A low-loss and low-crosstalk quad-mode supermode optical fiber

Abstract

The present invention proposes a low-loss and low-crosstalk four-mode supermode optical fiber; the optical fiber consists of six cores with a gradient refractive index distribution, a refractive index groove and a fluorine-doped silica cladding; it realizes supermode operation with high mode field density, low loss, low dispersion, low crosstalk and low DMGD, and uses a recessed refractive index cladding to effectively reduce bending loss.

Description

Technical Field

[0001] The present invention relates to a low-loss and low-crosstalk four-mode supermode optical fiber, which can be applied to the fields of optical fiber optics, optical fiber communication, optical fiber wireless access, optical information processing and new generation information technology. Background Art

[0002] In recent years, the traffic of various communication services has grown exponentially, and single-mode fiber communication has been faced with unprecedented challenges. The fiber-optic communication industry has achieved a breakthrough in the transmission capacity of communication networks around the physical dimension of space-division multiplexing (including core-type multiplexing and mode-division multiplexing and their combination); multi-core fiber and few-mode fiber in space-division multiplexing and their related devices and applications have become a frontier research hotspot [Guifang Li, Neng Bai, and Ningbo Zhao and Cen Xia, Space-division multiplexing: the next frontier in optical communication. Advances in Optics & Photonics, 2014, 6 (4): 5041-5046; Guifang Li, Magnus Karlsson, Xiang Liu, and Yves Quiquempois, Focus issue introduction: space-division multiplexing, Opt. Express 2014, 22, 32526-32527; He Wen, Hongjun Zheng et al. Few-mode fibre-optic microwave photonic links [J]. Light: Science and Engineering. Applications 2017, 6, 8; Zheng Hongjun, Li Xin, Bai Chenglin, Transmission of chirped pulses in optical fibers, Beijing: Science Press, 2018, 1-184; Dong Qiuhuan, Liu Yang, Zheng Hongjun, Li Xin, Bai Chenglin, Hu Weisheng, Chen Nanguang. Research on few-mode multiplexing (demultiplexing) technology in mode division multiplexing system [J]. Journal of Liaocheng University (Natural Science Edition), 2020, 33(2): 50-67; Wang Xiao, Zheng Hongjun* (corresponding author) Cen Xia, Neng Bai, Ibrahim Ozdur, et al., Supermodes for optical transmission, Optics Express, 2011, 19(17): 16653-16664; Cen Xia, Neng Bai, Rodrigo Amezcua-Correa, et al., A multi-core strongly coupled supermode fiber has also attracted everyone's attention; multi-core supermode fiber is essentially a few-mode fiber with a larger effective mode field area, high mode field density, low mode-dependent loss, low mode coupling and low differential mode group delay [Cen Xia, Neng Bai, Ibrahim Ozdur, et al., Supermodes for optical transmission, Optics Express, 2011, 19(17): 16653-16664; Cen Xia, Neng Bai, Rodrigo Amezcua-Correa, et al.,Supermodes in strongly-coupled multi-core fibers,OFC2013,OTh3K.5;Yu Ruyuan,Hongjun Zheng et al.A novel three-ring-core few-modefiber with large effective area and low nonlinear coefficient[J],Optoelectronics Letters.2018,14(1):30-35];Pure silica core can effectively reduce fiber attenuation and fusion loss, and is currently mostly used in single-mode optical fiber (T.Hasegawa et al.2016.Advances in ultra-low loss silica fibers[J].Frontiers in Optics,paper FTu2B.2;S.Ten.2016.UltraLow-loss Optical Fiber Technology[J].Optical Fiber Communication Conference,paper Th4E.5;Yoshiaki Tamura.2018.Ultra-low loss silica core fiber for longhaul transmission[J].Optical Fiber Communication Conference,paper M4B.1). Few-mode fibers with graded-index profiles can achieve low differential mode group delay (DMGD) and the same mode width in the far field and near field, and the effective refractive index difference (ERID) between different modes is greater than 0.5x10. -3Mode coupling can be avoided (Pierre Sillard et al. Few-Mode Fibers for Space-Division Multiplexed Transmissions [J], European Conference & Exhibition on Optical Communication, 2013.03 (A1): 1-3; Roland Ryf. Switching and Multiplexing Technologies for Mode-Division Multiplexed Networks, Optical Fiber Communication Conference & Exposition, 2017, Tu2c). In summary, if the concepts of pure silica core, gradient refractive index distribution and multi-solid core strongly coupled supermode fiber are organically integrated, it is expected to solve the current research challenges of few-mode fiber, which has important academic and application value, great research significance and broad application prospects. Summary of the invention

[0003] With the support of the National Natural Science Foundation of China (No. 61671227 and 61431009), the Natural Science Foundation of Shandong Province (ZR2011FM015), and the special funds for the "Taishan Scholars" construction project, the present invention proposes a low-loss and low-crosstalk four-mode supermode optical fiber; this optical fiber combines the advantages of a pure silica core, a gradient refractive index distribution, and a multi-solid-core supermode optical fiber, and provides important support for in-depth research in the fields of optical fiber optics, optical fiber communications, optical fiber wireless access, optical information processing, and new generation information technology.

[0004] The technical solution adopted by the present invention to solve its technical problem is:

[0005] The present invention proposes a low-loss, low-crosstalk four-mode supermode optical fiber; the optical fiber consists of six cores with a gradient refractive index distribution, a refractive index groove and a fluorine-doped silica cladding; the horizontal line shadow area in the figure is the six cores, and the radius is R = R 1 =2.5μm, the lower diagonal shaded area is the trench refractive index area, R 2 =15μm, R 3 =25μm, the rest of the white part is the cladding, the radius R 4 =62.5μm. The center coordinates of the six fiber cores are (5μm, 0), (-5μm, 0), The refractive indices of the six pure silica core centers, the trench refractive index region, and the fluorine-doped silica cladding are n 1 =1.444024,n2 =1.412774,n 3 =1.419024. Each fiber core is n=1.444024[1-2Δ(r / a) 2 ] 0.5 Gradient refractive index distribution, r is the distance from any point in the core to the axis, a is the outer diameter of the core, and the refractive index difference between the core center and the cladding When the optical fiber is excited by the incident light, the optical fiber modes in the six cores are strongly coupled, achieving supermode operation with low loss, low crosstalk, large effective mode field area, low nonlinear coefficient, high mode field density, and low differential mode group delay, thereby further improving the optical fiber transmission performance; the six cores of the optical fiber are distributed in a ring shape as a whole, showing the characteristics of a ring-core optical fiber supporting a single radial mode and a large effective refractive index difference between modes; the mode field characteristics of the supermode in the optical fiber can be changed by changing the size, position and refractive index distribution of the core, cladding and depressed refractive index cladding.

[0006] The beneficial effects of the present invention are as follows:

[0007] 1. The fiber modes in the six fiber cores are strongly coupled, achieving supermode operation with high mode field density, low dispersion, and low differential mode group delay, thereby further improving the fiber transmission performance;

[0008] 2. The fiber's depressed refractive index cladding can effectively reduce bending losses;

[0009] 3. This optical fiber combines the advantages of pure silica core, graded refractive index distribution and multi-solid core supermode fiber, providing important support for in-depth research in the fields of fiber optics, fiber optic communications, fiber optic wireless access, optical information processing, and new generation information technology.

[0010] 4. The mode field characteristics of the supermode in the optical fiber can be changed by changing the size, position and refractive index distribution of the core, cladding and depressed refractive index cladding. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic cross-sectional view of a low-loss, low-crosstalk four-mode supermode optical fiber of the present invention; the optical fiber consists of six cores with gradient refractive index distribution (horizontal line shaded parts), a refractive index groove (right oblique shaded part) and a fluorine-doped silica cladding (white part).

[0012] Figure 2The electric field distribution of four supermode x polarizations, LP01, LP11a / LP11b (degenerate mode), LP21a / LP21b (degenerate mode) and LP31a at 1.55μm wavelength is given. The potential lines in the figure represent the strength of the incident light electric field. The greater the density, the stronger the electric field. If polarization degeneracy is considered, the proposed optical fiber can support 12 supermodes as follows: LP01 X, LP01 Y, LP11a X, LP11a Y, LP11b X, LP11b Y, LP21a X, LP21a Y, LP21b X, LP21b Y, LP31a X and LP31a Y.

[0013] Figure 3 The effective refractive index of four supermodes is shown as a function of input wavelength. The solid lines with squares, stars, solid dots and diamonds are the LP01, LP11, LP21 and LP31 supermode cases, respectively.

[0014] Figure 4 The variation of DMGD of LP11, LP21 and LP31 supermodes with input wavelength is shown. The solid line, dotted line and dash-dot line represent the variation of DMGD of LP11, LP21 and LP31 modes respectively. DETAILED DESCRIPTION

[0015] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings, but the protection scope is not limited thereto.

[0016] Example 1 Figure 1 Schematic diagram of the cross section of a low-loss, low-crosstalk four-mode supermode optical fiber of the present invention; the optical fiber consists of six cores with a gradient refractive index distribution (the horizontal line shaded area), a refractive index groove (the lower diagonal shaded area) and a fluorine-doped silica cladding (the white part); the horizontal line shaded area in the figure is the six cores, and the radius is R = R 1 =2.5μm, the lower diagonal shaded area is the trench refractive index area, R 2 =15μm, R 3 =25μm, the rest of the white part is the cladding, the radius R 4 =62.5μm. The center coordinates of the six fiber cores are (5μm, 0), (-5μm, 0), The refractive indices of the six pure silica core centers, the trench refractive index region, and the fluorine-doped silica cladding are n 1 =1.444024,n 2 =1.412774,n 3 =1.419024. Each fiber core is n=1.444024[1-2Δ(r / a) 2 ] 0.5Gradient refractive index distribution, r is the distance from any point in the core to the axis, a is the outer diameter of the core, and the refractive index difference between the core center and the cladding By adjusting the core size and refractive index distribution in the optical fiber, the electromagnetic field characteristics in the optical fiber can be changed.

[0017] Figure 2 The electric field distribution of four supermodes x polarization, LP01, LP11a / LP11b (degenerate mode), LP21a / LP21b (degenerate mode) and LP31a at 1.55μm wavelength, is given. The potential lines in the figure represent the strength of the incident optical electric field. The greater the density, the stronger the electric field. If polarization degeneracy and spatial degeneracy are considered, the proposed optical fiber can support 12 supermodes as follows: LP01 X, LP01Y, LP11a X, LP11a Y, LP11b X, LP11b Y, LP21a X, LP21a Y, LP21b X, LP21b Y, LP31a X and LP31a Y. We obtained six supermodes formed by strong coupling between core modes, in which the core-to-core distance is much shorter than that of traditional multi-core optical fibers. The field distribution of the supermode can be regarded as the superposition and strong reconstruction of the independent modes of each core. Therefore, six-core few-mode fiber can support a larger effective area and higher mode density than conventional fiber, which can effectively reduce the nonlinear effects of optical fiber and optical communication. Since the effective refractive index (ERI), dispersion, effective area and nonlinear coefficient of the degenerate modes are similar, this paper discusses the characteristics of the four supermodes.

[0018] Figure 3 The effective refractive index of four supermodes is shown as a function of input wavelength. The solid lines with squares, stars, solid dots and diamonds are the LP01, LP11, LP21 and LP31 supermode cases, respectively. Figure 3 It can be seen that the effective refractive indexes of the four supermode modes decrease with the increase of the wavelength of the incident light, and the decreasing speed of the effective refractive indexes of LP01, LP11, LP21 and LP31 increases in turn; under the condition of a given wavelength λ, the effective refractive index of the LP01 mode is the largest, the LP31 mode is the smallest, and the effective refractive index of LP11 is larger than that of LP21. When the wavelength of the incident light is 1.55μm, the effective refractive indexes of the LP01, LP11, LP21 and LP31 modes are 1.428869, 1.427842, 1.42558825 and 1.423996 respectively; the effective refractive index difference between the LP01 and LP11 modes is 1.03×10 -3 The effective refractive index difference between LP11 and LP21 modes is 2.25×10 -3 , the effective refractive index difference between LP21 and LP31 modes is 1.59×10 -3 The effective refractive index differences of the four supermodes are all greater than 0.5×10 -3According to the literature [Pierre Sillard, Denis Molin. Few-mode fibers for space-division multiplexed transmissions [C]. European Conference & Exhibition on Optical Communication, 2013, 03 (A1): 1-3; Zheng Hongjun, Li Xin, Bai Chenglin, Transmission of chirped pulses in optical fibers, Beijing: Science Press, 2018, 1-184], low mode crosstalk can be achieved.

[0019] Figure 4 The variation of DMGD of LP11, LP21 and LP31 supermodes with input wavelength is shown. The solid line, dotted line and dash-dot line represent the variation of DMGD of LP11, LP21 and LP31 modes respectively. In the wavelength range of 1.4μm to 1.8μm, the DMGD of LP11 mode is the smallest and the DMGD of LP31 mode is the largest for a given input wavelength. As the wavelength of incident light increases, the DMGD of LP11 mode increases slightly linearly, while the DMGD of LP21 and LP31 modes gradually increases in the wavelength range of 1.4μm to 1.7μm and tends to be flat in the wavelength range of 1.7μm to 1.8μm. At a wavelength of 1.55μm, the DMGD of LP11 mode is 7.00ps / m, the DMGD of LP21 mode is 20.35ps / m, and the DMGD of LP31 mode is 33.01ps / m. In summary, the proposed optical fiber realizes four supermode operations of low loss, low dispersion, low crosstalk and low DMGD. It should be pointed out that the specific implementation is only a more representative example of the present invention. Obviously, the technical solution of the present invention is not limited to the above-mentioned embodiment, and there are many variations. Those skilled in the art who can obtain the information clearly disclosed by the present invention or obtained without objection based on the written description of the document should be considered as the scope of protection of this patent.

Claims

1. A low-loss, low-crosstalk four-mode supermode optical fiber; characterized in that: The optical fiber consists of six cores with graded refractive index distribution, refractive index grooves and fluorine-doped silica cladding; the six cores have a radius of m, where the refractive index groove, inner radius m, outer radius 25 m, the outer side of the refractive index groove is provided with a cladding layer, the outer radius 62.5 m; the center coordinates of the six fiber cores are (5 ,0),(2.5 ,2.5 ), (-2.5 , 2.5 ), (-5 ,0),(-2.5 ,-2.5 ), (2.5 ,-2.5 ); the refractive indices of the six pure silica core centers, refractive index grooves, and fluorine-doped silica claddings are = 1.444024, = 1.412774, =1.419024; each fiber core has n = 1.444024 Gradient refractive index profile, is the distance from any point in the fiber core to the axis, is the outer diameter of the fiber core, and the refractive index difference between the core center and the cladding ; In 1.4 In the wavelength range from 1.4 μm to 1.8 μm, for a given input wavelength, the DMGD of the LP11 mode is the smallest, and the DMGD of the LP31 mode is the largest. With the increase of the wavelength of the incident light, the DMGD of the LP11 mode increases slightly linearly, and the DMGD of the LP21 and LP31 modes increases within a range of 1.4 μm to 1.8 μm. m to 1.7 m wavelength range gradually increases, at 1.7 m to 1.8 It tends to be flat in the m wavelength range; The wavelength is 1.55 When the fiber is excited by the incident light, the fiber modes in the six cores are strongly coupled, achieving supermode operation with low loss, low crosstalk, large effective mode field area, and low differential mode group delay, thereby further improving the fiber transmission performance.

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