| Entering the“14th Five-Year-Plan”,the construction of new digital infrastructure will be accelerated,the integration and application of 5G will be deepened,and the ability of the communication industry to endow the digital transformation and upgrading of the economy and society will be comprehensively improved.However,the development of emerging technologies is often accompanied by the explosion of massive data.As an important national infrastructure service facility,the upgrade and expansion of the optical fiber communication system is imminent.Although the introduction of time,wavelength,polarization multiplexing and coherent modulation has helped the capacity of optical fiber transmission links to achieve a qualitative leap,due to the constraints of nonlinear effects,the remaining expandable range of single-mode fiber transmission capacity is relatively limited.Space division multiplexing fibers can significantly increase the transmission capacity of a single fiber through technical means such as mode multiplexing or core multiplexing,which are considered to be one of the important means to solve the capacity bottleneck of single-mode fibers.In recent years,transmission-type space division multiplexing fibers have received comprehensive attention from research institutions around the world,the key technical difficulties they face mainly include two aspects:one is the energy coupling among optical signal channels;the other is the parameter measurement of space division multiplexing fibers,and there is no mature commercial equipment for now.In view of the above technical difficulties,this thesis carries out related research on space division multiplexing fibers and their refractive index distribution measurement under the support of the NSFC’s National Major Scientific Research Instrument Development Project,the National Ministry of Science and Technology’s Key R&D Program and other projects.The improved design solutions are proposed for the limitations of the current weakly coupled few mode fibers and low crosstalk multi-core fibers,a measurement device for the refractive index distribution of space division multiplexing fibers is built,and the coupling characteristics of a commercial few mode fiber and a multi-core fiber are analyzed based on the experimental results.Specifically,the main innovations are as follows:1.Aiming at the inter-mode coupling problem of step-index few mode fibers,the variation of mode power distribution with the radial refractive index profile is studied,and a combined ring-core few-mode fiber is proposed.The double-layer composite ring-core structure effectively improves the isolation among six adjacent LP modes,and achieves an effective refractive index difference of greater than 1.63×10-3 in the C+L bands,which ensures the weak coupling characteristics of the fiber.To further optimize the refractive index distribution of the core,a nanopore-assisted double-cladding few mode fiber is proposed.The central power of specific LP modes is significantly reduced,and an effective refractive index difference of larger than 1.8×10-3 among six adjacent LP modes is achieved in the C+L bands,showing good weak coupling performance.Moreover,the bending resistance of the fiber is improved and the broadband properties is stable.2.Aiming at the problem of degenerate modes separation in mode groups of step-index few mode fibers,a dual candy-shaped ring-core 6-LP mode weakly coupled fiber scheme is proposed.The refractive index and mode field distribution of the transmission modes are adjusted by the ring-core and non-circularly symmetric dual candy-shaped structure,the effective refractive index difference between adjacent spatial modes and between adjacent LP modes are simultaneously greater than 1.93×10-4 and 1.51×10-3 at1550nm.This large mode spacing can further reduce the computational dimension of the required MIMO program in mode division multiplexing system from 4×4 to 2×2,thereby improving the overall link performance.3.Aiming at the problems of large crosstalk and low core multiplicity factor of multi-core fibers,a differential inner cladding heterogeneous 8-core single-mode fiber is proposed.The numerical analysis results show that the introduction of differential inner cladding structure produces a sufficiently large phase mismatch between adjacent core units,the inter-core crosstalk is less than-50d B/100km,the mode field area is higher than 90μm2 and the relative core multiplicity factor is 6.77 at 1550nm.The theoretical cut-off wavelength is less than 1350nm,and the bending loss at 30mm bending radius in the L band is smaller than 0.5d B/100turns,which meets the requirements of low crosstalk and low bending loss.4.Aiming at the comprehensive performance optimization of few mode multi-core fibers,an air-trench assisted graded-index profile heterogeneous 12-core 4-LP mode fiber is proposed.The air-trench structure is used to enhance the optical field confinement ability of core units,which achieves the mean crosstalk below-66d B/km and the relative core multiplicity factor of 41.7 at 1550nm.The differential mode group delay at C+L bands is less than 70ps/km,showing good transmission characteristics.To improve the compatibility of multi-core fibers with existing optical communication systems,a dense hole-assisted homogeneous 7-core 4-LP mode fiber scheme with standard cladding diameter is proposed.The differential air holes are surrounded to form the dense hole-assisted structure,which reduces crosstalk and makes the fiber insensitive to bending.The mean crosstalk is less than-30d B/100km and the core multiplicity factor is 84.92 at 1550nm,which satisfy the demands of large capacity and low crosstalk transmission links.5.Aiming at the common problems of existing fiber parameter analyzers such as large limitations,complex measurement process and long time,a refractive index distribution measurement device for space division multiplexing fibers is built based on quantitative phase microscopy and computed tomography.Combined with the optimization of related algorithms,the fast and accurate reconstruction of the refractive index distribution of a commercial few mode fiber and a multi-core fiber is realized.The measurement resolution of relative refractive index difference reaches the order of10-4,and the measurement error is less than 5%.Furthermore,the coupling characteristic of the commercial few mode fiber and the multi-core fiber are numerically analyzed according to the experimental data.The results show that the measurement data is accurate,the measurement device is stable and reliable. |
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