Few-mode multiplexer / demultiplexer based on step transmission main channel

Abstract

A few-mode multiplexer / demultiplexer based on a step type transmission main channel is characterized in that a transmission channel of the mode division multiplexer / demultiplexer is composed of 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 the 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 10.766 mm, and the lengths of the SMF1, the SMF2 and the SMF3 are respectively 10.766 mm, 8.3 mm and 8.872 mm; the mode field characteristics in the optical fiber can be changed by reasonably setting parameters such as the position, the size and the refractive index distribution of the fiber core; the mode degeneracy of LP11a / LP11b is broken, the multiplexing and demultiplexing of four super-modes with high practicability and 10mm magnitude length are realized, and the method can be used for MIMO-FREE operation.

Description

Technical Field

[0001] This patent relates to a few-mode multiplexer / demultiplexer based on a step-type transmission main channel, which can be applied to the fields of fiber optics, fiber optic communications, fiber optic wireless access, optical information processing and new generation information technology. Background Art

[0002] In recent years, the traffic volume of various communication services has grown exponentially, and single-mode fiber communications have faced unprecedented challenges. The optical fiber communication industry has achieved a breakthrough 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); the mode coupling between mode division multiplexing and few-mode fiber (Few-Mode Fiber, FMF) in space division multiplexing and its related devices and application research 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 lowloss 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 system[J]. Journal of Liaocheng University (Natural Science Edition), 2020, 33(2): 50-67; Wang Xiao, Zheng Hongjun* (corresponding author), Li Xin, Liu Yang, Yu Ruyuan, Bai Chenglin, Hu Weisheng, New progress in the study of few-mode fibers in mode division multiplexing system, Journal of Liaocheng University (Natural Science Edition), 2019.4, 32(2): 69-79]; pure silica core can effectively reduce optical fiber attenuation and fusion loss, and is currently mostly used in single-mode optical fiber [T.Hasegawa et al.2016.Advances in ultra-low loss silica fibers[J].Frontiersin 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,YYamamoto,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]; for optical components such as few-mode optical fiber transmission and mode division multiplexer / demultiplexer, a mode division multiplexing transmission method for suppressing mode crosstalk is proposed. [ARMay 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-coupled4-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; L Ma, S. Jiang, J. Du, C. Yang, W. Tong, and Z. He, "Ring-assisted 7-LP-mode Fiber with Ultra-low Inter-mode Crosstalk," in Asia Communications and Photonics Conference 2016, OSA Technical Digest (online) (Optica Publishing Group, 2016), paper AS4A.5] Weakly coupled step-index few-mode fiber (SI-FMF) has great potential in MDM applications; a new ring-assisted 7-LP mode fiber is proposed, which has ultra-low inter-mode crosstalk by increasing the minimum value of Δneff by 70% compared with the traditional SI-FMF design without significantly reducing other characteristics. [Ping L, Wei C, Junhao Z.Research on the Low-loss Low-crosstalk Bending. -insensitive Seven-core Fiber[J].Journal of Physics: Conference Series, 2023, 2617(1)] Low refractive index grooves are designed in multi-core optical fibers to reduce crosstalk and improve bending resistance. The produced multi-core optical fibers meet the requirements of long-distance and high-capacity transmission. [Sicheng J, Xiao S. Design and characteristics study of bend-resistant and low-crosstalk few-mode multi-core fiber[J].Optik, 2024, 297] Using a heterogeneous core solution, the cores are effectively isolated by grooves and air holes, significantly reducing crosstalk between cores. The cores are arranged in a square array to facilitate easier fusion splicing. The proposed optical fiber has good performance, meets the requirements of high-density and high-capacity transmission systems, and has great application potential. A key issue in mode division multiplexing systems is how to deal with degenerate non-circularly symmetric polarization modes. Asymmetric linear polarization modes contain dual spatial degenerate modes with the same propagation constant. However, due to the imperfections in fiber manufacturing and external disturbances, their spatial directions may randomly rotate along the propagation direction during transmission, resulting in a rapid deterioration of the bit error rate. One solution is to treat the two degenerate modes as overall linear polarization modes for single-channel transmission, which is compatible with traditional IM / DD systems; the second solution is to use structures such as elliptical cores to break the mode degeneracy. In recent years, supermode fibers have received widespread attention due to their many advantages. Supermode fibers utilize the coupling between multi-core fibers. The core-to-core distance of supermode fibers is much shorter than that of traditional multi-core fibers, which enables strong coupling between the modes of the fibers, thereby achieving supermode operation.The supermode fiber core has a large effective refractive index difference between modes, which can reduce the coupling between modes to reduce the crosstalk between modes. In addition, the characteristics of the supermode fiber core are related to the characteristics of each fiber core, so it has more design freedom [Xia Cen, Bai Neng, Ozdur Ibrahim, et al. Supermodes for optical transmission [J], Optics Express 2011, 19 (17): 16653-16664]; [Han X, Haisu L, Guobin R, et al. Polarization-Maintaining Supermode Fiber Supporting 20 Modes [J]. IEEE Photonics Technology Letters, 2017, 29(16)] proposed a polarization-maintaining supermode fiber consisting of the same core and quasi-elliptical arrangement; the high refractive index contrast between the core and the cladding prevents the high-order mode from being cut off and provides a compact structure; at the same time, the quasi-elliptical arrangement of the core introduces birefringence, thereby breaking the mode degeneracy. The proposed supermode fiber may be a promising candidate for mode division multiplexing to enhance optical transmission capacity. In summary, the advantages of pure silica core, elliptical core and supermode core are integrated to break the supermode mode degeneracy; the refractive index core of pure silica is used to reduce the loss; a large effective refractive index difference transmission channel is used and the position and length of the transmission channel are reasonably set to achieve low crosstalk and high extinction ratio performance, which can better separate spatial modes and reduce crosstalk between modes. It can be used for MIMO-FREE operation and is expected to solve the current research challenges of few-mode fiber multiplexers / demultiplexers. It has important academic value and application value, and the research is of great significance and has broad application prospects. Utility Model Content

[0003] With the support of the National Natural Science Foundation of China (No. 61671227 and 61431009), the Natural Science Foundation of Shandong Province (ZR2011FM015), and the special funds for the "Taishan Scholars" construction project, this patent proposes a few-mode multiplexer / demultiplexer based on a step-type transmission main channel; it breaks the mode degeneracy of LP11a / LP11b and realizes the multiplexing and demultiplexing of four modes: LP01, LP11a, LP11b and LP21b; the multiplexer combines the advantages of pure silica fiber core, elliptical core, supermode fiber core and step refractive index distribution fiber core, 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.

[0004] The technical solution adopted by this patent application to solve its technical problem is:

[0005] This patent proposes a few-mode multiplexer / demultiplexer based on a step-type transmission main channel, characterized in that: the mode division multiplexer / demultiplexer consists of a transmission channel consisting of a Four-Elliptical Core Supermode Fiber Core (FEC-SFC) and three single-mode fiber cores, wherein the elliptical four-core supermode fiber core at the center of the coordinate is used 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 for mode division multiplexing / demultiplexing, which is placed on the z axis, and its axis coincides 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. Other optical fields are incident from the left ends of the SMFs corresponding to LP11a, LP11b and LP21b respectively, and are coupled to the FEC-SFC in the main channel respectively. Modes LP11a, LP11b and LP21b are output in LP11a, LP11b and LP21b modes at the right end of the main channel FEC-SFC; mode multiplexing of the four modes LP01, LP11a, LP11b and LP21b in the FEC-SFC main channel is realized; if modes LP01, LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC fiber core, coupled demultiplexing is transmitted along the z direction; according to the coupled mode theory, mode LP01 is transmitted along the main channel FE C-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 the modes LP11a, LP11b and LP21b, realizing the 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 10.766 mm, the ellipticity of the elliptical core is 1.5, the length of the major semi-axis is 2.4 μm, and the length of the minor semi-axis is 1.6 μm; the channel center spacings of SMF1, SMF2, and SMF3 to the main channel FEC-SFC are and The lengths are 10.766 mm, 8.3 mm and 8.872 mm respectively, and the radii of SMF1, SMF2 and SMF3 are all 2 μm; the four cores of the optical fiber adopt a step refractive index distribution, which is higher than the refractive index of the surrounding cladding, and mainly use the high refractive index four-core parts and the single-mode core for light transmission; the refractive indices of the single-mode cores SMF1, SMF2 and SMF3 are 1.4446, 1.4421 and 1.4368 respectively; the four elliptical cores of the main transmission channel are made of pure silica material with a refractive index of n1=1.4440; the cladding part is made of fluorine-doped silica material with a refractive index of n2=1.4240; since FEC-SFC is to be connected to the transmission optical fiber in the transmission link, the length must be at least greater than or equal to the length of FEC-SFC, and FEC-SFC can be lengthened according to actual conditions; the mode field characteristics in the optical fiber can be changed by reasonably setting parameters such as the core and cladding position, size and refractive index distribution.

[0006] The beneficial effects of this patent application are as follows:

[0007] 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 step-index core of pure silica is used to achieve low loss performance.

[0008] 2. The optical fiber used in the multiplexer / demultiplexer combines the advantages of pure silica core, step refractive index distribution, elliptical core and supermode core. It adopts elliptical four-core supermode core, breaks the mode degeneracy of LP11a and LP11b, realizes the mode preservation function, and realizes the practical multiplexing and demultiplexing of four supermodes LP01, LP11a, LP11b and LP21b on a length of 10mm. It provides 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a cross-sectional schematic diagram of a few-mode multiplexer / demultiplexer based on a step-type transmission main channel of this patent, in which the four-core supermode fiber core at the coordinate center serves as the main transmission channel (ellipse, dark red part); the single-mode SMF fiber core (circle, gray part) is located on the X-coordinate axis and the Y-coordinate axis.

[0010] Figure 2 This is a three-dimensional stereogram of a few-mode multiplexer / demultiplexer based on a step-type transmission main channel of this patent.

[0011] Figure 3 The mode field distribution diagram of the supermode at a wavelength of 1.55μm is given. Figure 3 a. Figure 3 b. Figure 3 c. Figure 3 d Mode field distribution diagram corresponding to the LP01, LP11a, LP11b and LP21b supermodes of the four-core supermode fiber core.

[0012] Figure 4 The effective refractive index difference of each mode of the optical fiber at 1.53μm-1.565μm is given. The curves with circles, asterisks, squares and triangles represent the effective refractive index difference n between the LP01 mode and the LP11a mode. 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 .

[0013] Figure 5 The variation of the coupling efficiency of each spatial mode of the multiplexer / demultiplexer with the incident wavelength in the C band is given. The coupling efficiency of the LP01, LP11a, LP11b and LP21b modes are represented by triangles, asterisks, squares and circles, respectively.

[0014] Figure 6 The variation of the mode extinction ratio of the multiplexer / demultiplexer in the mode channel with the wavelength of the incident light is given. The extinction ratios of the LP01, LP11a, LP11b and LP21b modes are represented by triangles, asterisks, squares and circles respectively.

[0015] Figure 7 The intrinsic loss of the main channel FEC-SFC of the multiplexer / demultiplexer is given, and the intrinsic losses of the LP01, LP11a, LP11b and LP21b modes are represented by curves with circles, triangles, squares and stars, respectively. DETAILED DESCRIPTION

[0016] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings, but the protection scope is not limited thereto.

[0017] Example 1

[0018] Figure 1 This is a cross-sectional schematic diagram of a few-mode multiplexer / demultiplexer based on a step-type transmission main channel of this patent. Figure 2This is a three-dimensional diagram of a few-mode multiplexer / demultiplexer based on a step-type transmission main channel of the present patent; it is characterized in that: the mode division multiplexer / demultiplexer is composed of a transmission channel of a Four-Elliptical Core Supermode Fiber Core (FEC-SFC) and three single-mode fiber cores, wherein the elliptical four-core supermode fiber core at the center of the coordinate 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, and its axis coincides 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. Other optical fields are incident from the left ends of the SMFs corresponding to LP11a, LP11b and LP21b respectively, and are coupled to the FEC-SFC in the main channel respectively. Modes LP11a, LP11b and LP21b are output in LP11a, LP11b and LP21b modes at the right end of the main channel FEC-SFC; mode multiplexing of the four modes LP01, LP11a, LP11b and LP21b in the FEC-SFC main channel is realized; if modes LP01, LP11a, LP11b and LP21b are incident from the left end of the main channel FEC-SFC fiber core, coupled demultiplexing is transmitted along the z direction; according to the coupled mode theory, mode LP01 is transmitted along the main channel FE C-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 the modes LP11a, LP11b and LP21b, realizing the 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 10.766 mm, the ellipticity of the elliptical core is 1.5, the length of the major semi-axis is 2.4 μm, and the length of the minor semi-axis is 1.6 μm; the channel center spacings of SMF1, SMF2, and SMF3 to the main channel FEC-SFC are and The lengths are 10.766 mm, 8.3 mm and 8.872 mm respectively, and the radii of SMF1, SMF2 and SMF3 are all 2 μm; the four cores of the optical fiber adopt a step refractive index distribution, which is higher than the refractive index of the surrounding cladding, and mainly use the high refractive index four-core parts and the single-mode core for light transmission; the refractive indices of the single-mode cores SMF1, SMF2 and SMF3 are 1.4446, 1.4421 and 1.4368 respectively; the four elliptical cores of the main transmission channel are made of pure silica material with a refractive index of n1=1.4440; the cladding part is made of fluorine-doped silica material with a refractive index of n2=1.4240; since FEC-SFC is to be connected to the transmission optical fiber in the transmission link, the length must be at least greater than or equal to the length of FEC-SFC, and FEC-SFC can be lengthened according to actual conditions; the mode field characteristics in the optical fiber can be changed by reasonably setting parameters such as the core and cladding position, size and refractive index distribution.

[0019] Figure 3 The mode field distribution diagram of the supermode at a wavelength of 1.55μm is given. Figure 3 a. Figure 3 b. Figure 3 c. Figure 3 d Mode field distribution diagrams corresponding to the four supermodes LP01, LP11a, LP11b and LP21b, respectively. The modes are obtained by beam propagation method.

[0020] Figure 4 The effective refractive index difference of each mode in the four-core supermode fiber in the range of 1.53μm-1.565μm is given. The circle, asterisk, square and triangle represent the effective refractive index difference between LP01 mode and 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 4 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 4.11×10 -3 , and the refractive index difference gradually increases with the increase of wavelength. The refractive index difference between LP11a mode and LP11b mode is n LP11a -n LP11b Greater than 1.76×10 -3, and the refractive index difference gradually increases with the increase of wavelength. The refractive index difference n between modes LP11b and LP21b LP11b -n LP21b Greater than 3.17×10 -3 , and the refractive index difference also increases with the increase of wavelength. Cladding The refractive index difference n LP21b -n Cladding Greater than 3.07×10 -3 , the refractive index difference decreases gradually with the increase of wavelength. The refractive index difference n at 1545nm and 1550nm LP01 -n LP11a They are 4.1845×10 -3 and 4.208×10 -3 , n LP11a -n LP11b The refractive index difference at 1545nm and 1550nm is 1.794×10 -3 and 1.804×10 -3 , n LP11b -n LP21b The refractive index difference at 1545nm and 1550nm is 3.216×10 -3 and 3.2275×10 -3 , n LP21b -n Cladding The refractive index difference at 1545nm and 1550nm is 3.346×10 -3 and 3.2775×10 -3 .

[0021] Figure 5The variation of the coupling efficiency of each spatial mode of the multiplexer / demultiplexer with the incident wavelength in the C band is given. The coupling efficiency 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 about 0dB, and the coupling efficiency remains basically unchanged with the increase of wavelength. At 1550nm, the coupling efficiency of the LP11a mode reaches -0.0031dB, the coupling efficiency of the LP11b mode can reach -0.0142dB, and at 1555nm, the coupling efficiency of the LP21b mode can reach -0.0940dB. On the C-band on both sides of 1550nm, the coupling efficiency of the LP11a mode and the LP11b mode shows a downward trend, while the coupling efficiency of the LP21b mode shows a downward trend on the C-band on both sides of 1555nm. 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 the wavelength. On the entire C-band, the coupling efficiency of LP11a is higher than -0.4238dB, the coupling efficiency of LP11b is higher than -0.6508dB, the coupling efficiency of LP21b is higher than -1.4577dB, and the coupling efficiency of LP01 mode is the best.

[0022] Figure 6 The variation of the mode extinction ratio of the multiplexer / demultiplexer in the mode channel with the wavelength of the incident light is given. The extinction ratios of the LP01, LP11a, LP11b and LP21b modes are represented by curves with triangles, asterisks, squares and circles, respectively. The extinction ratio of mode LP01 is above 40.3504dB; the extinction ratio of LP11a can be maintained above 27.5296dB in the entire C-band, and the maximum value of 34.5467dB is achieved at 1555nm; the extinction ratio of the LP11b mode can be maintained above 24.6413dB in the entire C-band, and the maximum value of 27.5619dB is achieved at 1555nm. The extinction ratio of the LP21b mode can be maintained above 23.2593dB in the entire C-band, and the maximum value of 25.877dB is achieved at 1560nm.

[0023] Figure 7The intrinsic loss of the main channel FEC-SFC of the multiplexer / demultiplexer is given, and the intrinsic losses of the LP01, LP11a, LP11b and LP21b modes are represented by curves with circles, triangles, squares and stars, respectively. The intrinsic losses of the four modes in the C-band show a trend of first decreasing and then increasing. Overall, the intrinsic losses of the four modes are LP21b, LP11b, LP11a and LP01 from large to small. The intrinsic loss of mode LP01 can be maintained above 0.1577dB / km, and the maximum value is 0.1604dB / km at 1565nm; the intrinsic loss of mode LP11a can be maintained above 0.1596dB / km, and the maximum value is 0.1623dB / km at 1565nm; the intrinsic loss of mode LP11b can be maintained above 0.1609dB / km, and the maximum value is 0.1638dB / km at 1565nm; the intrinsic loss of mode LP21b can be maintained above 0.1656dB / km, and the maximum value is 0.1686dB / km at 1565nm.

[0024] In summary, the proposed multiplexer / demultiplexer breaks the degeneracy of modes LP11a and LP11b, realizes the mode-preserving function of four supermodes, and realizes the practical multiplexing and demultiplexing of four supermodes LP01, LP11a, LP11b and LP21b on a length of 10 mm.

Claims

1. A few-mode multiplexer / demultiplexer based on a step-type transmission main channel, characterized in that: 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, wherein the elliptical four-core supermode fiber core at the center of the coordinate is used 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 for mode division multiplexing / demultiplexing, which is placed on the z-axis, and its axis coincides with the z-axis, supporting supermode modes LP01, LP11a, LP 11b 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, and 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; other optical fields are incident from the left end of the SMF corresponding to LP11a, LP11b and LP21b respectively, and are coupled to the main channel F The modes LP11a, LP11b and LP21b in EC-SFC are output in the right end of the main channel FEC-SFC in the LP11a, LP11b and LP21b modes; 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, the transmission coupling demultiplexing is carried out along the z direction; according to the coupled mode theory, the mode LP01 is transmitted along the main channel FEC-SFC. The channel FEC-SFC is transmitted from the left end to the right end to realize 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 to the right end output of the SMF corresponding to the modes LP11a, LP11b and LP21b, realizing the 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 10.766 mm, the ellipticity of the elliptical core is 1.5, the length of the major semi-axis is 2.4 μm, and the length of the minor semi-axis is 1.6 μm; the channel center spacings of SMF1, SMF2, and SMF3 to the main channel FEC-SFC are and Their lengths are 10.766mm, 8.3mm and 8.872mm respectively, and the radii of SMF1, SMF2 and SMF3 are all 2μm; the four fiber cores of the main transmission channel adopt a step refractive index distribution, which is higher than the refractive index of the surrounding cladding, and mainly use the high refractive index four-core parts and single-mode fiber cores for light transmission; the refractive indices of the single-mode fiber cores SMF1, SMF2 and SMF3 are 1.4446, 1.4421 and 1.4368 respectively; the four elliptical cores of the main transmission channel are made of pure silica material with a refractive index of n1=1.4440; the cladding part is made of fluorine-doped silica material with a refractive index of n2=1.4240.

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