Low-loss and low-crosstalk few-mode multiplexer / demultiplexer based on six-core transmission channel

Abstract

The low-loss and low-crosstalk few-mode multiplexer / demultiplexer based on the six-core transmission channel is composed of a six-core super-mode fiber core supporting four modes and three two-mode ring core TMF, the six-core super-mode fiber core is composed of a pure silicon dioxide fiber core, and a cladding part is made of fluorine-doped silicon dioxide; the multiplexing / demultiplexing of the four modes of LP01, LP11, LP21 and LP31a in the FMF main channel mode is realized, and the multiplexer realizes low loss, low mode crosstalk and high extinction ratio, thereby solving the problem of signal deterioration caused by degenerate mode rotation in the transmission process.

Description

Technical Field

[0001] The present invention relates to a low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel, which can be applied to fields such as fiber optics, optical fiber communication, fiber wireless access, optical information processing, and new generation information technology. Background Art

[0002] In recent years, with the exponential growth of various communication service traffic, single-mode fiber communication has faced unprecedented challenges. The optical fiber communication industry has achieved breakthroughs in the transmission capacity of communication networks in the physical dimension of space-division multiplexing (including core multiplexing, mode-division multiplexing, and their combination); research on mode-division multiplexing, few-mode fiber (FMF), and their related devices and applications in space-division multiplexing has become a frontier 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 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, Transmission of chirped pulses 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 systems [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 fibers in mode division multiplexing systems, Journal of Liaocheng University (Natural Science Edition), 2019.4, 32(2):69-79]; A pure silica core can effectively reduce fiber attenuation and splicing loss and is currently mostly used in single-mode fibers [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]; In ring-core fibers, the propagation constant difference between adjacent azimuthal modes increases with the increase of the azimuthal mode, theoretically reducing the coupling between high-order modes; [Y. Yamamoto, Y. Kawaguchi and M. Hirano, "Low-Loss and Low-Nonlinearity Pure-Silica-Core Fiber for C-and L-band Broadband Transmission," in Journal of Lightwave Technology, vol. 34, no. 2, pp. 321-326, 15 Jan. 15, 2016, doi: 10.1109 / JLT.2015.2476837; X. Jin et al., "Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems," in Journal of Lightwave Technology, vol. 34, no. 14, pp. 3365-3372, 15 July 15, 2016, doi: 10.1109 / JLT.2016.2564991]; For optical components such as few-mode fiber transmission and mode division multiplexer / demultiplexer, a mode division multiplexing transmission method for suppressing mode crosstalk is proposed. [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 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, 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:10.1109 / 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-crosstalk double-ring-core FMF for weakly-coupled MDM transmission,” Optics Communications, vol.451, pp. 97–103, Nov. 2019]; A key issue in a mode-division multiplexing system is how to handle degenerate non-circularly symmetric polarization modes. Asymmetric linear polarization modes contain dual spatial degenerate modes with the same propagation constant. However, due to the imperfection of fiber manufacturing and external perturbations, their spatial directions may randomly rotate along the propagation direction during transmission, resulting in a rapid deterioration of the bit error rate. It is reasonable to transmit the two degenerate modes as an overall linear polarization mode in a single channel, which is compatible with the traditional IM / DD system; Since supermode fiber utilizes the coupling between multiple cores, the distance between cores is much shorter than that of traditional multi-core fibers, and strong coupling occurs between the modes of the fiber, thus achieving supermode operation. The supermode core has a large effective refractive index difference between modes, which can reduce inter-mode coupling to lower inter-mode crosstalk; The supermode core has advantages such as more design freedoms and a higher mode density; [Xia Cen, Bai Neng, Ozdur Ibrahim, et al. Supermodes for optical transmission [J], Optics Express 2011, 19(17): 16653-16664; Gao, Yuyang and Cui, Jian and Ge, Dawei and Jia, Junchi and Du, Chunyan and Xia, Cen and Liu, Yan and Li, Zhengbin and He, Yongqi and Chen, Zhangyuan and Li, Juhao and Li, Guifang, "A Degenerate-Mode-Selective Coupler for Stable DSP-free MDM Transmission," in Journal of Lightwave Technology, vol. 37, no. 17, pp. 4410-4420, 1 Sept. 1, 2019, doi: 10.1109 / JLT.2019.2925116]. In summary, the multiplexer combines the advantages of pure silica core, supermode fiber, and ring-core fiber, and uses the degenerate modes LP11a / LP11b and LP21a / LP21b as the overall linearly polarized modes for single-channel transmission, achieving mode multiplexing and demultiplexing of the degenerate output of four modes, namely LP01, LP11, LP21, and LP31. By using a pure silica refractive index core, it realizes low-loss performance; by using a large effective refractive index difference transmission channel and reasonably setting the position and length of the transmission channel, it realizes low crosstalk and high extinction ratio performance, solves the problem of signal degradation caused by the rotation of degenerate modes during transmission, is expected to solve the research challenges of current few-mode fiber multiplexers / demultiplexers, has important academic value and application value, and has great research significance and broad application prospects. Summary of the Invention

[0003] Supported by the National Natural Science Foundation of China (Grant Nos. 61671227 and 61431009), the Natural Science Foundation of Shandong Province (ZR2011FM015), and the Special Fund for the "Taishan Scholars" Construction Project, the present invention proposes a low-loss and low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel; the multiplexer combines the advantages of pure silica core, six-core supermode core, ring core, and step-index distribution core, providing important support for in-depth research in the fields of fiber optics, fiber communication, fiber wireless access, optical information processing, and new generation information technology.

[0004] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0005] The present invention proposes a low-loss and low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel. The multiplexer / demultiplexer consists of 4 different few-mode fiber core transmission channels. The six-core super-mode fiber core (SSFC) transmission channel SSFC is the main channel for mode division multiplexing / demultiplexing, supporting modes LP01, LP11a / b, LP21a / b, and LP31a. It is placed on the z-axis with its axis coinciding with the z-axis. The other three two-mode ring cores TMF1, TMF2, and TMF3 are placed on the positive half-axis of the y-axis, the positive half-axis of the x-axis, and the negative half-axis of the x-axis respectively. The axes of TMF1, TMF2, and TMF3 are parallel to the axis of SSFC starting from their respective starting points. TMF1, TMF2, and TMF3 are respectively used to achieve the demultiplexing output of LP11 a / b, LP21 a / b, and LP31a, and the mode LP01 is demultiplexed and output from the right end of SSFC. Each mode LP01, LP11 a / b, LP21 a / b, and LP31a is incident from the left end of the SSFC optical fiber and is coupled and demultiplexed along the z direction. According to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC to achieve demultiplexing output. The mode LP11 is directly coupled from SSFC to TMF1 on the positive half-axis of the y-axis. Among them, the degenerate modes LP11a / LP11b are incident from the left end of SSFC and are directly coupled to the modes LP11a / LP11b in TMF1 respectively, and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF1. The mode LP21 is directly coupled from SSFC to TMF2 on the positive half-axis of the x-axis. Among them, the degenerate modes LP21a / LP21b are incident from the left end of SSFC and are directly coupled to the modes LP11a / LP11b in TMF2 respectively, and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF2. The mode LP31a is incident from the left end of SSFC and is directly coupled to TMF3 on the negative half-axis of the x-axis, and is demultiplexed and output in the mode LP11a at the right end of TMF3. The demultiplexing of the four modes LP01, LP11, LP21, and LP31 from the main channel mode of SSFC is realized. If each mode LP01, LP11, LP21, and LP31 is incident from the left ends of SSFC, TMF1, TMF2, and TMF3 respectively and is coupled and multiplexed along the z direction. According to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC for multiplexing output, and the modes LP11, LP21, and LP31 are directly coupled from TMF1, TMF2, and TMF3 to SSFC for multiplexing output. Among them, LP11a / LP11b and LP21a / LP21b are incident from the left ends of their respective channels TMF1 and TMF2 and are multiplexed and output in a degenerate mode in SSFC. The multiplexing of the four modes LP01, LP11, LP21, and LP31 in the main channel mode of SSFC is realized.According to the variation relationship between the mode coupling length and the channel spacing, the channel center spacings of TMF1, TMF2, and TMF3 from the main channel SSFC are as follows; 12 μm and 14.5 μm, and their lengths are 7.752 mm, 6.6 mm, and 3.1 mm respectively, and the length of the main channel is 7.752 mm; reasonably setting the position and length of the transmission channel can adjust the performance of the mode multiplexer; the six cores of the optical fiber adopt a step refractive index distribution, which is higher than the refractive index of the periphery, and mainly use the six-core part with a high refractive index for light transmission; the mode field characteristics in the optical fiber can be changed by changing the sizes and refractive index distributions of the core, the inner and outer claddings; the six-core supermode core transmission channel consists of a core, an inner cladding, and an outer cladding. The core uses pure silica material with a refractive index of n1 = 1.444024, and the diameter of a single core is 4 μm; the centers of the six cores are distributed at the six corners of a regular hexagon, and their coordinates are: (4 μm, 0 μm), (-4 μm, 0 μm), and the refractive indices of its inner and outer claddings are n2 = 1.424024. For the TMF1 channel, the part with a diameter d < 0.9 μm is the inner cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 0.9 μm ≤ d ≤ 6.9 μm is the annular core with a refractive index of n3 = 1.450113; the part with d > 6.9 μm is the outer cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; for the TMF2 channel, the part with a diameter d < 2.5 μm is the inner cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 2.5 μm ≤ d ≤ 7 μm is the annular core with a refractive index of n4 = 1.445318; the part with d > 7 μm is the outer cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; for the TMF3 channel, the part with a diameter d < 4 μm is the inner cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 4 μm ≤ d ≤ 9 μm is the annular core with a refractive index of n5 = 1.435310; the part with d > 9 μm is the outer cladding, which uses fluorine-doped silica material with a refractive index of n2 = 1.424024; the radius of the outermost cladding is 62.5 μm.

[0006] The beneficial effects of the present invention are as follows:

[0007] 1. The multiplexer consists of a directional mode selection coupler. The large effective refractive index difference between the modes in the optical fiber ensures the low crosstalk characteristic between the modes; using a refractive index core of pure silica achieves low-loss performance;

[0008] 2. The multiplexer can simultaneously couple (de)multiplex four supermodes (LP11a / b and LP21a / b are degenerate modes, and LP01 and LP31 are non-degenerate modes), thus solving the problem of signal deterioration caused by the rotation of degenerate modes during transmission, and multiplexing / demultiplexing different modes by adjusting the lengths of different optical fibers in the multiplexer.

[0009] 3. The optical fiber used in this multiplexer combines the advantages of a pure silica core, a step refractive index distribution, and a six-core supermode core, providing important support for in-depth research in the fields of fiber optics, fiber communication, fiber wireless access, optical information processing, and new generation information technology. Brief Description of the Drawings

[0010] Figure 1 is a cross-sectional schematic diagram of a low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel according to the present invention, where the six-core supermode core at the coordinate center serves as the main transmission channel (vertical shaded part); the two-mode TMF cores located on the X-axis and Y-axis are the horizontal shaded parts.

[0011] Figure 2 is a three-dimensional perspective view of a low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel according to the present invention.

[0012] Figure 3 shows the mode field distribution diagrams of the LP01, LP11, LP21, and LP31 supermodes of a six-core supermode optical fiber at a wavelength of 1.55 μm.

[0013] Figure 4 shows the effective refractive index differences of each mode of the optical fiber in the range of 1.53 μm - 1.565 μm. The curves with circles, asterisks, solid dots, and diamonds respectively represent the effective refractive index differences n LP01 -n LP11 between the LP01 mode and the LP11 mode, the effective refractive index differences n LP11 -n LP21 between the LP11 mode and the LP21 mode, the effective refractive index differences n LP21 -n LP31 between the LP21 mode and the LP31 mode, and the effective refractive index differences n LP31 -n Cladding .

[0014] Figure 5(A) shows the variation of the coupling efficiency from the main transmission channel to the annular core with the coupling distance for the LP11a and LP11b modes at a wavelength of 1550 nm. Since LP11a / b outputs in a degenerate mode in the same annular core, when choosing the coupling distance, try to make the LP11a / LP11b modes reach the peak coupling efficiency simultaneously.

[0015] Figure 5 (B) shows the variation of the coupling efficiency from the main transmission channel to the annular core with the coupling distance for the LP21a and LP21b modes at a wavelength of 1550 nm. Since LP21ab also needs to output in a degenerate mode in another annular core, when choosing the coupling distance, also try to make the LP21a / LP21b modes reach the peak coupling efficiency simultaneously.

[0016] Figure 6 Shows the variation of the coupling efficiency of each spatial mode of the multiplexer / demultiplexer with the incident wavelength in the C band. The coupling efficiencies of the LP01, LP11, LP21, and LP31 modes are represented by circles, stars, solid dots, and diamonds respectively.

[0017] Figure 7 Shows the variation of the mode extinction ratio of the multiplexer / demultiplexer in the mode channel with the incident optical wavelength. The coupling efficiencies of the LP01, LP11, LP21, and LP31 modes are represented by circles, stars, solid dots, and diamonds respectively.

[0018] Figure 8 Shows the intrinsic loss of the six-core step-index fiber main channel of the multiplexer / demultiplexer. The intrinsic losses of the LP01, LP11, LP21, and LP31a modes are represented by curves with circles, stars, solid dots, and diamonds respectively. Detailed implementation mode

[0019] The technical solution of the present invention will be described in detail below in combination with embodiments and drawings, but the protection scope is not limited thereto.

[0020] Embodiment 1 Figure 1 Is a cross-sectional schematic diagram of a low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel of the present invention.

[0021] Figure 2It is a three-dimensional diagram of a low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel according to the present invention; the multiplexer / demultiplexer is composed of 4 different few-mode fiber core transmission channels; the six-core super-mode fiber core (SSFC) transmission channel SSFC is the main channel for mode division multiplexing / demultiplexing, supporting modes LP01, LP11a / b, LP21a / b, and LP31a, placed on the z-axis, and its axis coincides with the z-axis; the other three two-mode ring cores TMF1, TMF2, and TMF3 are respectively placed on the positive half-axis of the y-axis, the positive half-axis of the x-axis, and the negative half-axis of the x-axis, and the axes of TMF1, TMF2, and TMF3 are parallel to the axis of SSFC starting from their respective starting points; TMF1, TMF2, and TMF3 are respectively used to realize the demultiplexing output of LP11 a / b, LP21a / b, and LP31a, and the mode LP01 is demultiplexed and output from the right end of SSFC; each mode LP01, LP11 a / b, LP21 a / b, and LP31 is incident from the left end of the SSFC optical fiber and is coupled and demultiplexed along the z direction; according to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC to realize demultiplexing output; the mode LP11 is directly coupled from SSFC to TMF1 on the positive half-axis of the y-axis, where the degenerate modes LP11a / LP11b are incident from the left end of SSFC and are respectively directly coupled to the modes LP11a / LP11b in TMF1, and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF1; the mode LP21 is directly coupled from SSFC to TMF2 on the positive half-axis of the x-axis, where the degenerate modes LP21a / LP21b are incident from the left end of SSFC and are respectively directly coupled to the modes LP11a / LP11b in TMF2, and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF2; the mode LP31a is incident from the left end of SSFC and is directly coupled to TMF3 on the negative half-axis of the x-axis, and is demultiplexed and output in the mode LP11a at the right end of TMF3; the demultiplexing of four modes LP01, LP11, LP21, and LP31 from the SSFC main channel mode is realized; if each mode LP01, LP11, LP21, and LP31 is incident from the left ends of SSFC, TMF1, TMF2, and TMF3 respectively and is coupled and multiplexed along the z direction; according to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC for multiplexing output, and the modes LP11, LP21, and LP31 are directly coupled from TMF1, TMF2, and TMF3 to SSFC to realize multiplexing output, where LP11a / LP11b and LP21a / LP21b are incident from the left ends of their respective channels TMF1 and TMF2 and are multiplexed and output in a degenerate mode in SSFC; the multiplexing of four modes LP01, LP11, LP21, and LP31 in the SSFC main channel mode is realized;According to the variation relationship between the mode coupling length and the channel spacing, the channel center spacings of TMF1, TMF2, and TMF3 to the main channel SSFC are as follows; 12μm, 14.5μm, their lengths are 7.752mm, 6.6mm, 3.1mm respectively, and the length of the main channel is 7.752mm; reasonably setting the position and length of the transmission channel can adjust the performance of the mode multiplexer; the six cores of this optical fiber adopt a step refractive index distribution, higher than the surrounding refractive index, and mainly use the six-core part with a high refractive index for light transmission; the mode field characteristics in the optical fiber can be changed by changing the sizes and refractive index distributions of the core, inner cladding, and outer cladding; the six-core supermode core transmission channel consists of a core, an inner cladding, and an outer cladding. The core uses pure silica material with a refractive index of n1 = 1.444024, and the diameter of a single core is 4μm; the centers of the six cores are distributed at the six corners of a regular hexagon, and their coordinates are: (4μm, 0μm), (-4μm, 0μm), and the refractive indices of its inner and outer claddings are n2 = 1.424024. For the TMF1 channel, the part with a diameter d < 0.9μm is the inner cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 0.9μm ≤ d ≤ 6.9μm is the annular core with a refractive index of n3 = 1.450113; the part with d > 6.9μm is the outer cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; for the TMF2 channel, the part with a diameter d < 2.5μm is the inner cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 2.5μm ≤ d ≤ 7μm is the annular core with a refractive index of n4 = 1.445318; the part with d > 7μm is the outer cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; for the TMF3 channel, the part with a diameter d < 4μm is the inner cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; the part with 4μm ≤ d ≤ 9μm is the annular core with a refractive index of n5 = 1.435310; the part with d > 9μm is the outer cladding, using fluorine-doped silica material with a refractive index of n2 = 1.424024; the radius of the outermost cladding is 62.5μm. Since the SSFC needs to connect the transmission optical fiber in the transmission link, its length should be at least greater than or equal to the length of the SSFC, and the SSFC can be lengthened according to the actual situation.

[0022] Figure 3 The mode field distribution diagrams of four supermodes, LP01, LP11, LP21, and LP31, of the SSFC at a wavelength of 1.55μm are given, and the modes are obtained by studying with the beam propagation method.

[0023] Figure 4The effective refractive index differences of each mode of the six-core supermode fiber in the range of 1.53 μm - 1.565 μm are given. The variations of the effective refractive index differences of the four modes LP01, LP11, LP21, and LP31a in the six-core supermode fiber are represented by circles, asterisks, solid dots, and diamond curves, respectively. From Figure 4 it can be seen that in the wavelength range of 1530 - 1565 nm, the refractive index difference n LP01 -n LP11 between mode LP01 and mode LP11 is greater than 2.00×10 -3 , and the refractive index difference gradually increases with the increase of wavelength. The refractive index difference n LP11 -n LP21 between LP11 mode and LP21 mode is greater than 4.25×10 -3 , and the refractive index difference gradually increases with the increase of wavelength. The refractive index difference n LP21 -n LP31a between mode LP21 and LP31a is greater than 3.89×10 -3 , and the refractive index difference also gradually increases with the increase of wavelength. The refractive index difference n Cladding between mode LP31a and the cladding n LP31a -n Cladding is greater than 2.00×10 -3 , and the refractive index difference gradually decreases with the increase of wavelength. The refractive index differences n LP01 -n LP11 at 1545 nm and 1550 nm are 2.02×10 -3 and 2.04×10 -3 , respectively. The wavelengths of n LP11 -n LP21 at 1545 nm and 1550 nm are 4.33×10 -3 and 4.35×10 -3 , respectively. The wavelengths of n LP21 -n LP31a at 1545 nm and 1550 nm are 3.95×10 -3 and 3.98×10 -3 , respectively. The wavelengths of n LP31a -n Cladding at 1545 nm and 1550 nm are 2.33×10 -3 and 2.24×10 -3 .

[0024] Figure 5(A) shows the variation of the coupling efficiency from the main transmission channel to the ring core with the coupling distance for the LP11a and LP11b modes at a wavelength of 1550 nm. Since LP11a / b outputs in a degenerate mode in the same ring core, when choosing the coupling length, try to make the LP11a / LP11b modes reach the peak coupling efficiency simultaneously. Following this principle, the coupling channel length of LP11a / b is 7.752 mm.

[0025] Figure 5 (B) shows the variation of the coupling efficiency from the main transmission channel to the ring core with the coupling distance for the LP21a and LP21b modes at a wavelength of 1550 nm. Since LP21ab also needs to output in a degenerate mode in another ring core, when choosing the coupling length, also try to make the LP21a / LP21b modes reach the peak coupling efficiency simultaneously. Following this principle, the coupling channel length of LP11a / b is 6.6 mm.

[0026] Figure 6 Shows the variation of the coupling efficiency of each spatial mode of the multiplexer / demultiplexer with the incident wavelength in the C band. The coupling efficiencies of the LP01, LP11, LP21, and LP31a modes are represented by curves with circles, asterisks, solid dots, and diamonds respectively. The coupling efficiency of the LP01 mode is approximately -0.16 dB, and the coupling efficiency remains basically unchanged as the wavelength increases. At 1550 nm, the coupling efficiency of the LP11 mode reaches -0.18 dB, the coupling efficiency of the LP21 mode can reach -0.15 dB, and the coupling efficiency of the LP31a mode can reach -0.35 dB. On both sides of 1550 nm in the C band, the coupling efficiency shows a downward trend. The reason is that as the wavelength changes, the coupling period of each mode will change. Since the coupling length of the designed (de)multiplexer is fixed, it is impossible to reach the maximum coupling efficiency for each wavelength simultaneously and it changes with the wavelength. Throughout the C band, the coupling efficiency of LP11 is higher than -3.55 dB, the coupling efficiency of LP21 is higher than -0.58 dB, the coupling efficiency of LP31a is higher than -5.25 dB, and the coupling efficiency of the LP01 mode is the best.

[0027] Figure 7The variation of the mode extinction ratio of the multiplexer / demultiplexer in the mode channel with the incident optical wavelength is given. The extinction ratios of the LP01, LP11, LP21, and LP31a modes are represented by curves with circles, asterisks, solid dots, and diamonds, respectively. The extinction ratio of the LP01 mode is above 22.45 dB; throughout the C-band, the extinction ratio of LP11 can be maintained above 27.36 dB, with a maximum value of 30.48 dB at 1550 nm; the extinction ratio of the LP21 mode is above 27.43 dB; the extinction ratio of the LP31a mode can be maintained above 21.06 dB throughout the C-band, with a maximum value of 26.43 dB at 1555 nm.

[0028] Figure 8 The intrinsic losses of the six-core step-index fiber main channel of the multiplexer / demultiplexer are given. The intrinsic losses of the LP01, LP11, LP21, and LP31a modes are represented by curves with circles, asterisks, solid dots, and diamonds, respectively. The intrinsic losses of the four modes show a trend of first decreasing and then increasing in the C-band. Generally speaking, the intrinsic losses of the four modes from large to small are LP31, LP21, LP01, and LP11. The intrinsic loss of the LP01 mode can be maintained above 0.1585 dB / km, with a maximum value of 0.1612 dB / km at 1565 nm; the intrinsic loss of the LP11 mode can be maintained above 0.1572 dB / km, with a maximum value of 0.1600 dB / km at 1565 nm; the intrinsic loss of the LP21 mode can be maintained above 0.1589 dB / km, with a maximum value of 0.1619 dB / km at 1565 nm; the intrinsic loss of the LP31 mode can be maintained above 0.1607 dB / km, with a maximum value of 0.1640 dB / km at 1565 nm.

[0029] In summary, the proposed multiplexer / demultiplexer can simultaneously multiplex / demultiplex the LP01, LP11, LP21, and LP31 supermodes, thus solving the problem of signal deterioration caused by the rotation of degenerate modes during transmission.

Claims

1. A low-loss, low-crosstalk few-mode multiplexer / demultiplexer based on a six-core transmission channel, characterized in that: The multiplexer / demultiplexer consists of 4 different few-mode fiber core transmission channels; the six-core super-mode fiber core transmission channel SSFC is the main channel for mode division multiplexing / demultiplexing, supporting modes LP01, LP11a / b, LP21a / b, and LP31a, placed on the z-axis with its axis coinciding with the z-axis; the other three two-mode ring cores TMF1, TMF2, and TMF3 are placed on the positive half-axis of the y-axis, the positive half-axis of the x-axis, and the negative half-axis of the x-axis respectively. The axes of TMF1, TMF2, and TMF3 are parallel to the axis of SSFC starting from their respective starting points. TMF1, TMF2, and TMF3 are respectively used to realize the demultiplexing output of LP11 a / b, LP21 a / b, and LP31a, and the mode LP01 is demultiplexed and output from the right end of SSFC; each mode LP01, LP11 a / b, LP21 a / b, and LP31a is incident from the left end of the SSFC optical fiber and is transmitted, coupled, and demultiplexed along the z direction; according to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC to realize the demultiplexing output; the mode LP11 is directly coupled from SSFC to TMF1 on the positive half-axis of the y-axis, where the degenerate modes LP11a / LP11b are incident from the left end of SSFC and are respectively directly coupled to the modes LP11a / LP11b in TMF1 and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF1; the mode LP21 is directly coupled from SSFC to TMF2 on the positive half-axis of the x-axis, where the degenerate modes LP21a / LP21b are incident from the left end of SSFC and are respectively directly coupled to the modes LP11a / LP11b in TMF2 and are degenerate output in the mode LP11a / LP11b at the right end of the coupling channel TMF2; the mode LP31a is incident from the left end of SSFC and is directly coupled to TMF3 on the negative half-axis of the x-axis and is demultiplexed and output in the mode LP11a at the right end of TMF3; the demultiplexing of the four modes LP01, LP11, LP21, and LP31 from the main channel mode of SSFC is realized. If each mode LP01, LP11, LP21, and LP31 is incident from the left ends of SSFC, TMF1, TMF2, and TMF3 respectively and is transmitted, coupled, and multiplexed along the z direction; according to the coupled mode theory, the mode LP01 is transmitted from the left end to the right end along the main channel SSFC for multiplexing output, and the modes LP11, LP21, and LP31 are directly coupled from TMF1, TMF2, and TMF3 to SSFC to realize multiplexing output, where LP11a / LP11b and LP21a / LP21b are incident from the left ends of their respective channels TMF1 and TMF2 and are multiplexed and output in a degenerate mode in SSFC; the multiplexing of the four modes LP01, LP11, LP21, and LP31 in the main channel mode of SSFC is realized. According to the variation relationship between the mode coupling length and the channel spacing, the channel center spacings of TMF1, TMF2, and TMF3 to the main channel SSFC are successively 12 μm, 14.5 μm, their lengths are 7.752 mm, 6.6 mm, 3.1 mm respectively, and the length of the main channel is 7.752 mm; The six-core supermode fiber core transmission channel consists of a fiber core, an inner cladding, and an outer cladding. The fiber core is made of pure silica material with a refractive index of n1 = 1.444024, and the diameter of a single fiber core is 4 μm. The centers of the six fiber cores are distributed at the six corners of a regular hexagon, and their coordinates are: (4 μm, 0 μm), (-4 μm, 0 μm), and the refractive indices of its inner and outer claddings are n2 = 1.424024. The part of the TMF1 channel with a diameter d < 0.9 μm is the inner cladding, which is made of fluorine-doped silica material with a refractive index of n2 = 1.424024; The part where 0.9μm ≤ d ≤ 6.9μm is the annular core with a refractive index of n3 = 1.450113; the part where d > 6.9μm is the outer cladding made of fluorine-doped silica with a refractive index of n2 = 1.424024; the part where the channel diameter d of TMF2 < 2.5μm is the inner cladding made of fluorine-doped silica with a refractive index of n2 = 1.424024; the part where 2.5μm ≤ d ≤ 7μm is the annular core with a refractive index of n4 = 1.445318; the part where d > 7μm is the outer cladding made of fluorine-doped silica with a refractive index of n2 = 1.424024; the part where the channel diameter d of TMF3 < 4μm is the inner cladding made of fluorine-doped silica with a refractive index of n2 = 1.424024; the part where 4μm ≤ d ≤ 9μm is the annular core with a refractive index of n5 = 1.435310; the part where d > 9μm is the outer cladding made of fluorine-doped silica with a refractive index of n2 = 1.424024; the radius of the outermost cladding is 62.5μm.

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