Key Technologies Of Ultra-dense Wavelength Division Multiplexing Passive Optical Network

With the rapid development of new broadband services such as cloud computing,webcasting and online education,the bandwidth demand of access network is ever-growing.Fiber optical access technology is considered as the optimal solution for its capacity bottleneck,due to its advantages of wide coverage and high bandwidth.In view of the passive optical network(PON)evolution with large capacity,multiple users access,and wide coverage,ultradense wavelength division multiplexing passive optical network(UDWDM-PON)is believed as one of the most promising access technologies for future PON deployment.Since the access network is the closed to the end-user and is extremely sensitive to the cost,both architecture and technique of new fiber optical access system need to simultaneously achieve the capacity enhancement and cost management.To address such challenging,this thesis is focused on the simplified coherent detection based optical network unit(ONU),the low complexity receiver sensitivity enhancement and the simplified coherent detection based UDWDM-PON system architecture.The main innovative research outcomes are summarized as follows:(1)Aiming at the low-cost ONU for the UDWDM-PON,this thesis explores the local oscillator diversity(LO-D)and transmitted signal diversity(TS-D)simplified coherent receiver(SCR)enabled ONU schemes based on a single 3×3 optical coupler.In order to further reduce the required bandwidth of the simplified coherent receiver,a polarization independent ONU scheme based on dual 3×3 couplers is proposed.On this basis,the feasibility of those three simplified coherent detection enabled ONUs for the reception of fourlevel pulse amplitude modulation(PAM-4)signals is numerically verified.Several key parameters,including frequency offset and the bandwidth of low-pass filter,of three simplified coherent detection enabled ONUs are optimized.Finally,their performance is comprehensively evaluated,in terms of receiver sensitivity and spectrum efficiency,for the UDWDM-PON system.Moreover,we carry out the experimental research based on TS-D coherent receiver for PAM-4 on the silicon-on-insulator platform.The key indicators such as the receiver sensitivity are in line with expectations.(2)Focusing on the increasing power budget of the UDWDM-PON system,it has been discovered that the receiving sensitivity of ONUs is restricted by the shot noise stemming from the analysis of noise mechanisms of simplified coherent detection.Through simulation optimization of the PAM-4 signal level spacing,we propose an optimization strategy based on a low complexity gradient descent algorithm.The numerical simulation results validate the feasibility and efficacy of the PAM-4 signal level spacing optimization strategy.Additionally,the results confirm that the optimization outcomes for PAM-4 level spacing are impervious to changes in the signal polarization state and LO output power.Compared with the conventional equally-spaced PAM-4 signal,2 dB receiver sensitivity enhancement of unequally-spaced can be abtained by the simulation.(3)In order to satisfy the development of next-generation PON with large capacity,multiple users,and low cost,we propose a PAM-4 UDWDM-PON system architecture based on simplified coherent receiver(SCR).In view of small signal attenuation and high tolerance of time skew arising in the LO-D scheme,we experimentally demonstrate digital signal processing(DSP)free 8 × 10 Gbit/s PAM-4 downstream transmission over 25-km standard single mode fiber(SSMF)with a wavelength spacing of 20 GHz,leading to a receiver sensitivity of-26.0 dBm and a power budget of 29 dB.Considering the high sensitivity of the TS-D,we experimentally demonstrate a 4 × 10 Gbit/s PAM-4 downstream transmission over 20-km SSMF with a wavelength spacing of 25 GHz based on the gradient descent PAM-4 optimization algorithm,leading to a receiver sensitivity of-29.5 dBm and a power budge of 32.5 dB.