Based on the die-less multi-mode optical fiber technology, different modes in the fiber core are used as independent transmission channels. Combined with DWDM technology and polarization multiplexing technology, the SMF can break the capacity of single-mode optical fiber communication system. The coupling characteristics of different modes in a small mode fiber are directly related to the hardware implementation and software calculation complexity of the signal processing at the multiple input-multiple output (MIMO) receiving end. Therefore, it is necessary to optimize the design of the coupling characteristics of the mode in the small mode fiber.
Among the less-mode optical fibers with ordinary circular core structure, the LP11 mode group contains two modes of orthogonal orientation, and they are easily coupled to each other in the optical fiber because they have the same propagation constant. Considering the polarization state, at least 4 × 4 MIMO digital signal processing is needed to solve the problem of mode coupling and the complexity is high. Therefore, by optimizing the design of fiber structure parameters, the coupling of degenerate modes in the LP11 mode group is avoided Worth to study the problem.
Wuhan National Laboratory of Optoelectronics, Professor Fu Songnian of the School of Optics and Electronic Information, Ph.D. students Liang Junpeng, Mo Qi, Yu Dawei and others, broke the two orthogonally oriented LP11 modes by designing the fiber core into an elliptical shape Degeneracy, increase its mode interval, thus largely avoiding the mode coupling between the two modes of occurrence.
Laboratory research group found that when the ellipticity of the core is less than 0.264, the two orthogonally oriented LP11 modes can achieve the weak coupling condition (ie, the effective refractive index difference is greater than 0.5 × 10-4). On this basis, they successfully designed and prepared an elliptical core fiber that supports three spatial modes. This optical fiber only needs to carry on 3 independent 2 × 2 MIMO digital signal processing in the optical signal transmission of the mode-division multiplexing, thus can simplify the complexity of MIMO signal processing greatly; The two orthogonally oriented LP11 modes are modeless Therefore, the field distribution of the LP11 mode can be stabilized as it propagates in the fiber. With the phase plate-based mode conversion device, the elliptical core fiber is successfully used for the high-precision fiber distortion sensing measurement of 20 μW / ° and at 20 ° C. ~ 180 ℃ temperature range showed excellent temperature insensitivity.
For the two-mode fiber with a circular core, the research group experimentally tested the mode field distribution of the Rayleigh backscatter in the optical fiber. The results show that the backscattered light is present in all the guided modes in different proportions, and the intensity of Rayleigh backscattering in the multimode fiber is higher than that of the single mode fiber. This conclusion is instructive for the suppression of Rayleigh scattering in a few-mode fiber applications.
Fig.2 Experimental results of Rayleigh backscatter in a few-mode fiber
This series of research projects have been funded by the National 863 Program (No. 2013AA013402) and the National Natural Science Foundation of China (No. and No.). The research results have been published in Optics Letters in the paper "Design and fabrication of elliptical-core few-mode fiber for MIMO-less data transmission" and "Temperature-insensitive fiber twist sensor based on elliptical-core few-mode fiber" Vol. 41, No. 13, pp. 3058-3061, 2016 and Vol. 41, No. 20, pp. 4617-4620, 2016); the research result "Characterization of Rayleigh backscattering arising in various two-mode fibers" Optical Journal Optics Express (Vol. 24, No. 11, pp., 2016).
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