Graded refractive index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation and control

Abstract

A gradient refractive index type transmission main channel few-mode multiplexer / demultiplexer based on optical field regulation and control is characterized in that a transmission channel is formed by an elliptical four-core super-mode fiber core FEC-SFC and three single-mode fiber cores, the elliptical four-core super-mode fiber core at the coordinate center serves as a transmission main channel, and the single-mode fiber cores SMF1, SMF2 and SMF3 are transmission coupling channels; the length of the transmission main channel FEC-SFC is 5.05 mm, and the lengths of the SMF1, the SMF2 and the SMF3 are also 5.05 mm; a low-loss and low-crosstalk non-degenerate mode super-mode and mode-preserving optical fiber (SMOF) core is designed as a main channel of a multiplexer and a demultiplexer, mode degeneracy of LP11a / LP11b is broken, a mode-preserving function is realized, light field regulation and control are provided to break mode degeneracy, effective refractive index difference between modes is enhanced and increased to solve mode crosstalk, and the mode-preserving function is realized. The refractive index fiber core made of pure silicon dioxide is adopted to reduce loss; the four super-mode multiplexing and demultiplexing of the LP01, the LP11a, the LP11b and the LP21b with the same magnitude length and high practicability are realized, and MIMO-FREE operation is realized.

Description

TECHNICAL FIELD

[0001] The patent relates to a graded-index transmission main channel few-mode multiplexer / demultiplexer based on light field regulation, which can be applied to the fields of fiber optics, fiber communication, fiber wireless access, optical information processing and new generation information technology. BACKGROUND

[0002] In recent years, various communication traffic has increased exponentially, and single-mode optical fiber communication has been faced with unprecedented challenges. In order to improve the capacity of communication system, the space division multiplexing technology of few-mode fiber (FMF) and multi-core fiber (MCF) has attracted extensive attention. The optical fiber communication industry has achieved a breakthrough in the transmission capacity of communication network around the space division multiplexing (including core multiplexing and mode division multiplexing and their combination) of physical dimension; the mode division multiplexing in space division multiplexing and few-mode fiber and its related devices and application research have become a hot research frontier [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, ISSN 1068-5200, https: / / doi.org / 10.1016 / j.yofte.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), ISSN: 1672-6634, 2022, 35(1): 30-56; Zheng Hongjun, Li Xin, Bai Chenglin, Chirped pulse propagation in optical fibers, Beijing: Science Press, 2018, 1-184; Dong Qiuhuan, Liu Yang, Zheng Hongjun, Li Xin, Bai Chenglin, Hu Weisheng, Chen Nanguang. 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 core can effectively reduce the fiber attenuation and fusion loss, and is 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 can be better compatible with single-mode fiber, and the fusion loss of few-mode fiber applied to single-mode fiber system is lower. In addition, FMF is superior to SMF in terms of transmission capacity and effective mode field area (i.e. lower nonlinearity). However, in the mode division multiplexing system based on few-mode fiber, there is crosstalk between adjacent modes, which affects 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 few-mode fiber transmission and mode division multiplexer / demultiplexer optical elements, a mode division multiplexing transmission method is proposed to suppress mode crosstalk. [A. R. May and M. N. Zervas, "Few-mode fibers with improved mode spacing," 2015 European 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, and Zhangyuan Cheh, "Weakly-coupled 4-mode step-index FMF and demonstration of IM / DD MDM transmission," Opt. Express 26, 8356-8363 (2018); Jiang, Shoulin and Ma, Lin and Zhang, Zhaopeng and Xu, Xiao and Wang, Shuai and Du, Jiangbing and Yang, Chen and 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, 1 Dec. 1, 2018, doi: 10.2874526; Dawei Ge, Yuyang Gao, Yu Yang, Lei Shen, Zhengbin Li, Zhangyuan Chen, Yongqi He, Juhao Li, "A 6-LP-mode ultralow-modal-crosstalk double-ring-core FMF for weakly-coupled MDM transmission," Optics Communications, vol. 451, pp. 97-103, Nov. 2019]; but these circular core few-mode fibers still have large mode degeneracy, mode crosstalk and loss, mode degeneracy and mode crosstalk require the use of multiple-input multiple-output digital signal processing MIMO-DSP, for short distance communication, the more the mode, the more complex, the amount of calculation, the cost and other problems of MIMO-DSP; the loss of the optical fiber will affect the transmission distance of the optical communication system, and the loss of the optical fiber is an important parameter to determine the performance of the optical fiber communication system.

[0003] Compared with the above-mentioned circular 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 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; Yuxin Ding, Jianshe Li, Shuguang Li, Yu Qin, Zelin Zhang, XiaokaiWang, 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 Techno1.41, 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 SWlI.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 Techno1.41, 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] At the same time, in recent years, supermode fibers with multiple cores have attracted widespread attention in the field of optical communications. They can achieve strong coupling between different cores, have a larger effective area, lower fiber nonlinearity, higher mode density and additional design freedom [Cen Xia, Neng Bai, Ibrahim Ozdur, Xiang Zhou, and Guifang Li,"Supermodesfor optical transmission,"[J].Optics express, 2011, 19(17); Jin Luo, Mei Sang, Ning bo, et al.,"A Supermode Fiber with Strong Mode Coupling for Space-Division Multiplexing."Asia Communications and Photonics Conference 2015; Hongyao Chen, Yijin Chen, et al."Octagonal polarization-maintaining supermodefiber for mode division multiplexing system,"510.(2022); Lin Sun, Junwei Zhang, Gai Zhou, Bin Chen, Gordon Ning Liu, Yi Cai, Zhaohui Li, Chao Lu, and Gangxiang Shen,"Theoretical investigations of weakly and strongly-coupled multi-core fibers for the applications of optical submarine communications under power and fiber count limits,″[J].Optics express,2023,31(3).]In 2022, a polarization-maintaining supermode fiber was proposed to improve the light propagation performance in MDM system, thereby reducing the complexity of MIMO-DSP[Hongyao Chen,Yijin Chen,et al.”Octagonal polarization-maintaining supermode fiber for modedivision multiplexing system,″510.(2022);].In 2023, the application of weakly coupled and strongly coupled multi-core fibers (WC-MCF, SC-MCF) in submarine communications was studied, among which SC-MCF cables can improve the Q factor by 0.75-2dB compared with WC-MCF cables [Lin Sun, Junwei Zhang, Gai Zhou, Bin Chen, Gordon Ning Liu, YiCai, Zhaohui Li, Chao Lu, and Gangxiang Shen,"Theoretical investigations of weakly and strongly-coupled multi-core fibers for the applications of optical submarine communications under power and fiber count limits,"[J].Opticsexpress, 2023, 31(3).]. Strongly coupled supermode operation is the current research focus and has great advantages. This is the second consideration in the design of the main channel fiber core of our multiplexer and demultiplexer.

[0005] In order to effectively solve the problems of mode degeneracy, mode crosstalk and loss in mode division multiplexing, a novel few-mode multiplexer / demultiplexer is proposed by combining the advantages of pure silica fiber core, elliptical mode-preserving fiber core, graded refractive index fiber core and supermode fiber core to realize MIMO-FREE operation. It is expected to solve the current research challenges of few-mode fiber multiplexers / demultiplexers, has important academic and application value, and has broad application prospects. Utility Model Content

[0006] Under the support of National Natural Science Foundation (No. 61671227 and 61431009), Shandong Natural Science Foundation (ZR2011FM015) and "Taishan Scholar" Construction Engineering Special Funds, aiming at the mode degeneracy, mode crosstalk and loss in mode division multiplexing, and combining the advantages of pure silica core, elliptical mode-preserving core, graded-index core and supermode core, the patent proposes a graded-index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation, designs a low-loss, low-crosstalk non-degenerate mode supermode mode-preserving optical fiber (SMOF) core as the multiplexer / demultiplexer main channel, breaks the mode degeneracy of LP11a / LP11b, realizes the mode-preserving function, proposes light field regulation to strengthen the increase of the effective refractive index difference between modes to solve the mode crosstalk scheme, and uses a pure silica refractive core to reduce the loss; LP01, LP11a, LP11b and LP21b four supermode multiplexing and demultiplexing are realized on the same order length, MIMO-FREE operation is realized, and important support is provided for in-depth research in the fields of fiber optics, fiber communication, fiber wireless access, optical information processing and new generation information technology.

[0007] The technical scheme adopted by the patent application to solve its technical problems is:

[0008] The patent proposes a graded-index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation, which is characterized in that the mode division multiplexer / demultiplexer is composed of one elliptical four-core supermode fiber core (FEC-SFC) and three single-mode fiber cores as transmission channels, wherein the elliptical four-core supermode fiber core at the coordinate center is used as the transmission main channel, and the three circles located on the-X coordinate axis, the Y coordinate axis and the X coordinate axis are the cross sections of the single-mode fiber cores SMF1, SMF2 and SMF3, which are transmission coupling channels; the FEC-SFC transmission channel is the main channel of mode division multiplexing / demultiplexing, which is placed on the z-axis, the axis center of which coincides with the z-axis, supporting supermode modes LP01, LP11a, LP11b and LP21b; the other three single-mode fiber cores SMF1, SMF2 and SMF3 are placed on the negative half of the x-axis and the positive half of the y-axis and the positive half of the x-axis respectively, and the axis centers of SMF1, SMF2 and SMF3 are parallel to the axis center of FEC-SFC from the starting point of each; according to the coupled mode theory, the mode LP01 is transmitted along the main channel FEC-SFC from the left end to the right end to realize the multiplexing output function; other light fields are respectively incident from the left end of the corresponding SMF of LP11a, LP11b and LP21b, and are coupled into the modes LP11a, LP11b and LP21b in the main channel FEC-SFC, and are output in the modes LP11a, LP11b and LP21b at the right end of the main channel FEC-SFC; the mode multiplexing of four modes LP01, LP11a, LP11b and LP21b in the main channel FEC-SFC is realized; if the modes LP01, LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC core and transmitted along the z direction, the mode demultiplexing is realized; according to the coupled mode theory, the mode LP01 is transmitted along the main channel FEC-SFC from the left end to the right end to realize the demultiplexing; the modes LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC and are respectively transmitted and coupled into the right end of the corresponding SMF of the modes LP11a, LP11b and LP21b to realize the mode demultiplexing of four modes LP01, LP11a, LP11b and LP21b from the main channel FEC-SFC; according to the change relationship between the mode coupling length and the channel spacing, the four-ellipse core center coordinates of the transmission main channel FEC-SFC are respectively (0μm), (0μm), and​​​​ μm, the length of both is 5.05 mm, the radius of SMF1, SMF2 and SMF3 is 2 μm; the four-elliptical core adopts a graded refractive index distribution, the refractive index distribution of the elliptical core follows the formula: n(r) = n x *[1-2Δ(r / a) α ] 1 / 2 , r≤a; the center refractive index of the left and right cores of the elliptical four-core corresponds to x = 0, and the center refractive index of the upper and lower cores of the elliptical four-core corresponds to x = 1; n x in the formula represents the center refractive index of the elliptical core, the center of the left and right cores of the elliptical four-core for transmitting the main channel adopts pure silica material, and the refractive index is n0 = 1.4440, and the center of the upper and lower cores of the elliptical four-core for transmitting the main channel adopts fluorine-doped silica material, and the refractive index is n1 = 1.4426; r represents the distance of any point in the core to the axis, a represents the long half axis of the core is 2.4 μm, and α is the gradient parameter 2, n(r) = n2, the refractive index of the cladding when r > a; the step refractive indexes of the single-mode cores SMF1, SMF2 and SMF3 are 1.4351, 1.4324 and 1.4295 respectively; the cladding part of the mode division multiplexer / demultiplexer adopts fluorine-doped silica material, and the refractive index is n2 = 1.4090, and the outermost cladding radius is 62.5 μm; since the FEC-SFC needs to be connected to the transmission optical fiber in the transmission link, the length needs to be at least greater than or equal to the length of the FEC-SFC, and the FEC-SFC can be lengthened according to the actual situation; the mode field characteristics in the optical fiber can be changed by reasonably setting the core position, size and refractive index distribution and the like.

[0009] The beneficial effects of the present patent application are as follows:

[0010] 1. The multiplexer / demultiplexer is composed of a directional mode selection coupler, and the large effective refractive index difference between the modes in the optical fiber ensures the low crosstalk characteristics between the modes; the refractive index core of pure silica is adopted to realize the low loss performance.

[0011] 2. The optical fiber used in the multiplexer / demultiplexer combines the advantages of pure silica core, graded refractive index distribution and elliptical four-core supermode core, adopts the elliptical four-core supermode core, breaks the mode degeneracy of LP11a / LP11b, realizes the mode-preserving function of LP01, LP11a, LP11b and LP21b four supermodes and the high performance multiplexing and demultiplexing of the same order length of the four supermodes, and provides important support for the in-depth research in the fields of fiber optics, fiber communication, fiber wireless access and optical information processing, new generation information technology and the like.

[0012] 3. The light field regulation is proposed to break the degeneracy of the modes, strengthen and increase the effective refractive index difference between the modes to solve the mode crosstalk; by adjusting the refractive index of the left and right two cores and the upper and lower two cores of the four-elliptical-core supermode fiber, the light field regulation is realized. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 is a cross-sectional view of a graded-index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation according to the present patent, wherein the four-core supermode fiber core at the center of the coordinates is the transmission main channel (elliptical); the single-mode SMF core (circular) located on the X coordinate axis and the Y coordinate axis.

[0014] Figure 2 is a three-dimensional perspective view of a graded-index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation according to the present patent.

[0015] Figure 3 The mode field distribution diagrams of the supermodes at 1.55 μm wavelength are given, Figure 3 (1), Figure 3 (2), Figure 3 (3), Figure 3 (4) correspond to the mode field distribution diagrams of the LP01, LP11a, LP11b and LP21b supermodes of the four-core supermode fiber core, respectively.

[0016] Figure 4 The changes of the effective refractive index difference of each mode with the refractive index difference of the left and right two cores and the upper and lower two cores of the four-core supermode fiber core at 1.55 μm wavelength are given. The curves with squares, triangles, diamonds and circles represent the effective refractive index difference n LP01 -n LP11a , the effective refractive index difference n LP11a- n LP11b , the effective refractive index difference n LP11b- n LP21b and the effective refractive index difference n LP21b -n Cladiing of the LP01 mode and the LP11a mode, the LP11a mode and the LP11b mode, the LP11b mode and the LP21b mode, and the LP21b mode and the cladding, respectively.

[0017] Figure 5 The effective refractive index difference of each mode of the fiber at 1.53 μm-1.565 μm is given. The curves with circles, asterisks, squares and triangles represent the effective refractive index difference n Lp01 -n LP11a , the effective refractive index difference n LP11a -n LP11bThe effective refractive index difference n between LP11b mode and LP21b mode LP11b- n LP21b The effective refractive index difference n between LP21b mode and the cladding LP21b- n cladding .

[0018] Figure 6 The coupling efficiency of each spatial mode of the multiplexer / demultiplexer is given as a function of the incident wavelength in the C band. The coupling efficiency of LP01, LP11a, LP11b and LP21b modes are represented by triangles, stars, squares and circles, respectively.

[0019] Figure 7 The mode extinction ratio of the multiplexer / demultiplexer in the mode channel is given as a function of the incident wavelength. The extinction ratio of LP01, LP11a, LP11b and LP21b modes are represented by triangles, stars, squares and circles, respectively.

[0020] Figure 8 The intrinsic loss of the multiplexer / demultiplexer main channel FEC-SFC is given, and the intrinsic loss of LP01, LP11a, LP11b and LP21b modes are represented by curves with circles, triangles, squares and stars, respectively. DETAILED DESCRIPTION

[0021] The technical solutions of the present application will be described in detail below in conjunction with the embodiments and drawings, but the protection scope is not limited thereto.

[0022] Example 1

[0023] Figure 1 is a cross-sectional schematic diagram of a graded-index type transmission main channel few-mode multiplexer / demultiplexer based on light field regulation. Figure 2This is a three-dimensional diagram of a gradient refractive index transmission main channel few-mode multiplexer / demultiplexer based on optical field control of the present patent; it is characterized in that: the mode division multiplexer / demultiplexer is composed of a four-elliptical core supermode fiber core (FEC-SFC) and three single-mode fiber cores to form a transmission channel, wherein the four-elliptical core supermode fiber core at the coordinate center serves as the main transmission channel, and the three circles located on the -X coordinate axis, the Y coordinate axis and the X coordinate axis are the cross sections of the single-mode fiber cores SMF1, SMF2 and SMF3, which are transmission coupling channels; the FEC-SFC transmission channel is the main channel of mode division multiplexing / demultiplexing, placed on the z axis, with its axis coinciding with the z axis, and supports supermode modes LP01, LP11a, LP11b and LP21b; in addition The three single-mode fiber cores SMF1, SMF2 and SMF3 are placed on the negative half axis of the x-axis, the positive half axis of the y-axis and the positive half axis of the x-axis respectively. The axes of SMF1, SMF2 and SMF3 are parallel to the axis of FEC-SFC from their respective starting points. According to the coupled mode theory, mode LP01 is transmitted from the left end to the right end along the main channel FEC-SFC to realize the multiplexing output function. The other light fields are incident from the left end of the SMF corresponding to LP11a, LP11b and LP21b respectively, and are coupled to the mode The four modes LP01, LP11a, LP11b and LP21b are output at the right end of the main channel FEC-SFC in the mode of LP11a, LP11b and LP21b; the mode multiplexing of the four modes LP01, LP11a, LP11b and LP21b in the FEC-SFC main channel is realized; if the modes LP01, LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC fiber core, they are transmitted along the z direction for coupled demultiplexing; according to the coupled mode theory, the mode LP01 is transmitted along the main channel FEC -SFC is transmitted from the left end to the right end to achieve demultiplexing; modes LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC and are respectively transmitted and coupled to the right end output of the SMF corresponding to modes LP11a, LP11b and LP211b, realizing demultiplexing of the four modes LP01, LP11a, LP11b and LP21b from the FEC-SFC main channel mode; according to the relationship between the mode coupling length and the channel spacing, the center coordinates of the four elliptical cores of the transmission main channel FEC-SFC are and The length of FEC-SFC is 5.05min, the ellipticity of the elliptical core is 1.5, the major axis length is 2.4μm, and the minor axis length is 1.6μm; the channel center distances of SMF1, SMF2 and SMF3 to the main channel FEC-SFC are and The lengths of SMF1, SMF2, and SMF3 are all 5.05 mm, and the radii of SMF1, SMF2, and SMF3 are all 2 μm. The four cores of the optical fiber adopt a graded refractive index distribution, and the refractive index distribution of the elliptical core follows the formula: n(r) = n x * [1-2Δ(r / a) α ] 1 / 2 , r≤a; the refractive index of the left and right cores of the elliptical four-core corresponds to x=0, and the refractive index of the upper and lower cores of the elliptical four-core corresponds to x=1; n x Represents the refractive index of the center of the elliptical fiber core. The left and right core centers of the elliptical quad core of the main transmission channel are made of pure silica material with a refractive index of n0 = 1.4440, while the upper and lower core centers of the elliptical quad core of the main transmission channel are made of fluorine-doped silica material with a refractive index of n1 = 1.4426; r represents the distance from any point in the core to the axis, a represents the semi-major axis of the core is 2.4μm, α is the gradient parameter 2, and the parameter The refractive index of the cladding is n(r) = n2, and r > a. The step refractive indices of the single-mode fiber cores SMF1, SMF2, and SMF3 are 1.4351, 1.4324, and 1.4295, respectively. The cladding of the mode division multiplexer / demultiplexer is made of fluorine-doped silica with a refractive index of n2 = 1.4090 and an outermost cladding radius of 62.5 μm. Since the FEC-SFC is connected to the transmission fiber in the transmission link, its length must be at least greater than or equal to that of the FEC-SFC and can be extended according to actual conditions. The mode field characteristics in the optical fiber can be modified by properly setting parameters such as the core position, size, and refractive index distribution.

[0024] Figure 3 The mode field distribution diagram of the supermode at a wavelength of 1.55μm is given. Figure 3 (1) Figure 3 (2) Figure 3 (3) Figure 3 (4) Mode field distribution diagrams corresponding to the four supermodes LP01, LP11a, LP11b, and LP21b of FEC-SFC, respectively. The modes were obtained by beam propagation method. The mode preservation function was achieved.

[0025] Figure 4 The effective refractive index difference of each mode at 1.55μm wavelength is given as the refractive index difference between the left and right cores and the upper and lower cores of the four-core supermode fiber core changes. The curves with squares, triangles, diamonds and circles represent the effective refractive index difference n between the LP01 mode and the LP11a mode, respectively. LP01- n LP11a , the effective refractive index difference n between LP11a mode and LP11b mode LP11a -n LP11b, the effective refractive index difference n between LP11b mode and LP21b mode LP11b -n LP21b and the effective refractive index difference n between the LP21b mode and the cladding LP21b -n Cladding The refractive indices of the left and right cores and the upper and lower cores of the four-core supermode fiber are adjusted according to the intersection of the diamond curve and the triangle curve, thereby achieving light field control.

[0026] Figure 5 The effective refractive index difference of each mode in the range of 1.53μm to 1.565μm is given. The circles, asterisks, squares and triangles represent the effective refractive index difference n between the LP01 mode and the LP11a mode, respectively. LP01 -n LP11a , the effective refractive index difference n between LP11a mode and LP11b mode LP11a -n LP11b , the effective refractive index difference n between LP11b mode and LP21b mode LP11b -n LP21b and the effective refractive index difference n between the LP21b mode and the cladding LP21b -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 LP01 and mode LP11a is LP01 -n LP11a Greater than 3.14×10 -3 , and the refractive index difference increases gradually with the increase of wavelength. The refractive index difference n between LP11a mode and LP11b mode LP11a -n LP11b Greater than 2.13×10 -3 , and the refractive index difference increases gradually with the increase of wavelength. The refractive index difference n between modes LP11b and LP21b LP11b -n LP21b Greater than 2.08×10 -3 , and the refractive index difference also increases with the increase of wavelength. Cladding The refractive index difference n LP21b -n Cladding Greater than 7.84×10 -3 , the refractive index difference decreases gradually with the increase of wavelength. The refractive index difference n at 1545nm and 1550nm LP01 -n LP11a are 3.231×10 -3 and 3.26×10 -3 , n LP11a -n LP11bThe refractive index difference at 1545nm and 1550nm is 2.165×10 -3 and 2.175×10 -3 , n LP11b -n LP21b The refractive index difference at 1545nm and 1550nm is 2.149×10 -3 and 2.171×10 -3 , n LP21b -n Cladding The refractive index difference at 1545nm and 1550nm is 8.231×10 -3 and 8.135×10 -3 At 1.55 μm wavelength, the effective refractive index difference n between the LP11a mode and the LP11b mode before light field manipulation is LP11a -n LP11b is 1.322×10 -3 , the effective refractive index difference n between LP11a mode and LP11b mode after light field control LP11a- n LP11b is 2.175×10 -3 The effective refractive index difference after regulation is about 1.65 times that before regulation, which increases the effective refractive index difference between LP11a mode and LP11b mode.

[0027] Figure 6 The coupling efficiency of each spatial mode in the multiplexer / demultiplexer (DMUX) is shown as a function of incident wavelength in the C-band. The coupling efficiencies of the LP01, LP11a, LP11b, and LP21b modes are represented by curves with triangles, asterisks, squares, and circles, respectively. The coupling efficiency of the LP01 mode is approximately 0 dB and remains essentially constant with increasing wavelength. At 1550 nm, the coupling efficiency of the LP11b mode reaches -0.0289 dB, and that of the LP21b mode reaches -0.058 dB. At 1555 nm, the coupling efficiency of the LP11a mode reaches -0.0072 dB. In the C-band on either side of 1550nm, the coupling efficiency of the LP11b and LP21b modes shows a decreasing trend, while the coupling efficiency of the LP11a mode shows a decreasing trend in the C-band on either side of 1555nm. This is because the coupling period of each mode changes with wavelength. Since the coupling length of the designed (de)multiplexer is fixed, it is impossible to achieve the maximum coupling efficiency for each wavelength simultaneously, and the coupling efficiency varies with wavelength. Across the entire C-band, the coupling efficiency of LP11a is higher than -0.7145dB, the coupling efficiency of LP11b is higher than -0.5531dB, and the coupling efficiency of LP21b is higher than -1.2847dB. The coupling efficiency of the LP01 mode is the highest.

[0028] Figure 7 The variation of the modal extinction ratio in the modal channels of the multiplexer / demultiplexer as a function of the incident light wavelength is presented. The extinction ratios of the LP01, LP11a, LP11b, and LP21b modes are represented by the curves with triangles, asterisks, squares, and circles, respectively. The extinction ratio of mode LP01 is consistently above 36.5607 dB. The extinction ratio of mode LP11a remains above 31.3674 dB throughout the entire C-band, reaching a maximum of 35.4784 dB at 1560 nm. The extinction ratio of mode LP11b remains above 22.801 dB throughout the entire C-band, reaching a maximum of 26.6633 dB at 1565 nm. The extinction ratio of mode LP21b remains above 26.6016 dB throughout the entire C-band, reaching a maximum of 29.1928 dB at 1555 nm.

[0029] Figure 8 The intrinsic loss of the main channel FEC-SFC of the multiplexer / demultiplexer is given. The intrinsic losses of the LP01, LP11a, LP11b, and LP21b supermodes are represented by curves with circles, triangles, squares, and asterisks, respectively. The intrinsic losses of the four supermodes show a decreasing trend in the C-band. Overall, the intrinsic losses of the four modes are LP01, LP11a, LP11b and LP21b, from large to small. The intrinsic loss of mode LP01 can be maintained above 0.1802dB / km, with a maximum value of 0.1865dB / km at 1530nm; the intrinsic loss of mode LP11a can be maintained above 0.1779dB / km, with a maximum value of 0.1837dB / km at 1530nm; the intrinsic loss of mode LP11b can be maintained above 0.1765dB / km, with a maximum value of 0.1820dB / km at 1530nm; the intrinsic loss of mode LP21b can be maintained above 0.1747dB / km, with a maximum value of 0.1798dB / km at 1530nm.

[0030] In summary, the proposed multiplexer / demultiplexer breaks the degeneracy of modes LP11a and LP11b, and realizes the mode-preserving function of the four supermodes; the effective refractive index difference between supermodes LP11a and LP11b is increased by adopting light field control, and the refractive index of the left and right fiber cores and the upper and lower fiber cores of the four-elliptical core supermode fiber is adjusted to achieve light field control; and the practical and high-performance multiplexing and demultiplexing of the four supermodes LP01, LP11a, LP11b and LP21b at the same order of length is realized.

Claims

1. A gradient refractive index transmission main channel few-mode multiplexer / demultiplexer based on optical field control, characterized by: The few-mode multiplexer / demultiplexer consists of an elliptical four-core supermode fiber core FEC-SFC and three single-mode fiber cores to form a transmission channel. The elliptical four-core supermode fiber core at the center of the coordinate serves as the main transmission channel. The three circles on the -X coordinate axis, the Y coordinate axis, and the X coordinate axis are the cross-sections of the single-mode fiber cores SMF1, SMF2, and SMF3, which are transmission coupling channels. The FEC-SFC transmission channel is the main channel for mode division multiplexing / demultiplexing and is placed on the z axis with its axis coinciding with the z axis, supporting supermode mode. LP01, LP11a, LP11b and LP21b; the other three single-mode fiber cores SMF1, SMF2 and SMF3 are placed on the negative half axis of the x-axis, the positive half axis of the y-axis and the positive half axis of the x-axis respectively. The axes of SMF1, SMF2 and SMF3 are parallel to the axis of FEC-SFC from their respective starting points. According to the coupled mode theory, mode LP01 is transmitted from the left end to the right end along the main channel FEC-SFC to realize the multiplexing output function. The other optical fields are transmitted from LP11a, LP11b and LP11b respectively. The left end of the SMF corresponding to LP01 and LP11b is incident, and is coupled to the modes LP11a, LP11b and LP21b in the main channel FEC-SFC respectively, and is output at the right end of the main channel FEC-SFC in the modes LP11a, LP11b and LP21b; the mode multiplexing of the four modes LP01, LP11a, LP11b and LP21b in the FEC-SFC main channel is realized; if the modes LP01, LP11a, LP11b and LP21b are output from the main channel FEC-S The left end of the FC fiber core is incident and coupled and demultiplexed along the z direction. According to the coupled mode theory, mode LP01 is transmitted from the left end to the right end along the main channel FEC-SFC to achieve demultiplexing. Modes LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC and are respectively transmitted and coupled to the right end output of the SMF corresponding to modes LP11a, LP11b and LP21b, realizing the four modes LP01, LP11a, LP11b and LP21b from FEC-SFC. - Main channel mode demultiplexing; according to the relationship between the mode coupling length and the channel spacing, the center coordinates of the four elliptical cores of the main channel FEC-SFC are and The length of FEC-SFC is 5.05 mm, the ellipticity of the elliptical core is 1.5, the major axis length is 2.4 μm, and the minor axis length is 1.6 μm. The channel center distances of SMF1, SMF2, and SMF3 to the main channel FEC-SFC are and The lengths are all 5.05 mm, and the radii of SMF1, SMF2, and SMF3 are all 2 μm. The four elliptical cores adopt a graded refractive index distribution, and the refractive index distribution of the elliptical core follows the formula: n(r) = n x * [1-2Δ(r / a) α ] 1 / 2 , r≤a; the refractive index of the left and right cores of the elliptical four-core corresponds to x=0, and the refractive index of the upper and lower cores of the elliptical four-core corresponds to x=1; n x Represents the refractive index of the center of the elliptical fiber core. The left and right core centers of the elliptical quad core of the main transmission channel are made of pure silica material with a refractive index of n0 = 1.4440, while the upper and lower core centers of the elliptical quad core of the main transmission channel are made of fluorine-doped silica material with a refractive index of n1 = 1.4426; r represents the distance from any point in the core to the axis, a represents the semi-major axis of the core is 2.4μm, α is the gradient parameter 2, and the parameter The refractive index of the cladding when n(r)=n2 and r>a; the step refractive indices of the single-mode fiber cores SMF1, SMF2 and SMF3 are 1.4351, 1.4324 and 1.4295 respectively; the cladding part of the mode division multiplexer / demultiplexer is made of fluorine-doped silica material with a refractive index of n2=1.4090 and an outermost cladding radius of 62.5μm.

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