All-optical Wavelength Conversion For Mode Division Multiplexed Superchannels

As the data traffic is growing rapidly in the information age,mode division multiplexing(MDM)has emerged as a new dimension to help break the bottleneck of transmission capacity of conventional single mode fiber communication system and the requirement of higher capacity.On the other hand,in order to realize practical applications,MDM needs to be compatible with present multiplexing techniques,especially wavelength division multiplexing(WDM).All-optical wavelength conversion(AOWC)is a key element in WDM system,which can greatly improve the system flexibility.For multi-dimensional multiplexing system,AOWC needs to be upgraded to a higher level in which multimode operations can be applied instead of single mode only.A channel which contains several multiplexed modes with a single wavelength carrier is called mode division multiplexed superchannel,and combining all the multiplexed modes in a single wavelength channel for AOWC can effectively reduce the blocking probability in WDM systems,thus further enhance the transmission capacity of communication systems and raise the network communication rate.Therefore,investigating on the key issues of AOWC for mode division multiplexed superchannels is one of the important research interests in developing future high-speed communications.Due to the fact that dispersion optimization requires lots of efforts when the number of modes for parallel AOWC in mode division multiplexed superchannels is increasing,it's more difficult to design a multimode fiber which can meet the phase-matching condition for all the modes,thus there's a big challenge for mode scalability.In this study,a novel scheme of AOWC for mode division multiplexed superchannels is demonstrated,which firstly converts the multimode signal into multiple single mode singals,then uses single mode device in each channel to realize single mode AOWC,aiming at addressing the issue of dispersion optimization,and finally combines the single mode signals into multimode signal.Theoretical analysis and experimental research have been presented as follows.(1)Theoretical analysis has been given for the working principle of AOWC based on four-wave mixing(FWM).Mathematical expression of the FWM conversion efficiency has been derived from coupled mode equations.Polarization division multiplexing has been widely used in optics communication systems,which requires the independence ofpolarization of AOWC.Therefore,three schemes of polarization insensitive AOWC have been analyzed and comparisons between them have been made.The effects of frequency detuning and input power variation on the FWM conversion efficiency have been studied based on the scheme which has two pump lights with orthogonal polarizations.(2)An MDM-AOWC system has been constructed based on the FWM effect in semi-conductor optical amplifier(SOA),which utilizes photonic lanterns as MDM devices.In combination with multiple-input multiple-output(MIMO)digital signal processing(DSP),AOWC for six mode division multiplexed superchannels using orthogonal frequency division multiplexing(OFDM)signals with quadrature phase shift keying(QPSK)/16 quadrature amplitude modulation(QAM)has been achieved,resulting in an aggregate bit rate of 104.2 Gb/s for QPSK signals and 185.8 Gb/s for 16-QAM signals.Bit error rates(BERs)for the two signals are below the forward error correction(FEC)hard decision limit and soft decision limit,respectively.(3)The effects of conversion efficiency of each single mode channel and optical signal to noise ratio(OSNR)imbalance on the system performance have been investigated.Experiment results show that a 10 dB fluctuation of conversion efficiency of single mode channel can be tolerated providing that the BER performance of the system is guaranteed.In the case of OSNR imbalance,the system performance deteriorates due to OSNR degradation of single mode channel.(4)Functionalities of downconversion and upconversion in reconfigurable optical add/drop multiplexers have been realized in mode division multiplexed superchannels.Results show that the OSNR penalties of upconversion light and converted light are similar compared to back-to-back case,and this illustrates that the OSNR penalties caused by single mode channel non-linear devices are quite small.