring methods based on interferometry are proposed.Firstly,the guideline of choosing optical delay time to optimize the performance of an interferometry-based in-band OSNR monitor is proposed to reduce the OSNR monitoring error and expand the scope of OSNR monitoring.Experimental results show that the OSNR monitoring range is from 1 dB to 30 dB with error less than±0.5 dB for 112 Gb/s PDM QPSK signal and the method is compatible with different bandwidth of filters,shape of filters,modulation formats and bit rates.Secondly,we propose an OSNR monitoring method based on Lyot-Sagnac interferometer,which can effectively avoid the problem of uneven light distribution in the interference arm of MZI interferometer.By introducing two different delay values to fit the interference characteristics of signals,the OSNR measurement is carried out without signal information.Experimental results show that the OSNR monitoring range is from 7.5 dB to 25 dB with error less than±0.5 dB for 4 X 40 GBaud NRZ-QPSK with 200-GHz interval.Finally,we design and fabricate an integrated in-band OSNR monitor based on asymmetrical parallel-MZIs,which realizes miniaturization and integration of OSNR monitor.Due to the broadband characteristics of MMI,the OSNR monitoring can be applied in WDM system.Experimental results show that the OSNR monitoring range is from 7.7 dB to 24.8 dB with error less than±0.5 dB for 4 X 28 GBaud NRZ-QPSK WDM signals.