Investigation Of Multi-wavelength Regeneration And Key Transceiver Technologies For New Kinds Of Optically-modulated Signals

Modern optical transport network(OTN)is evolving towards desired features such as large capacity,high bit rate and network intelligence.Large capacity is enabled by wavelength-division-multiplexing(WDM)and high spectral-efficiency modulation.High date rate relies on all-optical signal processing devices and low-complexity digital signal processing(DSP)algorithms.Network intelligence is reflected in the flexibilities of switching nodes and optical transceivers.The realization of these features largely relies on new kinds of optically-modulated signals,which refer to all kinds of complex multidimensional modulation formats different from conventional two-level amplitudemodulated signals.This thesis mainly investigates multi-wavelength optical regeneration and several key transceiver technologies for new kinds of optically-modulated signals,including all-optical regeneration of multi-wavelength time-interleaved signals,electrical DSP compensation for fiber nonlinear crosstalk in polarization-division-multiplexing(PDM)-64 QAM signal transmission,and the adaptability of time domain hybrid QAM(TDHQ)signals to different channel conditions.The main content and innovations are summarized as follows:1.Nonlinear crosstalk of multi-wavelength channels and all-optical regeneration technologiesDense WDM is crucial for increasing the capacity of OTN,and all-optical regeneration is an important way to suppress inter-channel nonlinear crosstalk of multiwavelength channels.1)A theoretical model of multi-wavelength four-wave mixing(FWM)crosstalk is built,and the origin of different kinds of crosstalks in multiwavelength regeneration is analyzed.The characteristics of three crosstalk suppression methods,including time-interleaving,polarization multiplexing and bidirectional transmission,are investigated by simulation and experiment.It is shown that the three methods are closely associated with pulse duty cycle,fiber birefringence and the isolation degree of optical isolator,respectively.2)Based on the symmetry of the power transfer function for data pump FWM,a polarization-insensitive method for improving extinction ratio is proposed and applied to our experiment,in which the polarization diversity and bidirectional transmission are used to suppress crosstalk.It is demonstrated that,for randomly polarized signals,the extinction ratio(ER)is increased by 4.2d B and the Q factor is improved from 5.8 to 6.9.3)Combining all three crosstalk suppression methods,six-and eight-wavelength 2R regeneration experiments are conducted and 0.8-d B sensitivity improvement are obtained over all channels for time-interleaved on-off keying(OOK)signals.The limiting factors for further increasing the amount of regenerative wavelengths is analyzed and the regenerative capacity bounded by crosstalk is given by simulation.2.Low-complexity DSP-based nonlinear compensation algorithm for PDM-64 QAM signalsBesides suppressing the crosstalk in optical domain,a low-complexity DSP-based nonlinear compensation algorithm is proposed to compensate for the intra-and interchannel nonlinear crosstalk induced by self-phase modulation(SPM)and cross-phase modulation(XPM)for multi-level signals in high-capacity short-reach transmission.Unlike the conventional compensation method of alternately computing in time and frequency domains,the proposed algorithm is implemented between the chromatic dispersion compensation block and the adaptive equalization block in the optical receiver,which requires only a single nonlinear phase compensation by taking into account the dispersion-induced symbol correlation.It is shown that,the performance of nonlinear compensation can be improved by intentionally keeping some residual dispersion,and the complexity of DSP algorithm can further be reduced by using rectangular finite impulse response(FIR)filters.Using 35 Gbaud PDM-64 QAM signal as an example,the single-wavelength experiment and multi-wavelength simulation are conducted,which show that >60% and >40% nonlinear crosstalk can be suppressed,respectively.3.The link adaptability of time domain hybrid QAM(TDHQ)signals and the provisioning of flexible transceiversFlexible optical transceivers can adapt to diverse link conditions by changing the symbol rate,modulation formats and other parameters for the largest system capacity or transmission distance.Researches have shown that,fixed-rate transceivers can achieve the largerst transmission distance by optimizing the combination of symbol rate and modulation format in few specific cases.In this work,the adaptability of independently variable symbol rate and modulation format signals to channel noise and optical bandwidth is investigated,in which TDHQ enables a quasi-continuous tradeoff between modulation format and link noise,and the variable symbol rate balances the signal optical bandwidth and the dynamic filtering effect of cascaded reconfigurable optical add-anddrop multiplexers(ROADMs).The TDHQ signals are formed by time-interleaving different QAMs on a symbol by symbol basis.For the optical transceivers operating at different modes,the system capacity is investigated by simulation and experiment.It is shown that,compared with the current commercial transceiver with variable standard QAMs at a fixed symbol rate,the average capacity improvements can reach up to >10.4%,>14% and >17% using flexible transceivers with flexible TDHQs,flexible symbol rates or both features,respectively.A transceiver provisioning method based on artificial neural network(ANN)is proposed,and the feasibility is further verified by simulation.The proposed method uses the monitoring parameters from the coherent receivers as the input of ANN to predict the optimal symbol rate and modulation format for further provisioning the flexible transceivers.