Research On Key Technologies In Time And Wavelength Division Mltiplexing Optical Access Network

Passive optical networks(PONs)have been widely considered as the ultimate solution for future access networks.Nowadays,time division multiplexing(TDM)based Gigabit Ethernet PONs(GE-PONs)and Gigabit-capable PONs(GPONs)have been standardized and commercialized in several countries.Based on the ITU-T G.984 GPON standard and the IEEE 802.3ah GE–PON standard,GE-PON and GPON can deliver signal with a data rate up to 1.25/1.25Gb/s and 2.5/1.25Gb/s for downstream/upstream directions respectively.Since the whole bandwidth is shared by all users in the system,the bandwidth that each user could get access to is only several Mb/s.Meanwhile,a large number of newly emerging bandwidth-intensive and multimedia-rich applications such as video-on-demand,video/photo sharing,3-D TV etc are driving up the bandwidth demand of end-users,which will eventually outstrip the gigabit technologies and push forward the network upgrading progress.Therefore interested groups such as Full Service Access Network(FSAN),IEEE and ITU SG-15 have made efforts to draw the draft for next generation PON(NG-PON).The 10GE-PON and XG-PONs that standardized in 2009 are the first crops of upgrade systems,which supply a line rate no more than 10Gb/s in both directions.And closely follows the step of NG-PON1,the FSAN proposed the conception of NG-PON stage 2(NG-PON2).The standardization of this system is still in process and is expected to be completed soon.The basic requirements for the system include an aggregate capacity no less than 40-Gb/s and remains backward compatibility.In recent years,researchers had reported various evolving technology options for NG-PON2,including 40 G TDM-PON,WDM-PON,TWDM-PON and OFDM-PON.However,considering the backward compatibility and technical maturity,TWDM-PON was selected by FSAN as the primary architecture for NG-PON2 in Apr 2012.It consists of multiple XG-PONs stacked onto a common ODN using different wavelengths,which combines the benefits of TDM-PON and WDM-PON.If four wavelengths are used,the bandwidth would be four times of the XG-PON.TWDM-PON features plenty of inherent advantages,such as statistical sharing of bandwidth,i.e.customers can flexibly get access to a bandwidth ranging from several Mb/s to a peak of 10-Gb/s;backward compatibility,i.e.the splitter based ODN can be fully reused for TWDM-PON system,which reduces the cost and complexity in construction.However,despite that the technologies of XG-PONs are quite mature and can be directly used in TWDM-PON constructing to cut the implementation period,there are still some technology challenges to be addressed,which will be presented and discussed in detail in the following sections.The contents of this paper is organized as below:1.Experimental demonstration of N×10/1.25-Gb/s TWDM-PON system using RSOA based fiber ring laserIn TWDM-PON system,as the splitter is used in the ODN to distribute power,every ONU would receive signals of all wavelengths,so that each ONU must contain a device that could correctly select the downstream wavelength.Similarly,for the upstream deployment,ONU that will work on any wavelength,i.e.colorless ONU is intensively preferred by operators for easier network laying and maintaining.Therefore,optical network unit(ONU)is the key for TWDM-PON consruction.We propose a novel ONU configuration in which a single tunable optical filter(TOF)with relaxed requirements is used for both selecting downstream wavelength and generating upstream colorless light source.The proposed configuration is totally compatible with the traditional TDM-PON without making any change to ODN and the colorless ONU is realized by using directly modulated reflective semiconductor optical amplifier(RSOA)based fiber ring laser.The transmission performance is experimentally investigated,which shows a stable performance within the whole operating bandwidth and the immunity to back reflection and Rayleigh backscattering(RBS)induced crosstalk was demonstrated due to the self-phase modulated wavelength offset in RSOA.2.Experimental demonstration of symmetric 40Gb/s TWDM-PON using spectral-reshaped directly modulated laserLimited by the carrier lifetime,the modulation bandwidth of RSOA is generally limited to ~2GHz.For higher upstream data rate scenerios,we propose to use thermally tuned directly modulated laser(DML)as upstream laser souce.In order to mitigate the signal distortion caused by the interaction between laser chirp and fiber chromatic dispersion,we use optical filter to reshape the signal spectrum,thereby realizing 25 km fiber transmission of 10-Gb/s upstream signal.Meanwhile,the filter can be used for downstream wavelength selection,which further reduces the cost of ONU.As a result,4×10 Gb/s TWDM-PON is experimentally demonstrated with 25 km reach and 31 dB loss budget.For further loss budget improvement,we can remove the spectral reshaping filter to the OLT side,thereby eliminates the insertion loss of the filter and enables a higher launch power.The loss budget is increased to 39 dB in the new network architecture.Furthermore,we propose to use an SOA for upstream signal boosting.Due to the gain saturation and SPM effect induced OOK-BPSK format conversion,the pattern effect in SOA is eliminated.As a result,the loss budget for symmetric 40Gb/s TWDM-PON is increased to 53 dB.3.DM and DD based long reach PON supporting 100-km reach and 53-dB loss budgetWe demonstrate a long reach PON,where the reach distance is increased to 100 km and the loss budget is improved to 53 dB.Experimental results show that due to the frequency chirp induced spectral broadening,the directly modulated signal has a lower carrier component,thereby shows a higher tolerance to fiber nonlinearity.As a result,the launch power of directly modulated signal can be as high as 20 dBm while only 14 dBm is aloowed for externa modulated signal.Besides,by using a signle DI for multi-channel spectral reshaping,the chirp management of all upstream and downstream channels are realized.Finally,thanks to the high launch power and terminal amplification,we succesfully demonstrate 100-km transmission of symmetric 40-Gb/s TWDM-PON with 53-dB loss budget.No extender in the ODN is required,which is the record result of direct modulation and direct detection based long reach PONs.4.4×25-Gb/s TWDM-PON enabled by 10-GHz devicesIn order to realize a low cost and smooth capacity upgrade,we propose to use 10-GHz devicecs to realize 25Gb/s or higher modulation and detection.Serveral candidates for 100 G EPON are propsoed,including 25Gb/s OOK,28 Gb/s Duobinary and 25Gb/s PAM-4 formats.1)Taking advantage of the low-pass characteristic of commercial 10-GHz devices,the direct modulation and detection of 28-Gb/s duobinary signal is realized.In order to mitigate the chromatic dispersion induced signal distortion,an optical delay interferometer(DI)is employed to narrow down the signal spectrum,thereby realizing 40-km single mode fiber(SMF)transmission in C-band.2)The feasibility of using DI for the chirp management of 25Gb/s PAM-4 signal is investigated.Experimental results show that 40-km fiber transmission can be supported by using directly modulated laser combined with DI based spectral reshaping.3)By employing a delay-interferometer for simultaneous frequency-equalization and chirp-management,we realize 25-Gb/s OOK modulation/detection based on commercial DML and PIN both operating at 10-GHz bandwidth,achieving 37.5-dB loss budget with 40-km reach,providing a promising solution for 100 G PON.