1.State Key Laboratory of Metastable Materials Science & Technology, Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, China 2.School of Information Science and Engineering, Yanshan University, Qinhuangdao 066004, China 3.State Key Laboratory of Process Industry Integrated Automation, School of Information Science and Engineering, Northeast University, Shenyang 110004, China
Fund Project:Project is supported by the National Key Research and Development Project, China (Grant No. 2019YFB2204001), the National Natural Science Foundation of China (Grant No.12074331), the Natural Science Foundation of Hebei Province, China (Grant Nos. F2017203193, F2020203050, F2017203110), and the Postdoctoral Preferred Funding Research Project of Hebei Province, China (Grant No. B2018003008)
Received Date:02 November 2020
Accepted Date:30 December 2020
Available Online:09 May 2021
Published Online:20 May 2021
Abstract:Information technology has an increasingly strong demand for high-speed and large-capacity optical fiber networks. Space division multiplex(SDM) is a new generation of optical fiber communication technology which can be several times in communication capacity higher than the wavelength division multiplexing systems. In this paper, we present a kind of 13-core 5-mode fiber with double trench structure to meet the demand for high-speed and large-capacity information transmission in the future. In order to solve the crosstalk problem in SDM, a double-trench structure is adopted to better limit the light energy in the fiber core, thus reducing the crosstalk between cores and modes. The crosstalk and transmission characteristics of multi-core fiber are calculated and analyzed by the full vector finite element method and coupled power theory. After the optimization of structural parameters, the fiber can stably transmit LP01, LP11, LP21, LP02 and LP31 in the band of 1.3–1.7 μm; when the signal is transmitted at the 1.55 μm for 60 km, the inter-core crosstalks corresponding to the adjacent fiber cores in the above five modes are –122.37 dB, –114.76 dB, –106.28 dB, –100.68 dB and –92.813 dB, respectively; the effective refractive index difference between adjacent modes is greater than 1.026 × 10–3; inter-core and inter-mode crosstalk can be effectively suppressed. The corresponding non-linear coefficients of the 5-modes are 0.74 W–1·km–1, 0.82 W–1·km–1, 0.88 W–1·km–1, 1.26 W–1·km–1, 0.93 W–1·km–1, which can maintain low non-linear transmission. The structure of fiber is simple and compact, and the preform can be fabricated by vapor deposition method and stack method, then the 13-core five-mode fiber with low crosstalk and low nonlinear can be further drawn, which can be used in a long distance high-speed and large-capacity fiber transmission system. Keywords:space division multiplex/ a double-trench/ low-crosstalk/ low non-linearity
芯区半径对光纤的串扰和性能有着显著的影响, 分析芯区大小与光纤性能的关系是必不可少的步骤, 本文拟定对芯区半径a = 5—8 μm进行分析, 分析结果如图7所示. 图 7 在?1 = 0.015, 波长1.55 μm处5个模式的串扰、Aeff和Δneff与芯区大小的关系 (a)芯区大小和串扰的关系; (b)芯区大小和Aeff的关系; (c)芯区大小和模式折射率差的关系 Figure7. The relationship between crosstalk, Aeff, Δneff of five modes and core size at 1.55 μm: (a) The relationship between core size and crosstalk; (b) the relationship between core size and Aeff; (c) the relationship between core size and Δneff.
由图7(a)可以看出5个模式的芯间串扰呈现非线性变化, LP01模式的串扰随芯区大小变化具有一定的波动性, 芯区较小时LP01模式呈现低串扰特性, 但芯区半径超过7 μm后, LP01模式的串扰只有微小的变化; LP11模式的串扰呈现出随着芯区半径增大, 串扰缓慢增长的趋势; LP21和LP02两模式的串扰变化趋势基本相同, 但LP21模式串扰低于LP02, 随着芯区增大串扰在a = 6.5 μm处存在一最小值, 芯区半径超过7 μm后串扰变化趋于平缓; LP31模式的串扰随芯区半径变化最为明显, 随着芯区增大串扰迅速降低, 在芯区半径超过7 μm后, 串扰变化趋于平缓. 图7(a)中的串扰变化规律可以从模式的电磁能量分布角度解释, 同一大小的芯区对不同模式的能量束缚能力不同, 高阶模式的能量更容易泄露到纤芯外, 且高阶模的能量主要分布在芯区外围更容易引起能量耦合, 通过增大芯区可以增强对高阶模能量的束缚能力以达到减小高阶模式的芯间串扰的目的, 当芯区增大到一定程度后, 芯区对模式的束缚能力增长程度不在明显. 综上分析, 5个模式的串扰均有一个相同的特点: 芯区半径超过7 μm后, 串扰均能保持在较低水平, 并且串扰随芯区半径变化很小; 若想保持5个模式的串扰均保持在较低水平, 芯区半径最佳的选择范围是7—8 μm. 由图7(b)可知, 光纤中各模式的Aeff均与芯区半径成正比, 5个模式的Aeff增长趋势基本形同, 但LP02模式的Aeff大小却低于其他4个模式. 对于高折射率芯区, 其具有较强的集光能力, 光会被集中在芯区内传播, 当芯区半径较小时, 传输模式会被集中在小芯区内, 模式所占据的横向面积就会很小, 对应的有效模式面积也会较小; 当芯区半径增大时, 模式被集中在较大的芯区面积内, 模式的横向分布面积也会增大, 有效模式面积也会随之增大. 根据图7(b)分析, 若想获得较大的Aeff, 就要尽量扩大芯区半径. 图7(c)为相邻模式之间的有效折射率差, 随着芯区增大, 模式之间的有效折射率差减小. 从图7(c)中可以看出LP21和LP02模式之间的有效折射率很接近, 两者之间的差值相比于其他4个模式略低, 所以在考虑折射率差时应以LP21–LP02为标准. 根据上文所述, 各模式之间的有效折射率差大于10–3便可忽略模间串扰, 但过大的折射率差将会引起较大的MDGD, 所以应选择适宜的芯区半径以获得最佳的模间折射率差值, 从而获得足够小的模间串扰. 除芯区半径外, 芯区的折射率大小对光纤也有着重要的影响. 现对芯区的相对折射率差Δ1进行分析, 结果如图8所示. 图 8 在1.55 μm处5个模式的芯间串扰、Aeff和?neff与芯区相对折射率差Δ1的关系 (a) Δ1和芯间串扰的关系; (b) Δ1和Aeff的关系; (c) Δ1和模式折射率差的关系 Figure8. . The relationship between crosstalk, Aeff, ?neff of five modes and Δ1 at 1.55 μm: (a) The relationship between Δ1 and crosstalk; (b) The relationship between Δ1 and Aeff; (c) The relationship between Δ1 and ?neff.
表35个LP模式的串扰、有效模面积和MDGD(LPmn–LP01) Table3.Estimated values of crosstalk, effective area and MDGD(LPmn–LP01) for 5-LP modes at 1.55 μm.
图 9 5-LP横向模式剖面 Figure9. Transverse mode profile for 5-LP modes.
芯间串扰受波长影响较大, 如图10所示, 在1.3—1.7 μm波段, 芯间串扰随着波长逐渐增大. 在1.3 μm处传输60 km, LP31模式的芯间串扰可以保持在–140.61 dB左右, 但在1.7 μm处, LP31模式的芯间串扰却达到了–66.75 dB, 若要在传输过程中保持低串扰, 应尽量选择在较低波段进行传输. 图 10 双沟槽十三芯五模光纤芯间串扰与波长关系 Figure10. Relation between wavelength and core-to-core crosstalk for the double-trench assisted 13-core 5-LP mode fiber.
对于模间串扰, 根据图11分析, 优化后的光纤结构相邻模式之间可以保持较大的有效折射率差, 模式区分度大, 模间串扰可以被有效抑制, 当波长大于1.54 μm, 5个模式间的有效折射率差均大于10–3, 可以忽略模间串扰. 相邻模式间的模式差分群时延也可保持在适宜大小, 可以在接收端解复用. 图 11 5个LP模式之间的?neff和MDGD与波长的关系 (a) ?neff与波长的关系; (b)相邻模式之间差分群时延与波长的关系 Figure11. The relationship between ?neff, MDGD of five modes and wavelength: (a) The relationship between ?neff and wavelength; (b) the relationship between ?neff and wavelength.