Weakly-coupled multi-ring few-mode fiber with suppressed radial high-order modes

Abstract

The application discloses a weak-coupling multi-ring few-mode optical fiber for compressing a radial high-order mode, comprising a middle core, a first ring core, a second ring core and a cladding, wherein: the outer periphery of the middle core is provided with the first ring core, the outer periphery of the first ring core is provided with the second ring core, and the outer periphery of the second ring core is provided with the cladding; the refractive index of the first ring core is higher than the refractive index of the middle core and the refractive index of the second ring core. In one aspect, the application utilizes a multi-layer structure to regulate the high-order OAM mode to a suitable position, so that as many radial first-order OAMs as possible can reach the threshold condition of weak-coupling transmission. In another aspect, the multi-layer structure disperses the refractive index jump, and to a certain extent, improves the purity of the OAM mode.

Description

Technical Field

[0001] This invention relates to the field of optical fiber communication, and more specifically, to a weakly coupled multi-ring few-mode optical fiber with compressed radial higher-order modes. Background Technology

[0002] With the rapid development of the information age, projects and industries such as the Internet of Things, AI, smart cities, and "Eastern Data, Western Computing" require a large amount of data transmission and computing. As an important strategic resource, data centers carry a large amount of data storage, transmission, and computing. The demand for data traffic in data centers is growing rapidly in the information age. However, the capacity of traditional single-mode optical fibers used in data centers is approaching the physical limit defined by Shannon's theorem due to limitations such as fiber splice damage and nonlinear noise. In an era of rapid growth in information traffic, how to effectively improve the transmission capacity of optical fibers is one of the core issues in the field of optical fiber communication.

[0003] Space division multiplexing (SDM) technology offers a new direction for increasing fiber optic capacity. Among these technologies, few-mode fiber (FMF) offers the highest channel integration density per unit space and can be better integrated with single-mode fiber, making it an effective way to improve fiber optic transmission capacity. Mode division multiplexing in FMF typically employs two approaches: strongly coupled transmission and weakly coupled transmission. The former requires coherent detection and Multiple-Input Multiple-Output (MIMO) algorithms to demultiplex the transmitted signals. Therefore, as the number of channels increases, using strongly coupled transmission significantly increases the complexity of the receiving system. In contrast, each mode in the latter can function as an independent channel, without interference. Within data centers, Intensity Modulation and Direct Detection (IM / DD) technology is used for communication transmission, which is less complex than coherent optical communication. With the increase in transmission mode channels within the fiber, to reduce system complexity and better integrate with single-mode fiber systems within the data center, high-capacity weakly coupled FMF can better address the capacity limitations of data centers.

[0004] Orbital Angular Momentum (OAM) mode is a new mode supported by FMF. OAM modes with different topological charges are orthogonal to each other. In recent years, OAM mode has made significant breakthroughs in communication capacity density and compatibility. Radial first-order OAM mode is more conducive to integrated multiplexing and demultiplexing design due to its highly symmetrical intensity and phase distribution. Traditional OAM mode uses ring-core fiber for transmission. The core of the ring-core fiber is recessed into the cladding to completely suppress radial higher-order modes. By limiting the thickness of the ring core, the radial dimension of the OAM mode can be effectively controlled. However, as the number of transmission modes required by the fiber increases, the refractive index doping concentration of the fiber needs to be further increased to meet the suppression of higher-order modes and provide sufficient effective refractive index difference for all transmitted OAM modes. The literature (Optics Communications, 499 (2021) 127314) describes that while ring-core fiber suppresses radial higher-order modes, it also reduces the effective refractive index difference between radial first-order modes. Furthermore, according to the literature (OpticsExpress, 23(23), 29331, 2015), excessively high refractive index abrupt changes can increase the intrinsic crosstalk of the OAM mode and reduce the purity of the mode. Summary of the Invention

[0005] This invention provides a weakly coupled multi-ring few-mode fiber with compressed radial high-order modes, enabling high-capacity radial first-order OAM mode signal transmission under limited refractive doping concentration.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] A weakly coupled multi-ring few-mode fiber with compressed radial high-order modes includes a core, a first ring core, a second ring core, and a cladding, wherein:

[0008] The outer periphery of the core is provided with the first ring core, the outer periphery of the first ring core is provided with the second ring core, and the outer periphery of the second ring core is provided with the cladding layer;

[0009] The refractive index of the first ring core is higher than that of the middle core and the second ring core.

[0010] In the aforementioned technical means, the weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes has three doped regions with different refractive index concentrations, which are used to effectively compress the effective refractive index interval between higher-order modes, thereby releasing the effective refractive index difference between the required radial first-order mode and its adjacent modes, thus realizing weakly coupled transmission of more radial first-order OAM modes. The refractive index of the first ring core is higher than that of the middle core and the second ring core, which is used to ensure the number of fiber transmission modes.

[0011] In a further embodiment, the radius of the core is r = 3.43 ± 0.1 μm.

[0012] In a further embodiment, the radius of the first ring core is r = 7.02 ± 0.1 μm.

[0013] In a further embodiment, the radius of the second ring core is r = 9.75 ± 0.1 μm.

[0014] In a further embodiment, the outer diameter of the cladding is 62.5 μm, and the cladding material is fused silica.

[0015] In a further embodiment, the difference between the refractive index of the core and the refractive index of the cladding is Δ1 = 1.4307 ± 0.01%.

[0016] In a further embodiment, the difference between the refractive index of the first ring core and the refractive index of the cladding is Δ2 = 1.4978 ± 0.01%.

[0017] In a further embodiment, the difference Δ3 between the refractive index of the second ring core and the refractive index of the cladding is 1.1107 ± 0.01%.

[0018] In the aforementioned technical means, the refractive index of the central core and the second ring core is slightly lower than that of the first ring core, which is used to adjust the effective refractive index difference between the modes.

[0019] Among the aforementioned technical means, when the operating wavelength is 1310nm, the few-mode fiber can support 7 radial first-order transmission modes and the effective refractive index difference between it and the adjacent modes is greater than 0.1%.

[0020] In a further scheme, the radii and refractive indices of the central core, the first ring core, and the second ring core are adjusted according to the number of supported transmission modes.

[0021] As the number of supported modes increases, the effective refractive index difference between radially higher-order modes can be selectively compressed by adjusting the radius and refractive index doping of the central core, the first ring core, and the second ring core. This increases the allocatable amount of the effective refractive index difference between the radially first-order mode and its adjacent modes, achieving an effective refractive index difference of 1×10⁻⁶ between the radially first-order mode and its adjacent modes. -3 above.

[0022] Increasing the effective refractive index difference between the first-order radial mode and its adjacent modes, i.e., compressing the effective refractive index spacing between higher-order radial modes within a limited effective refractive index spacing (under certain doping concentration constraints), is crucial. As fiber doping concentration increases, the number of modes accommodated in the fiber increases, with higher-order radial modes far outnumbering first-order radial modes. If only first-order radial modes are used for transmission, compressing the effective refractive index spacing between higher-order radial modes can free up more allocation space, especially given the increased number of higher-order radial modes. Using three-layer or more structures can maximize the effective refractive index difference between more higher-order radial modes to be much less than 1×10⁻⁶. -3 This allows for the allocation of effective refractive index spacing between the radial first-order mode and adjacent modes.

[0023] In a further scheme, the effective refractive index between the radial first-order mode and its adjacent modes is extracted. The minimum effective refractive index difference is used as the target value for optimization by the optimization algorithm. The fiber structure is iteratively optimized, and different fiber structures are used during the iteration process to make the target value reach a threshold above the desired threshold. The weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes can support 7 radial first-order modules, and the effective refractive index difference between modules is greater than 1×10⁻⁶. -3 Weakly coupled module transmission can be used.

[0024] When transmitting OAM mode, the three-layer fiber structure disperses abrupt changes in refractive index, suppresses intrinsic crosstalk of the mode, and improves the purity of the transmitted OAM mode.

[0025] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0026] The weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes proposed in this invention has a three-ring structure. On one hand, the three-ring structure effectively adjusts the refractive index of the transmission modes within the fiber. Instead of completely suppressing unused radial higher-order modes, it compresses the effective refractive index intervals between them and places the entire set of higher-order modes in a position that does not interfere with the weakly coupled transmission of the radial first-order modes. This adjusts the higher-order OAM modes to appropriate positions, allowing as many radial first-order OAM modes as possible to reach the threshold condition for weakly coupled transmission. On the other hand, the three-ring structure mitigates abrupt changes in the overall refractive index of the fiber, improving the purity of the OAM modes to a certain extent. Attached Figure Description

[0027] Figure 1 The cross-sectional diagram and refractive index distribution diagram of the weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes proposed in this embodiment of the invention are shown.

[0028] Figure 2 This is a schematic diagram of the purity of OAM mode (HE mode synthesis) of weakly coupled multi-ring few-mode fiber with compressed radial high-order mode proposed in an embodiment of the present invention.

[0029] Figure 3 This is a schematic diagram of the purity of OAM mode (EH mode synthesis) of weakly coupled multi-ring few-mode fiber with compressed radial high-order mode proposed in an embodiment of the present invention.

[0030] In the diagram, 1 is the core, 2 is the first ring core, 3 is the second ring core, and 4 is the cladding. Detailed Implementation

[0031] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0032] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0033] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0035] Example 1

[0036] This embodiment provides a weakly coupled multi-ring few-mode fiber with compressed radial high-order modes, such as... Figure 1 As shown, it includes a central core 1, a first ring core 2, a second ring core 3, and a cladding layer 4, wherein:

[0037] The outer periphery of the core 1 is provided with the first ring core 2, the outer periphery of the first ring core 2 is provided with the second ring core 3, and the outer periphery of the second ring core 3 is provided with the cladding 4.

[0038] The refractive index of the first ring core 2 is higher than that of the middle core 1 and the second ring core 3.

[0039] In this embodiment of the invention, the weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes has three doped regions with different refractive index concentrations. These regions are used to effectively compress the effective refractive index interval between higher-order modes, thereby releasing the effective refractive index difference between the required radial first-order mode and its adjacent modes. The first ring core 2 mainly controls the number of modes accommodated inside the fiber. The middle core 1 and the second ring core 3 adjust the effective refractive index of each mode to different degrees. Under the premise of conforming to physical laws, the effective refractive index difference between radial higher-order modes is minimized as much as possible, so that the radial first-order mode has a larger effective refractive index difference with its adjacent modes, thereby realizing the weakly coupled transmission of the radial first-order mode. The refractive index of the first ring core 2 is higher than that of the middle core 1 and the second ring core 3, which is used to ensure the number of transmission modes in the fiber.

[0040] Example 2

[0041] This embodiment, based on Embodiment 1, continues to disclose the following content:

[0042] The radius of the core 1 is r1 = 3.43 ± 0.1 μm.

[0043] The radius of the first ring core 2 is r2 = 7.02 ± 0.1 μm.

[0044] The radius of the second ring core 3 is r3 = 9.75 ± 0.1 μm.

[0045] The outer diameter of the cladding 4 is 62.5 μm, and the material of the cladding 4 is fused silica.

[0046] The difference between the refractive index of the core 1 and the refractive index of the cladding 4 is Δ1 = 1.4307 ± 0.01%.

[0047] The difference between the refractive index of the first ring core 2 and the refractive index of the cladding 4 is Δ2 = 1.4978 ± 0.01%.

[0048] The difference between the refractive index of the second ring core 3 and the refractive index of the cladding 4 is Δ3 = 1.1107 ± 0.01%.

[0049] In the aforementioned technical means, the refractive index of the central core and the second ring core is slightly lower than that of the first ring core, which is used to adjust the effective refractive index difference between the modes.

[0050] Among the aforementioned technical means, when the operating wavelength is 1310nm, the few-mode fiber can support 7 radial first-order transmission modes and the effective refractive index difference between it and the adjacent modes is greater than 0.1%.

[0051] At a working wavelength of 1310 nm, this few-mode fiber can support seven radial first-order transmission modes, and the effective refractive index difference between each mode and its adjacent modes is greater than 0.1%.

[0052] Example 3

[0053] Based on Examples 1 and 2, this embodiment continues to disclose the following content:

[0054] Adjust the radius and refractive index of core 1, first ring core 2, and second ring core 3 according to the number of supported transmission modes.

[0055] As the number of supported modes increases, the effective refractive index difference between radially higher-order modes can be selectively compressed by adjusting the radius and refractive index doping of the central core, the first ring core, and the second ring core. This increases the allocatable amount of the effective refractive index difference between the radially first-order mode and its adjacent modes, achieving an effective refractive index difference of 1×10⁻⁶ between the radially first-order mode and its adjacent modes. -3 above.

[0056] Increasing the effective refractive index difference between the first-order radial mode and its adjacent modes, i.e., compressing the effective refractive index spacing between higher-order radial modes within a limited effective refractive index spacing (under certain doping concentration constraints), is crucial. As fiber doping concentration increases, the number of modes accommodated in the fiber increases, with higher-order radial modes far outnumbering first-order radial modes. If only first-order radial modes are used for transmission, compressing the effective refractive index spacing between higher-order radial modes can free up more allocation space, especially given the increased number of higher-order radial modes. Using three-layer or more structures can maximize the effective refractive index difference between more higher-order radial modes to be much less than 1×10⁻⁶. -3 This allows for the allocation of effective refractive index spacing between the radial first-order mode and adjacent modes.

[0057] The effective refractive index between the radial first-order mode and its adjacent modes is extracted. The minimum effective refractive index difference is used as the target value for optimization by the optimization algorithm. The fiber structure is iteratively optimized, and different fiber structures are used during the iteration process to make the target value reach a threshold above the desired threshold. The weakly coupled multi-ring few-mode fiber with compressed radial higher-order modes can support 7 radial first-order modules, and the effective refractive index difference between modules is greater than 1×10⁻⁶. -3 Weakly coupled module transmission can be used.

[0058] When transmitting OAM mode, the three-layer fiber structure disperses abrupt changes in refractive index, suppresses intrinsic crosstalk of the mode, and improves the purity of the transmitted OAM mode.

[0059] In the specific implementation process, the radii of the middle ring 1, the first ring core 2, and the second ring core 3 are r1, r2, and r3, respectively, and their refractive index differences with the cladding 4 are Δ1, Δ2, and Δ3, respectively. The material of the cladding 4 is fused silica.

[0060] A genetic algorithm is used for iterative optimization of the fiber structure. First, based on the required number of modes, a threshold is set using the normalized frequency V to determine the effective refractive index difference between the radius r3 of the second ring core 3 and the effective refractive index difference between the first ring core 2 and the cladding 4. The range of each structural parameter is adjusted to broaden the searchable structural range and improve algorithm diversity. The effective refractive index of each mode at 1310 μm is obtained through numerical calculation, and the fitness function is set as follows:

[0061] Fitness = min(Δn) eff径向一阶与相邻模式 )

[0062] The performance of each structure is judged based on its fitness. A higher fitness function indicates less coupling between overall patterns in that structure, resulting in better performance. When the fitness function reaches 1×10... -3 This indicates that all radial first-order modes accommodated by the optical fiber have reached the threshold condition for weakly coupled transmission.

[0063] Table 1 shows the range of all structural parameters that the genetic algorithm can traverse. Choosing an appropriate number of bits for each parameter is beneficial to improving the diversity and iteration speed of the algorithm.

[0064] Table 1. Parameter setting range for weakly coupled few-mode fibers with compressed radial higher-order modes.

[0065]

[0066]

[0067] During the algorithm iteration process, we focus on increasing the effective refractive index difference between the first radial order and adjacent orders, as shown in Tables 2 and 3, so as to select the optimal structure, namely, a multi-ring core structure that compresses the effective refractive index interval between higher radial orders as much as possible.

[0068] Table 2 Effective refractive index difference between the radial first-order module and adjacent modules

[0069] OAM01-OAM11 <![CDATA[1.47×10 -3 ]]> OAM11-OAM21 <![CDATA[2.08×10 -3 ]]> OAM21-OAM02 <![CDATA[1.01×10 -3 ]]> OAM02-OAM31 <![CDATA[1.52×10 -3 ]]> OAM31-OAM12 <![CDATA[1.40×10 -3 ]]> OAM12-OAM41 <![CDATA[1.45×10 -3 ]]> OAM41-OAM22 <![CDATA[1.41×10 -3 <!-- 5 -->]]> OAM03-OAM51 <![CDATA[1.35×10 -3 ]]> OAM51-OAM32 <![CDATA[1.24×10 -3 ]]> OAM13-OAM61 <![CDATA[1.38×10 -3 ]]> OAM61-OAM42 <![CDATA[1.08×10 -3 ]]> OAM04-OAM71 <![CDATA[1.00×10 -3 ]]> OAM71-OAM <![CDATA[1.02×10 -3 ]]>

[0070] Table 3 Effective refractive index difference between radial higher-order modules

[0071] OAM22-OAM03 <![CDATA[3.34×10 -4 ]]> OAM32-OAM13 <![CDATA[6.50×10 -4 ]]> OAM42-OAM23 <![CDATA[1.02×10 -3 ]]> OAM23-OAM04 <![CDATA[3.19×10 -4 ]]>

[0072] This fiber supports seven OAM radial first-order modes, and the effective refractive index difference between each supported mode and its adjacent modes is greater than 1×10⁻⁶. -3 This enables it to support weakly coupled transmission between seven OAM modules, and the purity of the OAM mode can reach 99.9%. Figure 2 It is the purity of the OAM mode synthesized from HE vector models. Figure 3 It is the purity of the OAM mode synthesized from EH vector modes.

[0073] The same or similar labels correspond to the same or similar parts;

[0074] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0075] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. 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 claims of the present invention.

Claims

1. A weakly coupled multi-ring few-mode optical fiber with compressed radial higher-order modes, characterized in that, It includes a core (1), a first ring core (2), a second ring core (3), and a cladding (4), wherein: The outer periphery of the core (1) is provided with the first ring core (2), the outer periphery of the first ring core (2) is provided with the second ring core (3), and the outer periphery of the second ring core (3) is provided with the cladding (4). The refractive index of the first ring core (2) is higher than that of the middle core (1) and the second ring core (3); The refractive index of the core (1) is higher than that of the second ring core (3), and the refractive index of the cladding (4) is lower than that of the core (1), the first ring core (2), and the second ring core (3); The radius of the core (1) is r1 = 3.430.1 μm; The radius r2 of the first ring core (2) is 7.020.1 μm; The radius r3 of the second ring core (3) is 9.75-0.1 μm; The outer diameter of the cladding (4) is 62.5 μm, and the material of the cladding (4) is fused silica; The difference Δ1 between the refractive index of the core (1) and the refractive index of the cladding (4) is 1.43070.01%; The difference between the refractive index of the first ring core (2) and the refractive index of the cladding (4) is Δ2 = 1.49780.01%; The difference between the refractive index of the second ring core (3) and the refractive index of the cladding (4) is Δ3 = 1.11070.01%.

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