Refractive index step type elliptical core few-mode optical fiber

Abstract

The utility model provides a refractive index step type elliptical core few-mode optical fiber, which is composed of a pure silicon dioxide elliptical fiber core with refractive index step distribution, a trench region and a cladding, and the ellipticity rho is equal to ax / ay equal to 1.6; spatial mode degeneracy is broken, mode-preserving operation of eight Hermite-Gaussian modes is realized, and the method has the advantages of low loss, low crosstalk and the like.

Description

Technical Field

[0001] This patent application relates to a novel optical fiber, proposing a step-index elliptical core few-mode optical fiber that can be applied to next-generation information technology fields such as mode division multiplexing. Background Technology

[0002] With the rapid development of various communication services, traditional single-mode optical fibers, limited by the nonlinear Shannon limit, can no longer meet the demands of more communication services. Mode division multiplexing (MDM) technology based on few-mode fibers has significantly improved the transmission capacity and spectral efficiency of single-mode fibers, becoming a hot topic in the field of optical fiber communication. As the transmission carrier of information in optical fiber communication systems, how to improve the transmission performance of few-mode fibers and reduce their loss and crosstalk is a pressing problem to be solved in optical transmission technology.

[0003] To address the limitation of single-mode fiber capacity, traditional circular-core few-mode fibers are continuously breaking through the transmission capacity or spectral efficiency of optical fibers. [Zhang Q, Han W, Xiong Z, et al. 1.3×34 Gb / s PDM-QPSK signalmode division multiplexing experiment based on fewmode fiber[J]. Optical Communication Technology, 2022, 46 (1): 77-80; RADEMACHER G, PUTTNAM BJ, LUIS RS, et al. 1.10.66 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-16QAMSignals[J].Journal of Lightwave Technology, 2020, 38: 2835-2841; WAKAYAMA Y, SOMAD, BEPPU S.266.1-Tbit / s Transmission over 90.4-km 6-Mode Fiber with InlineDual 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-BandWDM 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;] However, these round-core few-mode fibers suffer from mode degeneracy and crosstalk problems. Mode degeneracy and crosstalk require the use of multiple-input multiple-output digital signal processing (MIMO-DSP). As the number of modes increases, the complexity, computational load and cost of MIMO-DSP systems increase rapidly. To address this issue, a mode-preserving few-mode fiber was proposed. Mode-preserving few-mode fiber breaks mode degeneracy and effectively reduces mode crosstalk. Its systems can operate without MIMO-DSP, eliminating the need for multiple-input multiple-output digital signal processing, and are thus called MIMO-FREE or MIMO-LESS systems [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 Elliptical Core Optical Fibers,·inOptical Fiber Communication Conference, OSA Technical Digest(online)(OpticaPublishing Group, 2017), paper Tu2J.1; Yan G, Yanlei L, Xin L, et al. An Elliptical-Core Few-Mode Fiber with Low Loss and Low Crosstalk for the MIMO-FREEApplications[J].Frontiers in Physics, 2022, 9.]; In recent years, the study of mode-preserving few-mode optical fibers for MIMO-FREE systems has received widespread attention.

[0004] However, when the number of modes in a mode-preserving few-mode fiber in a MIMO-FREE system further increases, for example, to eight non-degenerate modes (described by Hermetic modes corresponding to elliptical cores: HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21), the large number of modes leads to severe mode degeneracy. For example, modes HG30 and HG02 exhibit severe degeneracy problems. Therefore, exploring changes to the core ellipticity ρ = a is necessary. x / a y The size further breaks the mode degeneracy, eliminates the complexity of multiple input multiple output digital signal processing (MIMO-DSP), and realizes MIMO-FREE applications. It has important academic and application value, and its research significance is great and its application prospects are broad. Summary of the Invention

[0005] Supported by the National Natural Science Foundation of China (Nos. 61671227 and 61431009), the Natural Science Foundation of Shandong Province (ZR2011FM015), and the "Taishan Scholar" Construction Project Funding, this patent application proposes a step-index elliptical core few-mode fiber. This fiber combines the advantages of a pure silica elliptical core and a trench structure, breaking the degeneracy of spatial modes, achieving mode preservation, eliminating complex MIMO-DSP processing, and realizing low-loss, low-crosstalk MIMO-FREE applications. It provides important support for in-depth research in the fields of fiber optics, fiber optic communication, fiber optic wireless access, optical information processing, and next-generation information technology.

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

[0007] A step-index elliptical core few-mode optical fiber, characterized in that: the fiber consists of a pure silica elliptical core with a step refractive index, a trench region, and a cladding; the horizontal radius of the elliptical core, i.e., the semi-major axis a, is... x = 6.96μm, vertical radius is the minor semi-axis a y =4.35μm, ellipticity ρ=a x / a y =1.6; where the trench cross-section is an elliptical ring structure, and the horizontal radius of the inner ellipse of the elliptical ring is the semi-major axis b. x =14.46μm, vertical radius (minor axis) by = 9.64μm, horizontal radius (major axis) of the outer ellipse c x =29.46μm, vertical radius (short semi-axis cy) =19.64μm, the rest is cladding, its outer cladding radius is R =62.5μm; the refractive indices of the pure silica core, cladding and trench region are n1 =1.4440, n2 =1.4236 and n3 =1.41949 respectively; this optical fiber uses an elliptical core to break the spatial mode degeneracy, and achieves low intrinsic loss, low crosstalk and low bending loss operation, thereby eliminating complex MIMO-DSP processing and realizing good transmission for MIMO-FREE applications.

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

[0009] 1. The elliptical fiber core and trench structure enable mode-preserving operation, break the degeneracy of spatial modes, eliminate complex MIMO-DSP processing, achieve good transmission in MIMO-FREE applications, and further improve fiber optic transmission performance.

[0010] 2. This optical fiber adopts an elliptical core structure, which improves the effective refractive index difference between modes and achieves low crosstalk characteristics between modes, enabling the optical fiber to transmit more modes.

[0011] 3. This optical fiber adopts a trench structure, which can effectively reduce bending loss.

[0012] 4. This optical fiber incorporates pure silicon dioxide as its core, achieving low loss and providing important support for in-depth research in fields such as fiber optics, fiber optic communication, fiber optic wireless access, optical information processing, and next-generation information technology. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the cross-section of a step-index elliptical core few-mode optical fiber according to this patent application; the optical fiber consists of an elliptical core (center shaded part) of pure silica, a trench (diagonal shaded part) and a cladding (white part).

[0014] Figure 2 The electric field distributions for X-polarization of the HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00 modes at a wavelength of 1.55 μm are presented. The isopotential lines in the figure characterize the strength of the incident electric field; the greater the density, the stronger the electric field.

[0015] Figure 3 The effective refractive index difference of eight modes at a wavelength of 1.55 μm is shown as a function of ellipticity ρ = a x / a y The changes are shown. The solid lines representing squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, and crosses are n respectively. HG00 -n HG10 n HG10 -n HG01 n HG01 -n HG20 n HG20 -n HG11 n HG11 -n HG30 n HG30 -n HG02 n HG02 -n HG21 and n HG21- n cladding How it changes with ellipticity.

[0016] Figure 4 This displays the variation of the effective refractive index with incident wavelength for eight modes. The solid lines with squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, crosses, and vertical lines represent the variations of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21, and the cladding CL, respectively.

[0017] Figure 5The diagram shows the variation of DMGD with incident wavelength for modes HG21, HG02, HG30, HG11, HG20, HG01, and HG10. The solid lines representing squares, hexagons, pentagrams, rhombuses, triangles, circles, and asterisks in the diagram represent the variations of DMGD for modes HG21, HG02, HG30, HG11, HG20, HG01, and HG10, respectively.

[0018] Figure 6 The intrinsic loss of eight Hermitian Gaussian modes varies with the input wavelength. The solid lines with squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, and crosses represent the intrinsic loss variations of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively.

[0019] Figure 7 The diagram shows how the bending loss of the HG21 mode changes with the bending radius. The solid lines with squares, asterisks, rhombuses, circles, and triangles represent the changes in bending loss with the bending radius when the angle θ between the major axis and the X-axis of the ellipse is 0°, 30°, 45°, 60°, and 90°, respectively.

[0020] Figure 8 The figure shows the variation of dispersion of Hermitian Gaussian modes HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00 with incident wavelength. The solid lines with circles, asterisks, and triangles in the figure represent the variations of material dispersion, waveguide dispersion, and total dispersion for each Hermitian Gaussian mode, respectively. Detailed Implementation

[0021] The technical solution of this patent application is described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto.

[0022] Example 1

[0023] Figure 1 This patent application discloses a step-index elliptical core few-mode optical fiber, characterized in that: the optical fiber consists of an elliptical core (central shaded portion) of pure silica, a trench (diagonally shaded portion), and a cladding (white portion); the horizontal radius of the elliptical core, i.e., the semi-major axis a... x = 6.96μm, vertical radius is the minor semi-axis a y =4.35μm, ellipticity ρ=a x / a y =1.6; the shaded area of ​​the outer elliptical ring is the Trench refractive index region, and the horizontal radius of the inner ellipse of the elliptical ring is the semi-major axis b. x =14.46μm, vertical radius (minor axis) by = 9.64μm, horizontal radius (major axis) of the outer ellipse cx = 29.46 μm, vertical radius is the minor semi-axis c y =19.64μm, the remaining white part is the cladding, and the radius of the outer cladding is R=62.5μm. The refractive indices of the pure silica fiber core, cladding and trench region are n1=1.4440, n2=1.4236 and n3=1.41949, respectively.

[0024] Figure 2 The electric field distributions for X-polarization of the HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00 modes at a wavelength of 1.55 μm are presented. The isopotential lines in the figure characterize the strength of the incident electric field; the greater the density, the stronger the electric field. The eight modes corresponding to this optical fiber are represented by Hermetic modes HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00. The proposed optical fiber has a clear mode-preserving function, realizing mode-preserving operation of the eight Hermetic modes HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00. The pure silica elliptical core and trench structure realize the operation of the eight Hermetic modes with large effective refractive index difference, low intrinsic loss, and low bending loss. The elliptical core structure breaks the mode degeneracy, realizes the mode-preserving function, eliminates the complex MIMO-DSP processing, and achieves good performance for MIMO-FREE applications.

[0025] Figure 3 The effective refractive index difference of the eight modes of this few-mode fiber at a wavelength of 1.55 μm is shown as a function of the core ellipticity ρ = a. x / a y The changes are shown in the diagram. The solid lines in the diagram representing squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, and crosses represent n respectively. HG00 -n HG10 n HG10 -n HG01 n HG01- n HG20 n HG20 -n HG11 n HG11 -n HG30 n HG30 -n HG02 n HG02 -n HG21 and n HG21 -n cladding The effect of ellipticity. It can be seen that the effective refractive index difference between the eight modes varies with ellipticity, at ellipticity ρ = a. x / a yWhen ρ = 1.58, the effective refractive index difference between all modes is greater than 1E-03. To better suit actual production, this patent selects the ellipticity of the optical fiber as ρ = a. x / a y =1.6, with ellipticity ρ=a x / a y =1.6, n HG00 -n HG10 n HG10 -n HG01 n HG01 -n HG20 n HG20 -n HG11 n HG11 -n HG30 n HG30 -n HG02 n HG02 -n HG21 and n HG21- n cladding The effective refractive index differences between the modes are 3.1555E-03, 2.7825E-03, 1.685E-03, 2.658E-03, 2.9535E-03, 1.286E-03, 1.019E-03, and 1.823E-03, respectively. The effective refractive index differences between the modes are all greater than 1E-03, further breaking the mode degeneracy and realizing MIMO-FREE applications.

[0026] Figure 4 The refractive index of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21, as well as the cladding CL, is shown as a function of wavelength. In the figure, squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, crosses, and vertical lines represent the variations of HG00, HG10, HG01, HG20, HG11, HG30, HG02, HG21, and the cladding CL mode as a function of incident wavelength, respectively. At 1.55 μm, the refractive indices of each mode and the cladding are 1.44096, 1.43781, 1.43502, 1.43334, 1.43068, 1.42773, 1.42644, 1.42542, and 1.4236, respectively. Among the eight modes in the C-band, HG00 has the maximum refractive index of 1.44101, and the HG21 mode has the minimum refractive index of 1.42517.

[0027] Figure 5The graph shows the variation of DMGD with incident wavelength for modes HG21, HG02, HG30, HG11, HG20, HG01, and HG10. The solid lines marked with squares, hexagons, pentagrams, rhombuses, triangles, circles, and asterisks represent the DMGD variations for modes HG21, HG02, HG30, HG11, HG20, HG01, and HG10, respectively. Within the wavelength range of 1.3μm to 1.4μm, the HG21 mode has the largest DMGD value; within the wavelength range of 1.45μm to 1.65μm, the HG30 mode has the largest DMGD value; within the wavelength range of 1.3μm to 1.6μm, the HG10 mode has the smallest DMGD value; and within the wavelength range of 1.61μm to 1.65μm, the HG02 mode has the smallest DMGD value. At a wavelength of 1.55μm, the DMGD values ​​of the HG21, HG02, HG30, HG11, HG20, HG01, and HG10 modes are 20.3234, 10.8017, 22.2697, 17.0651, 15.7605, 10.0887, and 6.96519 ps / m, respectively, all exhibiting significant differential mode group delays.

[0028] Figure 6 The intrinsic loss of eight Hermitian Gaussian modes varies with input wavelength. The solid lines marked with squares, asterisks, rhombuses, circles, triangles, pentagrams, hexagons, and crosses represent the intrinsic loss of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively. At a wavelength of 1.55 μm, the intrinsic losses of each mode are 0.146296, 0.148219, 0.151516, 0.151534, 0.15558, 0.15811, 0.166188, and 0.163181 dB / km, respectively. The proposed fiber intrinsic loss is significantly lower than that of other germanium-doped equivalent few-mode fibers.

[0029] Figure 7 The figure shows the bending loss of the HG21 mode as a function of the bending radius. The solid lines marked with squares, asterisks, rhombuses, circles, and triangles represent the bending loss as a function of the bending radius when the angle θ between the major axis of the ellipse and the X-axis is 0°, 30°, 45°, 60°, and 90°, respectively. The figure shows that the bending loss of the HG21 mode gradually decreases with increasing bending radius at 0°, 30°, 45°, 60°, and 90°. At a bending radius of 12 min, the bending losses of the HG21 mode at 0°, 30°, 45°, 60°, and 90° are 3.92897E-04, 1.63177E-03, 2.80324E-03, 2.54176E-03, and 1.68685E-03 dB / km, respectively, indicating that the proposed fiber exhibits low bending loss.

[0030] Figure 8 The figures show the dispersion of HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00 Hermitian Gaussian modes as a function of incident wavelength. Figures (a), (b), (c), (d), (e), (f), (g), and (h) show the dispersion of these modes, respectively. The solid lines marked with circles, asterisks, and triangles represent the variations in material dispersion, waveguide dispersion, and total dispersion for each Hermitian Gaussian mode, respectively. As can be seen from Figures (a), (b), (c), and (d), the total dispersion of modes HG21, HG02, HG30, and HG11 is relatively small in the wavelength range of 1.3 μm to 1.65 μm. As can be seen from Figures (e), (f), (g), and (h), the waveguide dispersion of modes HG20, HG01, HG10, and HG00 changes relatively smoothly. As can be seen from Figures (e), (f), (g), and (h), the material dispersion and total dispersion of modes HG20, HG01, HG10, and HG00 increase with the increase of incident wavelength. As can be seen from Figure (h), the total dispersion of mode HG00 gradually increases from 7.46508 ps / (nm·km) to 32.5574 ps / (nm·km). As can be seen from Figures (a), (b) and (c), the waveguide dispersion of HG21, HG02 and HG30 modes varies considerably in the wavelength range of 1.3 μm to 1.65 μm. For example, the waveguide dispersion of HG21 mode gradually decreases from 10.8608 ps / (nm·km) to -79.2142 ps / (nm·km).

[0031] In summary, the proposed optical fiber breaks spatial mode degeneracy, achieves multi-mode mode-preserving transmission, and has the advantages of low loss and low crosstalk. It should be noted that the specific implementation is merely a representative example of this technology; obviously, the technical solution of this technology is not limited to the above embodiments and many variations are possible. All methods explicitly disclosed in this technology or derived without objection from the written description in the documents by those skilled in the art should be considered within the scope of protection of this patent.

Claims

1. A step-index elliptical core few-mode optical fiber, characterized in that: The optical fiber consists of a pure silica elliptical core with a step refractive index, a trench region, and a cladding; the horizontal radius of the elliptical core is the semi-major axis a. x = 6.96μm, vertical radius is the minor semi-axis a y =4.35μm, ellipticity ρ=a x / a y =1.6; where the trench cross-section is an elliptical ring structure, and the horizontal radius of the inner ellipse of the elliptical ring is the semi-major axis b. x =14.46μm, vertical radius is the minor semi-axis b y = 9.64μm, the horizontal radius of the outer ellipse of the elliptical ring, i.e., the semi-major axis c x = 29.46 μm, vertical radius is the minor semi-axis c y =19.64μm, the rest is the cladding, and its outer cladding radius is R=62.5μm; the refractive indices of the pure silica core, cladding and trench region are n1=1.4440, n2=1.4236 and n3=1.41949, respectively; the optical fiber adopts an elliptical core to break the degeneracy of spatial modes, and achieves low intrinsic loss, low crosstalk and low bending loss operation, thereby eliminating complex MIMO-DSP processing and realizing good transmission for MIMO-FREE applications.

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