A negative curvature ring fiber mode selective coupler

Abstract

The present invention relates to the field of optical fiber technology, and more specifically to a negative curvature annular optical fiber mode selective coupler. The coupler is composed of two identical negative curvature annular optical fibers bonded together. The bonded portion is the coupling region, and the unbonded portion forms four ports serving as input and output ports. The power ratio between the collinear output end and the cross output end is determined by the coupling length L. The cross-sections of the two negative curvature annular optical fibers are polished and then bonded together using a refractive index matching glue. By rationally designing the optical fiber structural parameters and appropriately selecting the center spacing and coupling length of the two optical fibers, the coupler can generate high-order OAM modes, including OAM 01 、OAM 11 、OAM 21 、OAM 31 There are 12 OAM modes in total. Compared with traditional fiber couplers, the present invention can still achieve higher coupling efficiency in high-order modes. By adjusting the coupling distance and input mode, it can also achieve adjustable splitting ratio and coupling mode.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber technology, and in particular to a negative curvature annular optical fiber mode selective coupler. Background Art

[0002] Common fiber optic components used in fiber optic communication systems include couplers, polarization controllers, gratings, and multiplexers. The most classic fiber optic component is the fiber coupler, which plays a key role in lasers, fiber optic communications, and sensing. A coupler is a passive device used for transmission and light splitting, enabling coupling, interference, and separation within fiber optic links. Depending on the type of mode generated, common couplers can be categorized as those used to generate linear polarization modes (LP) and those used to generate orbital angular momentum (OAM).

[0003] Currently, the most common fiber structure used in OAM couplers is conventional ring fiber. The high-refractive-index region of a ring fiber can significantly separate the effective refractive indices of adjacent modes, preventing high-order vector modes from coupling into LP modes and thus ensuring stable OAM mode transmission. However, achieving this high-refractive-index region requires a relatively complex doping process, which not only complicates the fiber fabrication process but also results in high mode transmission losses. Furthermore, conventional ring fiber OAM couplers have poor stress-strain characteristics, making them susceptible to external forces during the manufacturing process. Summary of the Invention

[0004] The purpose of the present invention is to provide a negative curvature annular fiber mode selective coupler, providing a mode selective coupler structure with a stable structure, capable of generating high-order OAM modes and low transmission loss.

[0005] To achieve the above objectives, the present invention provides a negative curvature annular fiber mode selective coupler, which is composed of two identical negative curvature annular optical fibers bonded together. The bonded portion is a coupling region, and the unbonded portion forms four ports serving as input and output ports, respectively. The power ratio between the collinear output port and the cross output port is determined by the coupling length L.

[0006] The cut surfaces of the two negative curvature annular optical fibers are both ground and then bonded using refractive index matching glue.

[0007] The negative curvature annular optical fiber has a cladding air hole outside the ring core, 6 hollow capillaries with a thickness of t1 are arranged outside the cladding air hole, and the outer cladding is a hollow capillary with a thickness of t2.

[0008] The base material of the negative curvature annular optical fiber is pure silica, and the refractive index n0 is 1.444 at a wavelength of 1.55 μm.

[0009] The inner circle radius of the ring core is r0 = 1.2 μm, and the outer circle radius is r1 = 3.3 μm.

[0010] The rectangular air holes of the outer cladding of the ring core have a width of d1 = 1.6 μm, a height of d2 = 1.4 μm, a spacing between two air holes of k = 2 μm, and a radius of r2 = 4.8 μm.

[0011] The thickness t1 of the hollow capillary outside the cladding air hole is 0.72 μm, and the outer radius thereof is r3 = 4.8 μm.

[0012] The center spacing distance D between the two ring cores in the coupling area is adjustable, and D is 8-9.6 μm.

[0013] The present invention provides a negative curvature annular fiber mode selective coupler. The coupler is composed of two identical negative curvature annular optical fibers bonded together. The bonded portion is the coupling zone, and the unbonded portion forms four ports, serving as input and output ports. The power ratio between the collinear output end and the cross output end is determined by the coupling length L. The cross-sections of the two negative curvature annular optical fibers are polished and then bonded together using a refractive index matching glue. By rationally designing the optical fiber structural parameters and appropriately selecting the center spacing and coupling length of the two optical fibers, the coupler can generate high-order OAM modes, including OAM 01 (<0, ±1>), OAM 11 (<±1, 1>), OAM 21 (<±2, ±1>), OAM 31 Compared to conventional fiber couplers, this invention can achieve higher coupling efficiency even in higher-order modes. By adjusting the coupling distance and input mode, it can also achieve adjustable splitting ratio and coupling mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0015] Figure 1 It is a structural schematic diagram of a negative curvature annular optical fiber mode selective coupler of the present invention.

[0016] Figure 2 The OAM of the specific embodiment of the present invention 11 Mode field diagram, phase diagram, and coupling efficiency diagram of the mode.

[0017] Figure 3 The OAM of the specific embodiment of the present invention 21 Mode field diagram, phase diagram, and coupling efficiency diagram of the mode.

[0018] Figure 4 The OAM of the specific embodiment of the present invention 31 Mode field diagram, phase diagram, and coupling efficiency diagram of the mode. DETAILED DESCRIPTION

[0019] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0020] See also Figure 1 The present invention provides a negative curvature annular fiber mode selective coupler, which is composed of two negative curvature annular optical fibers of the same structure bonded together. The bonded portion is the coupling region, and the unbonded portion forms four ports serving as input and output ports, respectively. The power ratio between the collinear output port and the cross output port is determined by the coupling length L.

[0021] The cut surfaces of the two negative curvature annular optical fibers are both ground and then bonded using refractive index matching glue.

[0022] The negative curvature annular optical fiber has a cladding air hole outside the ring core, 6 hollow capillaries with a thickness of t1 are arranged outside the cladding air hole, and the outer cladding is a hollow capillary with a thickness of t2.

[0023] The base material of the negative curvature annular optical fiber is pure silicon dioxide, and the refractive index n0 is 1.444 at a wavelength of 1.55 μm.

[0024] The inner circle radius of the ring core is r0 = 1.2 μm, and the outer circle radius is r1 = 3.3 μm.

[0025] The rectangular air holes of the outer cladding of the ring core have a width of d1 = 1.6 μm, a height of d2 = 1.4 μm, a spacing between two air holes of k = 2 μm, and a radius of r2 = 4.8 μm.

[0026] The thickness t1 of the hollow capillary outside the cladding air hole is 0.72 μm, and its outer radius is r3 = 4.8 μm.

[0027] The center spacing distance D between the two ring cores in the coupling area is adjustable, and D is 8-9.6μm.

[0028] The following is further described with reference to specific embodiments:

[0029] In this embodiment, the center-to-center distance between the two ring-shaped fiber cores is D=8.3 μm.

[0030] Under the above parameters, the negative curvature annular fiber mode selective coupler of the present invention can support up to OAM 01 (<0, ±1>), OAM 11 (<±1, 1>), OAM 21 (<±2, ±1>), OAM 31 The stable generation and transmission of 12 OAM modes including (<±3, ±1>) are shown in the figure. Figures 2 to 4 shown.

[0031] Figure 2 The OAM generated by the negative curvature ring fiber mode selective coupler at different lengths 11 Mode field diagram, phase diagram, and coupling efficiency diagram of the (<+1,+1>) mode. The input wavelength is 1.55 μm, the spacing distance is D = 8.3 μm, and when the input port 1 inputs OAM 11 In mode, both output ports of the coupler (port 3 and port 4) can generate OAM 11 mode, the specific power distribution is related to the fiber coupling length L, and the coupling efficiency at the cross output end is as high as 99.3%.

[0032] Figure 3 The OAM generated by the negative curvature ring fiber mode selective coupler at different lengths 21 Mode field diagram, phase diagram, and coupling efficiency diagram of the (<+2,+1>) mode. The input wavelength is 1.55 μm, the spacing distance is D = 8.3 μm, and when the input port 1 inputs OAM 21 In the mode, both output ends of the coupler can generate OAM 21 mode, the specific power distribution is related to the fiber coupling length L, and the coupling efficiency at the cross output end is as high as 98.8%.

[0033] Figure 4 The OAM generated by the negative curvature ring fiber mode selective coupler at different lengths 31 Mode field diagram, phase diagram, and coupling efficiency diagram of the (<+3,+1>) mode. The input wavelength is 1.55 μm, the spacing distance is D = 8.3 μm, and when the input port 1 inputs OAM 31 In the mode, both output ends of the coupler can generate OAM 31 mode, the specific power distribution is related to the fiber coupling length L, and the coupling efficiency at the cross output end is as high as 80.6%.

[0034] In summary, the present invention has the following advantages:

[0035] 1. The structure is stable, which not only preserves the fiber structure originally used to transmit the OAM mode, but also does not affect the integrity of the mode in the coupling area.

[0036] 2. Compared with current fiber couplers, it can achieve higher coupling efficiency and mode purity in high-order OAM modes, and has low insertion loss.

[0037] 3. By adjusting the coupling distance and input mode, the splitting ratio and coupling mode can be adjusted, laying the foundation for the construction of an all-fiber MDM communication system.

[0038] The above disclosure is merely one or more preferred embodiments of the present invention, and certainly cannot be used to limit the scope of the present invention. A person skilled in the art can understand that all or part of the processes of the above embodiments and equivalent changes made in accordance with the claims of the present invention still fall within the scope of the invention.

Claims

1. A negative curvature annular fiber mode selective coupler, characterized in that: It consists of two negative curvature annular optical fibers of the same structure bonded together. The bonded portion is the coupling area, and the unbonded portion forms four ports as input and output ports. The power ratio between the collinear output port and the cross output port is determined by the coupling length L. The longitudinal sections of the two negative curvature annular optical fibers are both ground and then bonded using refractive index matching glue; The negative curvature annular optical fiber has a cladding air hole outside the ring core, 6 hollow capillaries with a thickness of t1 are arranged on the outer layer of the cladding air hole, and the outer cladding is a hollow capillary with a thickness of t2; The outer ring radius of the ring core is r1 = 3.3 μm; The rectangular air holes of the outer cladding of the ring core have a height of d2 = 1.4 μm; The center spacing distance D between the two ring cores in the coupling area is adjustable, and D is 8-9.6μm.

2. The negative curvature annular fiber mode selective coupler according to claim 1, wherein: The base material of the negative curvature annular optical fiber is pure silicon dioxide, and the refractive index n0 is 1.444 at a wavelength of 1.55 μm.

3. The negative curvature annular fiber mode selective coupler according to claim 2, wherein: The inner circle radius of the ring core is r0=1.2 μm.

4. The negative curvature annular fiber mode selective coupler according to claim 3, wherein: The rectangular air holes of the outer cladding of the ring core have a width of d1 = 1.6 μm, a spacing distance between two air holes is k = 2 μm, and a radius of r2 = 4.8 μm.

5. The negative curvature annular fiber mode selective coupler according to claim 4, wherein: The thickness t1 of the hollow capillary outside the cladding air hole is 0.72 μm, and its outer radius is r3 = 4.8 μm.

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