Heat-insulation low-loss vortex structure optical multiplexing demultiplexer and optical communication system

Abstract

The invention discloses an adiabatic low-loss vortex structure optical multiplexing and demultiplexing device and an optical communication system, and belongs to the field of optical communication, and the optical multiplexing and demultiplexing device comprises a plurality of optical fiber vortex mode selection couplers with different orders, which are respectively used for exciting vortex mode groups with different orders; the optical fiber vortex mode selection coupler is formed by parallel fused biconical taper of a multimode optical fiber with a transition cladding and a single-mode optical fiber subjected to pre-tapering, the multimode optical fiber and the single-mode optical fiber form a directional coupling structure, and a fundamental mode in the single-mode optical fiber is matched with a vortex mode group in the multimode optical fiber in phase; the multimode optical fiber with the transition claddings comprises a fiber core and a plurality of transition claddings, the width of each layer is gradually increased from the fiber core to the outermost transition cladding, and the refractive index difference between every two adjacent layers is gradually decreased; and a plurality of optical fiber vortex mode selection couplers with different orders are cascaded to form a multi-stage multiplexing and demultiplexing structure. The heat insulation loss of the optical fiber mode selection coupler can be reduced, so that the optical fiber mode selection coupler supports more vortex mode numbers.

Description

Technical Field

[0001] This invention belongs to the field of optical communication, and more specifically, relates to an adiabatic, low-loss vortex structure optical multiplexer / demultiplexer and an optical communication system. Background Technology

[0002] Currently, the utilization of traditional optical wave dimension resources relied upon by optical communication systems, such as amplitude, phase, frequency / wavelength, time, and polarization state, is approaching saturation, leading to a bottleneck in system capacity improvement and presenting a "capacity crisis" challenge. To overcome this limitation, academia and industry are turning their attention to spatial division multiplexing (SDM) technology, viewing it as a key approach to further enhance optical communication capacity. This technology significantly increases the number of transmission channels by tapping into the multiplexing potential of optical signals in the spatial dimension. Its implementation mainly relies on two technical paths: one is mode division multiplexing (also known as mode-mode multiplexing), based on few-mode fiber, multimode fiber, or ring fiber, utilizing different spatial modes as independent channels for parallel transmission; the other is core division multiplexing, based on multi-core fiber, expanding channel capacity by integrating multiple fiber cores within a single cladding. These technologies collectively offer broad prospects for addressing future optical communication capacity demands. Traditional mode division multiplexing technology requires the assistance of multiple-input multiple-output digital signal processing (MIMO-DSP) technology due to the influence of mode coupling during propagation. However, due to the increasing capacity demands of short-range interconnect applications in recent years, weakly coupled mode division multiplexing (MDD) technology without MIMO-DSP assistance has attracted much attention due to its low cost advantages, supporting intensity modulation and direct detection (IM / DD). Throughout the entire link of a weakly coupled system, inter-mode crosstalk must be strictly suppressed. Vortex modes (OAM), because their different orders are orthogonal and can transmit stably in optical fibers, are commonly used mode bases in MMD technology. Therefore, developing OAM mode multiplexing / demultiplexers with low insertion loss and low mode crosstalk is crucial.

[0003] Compared to free-space multiplexing / demultiplexing schemes, all-fiber OAM mode multiplexers / demultiplexers offer advantages such as high integration, low complexity, and good compatibility with fiber optic communication systems. One common structure involves cascading different-order fiber mode selection couplers to achieve multiplexing / demultiplexing of different mode groups. These fiber mode selection couplers are typically manufactured using a fused biconical tapering (FBT) process, where two fibers are tapered in parallel until the guided mode overflows from the core and is converted into the cladding mode. Energy conversion between modes is achieved through coupling between the cladding modes. In FBT couplers, the cladding mode in the tapered waist needs to be converted into the guided mode in the untapered core region via transmission through the tapered region. Adiabatic conversion between the cladding and guided modes can only be achieved in the tapered region when the steepness of the tapered region is less than a certain value. This is currently the main reason limiting the loss of FBT couplers. As the mode order increases, the adiabatic conditions become more stringent. This limits the generation of higher-order vortex modes by FBT couplers, further limiting the number of vortex modes that can be multiplexed.

[0004] In summary, how to develop a multiplexer / demultiplexer with low thermal loss and the ability to support more vortex modes to improve optical communication capacity remains a pressing technical challenge. Summary of the Invention

[0005] To address the shortcomings and improvement needs of existing technologies, this invention provides a thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer and optical communication system. Its purpose is to reduce the thermal loss of the fiber optic mode selection coupler, enabling it to support a greater number of vortex modes.

[0006] To achieve the above objectives, according to one aspect of the present invention, a thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer is provided, comprising: multiple fiber vortex mode selection couplers of different orders, each used to excite a group of vortex modes of different orders. The fiber vortex mode selection coupler is made by parallel fused taper of a multimode fiber with a transition cladding and a pre-tapered single-mode fiber. The multimode fiber and the single-mode fiber in the fiber vortex mode selection coupler form a directional coupling structure, and the fundamental mode in the single-mode fiber is phase matched with the vortex mode group in the multimode fiber. Multimode optical fiber with transition cladding includes a core and multiple transition cladding layers. From the core to the outermost transition cladding layer, the width of each layer gradually increases, and the refractive index difference between adjacent layers gradually decreases. In a fiber vortex mode selection coupler, one end of a single-mode fiber serves as the input / output port for the mode group optical signal, and multiple fiber vortex mode selection couplers of different orders are cascaded to form a multi-stage multiplexing and demultiplexing structure.

[0007] Furthermore, in a multimode fiber with a transition cladding, the refractive index of multiple transition cladding layers is distributed in a multi-layered stepped manner, and the width of each layer gradually increases from the core to the outermost transition cladding layer, while the refractive index difference between adjacent layers gradually decreases, thereby making the refractive index of the entire transition cladding layer approach an exponential distribution; or, in a multimode fiber with a transition cladding layer, the transition cladding layer exhibits an exponentially graded refractive index distribution.

[0008] Furthermore, in a multimode fiber with a transition cladding, all transition cladding layers are positively doped, or in a multimode fiber with a transition cladding, some inner transition cladding layers are positively doped, while the remaining transition cladding layers are negatively doped.

[0009] Furthermore, the positive doping uses silicon dioxide material doped with germanium dioxide, titanium dioxide, and phosphorus pentoxide; Negative doping uses silicon dioxide materials doped with boron pentoxide or fluorine.

[0010] Furthermore, the core of a multimode fiber with a transition cladding has a step or gradient refractive index distribution, or the core of a multimode fiber with a transition cladding has a single-layer or multi-layer annular refractive index distribution.

[0011] Furthermore, a single-mode fiber pigtail is fused to one end of the multi-level multiplexing and demultiplexing structure.

[0012] Furthermore, a loop-core fiber pigtail is fused to the other end of the multi-level multiplexing and demultiplexing structure.

[0013] Furthermore, the radial order of the vortex mode groups excited by multiple fiber vortex mode selection couplers of different orders is 1; Furthermore, the loop fiber pigtail only supports the first-order radial vortex mode.

[0014] Furthermore, the loop-core fiber pigtail is a single-layer or multi-layer loop-core structure fiber.

[0015] According to another aspect of the present invention, an optical communication system is provided, including the thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer provided by the present invention.

[0016] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects: (1) In the vortex structure optical multiplexer / demultiplexer provided by the present invention, the fiber vortex mode selection coupler is made by parallel fused tapering of a multimode fiber with a transition cladding and a single-mode fiber with pre-tapering. The multimode fiber with transition cladding includes a fiber core and multiple transition claddings. From the fiber core to the outermost transition cladding, the width of each layer gradually increases and the refractive index difference between adjacent layers gradually decreases. Through this design, the transition cladding can effectively limit the optical field, reduce the sudden change of the mode field diameter (MFD) during the fused tapering process, thereby reducing the transmission loss in the taper region and optimizing the thermal insulation performance of the multimode fiber. This overcomes the harsh thermal insulation conditions of the high-order fiber vortex mode selection coupler, reduces the loss of the high-order fiber vortex mode selection coupler, and enables the vortex structure optical multiplexer / demultiplexer to support cascaded multiplexing and demultiplexing of more mode groups.

[0017] (2) In this invention, the transition cladding of the multimode fiber adopts a multi-layer stepped design with an exponential refractive index distribution or a gradually changing exponential refractive index distribution design, which can further optimize the thermal insulation performance of the multimode fiber during tapering.

[0018] (3) In this invention, the refractive index of multiple transition cladding layers in multimode fiber can be adjusted by combining positive doping and negative doping. This doping method increases the refractive index span of the transition cladding layers, broadens the design space of the transition cladding layers, and can further optimize the thermal insulation performance.

[0019] (4) In this invention, a single-mode fiber pigtail is fused to one end of a multi-stage multiplexing-demultiplexing structure formed by cascading multiple fiber vortex mode selection couplers of different orders. This single-mode fiber pigtail can excite OAM. 01 The mode group, in conjunction with multiple fiber vortex mode selection couplers of different orders, can further increase the number of supported modes.

[0020] (5) Due to the degeneracy of the first-order radial vortex mode and some higher-order radial modes in multimode fiber, even with precise control of the tapering parameters, it is difficult to avoid coupling and generating higher-order radial modes, resulting in significant crosstalk. In this invention, the other end of the multi-stage multiplexing / demultiplexing structure is fused with a loop-core fiber pigtail that only supports the first-order radial vortex mode to filter out higher-order radial modes. This increases the mode purity of the first-order radial vortex mode excited by the multiplexer and reduces mode crosstalk during the multiplexing / demultiplexing process. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of an adiabatic, low-loss vortex structure optical multiplexer / demultiplexer provided in an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the mode conversion principle in an optical fiber vortex mode selection coupler provided in an embodiment of the present invention.

[0023] Figure 3 A schematic diagram of the cross-section of a multimode optical fiber with a multilayer stepped transition cladding having a near-exponential refractive index distribution, provided for an embodiment of the present invention.

[0024] Figure 4 The refractive index distribution diagram of a multimode optical fiber with a multilayer stepped transition cladding and a near-exponential refractive index distribution is provided for embodiments of the present invention.

[0025] Figure 5 The curve showing the variation of the effective mode field diameter of a multimode fiber vortex mode with a tapered ratio and a multilayer stepped transition cladding with a near-exponential refractive index distribution, provided in an embodiment of the present invention.

[0026] Figure 6 The curve showing the variation of the effective mode field diameter of the multimode fiber vortex mode without transition cladding as a function of the tapering ratio, provided in an embodiment of the present invention.

[0027] Figure 7 The refractive index distribution diagram of the ring-core fiber that only supports the radial first-order vortex mode is provided for the embodiments of the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the 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 merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0029] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0030] To reduce the thermal loss of fiber optic mode selection couplers and enable them to support more vortex modes, this invention provides a thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer and optical communication system, which includes multiple fiber optic vortex mode selection couplers of different orders, each used to excite vortex mode groups of different orders. The fiber vortex mode selection coupler is made by parallel fused taper of a multimode fiber with a transition cladding and a pre-tapered single-mode fiber. The multimode fiber and the single-mode fiber in the fiber vortex mode selection coupler form a directional coupling structure, and the fundamental mode in the single-mode fiber is phase matched with the vortex mode group in the multimode fiber. Multimode optical fiber with transition cladding includes a core and multiple transition cladding layers. From the core to the outermost transition cladding layer, the width of each layer gradually increases, and the refractive index difference between adjacent layers gradually decreases. In a fiber vortex mode selection coupler, one end of a single-mode fiber serves as the input / output port for the mode group optical signal, and multiple fiber vortex mode selection couplers of different orders are cascaded to form a multi-stage multiplexing and demultiplexing structure.

[0031] In this invention, the transition cladding of the multimode fiber with transition cladding used in the fiber vortex mode selective coupler can effectively confine the optical field, reduce the abrupt change in mode field diameter (MFD) during fused taper, thereby reducing the transmission loss in the taper region and optimizing the thermal insulation performance of the multimode fiber. This overcomes the stringent thermal insulation conditions of high-order fiber vortex mode selective couplers, reduces the loss of high-order fiber vortex mode selective couplers, and enables the vortex structure optical multiplexer / demultiplexer to support cascaded multiplexing / demultiplexing of more mode groups.

[0032] Without loss of generality, the following embodiments are illustrated using a multiplexer / demultiplexer comprising four fiber vortex mode-selective couplers of different orders. It is readily understood that using OAM... mn When describing the mode group of vortex structured light, m ​​represents the angular order and n represents the radial order.

[0033] The following is an example.

[0034] Example 1: A thermally adiabatic, low-loss vortex structured optical multiplexer / demultiplexer, such as... Figure 1 As shown, it includes: OAM 11 OAM 21 OAM 31 OAM 41 Four different orders of fiber vortex mode selective couplers were proposed. Each coupler is fabricated by parallel fused tapering of a multimode fiber with a transition cladding and a pre-tapered single-mode fiber. The multimode and single-mode fibers form a directional coupling structure, with the fundamental mode of the single-mode fiber phase-matched to the vortex mode group of the multimode fiber. The multimode fiber with the transition cladding optimizes the thermal insulation performance during the tapering process. OAM 11 OAM 21 OAM 31 OAM 41 Four different orders of fiber vortex mode selection couplers are cascaded in sequence to form a multi-stage multiplexing and demultiplexing structure.

[0035] Specifically, OAM 11 OAM 21 OAM 31 OAM 41Four different-order fiber vortex mode selective couplers were fabricated by parallel fused tapering of single-mode fibers with different pre-tapering ratios and multi-mode fibers with multilayer transition claddings having near-exponential refractive index distributions to below 0.08 of their original diameter. This achieves a good balance between coupling performance and stability. In each fiber vortex mode selective coupler, the pre-tapering ratio of the single-mode fiber can be determined through simulation based on the desired mode set to be excited. OAM 11 OAM 21 OAM 31 OAM 41 Single-mode fiber OAM in four different order fiber vortex mode selection couplers 01 The mode groups are respectively connected to the OAM in the multimode fiber. 11 OAM 21 OAM 31 OAM 41 Pattern group phase matching.

[0036] To further increase the number of supported vortex modes, such as Figure 1 As shown in this embodiment, a single-mode fiber pigtail is fused to one end of a multi-stage multiplexing / demultiplexing structure formed by cascading four different-order fiber vortex mode selection couplers. This single-mode fiber pigtail is used to excite OAM. 01 Pattern group.

[0037] The tapering process cannot separate the inherently degenerate radial first-order vortex modes from the radial higher-order modes in multimode fiber, thus introducing significant crosstalk. To reduce crosstalk, a preferred implementation method is... Figure 2 As shown in this embodiment, a loop-core fiber pigtail is fused to the other end of the multi-level multiplexing-demultiplexing structure to filter out radial higher-order modes generated by the multi-level multiplexing-demultiplexing structure, thereby reducing crosstalk.

[0038] like Figure 1 As shown, the vortex structured optical multiplexer / demultiplexer provided in this embodiment offers 5 single-mode fiber ports and 1 ring-core fiber port, enabling OAM (Optical Amplification and Demultiplexing). 01 OAM 11 OAM 21 OAM 31 OAM 41 Low-loss, low-crosstalk multiplexing and demultiplexing of five vortex modes. Specifically, when 5-channel OAM... 01 The mode group optical signals are input from five single-mode fiber ports and are multiplexed into OAM by a vortex structured optical multiplexer / demultiplexer. 01 OAM 11 OAM 21 OAM 31 OAM 41Five sets of vortex mode signals are output via a ring-core optical fiber. Conversely, when OAM... 01 OAM 11 OAM 21 OAM 31 OAM 41 Five sets of vortex mode signals are input from the ring-core fiber port, and after passing through a vortex structured optical multiplexer / demultiplexer, they are demultiplexed into 5 OAM channels. 01 The mode group optical signals are output from the five single-mode fiber ports respectively.

[0039] Figure 2 The diagram shown is a schematic of the mode conversion principle in the fiber vortex mode selection coupler provided in an embodiment of the present invention, with circularly polarized OAM input in a single-mode fiber. 01 The mode can be viewed as x-polarized and y-polarized HE. 11 by The phase difference is superimposed. HE modes with the same subscript have the same propagation constant in their two polarization directions. When the phase matching condition is met, the two linearly polarized HE modes... 11 The modes will be respectively associated with higher-order vector modes in multimode fiber. The odd and even modes are coupled, and because of the two linearly polarized HE... 11 Pattern carrying The phase difference means that the parity and even modes of the higher-order vector modes coupled from the multimode fiber will also have the same phase difference, thus resulting in a combined topological charge number in the fiber. The synthesis of higher-order circularly polarized vortex modes is not unique; higher-order circularly polarized vortex modes can also be synthesized by superimposing EH modes in multimode fiber.

[0040] Optionally, in this embodiment, the multimode optical fiber with transition cladding specifically includes three transition cladding layers and one outer cladding layer. Figure 3 The diagram shows a cross-sectional view of the multimode optical fiber. Specifically, from the inside out, it includes: fiber core 1, first transition cladding 2, second transition cladding 3, third transition cladding 4, and outer cladding 5.

[0041] In a preferred embodiment, the refractive index of the multiple transition cladding layers in the multimode fiber with transition cladding is distributed in a multi-layer stepped manner. From the fiber core to the outermost transition cladding layer, the width of each layer gradually increases, and the refractive index difference between adjacent layers gradually decreases. This makes the refractive index distribution of the entire transition cladding layer approach an exponential distribution, thereby further optimizing the thermal insulation performance of the multimode fiber during tapering. Figure 4The diagram shows the refractive index distribution of a multimode fiber with a near-exponential refractive index distribution and multiple stepped transition cladding layers provided in this embodiment. The fiber core 1 adopts a square-law gradient refractive index distribution, with a center refractive index of 1.459 and an edge refractive index of 1.446, and a core diameter of 25µm. Starting from the first transition cladding layer outside the fiber core, the width of each layer gradually increases, and the refractive index difference between adjacent layers gradually decreases, forming a near-exponential refractive index distribution. The first transition cladding layer 2 has a refractive index of 1.446 and an outer diameter of 28.2µm; the second transition cladding layer 3 has a refractive index of 1.441 and an outer diameter of 33.7µm; the third transition cladding layer 4 has a refractive index of 1.438 and an outer diameter of 43.1µm; and the outer cladding layer 5 has a refractive index of 1.437 and an outer diameter of 62.5µm. The fiber core 1 and the first transition cladding 2 are positively doped, and can be silicon dioxide materials doped with germanium dioxide, titanium dioxide, or phosphorus pentoxide. The second transition cladding 3, the third transition cladding 4, and the outer cladding 5 are negatively doped, and can be silicon dioxide materials doped with boron pentoxide or fluorine. By combining positive and negative doping, the refractive index range of the transition cladding is increased, and the design space of the transition cladding is broadened. Compared with existing positively doped transition claddings, the thermal insulation performance can be further optimized.

[0042] Figure 5 The figure shows the curve of the effective mode field diameter (MFD) of the vortex mode in a multimode fiber with a near-exponential refractive index distribution and a multilayer stepped transition cladding, as a function of the taper ratio, provided in this embodiment. Because the addition of the transition cladding restricts the optical field, it reduces the abrupt change in the mode field diameter (MFD) during fused tapering, thereby reducing transmission loss in the taper region and optimizing the thermal insulation performance of the multimode fiber. Figure 6 The figure shows the curve of the effective mode field diameter of a traditional multimode fiber vortex mode without a transition cladding, as a function of the taper ratio. (Comparison) Figure 5 and Figure 6 It can be seen that the first to fourth order vortex modes of the multimode fiber with a multilayer stepped transition cladding with a near-exponential refractive index distribution provided in this embodiment have good thermal insulation performance.

[0043] In other embodiments of the present invention, the refractive index of the transition cladding in the multimode fiber with transition cladding is exponentially distributed, and the refractive index in the transition cladding is gradually distributed, which can further optimize the thermal insulation performance of the multimode fiber during tapering.

[0044] Figure 7 The figure shown is a refractive index distribution diagram of a loop-core fiber pigtail that only supports the first radial vortex mode, provided by an embodiment of the present invention. Since the loop-core fiber pigtail has only one narrow loop core, it only supports the first radial vortex mode and can support up to the fifth vortex mode, which can meet the requirements of the vortex structure optical multiplexer / demultiplexer provided by the embodiment of the present invention.

[0045] It is easy to understand that when the type and number of vortex mode selection couplers change, the specific implementation of the vortex structured optical multiplexer / demultiplexer is similar to that in this embodiment.

[0046] Example 2: An optical communication system includes the thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer provided in Embodiment 1 above.

[0047] Because the vortex structure optical multiplexer / demultiplexer in the optical communication system provided in this embodiment can support more modes, the optical communication system provided in this embodiment has a larger optical communication capacity.

[0048] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thermally adiabatic, low-loss vortex structure optical multiplexer / demultiplexer, characterized in that, include: Multiple fiber vortex mode selection couplers of different orders are used to excite vortex mode groups of different orders respectively; The fiber vortex mode selection coupler is made by parallel fused taper of a multimode fiber with a transition cladding and a pre-tapered single-mode fiber. The multimode fiber and the single-mode fiber in the fiber vortex mode selection coupler form a directional coupling structure, and the fundamental mode in the single-mode fiber is phase-matched with the vortex mode group in the multimode fiber. The multimode fiber with transition cladding includes a core and multiple transition claddings. From the core to the outermost transition cladding, the width of each layer gradually increases, and the refractive index difference between adjacent layers gradually decreases. In the fiber vortex mode selection coupler, one end of the single-mode fiber serves as the input / output port of the mode group optical signal, and multiple fiber vortex mode selection couplers of different orders are cascaded to form a multi-stage multiplexing and demultiplexing structure.

2. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 1, characterized in that, The refractive index of the multiple transition cladding layers in the multimode fiber with transition cladding is distributed in multiple steps, and the width of each layer gradually increases from the core to the outermost transition cladding layer, while the refractive index difference between adjacent layers gradually decreases, so that the refractive index of the entire transition cladding layer approaches an exponential distribution; or, the transition cladding layers in the multimode fiber with transition cladding layer have an exponentially graded refractive index distribution.

3. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 2, characterized in that, In the multimode fiber with transition cladding, all transition cladding layers are positively doped, or in the multimode fiber with transition cladding layers, some inner transition cladding layers are positively doped, while the remaining transition cladding layers are negatively doped.

4. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 3, characterized in that, The positive doping uses silicon dioxide material doped with germanium dioxide, titanium dioxide, and phosphorus pentoxide; The negative doping is achieved using silicon dioxide material doped with boron pentoxide or fluorine.

5. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 1, characterized in that, The core of the multimode fiber with transition cladding has a step or gradual refractive index distribution, or the core of the multimode fiber with transition cladding has a single-layer or multi-layer annular refractive index distribution.

6. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in any one of claims 1 to 5, characterized in that, One end of the multi-level multiplexing and demultiplexing structure is fused with a single-mode optical fiber pigtail.

7. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 6, characterized in that, The other end of the multi-level multiplexing and demultiplexing structure is fused with a loop-core optical fiber pigtail.

8. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 7, characterized in that, The radial order of the vortex mode groups excited by the multiple fiber vortex mode selection couplers of different orders is 1. Furthermore, the loop-core fiber pigtail only supports the radial first-order vortex mode.

9. The adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in claim 8, characterized in that, The loop-core fiber pigtail is a single-layer or multi-layer loop-core structure fiber.

10. An optical communication system, characterized in that, Includes the adiabatic, low-loss vortex structure optical multiplexer / demultiplexer as described in any one of claims 1 to 9.

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