Microrod-based refractive index gradient elliptical core few-mode optical fiber

Abstract

The utility model provides a microrod-based refractive index gradient elliptical core few-mode optical fiber. The optical fiber is composed of an elliptical fiber core, a high-refractive-index microrod, an air hole and a cladding, wherein the elliptical fiber core is distributed in a gradient refractive index mode and takes a pure silicon dioxide core as a center; due to the large effective refractive index difference between modes and the structure of the elliptical fiber core and the air hole which are gradually distributed in the central refractive index of the pure silicon dioxide core, low-crosstalk, low-intrinsic-loss and low-bending-loss seven-Ermigauss mode operation is achieved, mode degeneracy is broken through through the elliptical fiber core and the microrod structure, the light field regulation and control and mode maintaining functions are achieved, and the optical fiber has a wide application prospect. Complex MIMO-DSP (Multiple Input Multiple Output-Digital Signal Processor) processing is eliminated, and good performance of MIMO-FREE application is realized.

Description

TECHNICAL FIELD

[0001] The patent application relates to a new type of optical fiber, and proposes a refractive index gradient type elliptical core few-mode optical fiber based on microrods, which can be applied to the field of new generation information technologies such as mode division multiplexing. BACKGROUND

[0002] With the rapid development of various communication services, the traditional single-mode optical fiber cannot meet the demand of more communication services due to the limitation of the nonlinear Shannon limit. The mode division multiplexing technology based on few-mode optical fiber can multiply the transmission capacity and spectral efficiency of single-mode optical fiber, and has become a hot spot in the field of optical fiber communication. As the transmission carrier of information in the optical fiber communication system, how to improve the transmission performance of the optical fiber and reduce the loss and crosstalk of the few-mode optical fiber is an urgent problem to be solved in optical transmission technology.

[0003] In view of the capacity limit of single-mode optical fiber, the traditional circular core few-mode optical fiber is constantly breaking through the transmission capacity or spectral efficiency of the optical fiber [Zhang Q, Han W, Xiong Z, et al. 3×34 Gb / s PDM-QPSK signal mode division multiplexing experiment based on few-mode fiber [J]. Optical Communication Technology, 2022, 46(1): 77-80; RADEMACHER G, PUTTNAM B J, R S, et al. 10.66 Peta-Bit / s Transmission over a 38-Core-Three-Mode Fiber. In Optical Fiber Communication Conference (OFC), OSA Technical Digest (Optical Society of America), 2020, Paper Th3H.1; BEPPU S, SOMA D, SUMITA S, et al. 402.7-Tb / s MDM-WDM Transmission over Weakly Coupled 10-Mode Fiber Using Rate-Adaptive PS-16QAM Signals [J]. Journal of Lightwave Technology, 2020, 38: 2835-2841; WAKAYAMA Y, SOMA D, BEPPU S. 266.1-Tbit / s Transmission over 90.4-km 6-Mode Fiber with Inline Dual C+L-Band 6-Mode EDFA [J]. Journal of Lightwave Technology, 2019, 37: 404-410; SOMA D, BEPPU S, WAKAYAMA Y, et al. 257-Tbit / s Weakly Coupled 10-Mode C+L-Band WDM Transmission [J]. Journal of Lightwave Technology, 2018, 36: 1375-1381; WEERDENBURG J, RYF R, ALVARADO-ZACARIAS J, et al. 138-Tb / s Mode-and Wavelength-Multiplexed Transmission over Six-Mode Graded-Index Fiber [J]. Journal of Lightwave Technology, 2018, 36: 1369-1374;], but these circular core few-mode fibers have the problems of mode degeneracy and crosstalk, and the mode degeneracy and crosstalk need to use multiple-input multiple-output digital signal processing (MIMO-DSP); with the increase of the number of modes, the complexity, the amount of calculation and the cost of the MIMO-DSP system increase rapidly.To solve this problem, a mode-preserving few-mode fiber has been proposed; the mode-preserving few-mode fiber breaks the mode degeneracy and effectively reduces mode crosstalk. Its system does not require MIMO-DSP, eliminating the need for multiple-input multiple-output digital signal processing, and is called a MIMO-FREE or MIMO-LESS system [Ezra Ip, Giovanni Milione, Ming - Jun Li, Neda Cvijetic, Konstantinos Kanonakis, Jeffery Stone, Gaozhu Peng, Xesús Prieto, Carlos Montero, Vicente Moreno, and Jesús "SDM transmission of real-time 10GbE traffic using commercial SFP+ transceivers over 0.5km elliptical-core few-mode fiber," Opt.Express 23, 17120-17126 (2015); G. Milione, E.Ip, P.Ji, Y.Huang, T.Wang, M.Li, J.Stone, and G.Peng, "MIMO-less Space Division Multiplexing with EllipticalCore Optical Fibers," in Optical Fiber Communication Conference, OSA TechnicalDigest(online)(Optica Publishing Group, 2017), paper Tu2J.1; Yan G, Yanlei L, XinL, et al.An Elliptical-Core Few-Mode Fiber with Low Loss and Low Crosstalk for the MIMO-FREE Applications[J].Frontiers in Physics, 2022, 9.]. The research on mode-preserving few-mode optical fiber for MIMO-FREE system has attracted widespread attention.

[0004] However, when the mode number of the MIMO-FREE system is further increased, for example, to seven non-degenerate modes (corresponding to Hermite-Gaussian modes HG00, HG10, HG01, HG20, HG11, HG02, HG30 with an elliptical core), the fiber will cause serious mode degeneration, such as the modes HG10 and HG01, and new mechanisms and methods for light field regulation must be explored to further break the mode degeneracy, eliminate complex MIMO-DSP, and achieve MIMO-FREE applications, which has important academic value and application value, and is of great significance and broad application prospects. Utility model content

[0005] Under the support of the National Natural Science Foundation (No. 61671227 and 61431009), the Shandong Provincial Natural Science Foundation (ZR2011FM015), and the special fund for the construction of the "Taishan Scholar", this patent application proposes a refractive index gradient type elliptical core few-mode fiber based on microrods. The fiber combines the advantages of an elliptical core with a pure silica core as the center of the refractive index gradient distribution, high refractive index microrods, and an elliptical core, breaks the degeneracy of spatial modes, realizes light field regulation and mode preservation, eliminates complex MIMO-DSP processing, realizes low-loss and low-crosstalk MIMO-FREE applications, and provides important support for in-depth research in the fields of fiber optics, fiber communication, fiber wireless access, optical information processing, and new generation information technology.

[0006] The technical solution adopted by this patent application to solve its technical problems is:

[0007] A refractive index gradient type elliptical core few-mode fiber based on microrods, characterized in that: the fiber is composed of an elliptical core with a graded refractive index distribution (diagonal line shaded part) with pure silica as the center, air holes (four point shaded circular hole parts on the XY axis), high refractive index microrods (horizontal line shaded part), and a cladding (white part); the horizontal radius (major axis) of the elliptical core is a x = 7.2 pm, the vertical radius (minor axis) is a y = 4.8 pm, and the ellipticity p = a x / a y= 1.5. The two circles on the X axis in the core are high refractive index microrod cross sections, the refractive indices of the two microrods are the same, and the radii of the microrods are completely the same, with a radius R1 = 0.72 μm; the four circles on the XY axis outside the core are air holes, in which the two holes on the X axis are completely the same, with a radius R3 = 7 μm; the two holes on the Y axis are also completely the same, with a radius R2 = 7.6 μm, and the rest is a cladding, with an outer cladding radius value R = 62.5 μm. The coordinates of the center of the core are (0, 0); the coordinates of the center of the high refractive index column on the X axis inside the core are (4.2E-6, 0) and (-4.2E-6, 0) respectively; the coordinates of the center of the two air holes on the X axis outside the core are (1.92E-5, 0) and (-1.92E-5, 0) respectively, and the coordinates of the center of the two air holes on the Y axis outside the core are (0, 1.74E-5) and (0, -1.74E-5) respectively; the refractive index distribution of the graded elliptical core follows the formula n(r) = n1*[1-2△(r / a) α ] 1 / 2 , (r≤a); in the formula, n1 is the pure silica core refractive index 1.4440 at the center of the elliptical core, r represents the distance of any point in the core to the axis, a represents the major axis of the core 7.2 μm, and α is the gradient parameter 2; the parameter△ = (n1 2 -n2 2 ) / (2n1 2 ); the refractive indices of the high refractive index microrod, the air hole refractive index region and the cladding on the X axis are n2 = 1.4600, n3 = 1 and n = 1.4178 respectively. The mode field characteristics of the spatial mode in the optical fiber can be changed by changing the refractive index distribution, size and position of the core and cladding.

[0008] The beneficial effects of the present patent application are as follows:

[0009] 1. The elliptical core and the high refractive index microrod break the degeneracy of the spatial mode, realize the light field regulation and mode preservation function, effectively improve the effective refractive index difference between the modes, are conducive to the low crosstalk transmission between the modes, eliminate the complex MIMO-DSP processing, realize the good transmission of MIMO-FREE application, and further improve the optical fiber transmission performance;

[0010] 2. The optical fiber adopts an air hole auxiliary region, which can effectively reduce the bending loss;

[0011] 3. The optical fiber fuses the pure silica core to realize low loss, and provides important support for in-depth research in the fields of optical fiber optics, optical fiber communication, optical fiber wireless access and optical information processing, new generation information technology and the like.

[0012] 4. The Hermite-Gaussian mode field characteristics in the optical fiber can be changed by changing the size, position and refractive index distribution of the core, cladding, high refractive index microrod and air hole. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a schematic cross-sectional view of a microrod-based graded-refractive-index elliptical-core few-mode optical fiber disclosed in this patent application; the optical fiber consists of an elliptical core (shaded portion with oblique lines) centered on a pure silica core with a graded-refractive-index distribution, air holes (the four dot-shaded circular holes on the XY axis), high-refractive-index microrods (shaded portion with horizontal lines), and a cladding (white portion).

[0014] Figure 2 The x-polarized electric field distributions for the HG30, HG02, HG11, HG20, HG01, HG10, and HG00 modes at 1.55 μm are shown. The contour lines in the figure represent the strength of the incident light's electric field; a higher density indicates a stronger electric field.

[0015] Figure 3 The graph shows how the effective refractive index of seven modes varies with input wavelength. The solid lines with squares, asterisks, diamonds, circles, triangles, six-pointed stars, and five-pointed stars represent the HG00, HG10, HG01, HG20, HG11, HG02, and HG30 modes, respectively.

[0016] Figure 4 The figure shows the variation of DMGD of the HG30, HG02, HG11, HG20, HG01, and HG10 modes with incident wavelength. The solid lines with hexagonal stars, pentagonal stars, triangles, circles, diamonds, and squares in the figure represent the variation of DMGD of the HG30, HG02, HG11, HG20, HG01, and HG10 modes, respectively.

[0017] Figure 5 The graph shows how the intrinsic loss of seven Hermite-Gaussian modes varies with input wavelength. The solid lines with hexagonal stars, pentagonal stars, triangles, circles, diamonds, asterisks, and squares represent the intrinsic loss variations of the HG30, HG02, HG11, HG20, HG01, HG10, and HG00 modes, respectively.

[0018] Figure 6 The dispersion of the Hermite-Gaussian modes of HG30, HG02, HG11, HG20, HG01, HG10, and HG00 varies with the incident wavelength. The solid lines with circles, asterisks, and triangles in the figure represent the variations in material dispersion, waveguide dispersion, and total dispersion for each Hermite-Gaussian mode, respectively. DETAILED DESCRIPTION

[0019] The technical solution of this patent application is described in detail below with reference to the embodiments and drawings, but the scope of protection is not limited thereto.

[0020] Example 1 Figure 1is a schematic diagram of a micro-rod-based graded-index elliptical-core few-mode fiber of the present patent application. The fiber is composed of an elliptical core (diagonal hatched part) with a graded-index distribution, air holes (four dot-hatched circular hole parts on the XY axis), high-index micro-rods (horizontal hatched part), and a cladding (white part) centered on a pure silica core; the horizontal radius (major semi-axis) of the elliptical core is a x = 7.2 μm, the vertical radius (minor semi-axis) is a y = 4.8 μm, the ellipticity p = a x / a y = 1.5. The two circles on the X axis in the core are high-index micro-rod cross sections, the two micro-rods have the same refractive index, and the micro-rod radii are exactly the same, with a radius R1 = 0.72 μm; the four circles on the XY axis outside the core are air holes, of which the two holes on the X axis are exactly the same, with a radius R3 = 7 μm; the two holes on the Y axis are also exactly the same, with a radius R2 = 7.6 μm, and the rest is the cladding, with an outer cladding radius value of R = 62.5 μm. The coordinates of the center of the core are (0, 0); the coordinates of the center of the high-index column on the X axis inside the core are (4.2E-6, 0) and (-4.2E-6, 0), respectively; the coordinates of the center of the two air holes on the X axis outside the core are (1.92E-5, 0) and (-1.92E-5, 0), respectively, and the coordinates of the center of the two air holes on the Y axis outside the core are (0, 1.74E-5) and (0, -1.74E-5), respectively; the graded-index elliptical core refractive index distribution follows the formula n(r) = n1*[1-2△(r / a) α ] 1 / 2 , (r≤a); in the formula, n1 is the refractive index of the pure silica core at the center of the elliptical core, 1.4440, r represents the distance of any point in the core to the axis, a represents the major semi-axis of the core, 7.2 μm, and a is the gradient parameter, 2; the parameter△= (n1 2 -n2 2 ) / (2n1 2 ); the refractive indices of the high-index micro-rod on the X axis, the air hole refractive index region, and the cladding are n2 = 1.4600, n3 = 1, and n = 1.4178, respectively. The mode field characteristics of the spatial mode in the fiber can be changed by changing the refractive index distribution, size, and position of the core and cladding.

[0021] Figure 2The x-polarization electric field distributions of HG30, HG02, HG11, HG20, HG01, HG10 and HG00 modes at 1.55 pm wavelength are given. The equipotential lines in the figure represent the strength of the incident light electric field, the greater the density, the stronger the electric field. HG30, HG02, HG11, HG20, HG01, HG10 and HG00 represent the corresponding seven modes of the optical fiber. The optical fiber proposed by us has clear mode maintaining function, realizing the mode maintaining operation of HG30, HG02, HG11, HG20, HG01, HG10 and HG00 seven Hermite-Gaussian modes; the large effective refractive index difference between Hermite-Gaussian modes, the gradually changing elliptical core with a pure silica core as the center and the air hole structure realize the low crosstalk, low intrinsic loss and low bending loss operation, the elliptical core and the micron rod structure break the mode degeneracy, realize the light field regulation and mode maintaining function, eliminate the complex MIMO-DSP processing, and realize the good performance of MIMO-FREE application.

[0022] Figure 3 The effective refractive indices of the seven modes of the few-mode optical fiber are shown as a function of the wavelength of the incident light. The solid lines with squares, stars, diamonds, circles, triangles, hexagons and pentagons represent the variation of HG00, HG10, HG01, HG20, HG11, HG02 and HG30 modes, respectively. Figure 3 As can be seen from the figure, the effective refractive indices of the seven modes decrease with the increase of the wavelength of the incident light, and the change of the effective refractive index is slow; for a given incident wavelength, the effective refractive index of HG00 mode is the largest, and the effective refractive index of HG30 mode is the smallest, and the effective refractive indices of HG00, HG10, HG01, HG20, HG11, HG02 and HG30 modes decrease in turn; when the wavelength of the incident light is 1.55 pm, the effective refractive indices of HG00, HG10, HG01, HG20, HG11, HG02 and HG30 modes are 1.43786, 1.43301, 1.43033, 1.42772, 1.42372, 1.42242 and 1.42006, respectively. When the wavelength of the incident light is 1.55 pm, the refractive index difference between HG10 and HG01 modes before regulation is 5.1300E-04, and the refractive index difference between the modes after regulation is 2.6825E-03, which is 5.23 times of the former; the refractive index difference between HG20 and HG11 modes before regulation is 6.8050E-04, and the refractive index difference between the modes after regulation is 3.9995E-03, which is 5.88 times of the former. The large effective refractive index difference between the modes realizes low mode crosstalk. The mode maintaining few-mode optical fiber with an elliptical core has many modes, which can cause serious mode degeneracy (such as the serious degeneracy problem of modes HG10 and HG01), and we introduce a high refractive index elliptical cylinder to realize light field regulation, further break the mode degeneracy, and realize MIMO-FREE application.

[0023] Figure 4 The variation of the DMGD of HG30, HG02, HG11, HG20, HG01 and HG10 modes with the incident wavelength is shown. The solid lines with hexagonal star, pentagonal star, triangle, circle, diamond and square in the figure respectively represent the variation of the DMGD of HG30, HG02, HG11, HG20, HG01 and HG10 modes. In the wavelength range of 1.3 μm to 1.5 μm, the DMGD value of HG30 mode is the largest, and the DMGD value of HG11 mode is the smallest; when the wavelength is 1.55 μm, the DMGD of HG30, HG02, HG11, HG20, HG01 and HG10 modes is respectively 21.7368 ps / m, 3.06342 ps / m, 1.55189 ps / m, 23.3712 ps / m, 2.72562 ps / m and 8.63374 ps / m, which has a large differential mode group delay.

[0024] Figure 5 It can be obtained that the intrinsic loss of each Hermite-Gaussian mode in the proposed few-mode fiber in the C-band is smaller than that of the germanium-doped equivalent fiber.

[0025] Figure 6 The variation of the dispersion of HG30, HG02, HG11, HG20, HG01, HG10 and HG00 Hermite-Gaussian modes with the incident wavelength is shown. The solid lines with circle, star and triangle in the figure respectively represent the variation of the material dispersion, waveguide dispersion and total dispersion of each Hermite-Gaussian mode. It can be seen from the figure that in the wavelength range of 1.3 μm to 1.6 μm, the waveguide dispersion of HG00 mode changes relatively smoothly, the material dispersion and total dispersion of HG00 mode increase with the increase of the incident wavelength, and the total dispersion of HG00 mode gradually increases from 4.41651 ps / (nm·km) to 26.3027 ps / (nm·km). Except for HG30 mode, the dispersion of other modes also changes relatively smoothly. In the wavelength range of 1.3 μm to 1.6 μm, the waveguide dispersion of HG30 mode changes greatly, the total dispersion of HG30 mode gradually decreases from -12.4831 ps / (nm·km) to -105.68 ps / (nm·km); at the wavelength of 1.55 μm, the total dispersion of HG30 mode is -72.7935.

[0026] In summary, the proposed optical fiber breaks the spatial mode degeneracy, realizes the light field modulation and mode maintaining functions, and has the advantages of low loss and low crosstalk. It should be pointed out that the specific embodiments are only relatively representative examples of the present technology, and obviously the technical solutions of the present technology are not limited to the above embodiments, but can also have many variations. Those skilled in the art, who obtain the scope of protection of the present patent without any doubt from the present technology explicitly disclosed or according to the written description of the file, should be considered as the scope of protection of the present patent.

Claims

1. A microrod-based graded-index elliptical-core few-mode optical fiber, characterized by: The optical fiber consists of an elliptical core with a gradient refractive index distribution, an air hole, a high refractive index micron rod and a cladding. The horizontal radius of the elliptical core, i.e., the long semi-axis, is a. x =7.2μm, the vertical radius, i.e. the short semi-axis, is a y =4.8μm, ellipticity ρ=a x / a y =1.5; the two circles on the X axis in the fiber core are cross sections of high refractive index microrods, which have the same refractive index and the same radius, R1 = 0.72 μm; the four circles on the X and Y axes outside the fiber core are air holes, of which the two holes on the X axis are exactly the same, with a radius of R3 = 7 μm; the two holes on the Y axis are also exactly the same, with a radius of R2 = 7.6 μm, and the rest is the cladding, whose outer cladding radius is R = 62.5 μm; the center coordinates of the fiber core are (0, 0); the center coordinates of the high refractive index column on the X axis inside the fiber core are (4.2E-6, 0) and (-4.2E-6, 0); the center coordinates of the two air holes on the X axis outside the fiber core are (1.92E-5, 0) and (-1.92E-5, 0), and the center coordinates of the two air holes on the Y axis outside the fiber core are (0, 1.74E-5) and (0, -1.74E-5); the refractive index distribution of the graded elliptical core follows the formula n(r) = n1*[1-2Δ(r / a) ɑ ] 1 / 2 , r≤a; n1 is the refractive index of the pure silica core at the center of the elliptical core is 1.4440, r is the distance from any point in the core to the axis, a is the semi-major axis of the core is 7.2μm, ɑ is the gradient parameter 2, parameter The refractive indices of the high-refractive-index microrods, air hole refractive-index region, and cladding on the X-axis are n2=1.4600, n3=1, and n=1.4178, respectively. The mode field characteristics of the spatial mode in the optical fiber can be changed by changing the refractive index distribution, size, and position of the core and cladding.

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