Eight-mode multiplexer / demultiplexer based on refractive index step type elliptical core main channel

Abstract

The utility model discloses an eight-mode multiplexer / demultiplexer based on a refractive index step type elliptical core main channel. According to the multiplexer / demultiplexer, a transmission channel is formed by one eight-mode elliptical fiber core (EC) and seven equal-length single-mode fiber cores; the length of a transmission main channel of the elliptical fiber core is 17.6 mm, the ellipticity rho of the elliptical core is 1.58, the length of a major semi-axis is 7.02 mu m, and the length of a minor semi-axis is 4.44 mu m; the length of the SMF-HG10, the length of the SMF-HG01, the length of the SMF-HG20, the length of the SMF-HG11, the length of the SMF-HG30, the length of the SMF-HG02 and the length of the SMF-HG21 are all 3.8 mm, and the SMF-HG10, the SMF-HG01, the SMF-HG20, the The multiplexer is used for demultiplexing a main channel of the multiplexer, the mode degeneracy of HG30 and HG02 is broken, the refractive index difference between modes is greater than 1 * 10 <-3 >, and the mode preserving function is realized; the refractive index fiber core made of pure silicon dioxide is adopted to reduce loss; the side core is divided into two sections for coupling, so that low-loss low-crosstalk multiplexing and demultiplexing in eight modes are realized, and MIMO-FREE operation is realized.

Description

Technical Field

[0001] This invention relates to an eight-mode multiplexer / demultiplexer based on a refractive index step-type master channel, which can be applied to fields such as fiber optics, fiber optic communication, fiber optic wireless access, optical information processing, and next-generation information technology. Background Technology

[0002] In the field of modern optical communication, with the exponential growth of data traffic, the demand for increased communication system capacity is becoming increasingly urgent. Traditional single-mode optical fiber communication systems are gradually facing unprecedented challenges, and few-mode fiber (FMF) technology has emerged to address this need. It transmits multiple modes of optical signals in a single optical fiber, which is expected to greatly improve the transmission capacity of optical fibers. The optical fiber communication industry has achieved breakthroughs in communication network transmission capacity around the physical dimension of spatial division multiplexing (including core-based multiplexing and mode-based multiplexing and their combination); research on mode-based multiplexing and few-mode fiber and related devices and applications in spatial division multiplexing has become a cutting-edge research hotspot [Sun Hyok Chang, Hwan Seok Chung, Nicolas K. Fontaine, Roland Ryf, Kyung Jun Park, Kwangjoon Kim, Jyung Chan Lee, Jong Hyun Lee, Byoung Yoon Kim, and Young Kie Kim, "Mode division multiplexed optical transmission enabled by all-fiber modemultiplexer," Opt. Express 22, 14229-14236 (2014); Yanlei Li, Xiao Wang, Hongjun Zheng, Xin Li, Chenglin Bai, Weisheng Hu, Yang Liu, Qiuhuan Dong, A novel six-core few-mode fiber with low loss and low crosstalk, Optical Fiber Technology, Volume 57, 2020, 102211; Gao Yan, Li Yanlei, Xing Huadong, Li Xin, Zheng Hongjun*, Bai Chenglin, Hu Weisheng, Xu Hengying, Yin Yingxin, Dong Qiuhuan, Research on Modular Division Multiplexing Optical Transmission Technology, Journal of Liaocheng University (Natural Science Edition), 2022, 35(1): 30-56; Zheng Hongjun, Li Xin, Bai Chenglin, Transmission of Chirped Pulses in Optical Fiber, Beijing: Science Press, 2018, 1-184; Dong Qiuhuan, Liu Yang, Zheng Hongjun, Li Xin, Bai Chenglin, Hu Weisheng, Chen Nan Guang. Research on few-mode multiplexing (demultiplexing) technology in mode division multiplexing system [J]. Journal of Liaocheng University (Natural Science Edition), 2020, 33(2): 50-67; Wang Xiao, Zheng Hongjun* (corresponding author), Li Xin, Liu Yang, Yu Ruyuan, Bai Chenglin, Hu Weisheng. New progress in the research of few-mode optical fiber in mode division multiplexing system, Journal of Liaocheng University (Natural Science Edition), 2019.4, 32(2): 69-79]; Pure silica fiber core can effectively reduce fiber attenuation and fusion splice loss, and is currently mostly used in single-mode optical fiber [T].Hasegawa et al. 2016. Advances in ultra-low loss silica fibers[J]. Frontiers in Optics, paper FTu2B.2; S.Ten. 2016. Ultra Low-loss Optical Fiber Technology[J]. Optical Fiber Communication Conference, paper Th4E.5; Y.Tamura, H.Sakuma, Y.Yamamoto, and T. Hasegawa, "Ultra-low loss silica core fiber for long haul transmission," in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optica Publishing Group, 2018), paper SF2K.3]; Compared to multi-core fibers, few-mode fibers support fewer transmission modes, and the transmission path differences between different modes are smaller. Furthermore, combining them with mode-division multiplexing technology in spatial division multiplexing can significantly improve transmission capacity. However, in mode division multiplexing systems based on few-mode fiber, as the number of modes increases, crosstalk between adjacent modes exists, affecting communication quality; [Jiajia Zhao, Ming Tang, Kyunghwan Oh, Zhenhua Feng, Can Zhao, Ruolin Liao, Songnian Fu, Perry Ping Shum, and Deming Liu, "Polarization-maintaining few mode fiber composed of a central circular-hole and an elliptical-ring core," Photon. Res. 5, 261-266 (2017); Jiwei Zhang, Guoru Wang, Han Zhang, et al. A weakly-coupled few-mode optical fiber with a graded concave high-index-ring[J]. IEEE Photonics Journal, 2021, 13: 7200710]; A mode division multiplexing transmission method to suppress mode crosstalk is proposed for few-mode fiber transmission and optical components such as mode division multiplexers / demultiplexers. [ARMay and MNZervas,″Few-mode fiberswith improved mode spacing,″2015European Conference on Optical Communication(ECOC),2015,pp.1-3,doi:10.1109 / ECOC.2015.7341706;Tao Hu,Juhao Li,Dawei Ge,Zhongying Wu,Yu Tian,Lei Shen,Yaping Liu,Su Chen,Zhenghin Li,Yongqi He,andZhangyuan Chen,″Weakly-coupled 4-mode step-index FMF and demonstration of IM / DD MDM transmissiOn,″Opt.Express 26,8356-8363(2018);Jiang,Shoulin and Ma,Linand Zhang,Zhaopeng and Xu,Xiao and Wang,Shuai and Du,Jiangbing and Yang,Chenand Tong,Weijun and He,Zuyuan,″Design and Characterization of Ring-AssistedFew-Mode Fibers for Weakly Coupled Mode-Division Multiplexing Transmission,″in Journal of Lightwave Technology,vol.36,no.23,pp.5547-5555,1Dec.1,2018,doi:10.1109 / JLT.2018.2874526;Dawei Ge,Yuyang Gao,Yu Yang,Lei Shen,Zhengbin Li,Zhangyuan Chen,Yongqi He,Juhao Li,″A 6-LP-mode ultralow-modal-crosstalkdouble-ring-core FMF for weakly-coupled MDM transmission,″OpticsCommunications,vo1.451,pp.97-103,Nov.

[2019] ; However, these round-core few-mode fibers still suffer from significant mode degeneracy, mode crosstalk, and loss. Mode degeneracy and crosstalk necessitate the use of Multiple-Input Multiple-Output Digital Signal Processing (MIMO-DSP). For short-distance communication, the more modes there are, the more severe the problems of complexity, computational load, and cost of MIMO-DSP become. Fiber loss affects the transmission distance of optical communication systems, and fiber loss is a crucial parameter determining the performance of optical fiber communication systems.

[0003] Compared to the aforementioned circular-core few-mode fibers, the references [Ezra Ip, Giovanni Milione, Ming-Jun Li, Neda Cvijetic, Konstantinos Kanonakis, Jeffery Stone, Gaozhu Peng, Xesús Prieto, Carlos Montero, Vicente Moreno, and Jesús]... "SDM transmission of real-time 10GbEtraffic using commercial SFP+transceivers over 0.5km elliptical-core few-mode fiber," Opt. Express 23, 17120-17126 (2015) proposes an elliptical-core mode-preserving fiber (MMOF) that can effectively reduce mode crosstalk and mode degeneracy. Without requiring MIMO-DSP, MMOF can be used to break mode degeneracy and solve mode crosstalk problems; this is known as MIMO-FREE or MIMO-LESS operation. MIMO-FREE operation has been a research hotspot in recent years [F.Parmigiani, et al. "MIMO-less Space Division Multiplexing Transmissionover 1km Elliptical Core Few Mode Fiber." in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optica Publishing Group, 2017), paperSW1I.1; Yuxin Ding, Jianshe Li, Shuguang Li, Yu Qin, Zelin Zhang, Xiaokai Wang, YingGuo, Xiaojian Meng, and Huijing Du, "Eight Modes Selective Elliptic-CorePhotonic Lantern in MIMO-Free Mode Division Multiplexing Systems at S+C+LBands," J. Lightwave Technol. 41, 739-744 (2023)].For example, in the paper [F. Parmigiani, et al.″MIMO-less Space Division Multiplexing Transmission over 1km Elliptical Core Few Mode Fiber.″in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optica Publishing Group, 2017), paper SW 1I.1.], the authors successfully demonstrated MIMO-FREE transmission over a three-mode elliptic core fiber at 1550nm. In the paper [Yuxin Ding, Jianshe Li, Shuguang Li, Yu Qin, Zelin Zhang, Xiaokai Wang, Ying Guo, Xiaojian Meng, and Huijing Du,″Eight Modes Selective Elliptic-Core Photonic Lantern in MIMO-Free Mode Division Multiplexing Systems at S+C+L Bands,″J. Lightwave Technol.41, 739-744(2023)], an elliptic core photonic lantern was proposed, in which MIMO-FREE operation can be implemented in an MDM system. MIMO-FREE operation is a current research hotspot and has significant advantages, which is our initial consideration for the design of the main channel fiber core of the multiplexer / demultiplexer.

[0004] As the number of modes in MIMO-FREE transmission on elliptical fiber cores increases, severe degeneracy occurs between modes, leading to a sharp decline in the performance of the corresponding multiplexers / demultiplexers and significantly reducing transmission quality. To address this issue, a novel eight-mode multiplexer / demultiplexer is proposed, which adjusts the ellipticity ρ = a of the main transmission channel on the elliptical fiber core. x / a y This breaks the mode degeneracy of the main transmission channel, making the refractive index difference between modes all within 1×10⁻⁶. -3 The above achieves mode preservation functionality, ensuring high-performance transmission of eight-mode multiplexing and demultiplexing. It is expected to solve the current research challenges of few-mode fiber multiplexers / demultiplexers, and has significant academic and application value with broad application prospects. Summary of the Invention

[0005] To address the issues of mode degeneracy, mode crosstalk, and loss in mode division multiplexing (MDF), this invention proposes an eight-mode few-mode multiplexer / demultiplexer with a refractive index-step elliptical core main channel, combining the advantages of pure silica fiber cores and elliptical mode-preserving fiber cores. The elliptical fiber core transmission main channel breaks spatial mode degeneracy, achieving low intrinsic loss and low crosstalk. The side cores are multiplexed to the elliptical core in two segments, avoiding coupling between side cores. It achieves multiplexing and demultiplexing of eight modes (HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21), realizing MIMO-FREE operation. This provides important support for in-depth research in fiber optics, fiber optic communication, fiber optic wireless access, optical information processing, and next-generation information technologies.

[0006] The technical solution adopted by this invention to solve its technical problem is:

[0007] This invention proposes an eight-mode multiplexer / demultiplexer based on a refractive index-step elliptical core main channel. The multiplexer / demultiplexer consists of a transmission channel composed of one eight-mode elliptical fiber core (EC) and seven equal-length single-mode fiber cores divided into two segments. The elliptical fiber core at the coordinate center serves as the main transmission channel, running through both segments. The second segment begins 10mm after the coupling length of the first segment. In the first segment, single-mode fiber cores SMF-HG10 are on the X-axis, SMF-HG01 on the Y-axis, SMF-HG20 on the -X-axis, and SMF-HG02 on the -Y-axis. In the second segment, single-mode fiber cores SMF-HG11 is on the first quadrant at 45°, SMF-HG30 on the -X-axis, and SMF-HG21 on the fourth quadrant at 30°. The elliptical fiber core transmission channel, which is the main channel for mode division multiplexing / demultiplexing, is placed on the z-axis, with its axis coinciding with the z-axis. Supported modes include HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21; the seven equal-length single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21, which serve as side cores, are all single-mode circular core transmission coupling channels, placed on the positive half-axis of the X-axis and the positive half-axis of the Y-axis, respectively. The negative half-axis of the X-axis and the negative half-axis of the Y-axis are 45° to the first quadrant and 30° to the fourth quadrant. The axes of SMF-HG10, SMF-HG01, SMF-HG20 and SMF-HG02 are parallel to the axis of the elliptical fiber core starting from their respective starting points; the axes of SMF-HG11, SMF-HG30 and SMF-HG21 are parallel to the axis of the elliptical fiber core starting 10 mm after the end of the first coupling length.According to coupled-mode theory, mode HG00 is transmitted from left to right along the main transmission channel of the elliptical fiber core to achieve multiplexing output function; other optical fields are incident from the left ends of the SMFs corresponding to HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively, and coupled to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel of the elliptical fiber core, respectively. At the right end of the main transmission channel of the elliptical fiber core, modes HG10, HG01, HG20, HG11, and HG30 are used for multiplexing output. HG02 and HG21 modes are output; mode multiplexing of eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel of the elliptical fiber core is achieved; if modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are incident from the left end of the main transmission channel of the elliptical fiber core, and are coupled and demultiplexed along the z-direction; according to the coupled mode theory, mode HG00 is demultiplexed by transmitting from the left end to the right end of the main transmission channel of the elliptical fiber core. Modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are transmitted from the left end of the main channel of the elliptical fiber core, and coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively. This achieves mode demultiplexing of the main channel transmission modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 from the elliptical fiber core; based on the changes in mode coupling length and channel spacing... The center coordinates of the main transmission channel of the elliptical fiber core are (0, 0), the length of the main transmission channel of the elliptical fiber core is 17.6 mm, the ellipticity ρ of the elliptical core is 1.58, the length of the major semi-axis is 7.02 μm, and the length of the minor semi-axis is 4.44 μm; the distances from the center of the circle to the center of the main transmission channel of the elliptical fiber core of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are 12.12, 10.74, 13.22, and 13.22, respectively. 12.62, 10.84 and The fiber cores are all 3.8 mm in length, and the radii of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are all 3 μm. The elliptical cores employ a step refractive index distribution, higher than the refractive index of the surrounding cladding. High-refractive-index elliptical cores and single-mode cores are primarily used for light transmission. The single-mode cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, and SMF-HG21 are all 3 μm in diameter. The step refractive indices of MF-HG02 and SMF-HG21 are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105 and 1.426017, respectively. The elliptical core transmission main channel uses pure silica material with a refractive index of n1 = 1.4440. The cladding uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics in the optical fiber can be changed by reasonably setting parameters such as the core position, size and refractive index distribution.

[0008] The beneficial effects of this invention are as follows:

[0009] 1. This multiplexer / demultiplexer consists of a directional mode selection coupler. The large effective refractive index difference between the modes in the optical fiber ensures low crosstalk between modes. The use of a pure silica refractive index fiber core achieves low loss performance.

[0010] 2. The optical fiber used in this multiplexer / demultiplexer combines the advantages of pure silica fiber core, step refractive index distribution, and elliptical fiber core. The use of elliptical fiber core breaks mode degeneracy, realizing mode preservation function and high-performance multiplexing and demultiplexing of eight modes: HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21. This provides important support for in-depth research in the fields of fiber optics, fiber optic communication, fiber optic wireless access, optical information processing, and next-generation information technology.

[0011] 3. The side core of this multiplexer / demultiplexer uses two segments coupled into the elliptical fiber core, which avoids coupling and crosstalk between modes, thereby realizing eight modes of transmission. Attached Figure Description

[0012] Figure 1 This is a cross-sectional schematic diagram of an eight-mode multiplexer / demultiplexer based on a refractive index step-type elliptical core main channel according to the present invention. The elliptical fiber core at the center of the coordinate system serves as the main transmission channel (ellipse); the single-mode SMF fiber cores (circular) are located on the X-axis, Y-axis, first quadrant, and fourth quadrant.

[0013] Figure 2This is a three-dimensional representation of an eight-mode multiplexer / demultiplexer based on a refractive index step-type elliptical core main channel according to the present invention.

[0014] Figure 3 The mode field distribution diagrams of HG21, HG02, HG30, HG11, HG20, HG01, HG10 and HG00 of the elliptical fiber core are given at a wavelength of 1.55 μm.

[0015] Figure 4 The variation of effective refractive index difference with ellipticity for each mode at a wavelength of 1.55 μm is presented. The curves marked with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 and HG10 modes. HG00 -n HG10 The effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 The effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 The effective refractive index difference n between HG20 mode and HG11 mode HG20 -n HG11 The effective refractive index difference n between HG11 mode and HG30 mode HG11 -n HG30 The effective refractive index difference n between HG30 mode and HG02 mode HG30 -n HG02 The effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 The effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding .

[0016] Figure 5 The effective refractive index differences for each mode in the optical fiber at 1.53 μm–1.565 μm are presented. The curves marked with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 and HG10 modes. HG00 -n HG10 The effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 The effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 The effective refractive index difference n between HG20 mode and HG11 mode HG20 -n HG11The effective refractive index difference n between HG11 mode and HG30 mode HG11 -n HG30 The effective refractive index difference n between HG30 mode and HG02 mode HG30 -n HG02 The effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 The effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding .

[0017] Figure 6 The coupling efficiency of each spatial mode of the multiplexer / demultiplexer in the C-band as a function of incident wavelength is presented. The coupling efficiencies of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are represented by black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons, respectively.

[0018] Figure 7 The mode extinction ratio of this multiplexer / demultiplexer in the mode channel is shown as a function of the incident light wavelength. The extinction ratios of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are represented by black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons, respectively.

[0019] Figure 8 The intrinsic loss of the main channel transmitted through the elliptical fiber core of the multiplexer / demultiplexer is presented. The intrinsic losses of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are represented by curves with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons and black hexagons, respectively. Detailed Implementation

[0020] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto.

[0021] Example 1 Figure 1 This is a cross-sectional schematic diagram of an eight-mode multiplexer / demultiplexer based on a refractive index step-type elliptical core main channel according to the present invention. The elliptical fiber core at the coordinate center serves as the main transmission channel (ellipse), with its major semi-axis a... x = 7.02μm, short semi-axis a y= 4.44μm; the single-mode SMF cores (circular) located on the X-axis, Y-axis, first quadrant, and fourth quadrant have a radius of 3μm. The distance between SMF-HG10 and the elliptical core is 12.12μm; the distance between SMF-HG01 and the elliptical core is 10.74μm; the distance between SMF-HG20 and the elliptical core is 13.22μm; the distance between SMF-HG11 and the elliptical core is... μm; the distance from the elliptical core of SMF-HG30 is 12.62 μm; the distance from the elliptical core of SMF-HG02 is 10.84 μm; the distance from the elliptical core of SMF-HG21 is μm. μm.

[0022] Figure 2This is a perspective view of an eight-mode multiplexer / demultiplexer based on a refractive index-step elliptical core main channel according to the present invention. Its characteristic is that the multiplexer / demultiplexer consists of a transmission channel composed of one eight-mode elliptical fiber core and seven equal-length single-mode fiber cores. The elliptical fiber core at the coordinate center serves as the main transmission channel. The seven single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21, located at 45° in the first quadrant and 30° in the fourth quadrant along the positive and negative half-axes of the X and Y axes respectively, are single-mode circular cores. Transmission coupling channel; the elliptical fiber core transmission channel is the main channel for mode division multiplexing / demultiplexing, placed on the z-axis with its axis coinciding with the z-axis, supporting modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21; the seven equal-length single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21, serving as side cores, are respectively placed on the positive and negative half-axis of the X-axis and Y-axis, at 45° in the first quadrant and 30° in the fourth quadrant. The axes of G10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are parallel to the axis of the elliptical fiber core, starting from their respective origins. According to coupled-mode theory, mode HG00 is transmitted along the main transmission channel of the elliptical fiber core from left to right, achieving multiplexing output. Other optical fields are incident from the left ends of the SMFs corresponding to HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively, and coupled to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel of the elliptical fiber core. The main transmission channel in the elliptical fiber core is output in modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 at the right end; mode multiplexing of eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel in the elliptical fiber core is realized; if modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are incident from the left end of the eight-mode elliptical fiber core transmission channel, coupling and demultiplexing are achieved along the z-direction; according to the coupled mode theory, mode HG00 is demultiplexed by transmitting from the left end to the right end along the main transmission channel in the elliptical fiber core;Modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are transmitted from the left end of the main channel of the elliptical fiber core, and are coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively. This achieves mode demultiplexing of eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 from the main channel of the elliptical fiber core; based on the variation of mode coupling length and channel spacing... The center coordinates of the main transmission channel of the elliptical fiber core are (0, 0), the length of the main transmission channel of the elliptical fiber core is 17.6 mm, the ellipticity of the elliptical core is 1.58, the length of the major semi-axis is 7.02 μm, and the length of the minor semi-axis is 4.44 μm; the distances from the center of the circle to the channel center of the elliptical fiber core of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are 12.12, 10.64, 13.12, and 13.12, respectively. 12.52, 10.74 and The fiber cores are all 3.8 mm in length, with a radius of 3 μm for SMF-HG10, SMF-HG01, SMFHG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21. The elliptical cores employ a step refractive index distribution, higher than the surrounding cladding refractive index, and primarily utilize high-refractive-index elliptical cores and single-mode cores for light transmission. The single-mode cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, and SMF-HG21 are all 3 μm in diameter. The step refractive indices of F-HG02 and SMF-HG21 are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105 and 1.426017, respectively. The elliptical core transmission main channel uses pure silica material with a refractive index of n1 = 1.4440. The cladding uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics in the optical fiber can be changed by reasonably setting parameters such as the core position, size and refractive index distribution.

[0023] Figure 3 Mode field distribution diagrams for HG21, HG02, HG30, HG11, HG20, HG01, HG10, and HG00 with an elliptical fiber core are presented at a wavelength of 1.55 μm. The modes were determined using the beam propagation method. By adjusting the ellipticity, the refractive index difference between the modes was ensured to be greater than 1 × 10⁻⁶. -3 This is to achieve the mold preservation function.

[0024] Figure 4The variation of the effective refractive index difference between each mode and the ellipticity of the elliptical fiber core at a wavelength of 1.55 μm is presented. The curves marked with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 and HG10 modes. HG00 -n HG10 The effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 The effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 The effective refractive index difference n between HG20 mode and HG11 mode HG20 -n HG11 The effective refractive index difference n between HG11 mode and HG30 mode HG11 -n HG30 The effective refractive index difference n between HG30 mode and HG02 mode HG30 -n HG02 The effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 The effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding The ellipticity of the elliptical core is selected based on the intersection of the red pentagram and the pink hexagon, determined by... Figure 4 It can be seen that when the ellipticity ρ = a x / a y When n = 1.58, the effective refractive index difference between the HG00 mode and the HG10 mode is... HG00 -n HG10 3.0935×10 -3 The effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 It is 2.641 × 10 -3 The effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 It is 1.743×10 -3 The effective refractive index difference n between HG20 mode and HG11 mode HG20 -n HG11 2.508×10 -3 The effective refractive index difference n between HG11 mode and HG30 mode HG11 -n HG30 2.995×10 -3 The effective refractive index difference n between HG30 mode and HG02 mode HG30 -n HG02 1.07×10-3 The effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 1.092×10 -3 The effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding It is 1.9115×10 -3 The effective refractive index difference between all modes is greater than 1×10⁻⁶. -3 This can effectively guarantee mode-preserving transmission in eight modes.

[0025] Figure 5 The effective refractive index differences for each mode in the elliptical fiber core range of 1.53 μm–1.565 μm are presented. The curves marked with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 and HG10 modes. HG00 -n HG10 The effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 The effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 The effective refractive index difference n between HG20 mode and HG11 mode HG20 -n HG11 The effective refractive index difference n between HG11 mode and HG30 mode HG11 -n HG30 The effective refractive index difference n between HG30 mode and HG02 mode HG30 -n HG02 The effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 The effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding .Depend on Figure 5 It can be seen that, within the wavelength range of 1530nm to 1565nm, the refractive index difference n between mode HG00 and mode HG10 is... HG00 -n HG10 Greater than 3.0265×10 -3 Furthermore, the refractive index difference gradually increases with increasing wavelength. The refractive index difference n between the HG10 mode and the HG01 mode... HG10 -n HG01 Greater than 2.596×10 -3 Furthermore, the refractive index difference gradually increases with increasing wavelength. The refractive index difference n between modes HG01 and HG20 is... HG01 -n HG20 Greater than 1.6985×10-3 Similarly, the refractive index difference gradually increases with increasing wavelength. The refractive index difference n between modes HG20 and HG11 is... HG20 -n HG11 Greater than 2.4705×10 -3 The refractive index difference gradually increases with increasing wavelength. The refractive index difference n between modes HG11 and HG30 is... HG11 -n HG30 Greater than 2.936×10 -3 The refractive index difference gradually increases with increasing wavelength. The refractive index difference n between modes HG30 and HG02 is... HG30 -n HG02 Greater than 1.0465×10 -3 The refractive index difference gradually decreases with increasing wavelength. The refractive index difference n between modes HG02 and HG21 is... HG02 -n HG21 Greater than 1.0435×10 -3 The refractive index difference gradually increases with increasing wavelength. HG21 and its cladding n Cladding refractive index difference n HG21 -n Cladding Greater than 1.6615 × 10 -3 The refractive index difference gradually decreases with increasing wavelength. The refractive index difference n between 1545 nm and 1550 nm is... HG00 -n HG10 They are 3.077×10 -3 and 3.0935×10 -3 n HG10 -n HG01 They are 2.6295×10 -3 and 2.641×10 -3 n HG01 -n HG20 They are 1.7315×10 -3 and 1.743×10 -3 n HG20 -n HG11 They are 2.499×10 -3 and 2.508×10 -3 n HG11 -n HG30 They are 2.98×10 -3 and 2.995×10 -3 n HG30 -n HG02 They are 1.077×10 -3 and 1.07×10 -3 n HG02 -n HG21 They are 1.0805×10 -3and 1.092×10 -3 n HG21- n Claddding They are 1.995×10 -3 and 1.9115×10 -3 .

[0026] Figure 6 The coupling efficiency of each spatial mode of the multiplexer / demultiplexer in the C-band as a function of incident wavelength is presented. The coupling efficiencies of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are represented by curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons, respectively. The coupling efficiency of mode HG00 is approximately 0 dB, and remains essentially constant with increasing wavelength. At 1550nm, the coupling efficiency of HG10 mode reaches -0.026605dB, HG01 mode reaches -0.0066676dB, HG20 mode reaches -0.059175dB, HG11 mode reaches -0.031164dB, HG30 mode reaches -0.1682dB, HG02 mode reaches -1.0726dB, and HG21 mode reaches -0.44815dB. In the C-band around 1550nm, the coupling efficiency shows a decreasing trend. This is because the coupling period of each mode changes with the wavelength. Since the coupling length of the designed (de)multiplexer is fixed, it is impossible to simultaneously achieve the maximum coupling efficiency for each wavelength, and the efficiency varies with the wavelength. Across the entire C-band, the coupling efficiency of HG10 is higher than -0.056206 dB, HG01 is higher than -0.30479 dB, HG20 is higher than -1.3175 dB, HG11 is higher than -2.9738 dB, HG30 is higher than -1.6025 dB, HG02 is higher than -2.5803 dB, and HG21 is higher than -1.9907 dB. The coupling efficiency of HG00 mode is the best.

[0027] Figure 7The extinction ratio of the multiplexer / demultiplexer mode as a function of incident light wavelength is presented. The extinction ratios of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are represented by curves with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons, and black hexagons, respectively. The extinction ratios of modes HG00 are all above 37.6421 dB; across the entire C-band, the extinction ratio of HG10 remains above 29.3292 dB, reaching a maximum of 32.2388 dB at 1545 nm; the extinction ratio of HG01 remains above 31.2806 dB, reaching a maximum of 36.7073 dB at 1550 nm; the extinction ratio of HG20 remains above 29.1136 dB, reaching a maximum of 37.0006 dB at 1540 nm; and the extinction ratio of HG11... The extinction ratios of all HG30 and HG02 can be maintained above 17.872dB, with a maximum value of 26.6399dB at 1555nm; the extinction ratios of HG30 can be maintained above 15.7615dB, with a maximum value of 19.2081dB at 1555nm; the extinction ratios of HG02 can be maintained above 11.2636dB, with a maximum value of 15.2420dB at 1565nm; and the extinction ratios of HG21 can be maintained above 9.8054dB, with a maximum value of 13.2594dB at 1550nm.

[0028] Figure 8The intrinsic loss of the main channel transmitted through the elliptical fiber core of the multiplexer / demultiplexer is presented. The intrinsic losses of modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are represented by curves with black squares, green asterisks, blue rhombuses, green circles, black triangles, red pentagrams, pink hexagons and black hexagons, respectively. Overall, the intrinsic losses of the eight modes, from highest to lowest, are HG02, HG21, HG30, HG11, HG20, HG01, HG10, and HG00. Mode HG00 maintains an intrinsic loss below 0.14892 dB / km, with a minimum of 0.14556 dB / km at 1535 nm; mode HG10 maintains an intrinsic loss below 0.15077 dB / km, with a minimum of 0.14745 dB / km at 1535 nm; mode HG01 maintains an intrinsic loss below 0.15385 dB / km, with a minimum of 0.1506 dB / km at 1535 nm; and mode HG20 maintains an intrinsic loss of 0.154 dB / km. The following intrinsic losses were achieved at 1535 nm: 0.15072 dB / km for mode HG11; 0.15455 dB / km for mode HG30; 0.15713 dB / km for mode HG02; 0.16491 dB / km for mode HG02; and 0.16202 dB / km for mode HG21.

Claims

1. An eight-mode multiplexer / demultiplexer based on a refractive index step-elliptic core main channel, characterized in that: The multiplexer / demultiplexer consists of a transmission channel composed of one eight-mode elliptical fiber core (EC) and seven equal-length single-mode fiber cores, divided into two segments. The elliptical fiber core at the center of the coordinate axis serves as the main transmission channel, running through both segments. The second segment begins 10mm after the coupling length of the first segment. In the first segment, single-mode fiber cores SMF-HG10 are on the X-axis, SMF-HG01 on the Y-axis, SMF-HG20 on the -X-axis, and SMF-HG02 on the -Y-axis. In the second segment, SMF-HG11 is on the 45° direction in the first quadrant, SMF-HG30 on the -X-axis, and SMF-HG21 on the 30° direction in the fourth quadrant. The elliptical fiber core transmission channel is the main channel for mode division multiplexing / demultiplexing, placed on the z-axis with its axis coinciding with the z-axis, supporting modes HG00, HG10, HG01, and HG20. HG11, HG30, HG02, and HG21; the seven equal-length single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21, serving as side cores, are all single-mode circular core transmission coupling channels, respectively placed at the positive and negative half-axis of the X-axis and Y-axis and the first quadrant at 45°, and at the negative half-axis of the X-axis and Y-axis and the fourth quadrant at 30°. The axes of SMF-HG10, SMF-HG01, SMF-HG20, and SMF-HG02 are parallel to the ellipse from their respective starting points. The fiber core axis; SMF-HG11, SMF-HG30, and SMF-HG21 begin parallel to the elliptical fiber core axis 10mm after the first coupling length; according to the coupled mode theory, mode HG00 is transmitted from the left end to the right end along the main transmission channel of the elliptical fiber core to achieve multiplexing output function; other optical fields are incident from the left end of the SMFs corresponding to HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively, and coupled to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel of the elliptical fiber core, respectively, in the main transmission channel of the elliptical fiber core. The right end of the channel outputs in modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21; mode multiplexing of eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 in the main transmission channel of the elliptical fiber core is realized; if modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are incident from the left end of the main transmission channel of the elliptical fiber core, coupling and demultiplexing are achieved along the z-direction; according to the coupled mode theory, mode HG00 is demultiplexed by transmitting from the left end to the right end of the main transmission channel of the elliptical fiber core;Modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21 are transmitted from the left end of the main channel of the elliptical fiber core, and coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02, and HG21, respectively. This achieves mode demultiplexing of eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 from the main channel of the elliptical fiber core; the mode coupling length is related to the channel spacing. The center coordinates of the 8-mode elliptical fiber core transmission main channel are (0, 0). The length of the elliptical fiber core transmission main channel is 17.6 mm, the ellipticity p of the elliptical core is 1.58, the length of the major semi-axis is 7.02 μm, and the length of the minor semi-axis is 4.44 μm. The distances from the center of the circle to the center of the elliptical fiber core transmission main channel of SMF-HG10, SMF-HG01, SMFHG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are 12.12, 10.74, 13.22, and 13.22, respectively. 12.62, 10.84 and All of them have a length of 3.8 mm, and the radii of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are all 3 μm. The elliptical core adopts a step refractive index distribution, which is higher than the refractive index of the surrounding cladding. High-refractive-index elliptical cores and single-mode cores are mainly used for light transmission. The single-mode cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 have a step refractive index distribution. The refractive indices are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105, and 1.426017, respectively. The elliptical core transmission main channel uses pure silica material with a refractive index of n1 = 1.4440; the cladding uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics in the optical fiber are changed by reasonably setting the core position, size, and refractive index distribution. This multiplexer and demultiplexer breaks the mode degeneracy of HG30 and HG02, and the refractive index difference between modes is greater than 1 × 10⁻⁶. -3 It achieves mold preservation function; uses pure silicon dioxide refractive index fiber core to reduce loss; and achieves low-loss, low-crosstalk multiplexing and demultiplexing of eight modes (HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21) by coupling the side core into two segments, thus realizing MIMO-FREE operation.

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