Eight-mode multiplexer / demultiplexer based on refractive index step type non-degenerate mode main channel

Abstract

An eight-mode multiplexer / demultiplexer based on a refractive index step type non-degenerate mode main channel is characterized in that a transmission channel is formed by one eight-mode elliptical fiber core (EC) and seven single-mode fiber cores, and the elliptical fiber core at the coordinate center serves as a transmission main channel and penetrates through two sections; the single-mode fiber core serving as a side core is a transmission coupling channel, two ends of the single-mode fiber core are coupled, and the second section is arranged at the rear end of the longest coupling length of the first section; the length of a transmission main channel of the elliptical fiber core is 11.37 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 is 8.112 mm, the length of the SMF-HG01 is 4.976 mm, the length of the SMF-HG20 is 3.258 mm, the length of the SMF-HG11 is 1.190 mm, the length of the SMF-HG30 is 1.626 mm, and the length of the SMF-HG21 is 1.964 mm; the refractive index difference between the main modes of the multiplexer and the demultiplexer is greater than 1 * 10 <-3 >, the mode degeneracy of HG30 and HG02 is broken, 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 and low-crosstalk multiplexing and demultiplexing of eight modes are realized, and MIMO-FREE operation is realized.

Description

Technical Field

[0001] The present invention relates to an eight-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel, which can be applied to the fields of optical fiber optics, optical fiber communication, optical fiber wireless access, optical information processing and new generation information technology. Background Art

[0002] In modern optical communications, with the exponential growth of data traffic, the demand for increased communication system capacity is becoming increasingly urgent. Traditional single-mode fiber communication systems are facing unprecedented challenges. Few-mode fiber (FMF) technology has emerged as a response to this challenge. By transmitting multiple modes of optical signals through a single fiber, it promises to significantly increase optical fiber transmission capacity. The optical fiber communication industry has achieved breakthroughs in the transmission capacity of communication networks around the physical dimension of space division multiplexing (including core multiplexing and mode division multiplexing and their combination); mode division multiplexing and few-mode fiber in space division multiplexing and their related devices and applications have become cutting-edge research hotspots [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 mode multiplexer," 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 Mode 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 research on few-mode 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 loss, and is currently mostly used in single-mode 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 with multi-core fiber, few-mode fiber supports fewer transmission modes, and the transmission path differences between different modes are small. In addition, the transmission capacity can be significantly improved by combining the mode division multiplexing technology in space division multiplexing. However, in the mode division multiplexing system based on few-mode fiber, as the number of modes increases, crosstalk between adjacent modes occurs, affecting the 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]; For optical components such as few-mode fiber transmission and mode division multiplexers / demultiplexers, a mode division multiplexing transmission method that suppresses mode crosstalk is proposed. [ARMay and MNZervas,"Few-mode fibers with 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,Zhengbin 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-Assisted Few-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,ZhangyuanChen,Yongqi He,Juhao Li,″A 6-LP-mode ultralow-modal-crosstalkdouble-ring-core FMF for weakly. -"[The text then abruptly shifts topics:] coupled MDM transmission," Optics Communications, vol. 451, pp. 97-103, Nov. 2019. However, these circular-core few-mode fibers still suffer from significant mode degeneracy, mode crosstalk, and loss. Mode degeneracy and mode crosstalk require the use of multiple-input, multiple-output (MIMO) digital signal processing (DSP). For short-distance communications, the more modes there are, the more severe the complexity, computational effort, and cost of MIMO-DSP. Fiber loss also affects the transmission distance of optical communication systems and is a key parameter in determining their performance.

[0003] Compared with the above-mentioned round core few-mode fiber, the literature [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 10GbE traffic using commercial SFP+ transceivers over 0.5km elliptical-core few-mode fiber," Opt. Express 23, 17120-17126 (2015)] proposed an elliptical-core mode-maintaining fiber (MMOF) that can effectively reduce mode crosstalk and mode degeneracy. When MIMO-DSP is not required, MMOF can be used to break mode degeneracy and solve the mode crosstalk problem, which is called 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 document [F. Parmigiani, et al. "MIMO-less Space Division Multiplexing Transmission over 1 km Elliptical Core Few Mode Fiber." In Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optica Publishing Group, 2017), paper SW1I.1.], the authors successfully demonstrated MIMO-FREE transmission on a three-mode elliptical core fiber at 1550 nm. In the document [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 elliptical core photonic lantern was proposed, in which MIMO-FREE operation can be achieved in an MDM system. MMOF with MIMO-FREE operation is a current research hotspot and has significant advantages. This is the initial consideration for the design of the main channel fiber core of our multiplexer and demultiplexer.

[0004] As the number of MIMO-FREE transmission modes on the elliptical core increases, the modes become severely degenerate, and the corresponding multiplexer / demultiplexer performance drops sharply, greatly reducing the transmission quality. In order to solve this problem, a novel eight-mode multiplexer / demultiplexer is proposed. By adjusting the ellipticity ρ=a of the main channel of the elliptical core transmission, the ellipticity of the elliptical core transmission channel is increased. x / a y , breaking the mode degeneracy of the main transmission channel, making the refractive index difference between the modes within 1×10 -3 The above has achieved the mode preservation function and ensured the high-performance transmission of eight-mode multiplexing and demultiplexing, which is expected to solve the current research challenges of few-mode optical fiber multiplexers / demultiplexers. It has important academic and application value and broad application prospects. Summary of the Invention

[0005] Aiming at the problems of mode degeneracy, mode crosstalk and loss in mode division multiplexing, the present invention combines the advantages of pure silica fiber core and elliptical mode-preserving fiber core, and proposes an eight-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel. The elliptical fiber core transmission main channel adopted breaks the spatial mode degeneracy and achieves low intrinsic loss and low crosstalk. The side core adopts two-segment multiplexing to the elliptical fiber core to avoid coupling between the side cores; it realizes the multiplexing and demultiplexing of eight modes of HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21, and realizes MIMO-FREE operation, providing important support for in-depth research in the fields of fiber optics, fiber-optic communications, fiber-optic wireless access, optical information processing and new generation information technology.

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

[0007] The present invention proposes an octahedral multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel, characterized in that: the multiplexer / demultiplexer is composed of an octahedral elliptical fiber core (EC) and 7 single-mode fiber cores, which are divided into two sections to form a transmission channel. The elliptical fiber core at the coordinate center serves as the main transmission channel and runs through the two sections; the second section starts at the rear end of the longest coupling length of the first section; in the first section, the single-mode fiber core SMF-HG10 is on the X coordinate axis, SMF-HG01 is on the Y coordinate axis, SMF-HG20 is on the -X coordinate axis, and SMF-HG02 is on the -Y coordinate axis; in the second section, the single-mode fiber core SMF-HG11 is in the 45° direction of the first quadrant, SMF-HG30 is on the -X coordinate axis, and SMF-HG21 is in the 45° direction of the fourth quadrant; the elliptical fiber core transmission channel is the main channel for mode division multiplexing / demultiplexing, is placed on the z axis, and its axis coincides with the z axis. The seven single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 as the side cores are all single-mode circular core transmission coupling channels, which are placed on the positive half axis of the X axis and the positive half axis of the Y axis respectively. The positive semi-axis and the negative semi-axis of the X-axis are 45° to the first quadrant, and the negative semi-axis of the X-axis and the negative semi-axis of the Y-axis are 45° 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 from their respective starting points; SMF-HG11, SMF-HG30 and SMF-HG21 are parallel to the axis of the elliptical fiber core from the end of the first longest coupling length.According to the coupled mode theory, mode HG00 is transmitted from the left end to the right end along the elliptical core transmission main channel to realize the multiplexing output function; other light fields are incident from the left end of the SMF corresponding to HG10, HG01, HG20, HG11, HG30, HG02 and HG21, respectively, and are coupled to the modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the elliptical core transmission main channel, and are transmitted from the right end of the elliptical core transmission main channel to the modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21. HG02 and HG21 mode output; realize the mode multiplexing of 8 modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the elliptical fiber core transmission main channel; if the modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are incident from the left end of the elliptical fiber core transmission main channel, they are transmitted along the z direction for coupled demultiplexing; according to the coupled mode theory, the mode HG00 is transmitted from the left end to the right end along the elliptical fiber core transmission main channel to achieve demultiplexing ; Modes 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 respectively coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21, realizing mode demultiplexing of the 8 modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 from the main transmission channel of the elliptical fiber core; according to the change of mode coupling length and channel spacing relationship, the center coordinates of the elliptical fiber core transmission main channel are (0, 0), the length of the elliptical fiber core transmission main channel is 11.37 mm, the ellipticity p of the elliptical core is 1.58, the major semi-axis length is 7.02 μm, and the minor semi-axis length 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 13.02, 10.94, 13.52, respectively. 12.52, 11.44 and Their lengths are 8.112, 4.520, 4.976, 3.258, 1.190, 1.626 and 1.964 mm respectively. The radius of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are all 3 μm. The elliptical fiber core adopts a step-index refractive index distribution, which is higher than the refractive index of the surrounding cladding. High-refractive-index elliptical fiber cores and single-mode fiber cores are mainly used for light transmission. Single-mode core SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are all 3 μm. The step refractive indices of MF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105, and 1.426017, respectively. The main transmission channel of the elliptical core fiber uses pure silica material with a refractive index of n1 = 1.4440; the cladding part uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics of the optical fiber can be changed by properly setting parameters such as the core position, size, and refractive index distribution.

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

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

[0010] 2. The optical fiber used in this multiplexer / demultiplexer combines the advantages of a pure silica core, a step-index profile, and an elliptical core. The elliptical core overcomes mode degeneracy, achieving mode-preserving functionality 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 fiber optics, fiber-optic communications, fiber-optic wireless access, optical information processing, and next-generation information technology.

[0011] 3. The side core of the multiplexer / demultiplexer uses two sections of coupled elliptical fiber core to avoid coupling and crosstalk between modes, thereby realizing eight-mode transmission. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0013] Figure 2 It is a three-dimensional stereogram of an eight-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel of the present invention.

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

[0015] Figure 4 The variation of the effective refractive index difference of each mode with ellipticity at a wavelength of 1.55μm is shown. The curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 mode and the HG10 mode. 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 -nHG02, the effective refractive index difference between HG02 mode and HG21 mode n HG02 -n HG21 and the effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding .

[0016] Figure 5 The effective refractive index difference of each mode of the optical fiber at 1.53μm-1.565μm is given. The curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons and black hexagons represent the effective refractive index difference n between the HG00 mode and the HG10 mode. 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 -nHG02, the effective refractive index difference between HG02 mode and HG21 mode n HG02 -n HG21 and the effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding .

[0017] Figure 6 The variation of the coupling efficiency of each spatial mode of the multiplexer / demultiplexer with the incident wavelength in the C-band is shown. The coupling efficiency of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes are represented by black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively.

[0018] Figure 7 The variation of the modal extinction ratio in the modal channels of the multiplexer / demultiplexer with the incident light wavelength is shown. The extinction ratios of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes are represented by black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively.

[0019] Figure 8 The intrinsic loss of the main transmission channel of the elliptical fiber core of the multiplexer / demultiplexer is given. The intrinsic losses of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes are represented by the curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively. DETAILED DESCRIPTION

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

[0021] Example 1 Figure 1 This is a cross-sectional diagram of an eight-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel of the present invention, wherein the elliptical fiber core at the coordinate center serves as the main transmission channel (ellipse), and its major semi-axis a x =7.02μm, semi-minor axis a y=4.44μm; the single-mode SMF cores (circular) located on the X-axis, Y-axis, first quadrant, and fourth quadrant are all side cores with a radius of 3μm. The distance between SMF-HG10 and the elliptical core is 13.02μm; the distance between SMF-HG01 and the elliptical core is 10.94μm; the distance between SMF-HG20 and the elliptical core is 13.52μm; the distance between SMF-HG11 and the elliptical core is The distance between SMF-HG30 and the elliptical fiber core is 12.52μm; the distance between SMF-HG02 and the elliptical fiber core is 11.44μm; the distance between SMF-HG21 and the elliptical fiber core is

[0022] Figure 2It is a stereoscopic diagram of an eight-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel of the present invention; it is characterized in that: the multiplexer / demultiplexer is composed of an eight-mode elliptical fiber core and seven single-mode fiber cores to form a transmission channel, wherein the elliptical fiber core at the center of the coordinate is used as the transmission main channel, and the seven single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 located on the positive half axis of the X axis, the positive half axis of the Y axis, the negative half axis of the X axis and 45° of the first quadrant, and the negative half axis of the X axis, the negative half axis of the Y axis and the fourth quadrant are single-mode circular Core transmission coupling channel; The elliptical core transmission channel is the main channel for mode division multiplexing / demultiplexing, which is placed on the z-axis with its axis coinciding with the z-axis, and supports modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21: seven single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are placed on the positive half axis of the X axis and the positive half axis of the Y axis and the negative half axis of the X axis and the first quadrant 45° and the negative half axis of the X axis and the negative half axis of the Y axis and the fourth quadrant 45°, respectively. SMF-HG10, The axes of SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are parallel to the axis of the elliptical fiber core from their respective starting points; 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 realize the multiplexing output function; other light fields are incident from the left end of the SMF corresponding to HG10, HG01, HG20, HG11, HG30, HG02 and HG21, and are coupled to the modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the main transmission channel of the elliptical fiber core, respectively. The right end of the elliptical core transmission main channel outputs in the HG10, HG01, HG20, HG11, HG30, HG02 and HG21 modes; mode multiplexing of the eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the elliptical core transmission main channel is realized; if the modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are incident from the left end of the elliptical core transmission main channel, they are coupled and demultiplexed along the z direction; according to the coupled mode theory, the mode HG00 is transmitted from the left end to the right end along the elliptical core transmission main channel to achieve demultiplexing;Modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are incident from the left end of the elliptical core transmission main channel and are respectively coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21, realizing mode demultiplexing of the 8 modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 from the elliptical core transmission main channel; according to the change of mode coupling length and channel spacing, The relationship between the center coordinates of the elliptical fiber core transmission main channel is (0, 0), the length of the elliptical fiber core transmission main channel is 11.37 mm, the ellipticity of the elliptical core is 1.58, the major semi-axis length is 7.02 μm, and the minor semi-axis length 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, SMFHG20, SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are 13.02, 10.94, 13.52, respectively; 12.52, 11.44 and μm, and their lengths are 8.112, 4.520, 4.976, 3.258, 1.190, 1.626 and 1.964 mm respectively. The radius of SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are all 3 μm; the elliptical fiber core adopts a step-index refractive index distribution, which is higher than the refractive index of the surrounding cladding. High-refractive-index elliptical fiber cores and single-mode fiber cores are mainly used for light transmission; single-mode core SMF-HG10, SMF-HG01, SMF-HG20, The step refractive indices of SMF-HG11, SMF-HG30, SMF-HG02, and SMF-HG21 are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105, and 1.426017, respectively. The main transmission channel of the elliptical core fiber uses pure silica material with a refractive index of n1 = 1.4440; the cladding part uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics of the optical fiber can be changed by properly setting parameters such as the core position, size, and refractive index distribution.

[0023] Figure 3 The mode field distribution diagrams of HG21, HG02, HG30, HG11, HG20, HG01, HG10 and HG00 with elliptical core at 1.55μm wavelength are given. The modes are obtained by beam propagation method. By setting the ellipticity, the refractive index difference between the modes is adjusted to be greater than 1×10 -3 , to achieve the mold preservation function.

[0024] Figure 4 The graph shows how the effective refractive index difference of each mode changes with the ellipticity of the elliptical fiber core at a wavelength of 1.55 μm. The curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons represent the effective refractive index difference n between the HG00 mode and the HG10 mode. 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 and 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 five-pointed star and the pink hexagon. Figure 4 It can be seen that when the ellipticity p = a x / a y =1.58, the effective refractive index difference n between HG00 mode and HG10 mode HG00 -n HG10 is 3.0935×10 -3 , the effective refractive index difference n between HG10 mode and HG01 mode HG10 -n HG01 is 2.641×10 -3 , the effective refractive index difference n between HG01 mode and HG20 mode HG01 -n HG20 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 HG021.07×10 -3 , the effective refractive index difference n between HG02 mode and HG21 mode HG02 -n HG21 1.092×10 -3 and the effective refractive index difference n between the HG21 mode and the cladding HG21 -n cladding is 1.9115×10 -3 The effective refractive index difference between each mode is greater than 1×10 -3 , which can effectively guarantee the number of modes and realize eight-mode mode-preserving transmission.

[0025] Figure 5 The effective refractive index difference of each mode in the elliptical core in the range of 1.53μm-1.565μm is given. The curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons and black hexagons represent the effective refractive index difference n between the HG00 mode and the HG10 mode. 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 and 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 in 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 , and the refractive index difference gradually increases with the increase of wavelength. The refractive index difference between HG10 mode and HG01 mode is n HG10 -n HG01 Greater than 2.596×10 -3 , and the refractive index difference increases gradually with the increase of wavelength. The refractive index difference n between mode HG01 and HG20 HG01 -n HG20Greater than 1.6985×10 -3 , and the refractive index difference also increases with the increase of wavelength. The refractive index difference n between modes HG20 and HG11 is HG20 -n HGG11 Greater than 2.4705×10 -3 , the refractive index difference increases gradually with the increase of wavelength. The refractive index difference n between mode HG11 and HG30 is HG11 -n HG30 Greater than 2.936×10 -3 , the refractive index difference increases gradually with the increase of wavelength. The refractive index difference n between mode HG30 and HG02 HG30 -n HG02 Greater than 1.0465×10 -3 , the refractive index difference decreases gradually with the increase of wavelength. The refractive index difference n between mode HG02 and HG21 HG02 -n HG21 Greater than 1.0435×10 -3 , the refractive index difference increases gradually with the increase of wavelength. Cladding The refractive index difference n HG21 -n Cladding Greater than 1.6615×10 -3 , the refractive index difference decreases gradually with the increase of wavelength. The refractive index difference n at 1545nm and 1550nm HG00 -n HGG10 are 3.077×10 -3 and 3.0935×10 -3 , n HG10 -n HG01 are 2.6295×10 -3 and 2.641×10 -3 , n HG01- n HG20 are 1.7315×10 -3 and 1.743×10 -3 , n HG20 -n HG11 are 2.499×10 -3 and 2.508×10 -3 , n HG11 -n HG30 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 HHG21They are 1.0805×10- 3 and 1.092×10 -3 , n HG21 -n Cladding 1.995×10 -3 and 1.9115×10 -3 .

[0026] Figure 6 The coupling efficiency of each spatial mode of the multiplexer / demultiplexer is shown as a function of the incident wavelength in the C-band. The coupling efficiencies of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes are represented by the curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively. The coupling efficiency of the HG00 mode is approximately 0 dB and remains essentially constant with increasing wavelength. At 1550nm, the coupling efficiency of the HG10 mode reaches -0.0060622dB, the coupling efficiency of the HG01 mode reaches -0.002836dB, the coupling efficiency of the HG20 mode reaches -0.018933dB, the coupling efficiency of the HG11 mode reaches -0.021372dB, the coupling efficiency of the HG30 mode reaches -0.09307dB, the coupling efficiency of the HG02 mode reaches -0.52845dB, and the coupling efficiency of the HG21 mode reaches -0.39268dB. In the C-band on both sides of 1550nm, the coupling efficiency shows a downward trend. The reason is that with the change of wavelength, the coupling period of each mode will change. Since the coupling length of the designed (de)multiplexer is fixed, it is impossible to achieve the maximum coupling efficiency of each wavelength at the same time and it changes with wavelength. Across the entire C-band, the coupling efficiency of HG10 is higher than -0.089021dB, that of HG01 is higher than -0.3675dB, that of HG20 is higher than -1.8095dB, that of HG11 is higher than -2.0723dB, that of HG30 is higher than -0.88674dB, that of HG02 is higher than -1.5331dB, and that of HG21 is higher than -1.9866dB. The HG00 mode has the best coupling efficiency.

[0027] Figure 7The variation of the modal extinction ratio of the multiplexer / demultiplexer as a function of the incident light wavelength is shown. The extinction ratios of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes are represented by the curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively. The extinction ratio of mode HG00 is above 43.2359dB. In the entire C band, the extinction ratio of HG10 can be maintained above 34.9875dB, and the maximum value is 40.5087dB at 1545nm. The extinction ratio of HG01 can be maintained above 31.0093dB, and the maximum value is 38.5081dB at 1550nm. The extinction ratio of HG20 can be maintained above 26.7094dB, and the maximum value is 38.6356dB at 1550nm. The optical ratio can be maintained above 20.2871dB, and the maximum value is 25.4446dB at 1550nm; the extinction ratio of HG30 can be maintained above 17.41dB, and the maximum value is 19.5387dB at 1560nm; the extinction ratio of HG02 can be maintained above 19.341dB, and the maximum value is 21.0968dB at 1545nm; the extinction ratio of HG21 can be maintained above 13.0091dB, and the maximum value is 14.2039dB at 1560nm.

[0028] Figure 8The intrinsic loss of the main transmission channel of the multiplexer / demultiplexer's elliptical fiber core is shown. The intrinsic loss of the HG00, HG10, HG01, HG20, HG11, HG30, HG02, and HG21 modes is represented by the curves with black squares, green asterisks, blue diamonds, green circles, black triangles, red five-pointed stars, pink hexagons, and black hexagons, respectively. Overall, the intrinsic loss of the eight modes, from highest to lowest, is HG02, HG21, HG30, HG11, HG20, HG01, HG10, and HG00. In the C-band, the intrinsic loss of mode HG00 can be kept below 0.14892dB / km, with a minimum of 0.14556dB / km at 1535nm; the intrinsic loss of mode HG10 can be kept below 0.15077dB / km, with a minimum of 0.14745dB / km at 1535nm; the intrinsic loss of mode HG01 can be kept below 0.15385dB / km, with a minimum of 0.1506dB / km at 1535nm; the intrinsic loss of mode HG20 can be kept below 0.154dB / km, with a minimum of 0.15072dB / km at 1535nm. ; The intrinsic loss of mode HG11 can be kept below 0.15776dB / km, and the minimum value is 0.15455dB / km at 1535nm; the intrinsic loss of mode HG30 can be kept below 0.16054dB / km, and the minimum value is 0.15713dB / km at 1535nm; the intrinsic loss of mode HG02 can be kept below 0.16833dB / km, and the minimum value is 0.16491dB / km at 1535nm; the intrinsic loss of mode HG21 can be kept below 0.16557dB / km, and the minimum value is 0.16202dB / km at 1535nm.

Claims

1. An octal-mode multiplexer / demultiplexer based on a refractive index step-type non-degenerate mode main channel, characterized by: The multiplexer / demultiplexer consists of an octamode elliptical core (EC) and seven single-mode cores divided into two sections to form the transmission channel. The elliptical core at the coordinate center serves as the main transmission channel running through the two sections. The second section starts at the end of the longest coupling length of the first section. In the first section, the single-mode fiber core SMF-HG10 is on the X coordinate axis, SMF-HG01 is on the Y coordinate axis, SMF-HG20 is on the -X coordinate axis, and SMF-HG02 is on the -Y coordinate axis. In the second section, the single-mode fiber core SMF-HG11 is in the 45° direction of the first quadrant, SMF-HG30 is on the -X coordinate axis, and SMF-HG21 is in the 45° direction of the fourth quadrant. The elliptical fiber core transmission channel is the main channel for mode division multiplexing / demultiplexing, placed on the z axis, and its axis coincides with the z axis, supporting modes HG00, HG10, HG01, HG 20, HG11, HG30, HG02 and HG21; the seven single-mode fiber cores SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are all single-mode circular core transmission coupling channels, which are placed at the positive half axis of the X axis, the positive half axis of the Y axis, the negative half axis of the X axis and the first quadrant 45°, and the negative half axis of the X axis, the negative half axis of the Y axis and the fourth quadrant 45°, respectively. The axes of SMF-HG10, SMF-HG01, SMF-HG20 and SMF-HG02 are parallel to the elliptical core from their respective starting points. axis; SMF-HG11, SMF-HG30 and SMF-HG21 start to be parallel to the axis of the elliptical core at the end of the first longest 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 core to realize the multiplexing output function; other light fields are incident from the left end of the SMF corresponding to HG10, HG01, HG20, HG11, HG30, HG02 and HG21, and are coupled to the modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the main transmission channel of the elliptical core, and are transmitted to the right end of the main transmission channel of the elliptical core. The end outputs in HG10, HG01, HG20, HG11, HG30, HG02 and HG21 modes; it realizes the mode multiplexing of 8 modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 in the main transmission channel of the elliptical fiber core; if the 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, they are coupled and demultiplexed along the z direction; according to the coupled mode theory, the mode HG00 is transmitted from the left end to the right end along the main transmission channel of the elliptical fiber core to achieve demultiplexing;Modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21 are incident from the left end of the eight-mode elliptical fiber core transmission main channel, and are respectively coupled to the right end output of the SMF corresponding to modes HG10, HG01, HG20, HG11, HG30, HG02 and HG21, realizing mode demultiplexing of the eight modes HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21 from the elliptical fiber core transmission main channel; according to the change of mode coupling length and channel spacing, The relationship between the center coordinates of the elliptical fiber core transmission main channel is (0, 0), the length of the elliptical fiber core transmission main channel is 11.37 mm, the ellipticity of the elliptical core is 1.58, the major semi-axis length is 7.02 μm, and the minor semi-axis length 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 13.02, 10.94, 13.52, respectively; 12.52, 11.44 and Their lengths are 8.112, 4.520, 4.976, 3.258, 1.190, 1.626 and 1.964 mm respectively. The radius of SMF-HG10, SMF-HG01, SMFHG20, SMF-HG11, SMF-HG30, SMF-HG02 and SMF-HG21 are all 3 μm. The elliptical fiber core adopts a step-index refractive index distribution, which is higher than the refractive index of the surrounding cladding. High-refractive-index elliptical fiber core and single-mode fiber core are mainly used for light transmission. Single-mode core SMF-HG10, SMF-HG01, SMF-HG20, SMF-HG11, SMF-HG30, SMF- The step refractive indices of HG02 and SMF-HG21 are 1.437988, 1.435368, 1.433626, 1.431141, 1.428153, 1.427105, and 1.426017, respectively. The main transmission channel of the elliptical core fiber uses pure silica material with a refractive index of n1 = 1.4440; the cladding part uses fluorine-doped silica material with a refractive index of n2 = 1.4240. The mode field characteristics of the optical fiber can be changed by reasonably setting parameters such as the core position, size, and refractive index distribution. The multiplexer and demultiplexer break the mode degeneracy of HG30 and HG02, and the refractive index difference between the modes is greater than 1×10 -3 , realizing the mode preservation function, and using the refractive index core of pure silica to reduce loss; through the side core being divided into two sections for coupling, it realizes the low-loss and low-crosstalk multiplexing and demultiplexing of eight modes of HG00, HG10, HG01, HG20, HG11, HG30, HG02 and HG21, and realizes MIMO-FREE operation, which is expected to solve the current research challenges of few-mode optical fiber multiplexers / demultiplexers, has important academic and application value, and has broad application prospects.

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