Key Technologies In High-Speed Direct-Modulation Direct-Detection Optical Access Networks

The emerging new applicatons such as 4K/8K TV,online gaming and cloud computing are now driving the swift increase of bandwidth demand from the user side.Optical access networks are regarded as the primary and almostly the only way to meet this demand.Meanwhile,new network facilities such as the mobile fronthaul,Internet of things(IoT)demand the carrying of optical access networks,which requires the future optical access network converged for heterogenous services.The potential cost of the optical access network will be dramatically increased by the increase in the capacity and the extending in the coverage.Direct-moduation and direct-detection transmission scheme is a primary choice in optical access networks due to its low-cost,but the increase in the capacity and the service coverage prohibitively induce impairments on the performance.This will increase the cost by introducing additional impairment mitigation methods,which will devite the original low-cost intention of direct-modulation and direct-detection.Also,because the user side of optical access networks is extremely cost-sensitive,it is desirable to simplify the design of the ONU.In addition,one central office usually has a large amount of OLTs,which offers the convenience for statistical multiplexing of OLT opto-electronic hardwares.In this thesis,aiming at decreasing the cost and enhancing the performance,comprehensive studies on the enabling schemes in direct-modulation and direct-detection optical access networks are conducted.Contributions of this thesis are outlined as follows.1.Non-aided schemes for chirp impairments suppression in optical access systemsIn direct-modulation laser(DML)based optical access networks,the transmission performance is severely influenced by the interaction between the large chirp of DML and the chromatic dispersion(CD)in the fiber.This requires additional methods to suppress the distortions,which are usually not cost-effective.This thesis researches on chirp distortion mitigation schemes without additional devices,namely non-aided schemes.Two non-aided schemes are proposed and investigated,(1)the chirp optimization scheme based on bias current adaption and(2)the filtering scheme with existing(de)multiplexers in the optical communication system.The main contributed work includes:(1)A comprehensive study on the relationship between DML power fading and the gain bias is conducted.Our study shows that,the power fading has either an overcompensated phenomenon or an under-compensated phenomenon during transmission,which is influenced by the bias current.Balanced should be achieved for each distance.We report more than 10 dB power budget improvement with the bias current optimization scheme and achieve uniform transmission performance with less than 2.2dB power penalties for distances from 0 to 100 km.MAC protocol to automatically obtain the optimal bias current is proposed according to the ranging procedure.(2)We propose to use the existing(de)multiplexers in optical communication systems to manage the frequency chirp.The feasibility is experimentally exmamined.The robustness and compatibility is comprehasively investigated.In particular,we report that the allowable optical signal's frequency deviation ranges from-7.5GHz to 7GHz,and the induced additional crosstalk to(De)Mux's adjacent port due to a deliberate frequency offset is far below system design limit.In addition,the scheme is compatible with common 50 GHz,100GHz and 200 GHz channel spacing(De)Mux.2.Low-complexity equalization for chirped signals in optical access systemsThe mature of high-speed digital-to-analog or analog-to-digital converters(DAC/ADC)allows the digtial signal processing(DSP)for high-speed optical transmissions.The last decade has witnessed a great success of DSP applications in long-haul coherent systems.DSP in optical access network is also been actively studied in recent years.In this work,we consider the specific distortions caused by the DML,and propose a novel equalizer architecture for DML-based systems.The main contributed work includes:(1)Unlike most equalization schemes in IM/DD systems that directly adopt existing equalizers for equalization,the proposed equalizer considers how the distortion is introduced in DML-based systems.In particular,the equalizer is based on the fact that the directly modulated PAM symbols with different intensity levels have different adiabatic chirp frequencies,which will lead to different inter-symbol interference contributions to their adjacent symbols due to the velocity difference caused by chromatic dispersion.Therefore,the proposed equalizer employs multiple sets of tap coefficients according to the intensity levels of PAM signals.(2)We demonstrate a 56Gb/s PAM4 transmission over a record 43 km transmission in dispersion unmanaged fiber link.This result shows a 115% distance increase compared with using a conventional FFE/DFE(20km as reported previously),and also outperforms the systems based on MZM or EMLs(less than 30 km as reported previously)in which not only the transmitter its own is costlier than DML but also the complicated MLSE is even adopted.(3)We conduct a 40Gb/s PAM4 transmission over 63 km transmission for long reach optical access networks with the proposed equalizer.These results even show superiority to multi-carrier optical access systems.3.Reduction of user side cost: DSP-free ONU in high-speed direct-modulation direct-detection optical access systemsDSP relies on high-speed DAC/ADC and numerical operations,which are not costeffective especially for the optical network units(ONUs)located at the user side.For example,the 50Gb/s PAM4 PON requires ADCs with a sampling rate > 25GS/s at the ONU side.Therefore,high-speed PON systems with both DSP-free ONUs and low-bandwidth optics will be of great interest to the development of future cost-effective optical access networks.The main contributed work includes:(1)A PON system in the O-band with DSP-free ONUs is proposed.In this system all DSPs are put in the central office,which include Nyquist pulse shaping and finite impulse response(FIR)filter pre-equalization for downlink,FIR filter post-equalization for uplink.(2)In a direct-modulation and direct direction system with an end-to-end bandwidth of only 8.5GHz,50Gb/s PAM4 PON is demonstrated with 29 dB power budget.Experimental investigations on the DSP parameters are conducted,which include the tap number and the pulse shaping roll off factor.(3)NRZ-OOK and duobinary signals are also investigated with this scheme.In particular,with an end-to-end bandwidth of only 8.5GHz,the experimental results show that the duobinary format is suitable for 40Gb/s per wavelength PON,and the NRZ-OOK format is suitable for 25Gb/s per wavelength PONs.4.Reduction of central office side cost: Resource sharing and statistical multiplexing in the OLT based on AWGR and directly modulated tunable lasers.The heterogenous feature of future network services requires massive deployments of optical access networks.On the other hand,the point to multi-point(PtMP)tree structure of optical access network put all the hardware resources in the central office together by nature.One central office would support massive sub-optical access networks.This provide the potential for statistically multiplexing of opto-electronic hardware resources,which include the transceivers,DSPs,MAC,etc.In this work,we propose to employ a novel asymmetric AWGR and fast tunable distributed Bragg reflector(DBR)DMLs to enable resource sharing in the central office.The contributed works include:(1)An × ( > )partitioned asymmetric AWGR using a symmetric × AWGR and a few couplers is designed.This structure shows a partitioned cyclic routing feature.As a result,we achieve the following benefits:(i)it can support contention-free transceiver assignment;(ii)the required tuning range of the transmitter laser is significantly reduced;(iii)the diversity of tunable lasers is reduced from different types to / different types,which can reduce deployment complexity and operation cost.(2)We fabricated a fully packaged directly modulated DBR laser,which has a tuning range of >10nm,a tuning speed of <50ns,and a direct-modulation bandwidth of >10GHz.By incorporating this tunable laser with AWGR,we demonstrated the optical switching function.