Investigation Of Key Techniques For Short-to-medium Reach Optical Fiber Communication System

With the advent and booming of the emerging big data traffic-driven internet services,such as cloud computing,Internet of things(IoT),high-definition television(HDTV)and virtual reality(VR)etc.,the traffic distribution of optical network has changed and the current situation is that the short-to-medium range metro traffic has surpassed long-haul traffic.In the future,short-to-medium reach optical fiber communication systems,including metropolitan area network(MAN),datacenter network(DCN)and access network(AN),will play more and more important roles in transmission and supporting big-data featured 5G and IoT services.Therefore,investigations on modulation,multiplexing and transmission techniques in short-to-medium reach(1?100km)optical fiber communication systems will provide high theoretical and practical value for the upcoming big data transmission and information interaction and it may be helpful for achieving flexible optical network with lower cost and larger capacity.In this dissertation,aiming at solving the problems in short-to-medium reach optical fiber communication systems,we conducted simulations and experiments to achieve a low-cost,large-capacity and efficiently flexible optical transmission based on multi-dimensional hybrid multiplexing techniques including frequency division multiplexing(FDM),wavelength division multiplexing(WDM)and space division multiplexing(SDM).The main research contents and the novelties are summarized as following.Firstly,based on electric-optical modulation technique,we experimentally generated various high-speed advanced modulation formats,such as single carrier polarization-division multiplexing QPSK,QPSK-OFDM,16QAM-OFDM and 32QAM-FMBC etc.In addition,the subchannel signal to ratio(SNR)monitoring technique for multi-carrier modulated systems is also explored in this thesis.It not only gives an objective and quantitative indicator for systems performance evaluation and optimization,but also will lay a solid foundation for realizing adaptive modulations and flexible optical network with user defined capacity and transmission performance.We verified the subchannel SNR monitoring method and adaptive modulation techniques in an upstream transmission experiment of access network scenario.The results show that the upstream data rate can be increased by 25%with adaptively assigning modulation formats and power levels to every subcarrier based on Chow's bit-power loading algorithm.Secondly,in order to solve the frequency selective power fading problem induced by chromatic dispersion(CD)and bandwidth limitation in direct detection optical(DDO)OFDM system,a channel and modulation format independent precoding technique was proposed,which introduces frequency diversity among different sub-carriers and may resist the power fading in high frequency range.We theoretically analyzed how the precoding technique can help to reduce peak to average power ratio(PAPR)of OFDM signal and equalize the SNR imbalance among subcarriers.Simulation and experimental results proved that two specific precoding matrixes we used can be able to improve the transmission performance of DDO-OFDM systems in terms of PAPR reduction,receiver sensitivity and CD tolerance improvement,as well as subcarrier SNR equalization.Moreover,we also proposed and verified a novel simplified adaptive modulation scheme which can greatly reduce the complexity compared with traditional adaptive modulation.Thirdly,we established a high capacity transmission platform for short-to-medium reach optical fiber communication system.In order to explore new methods for increasing the capacity and decreasing the cost of the system,we studied multi-dimensional multiplexing and high-capacity transmission techniques based on hybrid space and wavelength division multiplexing and multi-carrier modulation and demodulation.Combining the low-crosstalk 7-core fiber and high order modulation formats,we proposed and experimentally demonstrated a novel hybrid wavelength space division multiplexing(WSDM)optical access network architecture.Meanwhile,we have created a series of breakthroughs about capacity record of multicore fiber(MCF)based short-to-medium reach transmission systems,(a)Taking advantage of MCF,we have achieved mobile front-haul compatible optical access network with 300 Gb/s downstream data rate for the first time in the world.The capacity can be further upgraded to 1 Tb/s by using 16QAM-OFDM modulation format.(b)Based on wavelength centralized distribution of optical frequency comb in the optical line terminal(OLT),bidirectional symmetric access with 1 Gb/s per optical network unit(ONU)user can be realized.(c)With local oscillator light(LO)distributing from the inner core of the MCF,we have demonstrated a cost-efficient utra-capacity MCF based hybrid WSDM bidirectional optical access network with self-homodyne coherent detection(SHCD)based 16QAM-OFDM downstream and direct detection based 32QAM filter bank multicarrier(32QAM-FBMC)upstream,with aggregate data rate of 4.8 Tb/s and 120 Gb/s,respectively.(d)For practical applications,we even experimentally demonstrated a real-time 400G OOK quasi-duplex bidirectional optical interconnection over 20-km 7-core fiber with the help of using an optical delay interferometer for chirp and CD managements.This results setup a new distance and capacity records for high speed optical interconnection using C-band wavelengths.Fourthly,we explored the power domain non-orthogonal multiplexing technique and proposed two kinds of models for power division multiplexing optical fiber transmission systems.In order to prove the feasibility of simultanous transmission of two signals occupied the same frequency band in the same communication channel,proof-of-concept experiments based digital domain power division multiplexing and optical domain power division multiplexing were carried out using two QPSK-OFDM signals.We also investigated the relationship between the power division ratio(PDR)and transmission performance of each signal.To avoid the influence from the coherent beating interference between the two optical carriers in optical domain power division multiplexing scenario,we further proposed a modified scheme to enable two spectrally overlaid signals transmission with indentical wavelength using orthogonal polarizations combination method instead of optical field coupling,which fully confirmed the feasibility of our proposed optical power division multiplexing scheme.Power division multiplexing at the same wavelength can not only provide a new solution to realize low-cost multiplexing or multiple access,but also can be used to achieve BER performance adaptable flexible optical transmission by simply adjusting the PDR of the power multiplexed signals according to users' quality of service(QoS)requirements.These research results above are expected to provide technical supports for achieving cost-efficient,high-capacity and flexible short-to-medium reach optical fiber transmission systems.