Weakly-coupled Space Division Multiplexing Optical Fibers And Key Components For Large-capacity Optical Fiber Communication

Since the introduction of“optical fibers”by Charles Kao in 1966,the transmission capacity of optical fibers has increased dramatically thanks to the development of fiber fabrication technique,wavelength-division multiplexing（WDM）,erbium-doped fiber amplifier（EDFA）,coherent detection,digital signal processing,and so on.Nowadays,optical fiber has become the foundation of global communication network.As the quick development of 5G,big data,cloud storage,cloud calculation,and high-definition video,the internet traffic is expected to keep growing at an annual rate about 40%.The new era of“Internet of Everything”requires larger capacity of optical fiber.However,the transmission capacity of conventional single-mode fiber（SMF）is believed to approach its theoretical limit due to Shannon nonlinear limit and fiber fuse phenomenon.Space division multiplexing（SDM）,utilizing multiple orthogonal modes or multiple cores within the same cladding,is expected to further extend the transmission capacity of fiber dramatically.In this thesis,the specialty fibers and key components for low-cost weakly-coupled SDM transmission are investigated,including the design and characterization of weakly-coupled few-mode fibers,the design of multicore fibers with low crosstalk for wideband transmission,the design and fabrication of key components for SDM.Besides,fiber fuse phenomenon,which threatens the safety of optical communication systems,is investigated in experiment.The main achievements and innovations are listed as follows:1)Ring-assisted（RA）structure is proposed to avoid strong mode coupling between LP₂₁and LP₀₂ mode in step-index few-mode fibers（SI-FMFs）.A RA four-mode fiber was designed and fabricated with Min（35）n_eff of 1.8×10^-3,which is 2.2 times larger than the value in the SI-FMF.The fabricated RA 4-mode fiber has a low mode-average loss and a low mode dependent loss of 0.23 and 0.02 dB/km at 1550-nm wavelength,respectively.The excess loss caused by the high-index ring is as low as 0.01 dB/km.The distributed mode coupling was measured using phase-noise-compensated swept-wavelength interferometry in 23-km-long RA-FMF and SI-FMF.The results show that RA-FMF has better robustness than SI-FMF and the mode coupling between LP₂₁ and LP₀₂ mode in RA-FMFs is well suppressed with large winding tension.Furthermore,the strategy was adopted to design a ring-assisted 7-mode fiber which supports up to 12 spatial channels with Min（35）n_eff of 1.7×10^-3.2)The reported high-density multicore fibers（MCFs）with（43）<35μm and XT<-30dB/100 km usually show long cut-off wavelength and narrow available bandwidth.By optimizing trench parameters for heterogeneous cores,the trade-off among the effective area,cut-off wavelength,and crosstalk is well balanced.We detailed the core selection strategies within 125-μm diameter cladding and designed a 6-core fiber and an 8-core fiber.The 6-core fiber was designed with ultra-low crosstalk（XT<-30 dB/100 km）and available wavelength bandwidth from 1300 nm to 1625 nm.Its available wavelength bandwidth is about 3 times larger than the reported MCFs with similar XT.The 8-core fiber was designed with XT<-30dB/km and with available wavelength bandwidth from 1260 nm to 1625 nm.It is expected to be suitable for short-reach SDM transmission in access network and data center.3)Wideband LP₁₁ mode selective coupler was fabricated with low insertion loss and high mode purity.The lowest insertion loss is only 0.3 dB.Clear mode patterns and insertion loss lower than 0.6 dB have been achieved from 1480-1640 nm.25 Gb/s NRZ and 20 Gb/s PAM4 MDM transmission have been demonstrated in 15-km-long FMF.Besides,orientation-insensitive azimuthally asymmetric mode rotators（OI-AAMR）using chirally-coupled-core fiber is proposed based on three-mode coupling model and simulated by using beam propagation method.In the traditional mode rotators based on polarization-maintaining few-mode fibers,the angle between the incident mode lobe orientation and the fast axis is required to be carefully controlled.The proposed OI-AAMR can convert azimuthally asymmetric mode(LP_mn mode,m≠0)to the orthogonal degenerate mode without the requirement for angle alignment.An LP₁₁ mode rotator with a rotation efficiency and an extinction ratio as high as 97%and 17 dB over C-band respectively,has been successfully demonstrated for any incident lobe orientation.It has good scalability to higher-order modes and is promising for mode division multiplexing using degenerate modes.In summary,this dissertation aims at the improvement of weakly-coupled SDM technologies by improved fiber and key components.This topic has been systematically studied from the prospects of coupled mode theory,design and characterization of optical fiber,design and fabrication of key components.The developed optical fibers and componets are promising for the practical use of weakly-coupled SDM technology.