Research On Novel Few-Mode Fibers For Short-reach Mode Division Multiplexing Communication Systems

In recent years,the demand for high capacity in short-reach optical fiber communication has been rising rapidly with the development of emerging big data and cloud computing services.The mode division multiplexing(MDM)is regarded a promising solution to enhance transmission capacity for short-reach communications.Multi-input-multi-output digital signal processing technology is often used at MDM systems to deal with modal crosstalk caused by mode coupling,which increases the system complexity and cost.Therefore,the scheme of applying weakly-coupled few-mode fiber to simplify or eliminate complex equalization modules is proposed,which can reduce the system cost while ensuring the transmission capacity.Weakly-coupled few-mode fibers have become the core component of short-reach MDM systems.The few-mode fibers used in MDM systems still have the defects of not enough mode channels,strong mode coupling and short transmission distance.Based on the above problems,this dissertation mainly studies the weakly-coupled few-mode fibers and their applications in short-reach MDM systems,proposing novel few-mode fibers based on total reflection and photonic bandgap theories respectively,which have the advantages of weakly-coupling,bend resistance,large mode field area,and so on.The proposed fibers can effectively improve the transmission capacity and reduce mode crosstalk,so as to ensure the transmission stability and control the complexity of short-reach communication systems.The main works of this paper are as follows:1.Research on weakly-coupled few-mode fiber of total internal reflectionA weakly-coupled step-index circular core fiber is optimized to support four linear polarization modes,which has the effective mode area of more than 180?m2,and the minimum effective refractive index difference of 0.00055 between adjacent modes in C-band.Besides,a five-mode trench-assisted M-type fiber is designed and numerically investigated,taking into account the advantages of double-cladding W-type fiber and M-type fiber.The proposed M-type fiber can achieve a large effective mode area of more than 200 ?m2 and good bending resistance,while having the minimum effective refractive index difference of more than 0.001 between adjacent modes.It can simplify MIMO-DSP at the receiving end of short-reach systems.2.Research on weakly-coupled dispersion-flattened few-mode photonic crystal fiberNumerical simulations for weakly-coupled photonic crystal fibers are carried out.Based on the high birefringence,controllable chromatic dispersion and high nonlinearity of these fibers,a dispersion-flattened weakly-coupled photonic crystal fiber which can support 10 vector modes is proposed.The fiber has a maximum effective index difference of more than 9.0×10-3 between adjacent modes,and can improve the utilization of vector modes.The proposed fiber can achieve dispersion flatness of 10 modes in C-band,and has the potential for applying to large-capacity WDM-MDM communication systems.3.Research on weakly-coupled few-mode photonic bandgap Bragg fiberThe possibility of Bragg fibers applying to short-reach MDM communication systems is explored.Two kinds of novel few-mode Bragg fiber are put forward,and their transmission theories and loss performance are numerically analyzed,including an all-solid elliptical core Bragg fiber capable of working in the O+C+L-band,which has the effective refractive index difference of more than 4×10-4 and small confinement loss;A hollow-core coaxial Bragg fiber supporting 16 spatial modes across the whole C-band,which greatly improves fiber capacity and utilization of vector modes.The research raises the possibility of using one-dimensional photonic bandgap fibers in MDM systems,while providing potential candidates for short-reach transmission.4.Basic confirmatory transmission experiment in MDM passive optical networkA few-mode transmission experiment in short-reach passive optical network is realized,using the step-index fiber optimized in Work 1 and two all-fiber multiplexers/demultiplexers.The power of each mode channel is measured at the receiving end.The mode crosstalk caused by mode coupling is less than 13dB.The bit error rate and eye diagram are also measured,with the receiver sensitivity of LP01,LP11 and LP21 modes to be-30.1 dBm,-28.8dBm and-27.9dBm,respectively.MIMO-DSP modules are removed in the system.The experiment provides the feasibility for weakly-coupled fibers to be applied in short-reach MDM systems.