Four-core few-mode optical fiber with gradient refractive index distribution

Abstract

The invention provides a four-core few-mode optical fiber with gradient refractive index distribution. The optical fiber is composed of an elliptical fiber core with gradient refractive index distribution, a groove area and a cladding. The formed supermode breaks the degeneracy of a spatial mode, realizes a mode-preserving function, and has the advantages of low loss and low crosstalk.

Description

Technical Field

[0001] The present invention provides a four-core few-mode fiber with a graded refractive index distribution, which can be applied to new-generation information technology fields such as mode division multiplexing. Background Art

[0002] With the rapid development of various communication services, traditional single-mode fibers are no longer able to meet the needs of more communication services due to the limitation of the non-linear Shannon limit [ELLIS A D, ZHAO J, COTTER D. Approaching the Non-Linear Shannon Limit[J]. Journal of Lightwave Technology, 2010, 28(4): 423 - 433]. The mode division multiplexing technology based on few-mode fibers doubles the transmission capacity and spectral efficiency of single-mode fibers, becoming a hot topic in the current field of optical fiber communication. As the information transmission carrier in an optical fiber communication system, how to improve the transmission performance of the optical fiber and reduce the loss and crosstalk of the few-mode fiber are urgent problems to be solved in optical transmission technology.

[0003] Regarding the limit problem of the capacity of single-mode fibers, the currently proposed traditional circular-core few-mode fibers are continuously breaking through the transmission capacity or spectral efficiency of the optical fiber [RADEMACHER G, R S, PUTTNAM B J, et al. 93.34 Tbit / s / Mode (280 Tbit / s) Transmission in a 3-Mode Graded-Index Few-Mode Fiber. In Optical Fiber Communication Conference, OSA Technical Digest (Online) (Optical Society of America), 2018, Paper W4C.3; SOMAD, 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; 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; BEPPU S, SOMA D, SΜMITA 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; 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; 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]. However, the crosstalk and loss between modes of these round-core few-mode fibers still exist. To address the crosstalk between modes, multiple-input multiple-output digital signal processing (MIMO-DSP) is required. For short-distance communication, the more modes there are, the more serious the problems of the complexity, computational amount, and cost of MIMO-DSP become. The loss of the optical fiber will affect the transmission distance of the optical communication system, and the loss of the optical fiber is an important parameter determining the performance of the optical fiber communication system. To solve the crosstalk and loss in the optical fiber, elliptical-core few-mode fibers [PARMIGIANI F, JUNG Y, GRüNER-NIELSEN L, et al. Elliptical Core Few Mode Fibers for Multiple-Input MultipleOutput-Free Space Division Multiplexing Transmission[J]. IEEE Photonics Technology Letters, 2017, 29(21): 1764-1767; RIESEN N, LOVE J D, ARKWRIGHT J W. Few-Mode Elliptical-Core Fiber Data Transmission[J]. IEEE Photonics Technology Letters, 2012, 24(5): 344-346; IP E, MILIONE G, LI M, et al. SDM Transmission of Real-Time 10GbE Traffic using Commercial SFP+ Transceivers over 0.5km Elliptical-Core Few-Mode Fiber[J].Optics Express, 2015, 23(13): 17120 - 17126; TAN H, ZHANG J, LIU J, et al. Low-Loss Ring-Core Fiber Supporting 4 Mode Groups. In Conference on Lasers and Electro-Optics, OSA Technical Digest, 2019, Paper SM2L.4; SONG W, CHEN H, WANG J, et al. Panda Type Elliptical Ring Core Few-Mode Fiber[J]. Optical Fiber Technology, 2020, 60: 1 - 5; YANG T, ZHANG H, XI L, et al. Design of a Novel Bow-Tie Polarization Ring-Core Few-Mode Fiber for MIMO-Free MDM System. In 26th Optoelectronics and Communications Conference, P. Alexander Wai, H. Tam, and C. Yu, eds., OSA Technical Digest, 2021, Paper JS3C.3], elliptical ring-core few-mode fiber [WANG L, NEJAD R M, CORSI A, et al. Linearly Polarized Vector Modes: Enabling MIMO-Free Mode-Division multiplexing[J]. Optics Express, 2017, 25(10): 11736 - 11748; WANG L, LAROCHELLE S. Design of Eight-Mode Polarization-Maintaining Few-Mode Fiber for Multiple-Input Multiple-Output-Free Spatial Division Multiplexing[J]. Optics Letters, 2015, 40(24): 5846 - 5849] and panda-type few-mode fiber [YAN H, LI S, XIE Z, et al. Design of PAND Ring-Core Fiber with 10 Polarization-Maintaining Modes[J].Photonics Research, 2017, 5(1): 1-5; CAO Y, ZHAO Y, YU X, et al. Design and Characterization of 16-Mode PANDA Polarization-Maintaining Few-Mode Ring-Core Fiber for Spatial Division Multiplexing[J]. Optical Engineering, 2017, 56: 116102] has been proposed and proven to have the ability to separate spatial modes and reduce mode coupling.

[0004] Although the above-mentioned elliptical core few-mode fibers have achieved separation of spatial modes to a certain extent, the inter-mode crosstalk is still relatively large and they have relatively large intrinsic losses; the above-mentioned elliptical ring-core few-mode fibers are all polarization-maintaining few-mode fibers, where the x and y components in the modes are separated, and the relative refractive index difference between the modes is greater than 10 -4 order of magnitude, which can reduce the crosstalk between the xy components of the modes to a certain extent; if compensated by digital signal processing at the receiving end, it will still increase the complexity, computational load, cost, etc. of the short-distance transmission system. To address the above problems and considering the advantages of supermode fibers at the same time, the present invention proposes a four-core few-mode fiber with a graded refractive index distribution, which forms supermodes by using the strong coupling effect between the four cores and the elliptical core structure, quadrupling the transmission capacity and transmission spectral efficiency of a single fiber, and having lower bending losses and intrinsic losses, which can better separate spatial modes and reduce crosstalk between modes, and is applied to MIMO-FREE operation without multiple-input multiple-output. Summary of the Invention

[0005] Supported by the National Natural Science Foundation of China (Grant Nos. 61671227 and 61431009), the Natural Science Foundation of Shandong Province (ZR2011FM015), and the special funds for the "Taishan Scholars" construction project, this patent application proposes a four-core few-mode fiber with a graded refractive index distribution. This fiber combines the advantages of pure silica elliptical cores, supermode fibers, graded refractive index distributions, and trench assistance, breaks the degeneracy of spatial modes, eliminates complex MIMO-DSP processing, and realizes MIMO-FREE applications with low loss, low crosstalk, and low bending losses, providing 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 the present invention to solve its technical problems is as follows:

[0007] The present invention proposes a four-core few-mode optical fiber with a graded refractive index distribution; the optical fiber is composed of four elliptical cores, auxiliary grooves, and a cladding; the horizontal radius (major axis) of each elliptical core (the cross-hatched part) is a x = 2.25 μm, and the vertical radius (minor axis) is a y = 1.5 μm, and the ellipticity ρ = a x / a y = 1.5. The four elliptical cores are externally surrounded by a groove region with an elliptical ring structure (the right-hatched part), and the internal elliptical horizontal radius is R 1x = 14.4 μm, and the vertical radius is R 1y = 9.6 μm; the external elliptical horizontal radius is R 2x = 23.4 μm, and the vertical radius is R 2y = 15.6 μm, and the rest is the cladding (the white part), and the outer cladding radius value is R = 62.5 μm; the central coordinates of the four cores are (0, ), (0, ), ([[]] 0) and ([[]] 0); each core of the optical fiber adopts a graded refractive index distribution, following the formula: n(r) = n1·[1 - 2Δ(r / a) α ) 1 / 2 , (r ≤ a); in the formula, n1 represents the refractive index of pure silica at the core center, which is 1.4440, r represents the distance from any point in the core to the axis, a represents the major axis of the core, which is 2.25 μm, α is the graded parameter 2, and the parameter The refractive indices of the groove region and the cladding are n2 = 1.4055 and n3 = 1.4090 respectively; the spatial mode degeneracy of the four supermodes formed is broken, realizing four non-degenerate supermode operations with low intrinsic loss, low crosstalk, and low bending loss, so that complex MIMO-DSP processing can be eliminated, and good transmission for MIMO-FREE applications can be achieved; the mode field characteristics of the supermodes in the optical fiber can be changed by changing the refractive indices of the core and the cladding or the size, position, and refractive index distribution of the cladding. The beneficial effects of the invention are as follows:

[0008] 1. The elliptical core optical fiber forms supermodes, breaks the spatial mode degeneracy, realizes the mode-preserving function, can eliminate complex MIMO-DSP processing, realizes good transmission for MIMO-FREE applications, and further improves the optical fiber transmission performance.

[0009] 2. The optical fiber combines a pure silica core and a large effective refractive index difference distribution to achieve low loss and low mode crosstalk, providing important support for in-depth research in the fields of fiber optics, optical fiber communication, fiber wireless access, optical information processing, new generation information technology, etc.

[0010] 3. The modal field characteristics of the non-degenerate supermodes in this optical fiber can be changed by varying the refractive indices of the core, cladding, and trench regions, as well as the size, position, and refractive index distribution of the cladding. Description of the Drawings

[0011] Figure 1 is a schematic cross-sectional view of a four-core few-mode optical fiber with a graded refractive index distribution; the optical fiber consists of a pure silica elliptical core (the horizontally shaded part) with a graded refractive index distribution, a trench region (the outer slanted shaded circular ring region of the core), and a cladding (the white part).

[0012] Figure 2 gives the electric field distribution of the X polarization of four modes, namely LP01, LP11a, LP11b, and LP21b, at a wavelength of 1.55 μm. The equipotential lines in the figure characterize the strength of the incident optical electric field. The greater the density, the stronger the electric field.

[0013] Figure 3 shows the variation of the effective refractive indices of four spatial modes with the input wavelength. The solid lines with squares, triangles, diamonds, and circles are the variation curves of the LP01, LP11a, LP11b, and LP21b modes, respectively.

[0014] Figure 4 shows the variation of the DMGD of the LP11a, LP11b, and LP21b modes with the incident wavelength. The solid lines with circles, asterisks, and triangles in the figure represent the variation curves of the DMGD of the LP21b, LP11b, and LP11a modes, respectively.

[0015] Figure 5 shows the variation of the intrinsic losses of four spatial modes with the input wavelength. The solid lines with squares, diamonds, circles, and triangles are the variation curves of the intrinsic losses of the LP01, LP11a, LP11b, and LP21b modes, respectively.

[0016] Figure 6 shows the variation of the dispersion of the LP01, LP11a, LP11b, and LP21b modes with the incident wavelength. The solid lines with circles, asterisks, and triangles in the figure represent the variation curves of the material dispersion, waveguide dispersion, and total dispersion of each supermode, respectively.

[0017] Figure 7 shows the variation of the bending loss of the LP21b mode with the bending radius. The solid lines with squares, crosses, circles, diamonds, and pentagrams in the figure represent: the variation curves of the bending loss with 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. Detailed Embodiments

[0018] The technical solution of the present invention will be described in detail below in conjunction with embodiments and drawings, but the protection scope is not limited thereto.

[0019] Embodiment 1 Figure 1 is a schematic diagram of a four-core few-mode fiber with a graded refractive index distribution; the fiber consists of four elliptical cores, auxiliary grooves, and a cladding; the horizontal radius (major semi-axis) of each elliptical core (the cross-hatched part) is a x = 2.25 μm, and the vertical radius (minor semi-axis) is a y = 1.5 μm, and the ellipticity ρ = a x / a y = 1.5. Outside the four elliptical cores is a groove region (the right-slanted hatched part) with an elliptical ring structure. The inner elliptical horizontal radius is R 1x = 14.4 μm, and the vertical radius is R 1y = 9.6 μm; the outer elliptical horizontal radius is R 2x = 23.4 μm, and the vertical radius is R 2y = 15.6 μm. The rest is the cladding (the white part), and its outer cladding radius value is R = 62.5 μm; the central coordinates of the four cores are (0, ), (0, ), ( 0), and ( 0); each core of the fiber adopts a graded refractive index distribution, following the formula: n(r) = n1·[1 - 2Δ(r / a) α ) 1 / 2 , (r ≤ a); in the formula, n1 represents the refractive index of pure silica at the core center, which is 1.4440, r represents the distance from any point in the core to the axis, a represents the major semi-axis of the core, which is 2.25 μm, α is the graded parameter 2, and the parameter The refractive indices of the groove region and the cladding are n2 = 1.4055 and n3 = 1.4090 respectively; the spatial mode degeneracy of the formed supermodes is broken, and four non-degenerate supermode operations with low intrinsic loss, low crosstalk, and low bending loss are realized, so that complex MIMO-DSP processing can be eliminated and good transmission of MIMO-FREE applications can be achieved; the mode field characteristics of the supermodes in the fiber can be changed by changing the refractive indices of the core and the cladding or the size, position, and refractive index distribution of the cladding.

[0020] Figure 2The electric field distributions of the X polarization of four spatial supermodes, namely LP01, LP11a, LP11b, and LP21b, at a wavelength of 1.55 μm are given. The equipotential lines in the figure characterize the strength of the incident optical electric field. The greater the density, the stronger the electric field. In the case of fewer modes, since the mode distribution of the elliptical core few-mode fiber (EFMF) is similar to that of the traditional circular core few-mode fiber (CFMF), the mode symbols LP01, LP11a, LP11b, and LP21b of the CFMF are still used to represent the four corresponding modes of the EFMF. According to the definition of the mode-preserving function of the few-mode fiber in the literature [MILIONE G, IP E, JIP, et al. MIMO-less Space Division Multiplexing with Elliptical Core Optical Fibers. In Optical Fiber Communication Conference, OSA Technical Digest (Online), 2017; LIANG J, MO Q, FU S, et al. Design and Fabrication of Elliptical-Core Few-Mode Fiber for MIMO-Less Data Transmission[J]. Optics Letters, 2016, 41: 3058-3061], the EFMF we proposed has a clear mode-preserving function and realizes the mode-preserving operation of four non-degenerate modes, namely LP01, LP11a, LP11b, and LP21b.

[0021] Figure 3 Shows the variation of the effective refractive indices of the four supermodes of the few-mode fiber with the incident optical wavelength. The solid lines with squares, triangles, diamonds, and circles in the figure respectively represent the variation of the effective refractive indices of the LP01, LP11a, LP11b, and LP21b supermodes. Figure 3 It can be seen from [reference] that the effective refractive indices of the four supermodes all decrease with the increase of the incident optical wavelength, and the change of the effective refractive index is slow; for a given incident wavelength, the effective refractive index of the LP01 mode is the largest, and the effective refractive indices of the LP01, LP11a, LP11b, and LP21b modes decrease in turn; when the incident optical wavelength is 1.55 μm, the effective refractive indices of the LP01, LP11a, LP11b, and LP21b modes are 1.4245, 1.4202, 1.4187, and 1.4155 respectively. The large effective refractive index difference between the supermodes realizes low mode crosstalk.

[0022] Figure 4Shows the variation of DMGD of LP11a, LP11b and LP21b supermodes with the input wavelength. The solid lines with circles, asterisks and triangles respectively represent the variation of DMGD of LP21b, LP11b and LP11a supermodes. In the wavelength range from 1.3μm to 1.5μm, for a given input wavelength, the DMGD of the three supermodes increases with the increase of the input optical wavelength; in the wavelength range from 1.5μm to 1.66μm, the DMGD of the three supermodes decreases with the increase of the input wavelength; when the wavelength is 1.55μm, the DMGD of LP11a, LP11b and LP21b supermodes are 17.26ps / m, 25.55ps / m and 38.47ps / m respectively, having a relatively large differential mode group delay.

[0023] Figure 5 It can be obtained that the intrinsic losses of each supermode in the proposed elliptical four-core tapered few-mode fiber are small, and the intrinsic losses of LP01, LP11a, LP11b, and LP21b supermodes decrease in turn; the intrinsic loss of the proposed fiber is less than that of the elliptical four-core GeO₂-doped few-mode fiber.

[0024] Figure 6 Shows the variation of the dispersion of LP01, LP11a, LP11b and LP21b supermodes with the incident wavelength. The solid lines with circles, asterisks and triangles in the figure respectively represent the variation of material dispersion, waveguide dispersion and total dispersion of each spatial supermode. It can be seen from the figure that in the wavelength range from 1.3um to 1.65um, the waveguide dispersion of LP01 supermode changes relatively smoothly, and the material dispersion and total dispersion of LP01 supermode increase with the increase of the incident wavelength. In the wavelength range of the C band (1.53um to 1.565um), the total dispersion of LP01 supermode gradually increases from 0.26ps / (nm·km) to 3.29ps / (nm·km). In the wavelength range from 1.3um to 1.65um, the waveguide dispersion of LP11a supermode gradually decreases, and the material dispersion and total dispersion of LP11a supermode increase with the increase of the incident wavelength. In the wavelength range of the C band, the total dispersion of LP11a supermode gradually increases from 0.19ps / (nm·km) to 1.62ps / (nm·km). In the wavelength range from 1.3um to 1.65um, the waveguide dispersion of LP11b gradually decreases, the material dispersion gradually increases, and the total dispersion changes relatively smoothly; in the wavelength range of the C band, the total dispersion of LP11b supermode gradually decreases from -2.69ps / (nm·km) to -3.07ps / (nm·km). In the wavelength range from 1.3um to 1.65um, the waveguide dispersion and total dispersion of LP21b supermode gradually decrease, and the material dispersion gradually increases; in the wavelength range of the C band, the total dispersion of LP21b supermode gradually decreases from -6.11ps / (nm·km) to -10.40ps / (nm·km).

[0025] Figure 7 The figure shows how the bending loss of the LP21b supermode changes with the bending radius. The solid lines with squares, crosses, circles, diamonds and five-pointed stars in the figure respectively represent how the bending loss changes with 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°. It can be seen from the figure that the bending loss of the LP21b supermode gradually decreases with the increase of the bending radius at 0°, 30°, 45°, 60° and 90°; in the range of bending radius from 1mm to 3mm, as the angle θ increases, the bending loss of the LP21b supermode gradually decreases; when the bending radius is greater than 2mm, the corresponding bending losses at different angles θ are close to each other. The bending loss of the LP21b supermode at each angle decreases with the increase of the bending radius.

[0026] In summary, the proposed optical fiber realizes the operation of four non-degenerate supermodes with low intrinsic loss, low crosstalk and low bending loss. 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 four-core few-mode optical fiber with a graded refractive index distribution, characterized in that, The optical fiber consists of four elliptical cores, auxiliary grooves and a cladding; the horizontal radius (major semi-axis) of each elliptical core is a x = 2.25 μm, and the vertical radius (minor semi-axis) is a y = 1.5 μm, and the ellipticity ρ = a x / a y = 1.5; outside the four elliptical cores is a groove area with an elliptical ring structure. The inner elliptical horizontal radius is R 1x = 14.4 μm, and the vertical radius is R 1y = 9.6 μm; the outer elliptical horizontal radius is R 2x = 23.4 μm, and the vertical radius is R 2y = 15.6 μm. The rest is the cladding, and the outer cladding radius value is R = 62.5 μm; the central coordinates of the four cores are respectively and The refractive index distribution of each core of the optical fiber is graded and follows the formula: n(r) = n1·[1 - 2Δ(r / a) α 1 / 2 , (r ≤ a); in the formula, n1 represents the refractive index of pure silica at the core center, which is 1.4440, r represents the distance from any point in the core to the axis, a represents the major semi-axis of the core, which is 2.25 μm, α is the graded parameter 2, and the parameter The refractive indices of the groove area and the cladding are n2 = 1.4055 and n3 = 1.4090 respectively.​

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