Graded refractive index distribution topology constraint ring core optical fiber

Abstract

The invention discloses a graded index distribution topology constraint ring core optical fiber, and belongs to the field of optical fiber communication. By designing the annular fiber core area to be in gradient refractive index distribution, the problem of effective mode degradation in the step refractive index distribution topology constraint ring core fiber is effectively solved, so that better performance is shown in the aspect of reducing nonlinearity, and low-loss transmission of a plurality of cut-off orbital angular momentum modes is supported at 1550 nm.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber communication, and more specifically, relates to a graded refractive index distribution topological constraint ring-core optical fiber. Background Art

[0002] In the pursuit of improving optical communication capacity, great efforts have been made in exploring wavelength, frequency, and phase multiplexing technologies. In the past decade, researchers have attempted to further expand the capabilities of optical fiber communication through space-division multiplexing. Space-division multiplexing increases the communication capacity by increasing the number of available data transmission paths in a single optical fiber, including two methods: core-division multiplexing and mode-division multiplexing. Although significant progress has been made in core-division multiplexing, its capacity and the number of cores are gradually approaching saturation. In theory, mode-division multiplexing can achieve the transmission of an infinite number of modes by utilizing orthogonal states with multiple overlapping spaces, and mode-division multiplexing based on orbital angular momentum modes has been widely studied. Researchers have proposed many multimode optical fiber structures that support the transmission of orbital angular momentum modes, including hollow-core optical fibers, photonic crystal fibers, and ring-core optical fibers. However, mode coupling occurs between two modes with the same effective refractive index, which will affect the stability of mode transmission. This problem can be solved by increasing the effective refractive index difference between the two modes. In addition, the mode coupling problem can also be effectively solved by multi-input multi-output digital signal processing. However, when more modes and a larger inter-mode time delay difference are involved, the complexity of mode coupling and signal processing will increase significantly.

[0003] In 2023, Zelin Ma of Boston University discovered a new photoconduction mechanism beyond traditional cut-off - the topological constraint mechanism, and verified the practical utility of these modes at a wavelength of 1550 nm. This mechanism supports the low-loss transmission of multiple orbital angular momentum cut-off modes in a step refractive index distribution ring-core optical fiber without using multi-input multi-output digital signal processing technology, and the mode coupling is unexpectedly naturally reduced. These modes are also called topological constraint modes. However, there is a problem of deterioration of the effective mode area in this type of refractive index optical fiber, which poses a challenge to increasing the non-linear resistance and there is still room for improvement. Summary of the Invention

[0004] Aiming at the defects existing in the prior art, the purpose of the present invention is to propose a graded refractive index distribution topological constraint ring-core optical fiber, aiming to solve the problem of deterioration of the effective mode in the step refractive index distribution topological constraint ring-core optical fiber.

[0005] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0006] The present invention provides a graded refractive index distribution topological confinement ring-core optical fiber, which includes a central region, an annular core, and a cladding region. The annular core region has a graded refractive index distribution. The central region and the cladding region have the same refractive index, and the refractive index of the annular core is greater than that of the central and cladding regions.

[0007] Furthermore, the refractive index distribution of the graded refractive index distribution topological confinement ring-core optical fiber can be expressed as:

[0008]

[0009] where n co is the maximum refractive index of the core, n cl is the refractive index of the central region and the cladding region, r1 is the inner ring radius, r2 is the outer ring radius, is the relative refractive index difference.

[0010] Furthermore, the outer ring radius range of the annular core is 25 - 35 μm, the inner ring radius range is 5 - 15 μm, and the refractive index difference between the maximum refractive index of the core and the refractive index of the cladding region is not less than 0.04.

[0011] Furthermore, the mode basis in the graded refractive index distribution topological confinement ring-core optical fiber is the optical vortex mode.

[0012] Furthermore, the materials of the central region and the cladding region are fluorine-doped silica, and the material of the annular core region is a combination of germanium-doped silica and fluorine-doped silica with a gradually changing fluorine doping concentration and germanium doping concentration.

[0013] Furthermore, the graded refractive index distribution topological confinement ring-core optical fiber supports multiple cut-off orbital angular momentum modes without mode coupling.

[0014] Furthermore, the effective refractive index difference between the cut-off spin-orbit alignment mode and the spin-orbit anti-alignment mode in the graded refractive index distribution topological confinement ring-core optical fiber is > 10 -4 .

[0015] Furthermore, for the cut-off orbital angular momentum mode in the graded refractive index distribution topological confinement ring-core optical fiber, its confinement loss is almost negligible.

[0016] Furthermore, for the cut-off orbital angular momentum mode in the graded refractive index distribution topological confinement ring-core optical fiber, its effective mode area shows a stable trend.

[0017] Generally speaking, compared with the prior art by the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

[0018] (1) The present invention proposes a novel graded refractive index distribution topological constraint ring-core optical fiber. By designing the annular core region with a graded refractive index distribution, the problem of effective mode degradation in the step refractive index distribution topological constraint ring-core optical fiber is effectively solved, thereby showing better performance in reducing non-linearity and supporting low-loss transmission of multiple cutoff orbital angular momentum modes at 1550 nm.

[0019] (2) The graded refractive index distribution topological constraint ring-core optical fiber provided by the present invention can effectively suppress the mode coupling between the cutoff spin-orbit aligned mode and the spin-orbit anti-aligned mode, and their effective refractive index differences are all > 10 -4 . Brief Description of the Drawings

[0020] Figure 1 is a cross-sectional view of a graded refractive index distribution topological constraint optical fiber with different refractive index distribution parameters.

[0021] Figure 2 is a refractive index distribution diagram of a graded refractive index distribution topological constraint optical fiber with different refractive index distribution parameters.

[0022] Figure 3 is a curve of the confinement loss variation of the topological constraint modes supported by a graded refractive index distribution topological constraint optical fiber with different refractive index distribution parameters.

[0023] Figure 4 is a curve of the effective refractive index difference variation between the spin-orbit aligned mode and the spin-orbit anti-aligned mode of a graded refractive index distribution topological constraint optical fiber with different refractive index distribution parameters.

[0024] Figure 5 is a curve of the effective mode area variation of the topological constraint modes supported by a graded refractive index distribution topological constraint optical fiber with different refractive index distribution parameters. Detailed Embodiments

[0025] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] The present invention provides a graded refractive index distribution topologically constrained optical fiber, including a central region, an annular core, and a cladding region. The annular core region has a graded refractive index distribution, the central region and the cladding region have the same refractive index, and the refractive index of the annular core is greater than that of the central and cladding regions. This optical fiber can not only support low-loss transmission of multiple cutoff orbital angular momentum modes at 1550 nm, but also effectively solve the problem of effective mode degradation in step refractive index distribution topologically constrained ring core optical fibers. In addition, this optical fiber structure can effectively increase the effective refractive index difference between the cutoff spin-orbit aligned mode and the spin-orbit anti-aligned mode to suppress the mode coupling between them.

[0027] Specifically, the refractive index distribution of the graded refractive index distribution topologically constrained ring core optical fiber can be expressed as:

[0028]

[0029] where n co is the maximum refractive index of the core, n cl is the refractive index of the central region and the cladding region, r1 is the inner ring radius, r2 is the outer ring radius, is the relative refractive index difference.

[0030] Specifically, the outer ring radius of the annular core ranges from 25 to 35 μm, the inner ring radius ranges from 5 to 15 μm, and the refractive index difference between the maximum refractive index of the core and the refractive index of the cladding region is not less than 0.04.

[0031] Embodiment

[0032] In this embodiment, the inner ring radius r1 of the annular optical fiber is 15 μm, the outer ring radius is r2 = 25 μm, and the refractive index difference between the maximum refractive index of the core and the refractive index of the cladding region is n co -n cl = 0.04. The value of g ranges from 1 to 4, and its optical fiber cross-section and refractive index distribution are as shown in Figure 1 and Figure 2 shown.

[0033] The refractive index distribution of the graded refractive index distribution topologically constrained optical fiber with different refractive index distribution parameters can be written as:

[0034]

[0035] where g is the refractive index distribution parameter, n co is the maximum refractive index of the core, n cl is the refractive index of the central region and the cladding region, r1 is the inner ring radius, r2 is the outer ring radius, is the relative refractive index difference.

[0036] The variation curves of the confinement losses of the topologically confined modes supported by the graded-index distributed topologically confined fiber with different refractive index distribution parameters are as Figure 3 shown. Where Lc is the topological charge number of the last confined mode. For g = 1, 2, 3, 4, |Lc| = 19, 22, 24, 25. As g increases, the topological charge number of the first topologically confined mode becomes larger, and the number of topologically confined modes becomes more.

[0037] The variation curves of the effective refractive index differences between the cutoff spin-orbit aligned mode and the spin-orbit anti-aligned mode in the graded-index distributed topologically confined fiber with different refractive index distribution parameters are as Figure 4 shown. As g increases, the effective refractive index difference gradually becomes larger and shows an increasing trend. When g = 3, the effective refractive index differences are all greater than 1×10 -4 .

[0038] The variation curves of the effective mode areas of the topologically confined modes supported by the graded-index distributed topologically confined fiber with different refractive index distribution parameters are as Figure 5 shown. As g increases, the effective mode area initially shows an increasing trend until g = 3, when the mode area stabilizes at 695 μm 2 , and then the mode area shows a decreasing trend. When g = ∞, the graded-index distribution becomes a step-index distribution, and the decreasing trend of the effective mode area intensifies. Considering the above, when g is 3, the performance of the graded-index distributed topologically confined ring-core fiber is the best. Therefore, the graded-index distributed topologically confined fiber proposed in this embodiment effectively solves the problem of the reduction of the effective mode area in the step-index distributed topologically confined fiber while maintaining good characteristics.

[0039] Those skilled in the art can easily understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A topologically constrained ring-core optical fiber with a gradient refractive index distribution, characterized in that: From the inside to the outside, it includes a central area, an annular core area and a cladding area. The annular core area has a gradient refractive index distribution. The central area and the cladding area have the same refractive index, and the refractive index of the annular core is greater than the refractive index of the central area and the cladding area.

2. The graded refractive index profile topologically constrained ring core optical fiber according to claim 1, characterized in that: The refractive index distribution expression of the optical fiber is: Where n co is the maximum refractive index of the ring core, n cl is the refractive index of the central region and the cladding region, r1 is the inner radius of the annular core, r2 is the outer radius of the annular core, is the relative refractive index difference.

3. The graded refractive index profile topologically constrained ring core optical fiber according to claim 2, characterized in that: The outer ring radius of the annular fiber core is in the range of 25-35 μm, and the inner ring radius is in the range of 5-15 μm.

4. The graded refractive index profile topologically constrained ring core optical fiber according to claim 2, characterized in that: The difference between the maximum refractive index of the annular core and the refractive index of the cladding region is not less than 0.

04.

5. The graded refractive index profile topologically constrained ring core optical fiber according to claim 1, characterized in that: The mode base is the cut-off optical vortex mode.

6. The graded refractive index profile topologically constrained ring core optical fiber according to claim 1 or 5, characterized in that: The material of the central region and the cladding region is fluorine-doped silica, and the material of the annular core region is a combination of germanium-doped silica and fluorine-doped silica with gradient fluorine and germanium doping concentrations.

7. The graded refractive index profile topologically constrained ring core optical fiber according to claim 5, characterized in that: The optical fiber supports multiple cutoff optical vortex modes without mode coupling, and the effective refractive index of the cutoff spin-orbit alignment mode and the cutoff spin-orbit anti-alignment mode in the optical fiber is both greater than 10 -4 .

8. The graded refractive index profile topologically constrained ring core optical fiber according to claim 5, characterized in that: The effective area of ​​the mode in the optical fiber presents a stable trend as the topological charge number increases.

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