The Key Technologies Of Weakly-Coupled Multicore Fiber-Based Optical Transmission Systems

In the future,with the emergence of new technologies such as the Internet of Things,smart phones,cloud computing,and smart cities,optical fiber communication systems mainly based on the existing single-mode optical fibers will further improve transmission capacity in the next five to ten years by using multi-band wavelength-division multiplexing transmission.However,this approach is very difficult to achieve Pbit/s transmission capacity in the next twenty to thirty years.Space division multiplexing(SDM)technology aims to realizes the rapid increase of transmission capacity by increasing the spatial channel density of the optical fiber.Combined with integrated optical amplification and integrated optical transceiver technology,SDM can achieve a balance between system cost and energy efficiency,so it has gained widespread attention and made important progress in the past decade.This dissertation focuses on the key technologies in the weakly coupled multi-core optical fiber-based(WC-MCF-based)SDM optical transmission system,including the modeling of the WC-MCF channel and communication system,the design of the WC-MCF multiplexer/demultiplexer(MUX/de-MUX),and the experimental and simulation investigation of the WC-MCF-based short-reach intensity-modulation/direct-detection(IM/DD)and short-reach self-coherent system.The main research results of the thesis include:Firstly,considering the WC-MCF under bending and twisting conditions,a simulation model of inter-core crosstalk(IC-XT)based on the equivalent refractive index model and the mode coupling theory is established,and the mechanism and evolution law of the frequency-dependent characteristics of IC-XT are revealed.The analysis indicates that the frequency-dependent characteristics of IC-XT mainly depend on the significant difference in phase accumulation of optical fields at different optical frequencies during transmission.Furthermore,through experiments and simulations,it has been proved that the de-correlation bandwidth of IC-XT in the frequency domain decreases with the relative time delay between cores(walk-off×transmission distance)as a fractional linear function.Secondly,for the WC-MCF,a channel model based on the coupled nonlinear Schr?dinger equation is established.The IC-XT can be accurately described using a coupling matrix and a phase transfer function.Furthermore,the calculation speed of the WC-MCF channel model is increased by about 10 times by down-sampling the IC-XT coupling matrix in the frequency domain,which improves the simulation efficiency of the WC-MCF-based communication system.Simulations also verify the applicability of this channel model to randomly coupled MCF.Thirdly,in order to accurately investigate the IC-XT in WC-MCF,a trench-assisted multi-cladding bridge fiber with a cladding diameter of 125?m is designed and fabricated,which can be applied to the fabrication of MCF MUX/de-MUX with MCF's core pitch larger than 30?m.Based on this bridge fiber,a low-crosstalk fused tapered type 7-core MCF MUX/de-MUX is designed and fabricated to achieve low XT of less than-62dB,insertion loss of less than 1.2dB,and small Fresnel reflection of less than-58dB.Fourthly,a WC-MCF-based short-reach intensity-modulation/direct-detection transmission system is investigated,and the effect of IC-XT on the signal-to-noise ratio of the pulse amplitude modulation(PAM)format under different conditions is discussed.For two typical transmission schemes,through experiments and simulation analysis,it is confirmed that the carrier beating impairment(CBI)caused by IC-XT will cause additional degradation of the SNR.At the worst 1-d B signal-to-noise ratio degradation threshold,the IC-XT tolerance threshold decreases by 20dB.Furthermore,the tolerance characteristics of the CBI to three modulation formats including PAM,carrier-less amplitude and phase(CAP),and discrete multitone modulation(DMT)are experimentally investigated.The results show that CAP has good resistance to the CBI.The related results are helpful for the analysis and optimization of the WC-MCF itself and its related IM/DD transmission system.Fifthly,a WC-MCF-based short-reach self-coherent detection transmission system is investigated.The effect of the local oscillator optical power on the self-coherent detection transmission system is studied.It is pointed out that the minimum local oscillator optical power after MCF transmission should be higher than-13dBm under the 1dB SNR penalty,corresponding to a 17dB local oscillator optical power budget.The effect of phase noise caused by the relative time delay of the WC-MCF is further studied.It is pointed out that the relative time delay between the cores of the WC-MCF should be less than 1 ns to achieve the phase noises of less than 0.1 rad for the DFB laser.The quadrature phase shift keying(QPSK)modulation format and 64-quadrature amplitude modulation(64-QAM)format are used to investigate the impact of IC-XT on local oscillator,optical signal,and the both of the local oscillator and the optical signal.The results indicate that the IC-XT can be approximated as additive white Gaussian noise in this transmission system.The related simulation and theoretical analysis results are consistent with the experiment results,and the simulation is helpful for the analysis and optimization of the WC-MCF itself and its related self-coherent detection optical transmission system.