Non-zero dispersion displacement ring core optical fiber based on groove assistance

Abstract

The invention discloses a non-zero dispersion displacement ring core optical fiber based on groove assistance, and the non-zero dispersion displacement ring core optical fiber is a single-ring-core gradient refractive index ring core optical fiber, and comprises an annular fiber core, an annular groove, and an optical fiber cladding. The optical fiber cladding comprises a central cylinder, an annular fiber core, an annular groove interlayer and an outer optical fiber cladding, and all the layers have the same refractive index; the annular fiber core with the high refractive index is sleeved on the central cylinder, the annular fiber core and the annular groove interlayer are sleeved on the annular fiber core, the annular groove with the low refractive index is sleeved on the annular fiber core and the annular groove interlayer, and the outer fiber cladding is sleeved on the annular groove. The invention aims to provide a low-crosstalk optical fiber structure so as to meet the requirement of low-dispersion transmission in a plurality of orbital angular momentum modes.

Description

Technical Field

[0001] The invention relates to the technical field of non-zero dispersion shifted ring core optical fiber, and in particular to a groove-assisted non-zero dispersion shifted ring core optical fiber. Background Art

[0002] In recent years, fiber dispersion and nonlinear effects have restricted the development of optical communications. The industry believes that single-mode fiber communication systems are close to the Shannon limit of 100Tb / s. The capacity crisis is still a huge challenge facing optical communications, and the research results of the spatial dimension provide new solutions for optical communications. The fiber communication system based on space division multiplexing has broken through the capacity limit of single-mode fiber, and its potential communication capacity is extremely large, which is expected to meet the current rapidly expanding traffic demand and realize the sustainable expansion of the fiber communication system.

[0003] For optical fiber communication systems, dispersion is one of the important factors that limit long-distance transmission. Higher dispersion will lead to a significant increase in the communication bit error rate. When the dispersion is small enough to meet the phase matching conditions, it will produce a four-wave mixing effect, aggravating the signal crosstalk and distortion of the system. At present, it is difficult to achieve low-cost, low-complexity, high-speed optical fiber communication systems.

[0004] Currently, there is little research on non-zero dispersion-shifted optical fibers with low dispersion, low nonlinearity, and low crosstalk. Summary of the invention

[0005] In order to overcome the defects of the above prior art, the present invention provides a groove-assisted non-zero dispersion-shifted ring-core fiber, which aims to provide a low-crosstalk fiber structure to meet the needs of low-dispersion transmission of multiple orbital angular momentum modes. The structure can well match the ring intensity profile of the OAM mode and has the characteristics of low dispersion and large effective mode field area.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A groove-assisted non-zero dispersion-shifted ring-core optical fiber is a graded-refractive-index single-ring-core optical fiber, comprising an annular core, an annular groove and an optical fiber cladding; the refractive index of the annular core is greater than the refractive index of the optical fiber cladding, and the refractive index of the optical fiber cladding is greater than the refractive index of the annular groove;

[0008] The optical fiber cladding includes a central cylinder, an interlayer and an outer optical fiber cladding, each layer having the same refractive index;

[0009] The annular fiber core is sleeved on the central cylinder, the interlayer is sleeved on the annular fiber core, the annular groove is sleeved on the interlayer, and the outer optical fiber cladding is sleeved on the annular groove.

[0010] The annular core and the annular groove use the same refractive index distribution coefficient α, and the refractive index distribution expression is determined by the following formula. When α=2, it is a parabolic refractive index, and when α=∞, it is a step refractive index;

[0011] The refractive index of the annular groove area is lower than that of the optical fiber cladding, which has the functions of limiting the range of the light field and suppressing crosstalk.

[0012] The width of the ring core d = r2-r1, where r1 is the inner diameter of the ring core area, and r2 is the outer diameter of the ring core area. The width of the ring core is the main reason affecting the radial order. A larger ring core width will increase the probability of the occurrence of radial high-order modes. The empirical formula for radial single mode is used to estimate the width range of the ring core:

[0013]

[0014] Wherein, n1 is the refractive index at the position of the maximum concentration of germanium doping in the annular core region, n2 is the refractive index at the position of the maximum concentration of fluorine doping in the annular core region, and λ is the communication wavelength.

[0015] The materials of the optical fiber cladding, the annular core, and the annular groove are a combination of silica, germanium-doped silica, and fluorine-doped silica materials under the condition of satisfying the above-mentioned refractive index distribution;

[0016] The center cylinder, interlayer and outer optical fiber cladding are made of silica, the annular core material is germanium-doped silica with a gradient doping concentration of germanium dioxide, and the annular groove material is fluorine-doped silica with a gradient doping concentration of calcium fluoride; the gradient rule is determined by the refractive index distribution expression.

[0017] The maximum value of the germanium doping concentration of the annular fiber core is in the range of 8-16 mol%, corresponding to the refractive index n1 range of 1.456-1.468;

[0018] The fluorine doping concentration of the annular groove ranges from 0 to 100 mol%. Since the doping concentration has little effect on dispersion, the fluorine doping concentration of the graded-index single-ring core optical fiber is uniformly set to mf_CaF2=80 mol%, corresponding to a refractive index n2=1.4303; the refractive index distribution expressions of the graded-index single-ring core optical fiber in the annular core and the annular groove region are respectively:

[0019]

[0020] Wherein, r is the distance from the center of the gradient refractive index single ring core optical fiber, r1 and r2 are the inner diameter and outer diameter of the ring core respectively, r3 and r4 are the inner diameter and outer diameter of the ring groove respectively, α is the refractive index distribution parameter, n1 is the refractive index at the position of the maximum concentration of germanium doping in the ring core region, n2 is the refractive index at the position of the maximum concentration of fluorine doping in the ring core region, and n0 is the refractive index of pure silica in the optical fiber cladding region. The multi-ring core gradient refractive index optical fiber includes seven gradient refractive index single ring core optical fibers, and its structure is centrally symmetrically distributed, with six outer ring cores surrounding a central ring core, and the core spacings of two adjacent ring cores are the same.

[0021] The core spacing ranges from 40 to 48 μm.

[0022] Compared with step-index optical fiber, gradient-index optical fiber has lower contrast and lower mode restriction. The present invention improves the optical field by adding a fluorine-doped groove structure on the outside of the annular core, thereby effectively restricting the light field in the core and binding the mode field, thereby reducing restriction loss and improving transmission efficiency.

[0023] Under certain structural parameters, the optical fiber of the present invention can provide lower dispersion while maintaining a larger effective mode field area and a smaller nonlinear coefficient to reduce nonlinear effects.

[0024] In order to further increase the number of OAM modes supported by optical fiber and improve the information transmission density, the present invention further designs a graded-index multi-ring core optical fiber based on the graded-index single-ring core optical fiber by using the same fluorine-doped groove structure as above. Its structure is centrally symmetrically distributed, with six outer cores surrounding a central core, and the core spacings of two adjacent cores are the same. This structure can improve the utilization rate of the optical fiber and maximize the number of transmitted modes.

[0025] A method for adjusting a non-zero dispersion shifted ring core optical fiber based on a groove auxiliary comprises the following steps:

[0026] Determine the refractive index distribution coefficient, the refractive index distribution coefficient ranges from 2 to 10, and the refractive index distribution coefficient that meets the low dispersion requirement is selected according to the optical fiber geometry;

[0027] Determine the radius of the central cylinder of the optical fiber cladding, which is in the range of 45-53μm. The high-order OAM mode is greatly affected by the radius of the central cylinder. The radius of the central cylinder can be appropriately increased while meeting the ITU-T G.655.C standard.

[0028] Determine the width of the annular core, which ranges from 1.2 to 2.5 μm. Select the width of the annular core region that meets the low dispersion requirements based on the radius of the central cylinder, the single-ring core and the multi-ring core structure. When radial high-order modes appear, the width of the annular core region should be reduced to ensure that it is less than d determined by the radial single-mode empirical formula;

[0029] Determine the maximum concentration of germanium doping in the annular core region, the maximum concentration of germanium doping in the annular core region is in the range of 8-16 mol%, corresponding to the refractive index n1 range of 1.456-1.468;

[0030] High germanium doping concentration can support more OAM mode transmission. When the low-order mode dispersion is less than 1ps / nm / km, the germanium doping concentration should be reduced to ensure that it meets the ITU-T G.655.C standard.

[0031] The refractive index distribution coefficient of a single core in the seven-core optical fiber, the radius of the central cylinder, the width of the annular core region, and the maximum germanium doping concentration in the annular core region are determined, and the dispersion variation law of the graded refractive index single ring core optical fiber is referred to so that as many OAM modes as possible meet the ITU-T G.655.C standard.

[0032] Determine the core spacing of the seven-core optical fiber. The core spacing ranges from 40 to 48 μm. Increasing the core spacing can reduce crosstalk and is generally selected based on the single-ring core geometry.

[0033] Beneficial effects of the present invention:

[0034] By selecting the materials of the annular core region and the annular groove region, adjusting the width and position of the annular core and the annular groove, and adjusting the refractive index distribution coefficient, the dispersion value in the C band can be adjusted so that the dispersion of multiple orbital angular momentum modes meets the ITU-T G.655.C standard.

[0035] By adjusting the inner diameter of the ring core and the core spacing, the single ring core fiber can be applied to the seven-core fiber, maintaining low crosstalk while meeting the dispersion standard of ITU-T G.655.C, further improving the information transmission density. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the optical fiber cross section and refractive index distribution of the present invention

[0037] Figure 2 a is the variation of the dispersion of each order OAM mode synthesized by the HE mode with wavelength, Figure 2 b is the variation of dispersion of each order OAM mode synthesized by the EH mode with wavelength.

[0038] Figure 3The different refractive index distribution coefficients α at 1550nm when the inner diameter of the ring core is 35μm and the width of the ring core is 2.2μm, HE 2,1 and HE 5,1 Dispersion and dispersion slope of the mode.

[0039] Figure 4 In the optical fiber structure of the present invention, HE 2,1 , HE 4,1 , HE 6,1 , HE 8,1 , HE 10,1 , HE 12,1 , HE 14,1 Mode dispersion at 1550nm.

[0040] Figure 5 In the optical fiber structure of the present invention, HE 2,1 , EH 1,1 , HE 14,1 , EH 12,1 Dispersion and dispersion slope of the modes at 1550 nm.

[0041] Figure 6 In the optical fiber structure of the present invention, HE under different germanium doping concentrations 2,1 , EH 1,1 , HE 14,1 , EH 12,1 Dispersion and dispersion slope of the modes at 1550 nm.

[0042] Figure 7 This is a schematic diagram of the cross section and refractive index distribution of the seven-ring core graded-index optical fiber.

[0043] Figure 8 When the structural parameters are r1=10μm, r2=12.4μm, r3=13.6μm, r4=15.2μm, r5=62.5μm, mf_GeO2=10mol%, and mf_CaF2=20mol%, the optical fiber corresponds to HE 2,1 , HE 3,1 , HE 4,1 , HE 5,1 , EH 1,1 , EH 2,1 , EH 3,1 Schematic diagram of how the mode dispersion varies with wavelength.

[0044] Fig. 9 In the invented optical fiber structure, when the transmission distance L = 100km and the core spacing Λ = 46μm, the OAM synthesized by the HE mode at different wavelengths 1,1 、OAM 2,1 、OAM 3,1、OAM 4,1 Schematic diagram of crosstalk between modes. DETAILED DESCRIPTION

[0045] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0046] Figure 1 It is a schematic diagram of the optical fiber cross section and refractive index distribution of the present invention. The color depth represents the refractive index value under different material doping concentrations. The darkest color part corresponds to the silica material with the highest germanium doping concentration, and the lightest color part corresponds to the silica material with the highest fluorine doping concentration.

[0047] The refractive index n of the annular core and annular trench regions Ge and n F The following equation is satisfied:

[0048]

[0049] Wherein, r is the distance from the center of the optical fiber, r1 and r2 are the inner diameter and outer diameter of the ring core respectively, r3 and r4 are the inner diameter and outer diameter of the groove respectively, α is the refractive index distribution parameter, n1 is the refractive index at the position of the maximum concentration of germanium doping in the ring core region, which is 1.4605 at a wavelength of 1550nm, n2 is the refractive index at the position of the maximum concentration of fluorine doping in the ring core region, which is 1.4303 at a wavelength of 1550nm, and n0 is the refractive index of pure silica in the cladding region, which is 1.4440 at a wavelength of 1550nm.

[0050] In this example, the OAM mode is used as the signal transmission carrier. The OAM beam is a special beam that carries orbital angular momentum and has the characteristics of a spiral phase wavefront and a central phase singularity. In the field of optical communications, since OAM multiplexing is independent of polarization and wavelength, it can be combined with other multiplexing methods such as wavelength division multiplexing and polarization division multiplexing, and has great potential in improving the channel capacity and spectrum efficiency of optical fiber communication systems. Light with a phase factor exp(ilθ) carries OAM, which is Where l is the topological charge, and the OAM mode is the eigenmode in cylindrical coordinates. Theoretically, the OAM topological charge l has infinite values ​​and is orthogonal, and can be used as different communication channels. The synthesis formula is:

[0051]

[0052] The positive and negative signs in the subscripts of OAM represent right-handed circular polarization and left-handed circular polarization, respectively. The positive and negative signs before the l represent the rotation direction of the spiral phase wavefront of the OAM mode. The direction of the spin angular momentum of the HE mode is consistent with the direction of OAM, and the direction of the spin angular momentum of the EH mode is opposite to the direction of OAM.

[0053] Figure 2 (a) is the OAM synthesized by the HE mode when the structural parameters are r1 = 51.5 μm, r2 = 53.7 μm, r3 = 55.5 μm, r4 = 60 μm, r5 = 62.5 μm, α = 4, mf_GeO2 = 11 mol%, and mf_CaF2 = 80 mol%. 1,1 、OAM 2,1 、OAM 3,1 、OAM 4,1 、OAM 5,1 、OAM 6,1 、OAM 7,1 、OAM 8,1 、OAM 9,1 、OAM 10,1 、OAM 11,1 、OAM 12,1 、OAM 13,1 The dispersion of the mode varies with wavelength.

[0054] Figure 2 (b) OAM synthesized from the EH mode 2,1 、OAM 3,1 、OAM 4,1 、OAM 5,1 、OAM 6,1 、OAM 7,1 、OAM 8,1 、OAM 9,1 、OAM 10,1 、OAM 11,1 、OAM 12,1 、OAM 13,1 The dispersion of the mode varies with wavelength. mf_GeO2 and mf_CaF2 are the maximum concentrations of germanium doping and fluorine doping.

[0055] Figure 3 The different refractive index distribution coefficients α at 1550nm when the inner diameter of the ring core is 35μm and the width of the ring core is 2.2μm, HE 2,1 and HE 5,1 Dispersion and dispersion slope of the mode.

[0056] Figure 4 In the optical fiber structure of the present invention, HE 2,1 , HE 4,1 , HE 6,1 , HE 8,1 , HE 10,1 , HE 12,1 , HE 14,1 Mode dispersion at 1550nm.

[0057] Figure 5In the optical fiber structure of the present invention, HE 2,1 , EH 1,1 , HE 14,1 , EH 12,1 Dispersion and dispersion slope of the modes at 1550 nm.

[0058] Figure 6 In the optical fiber structure of the present invention, HE under different germanium doping concentrations 2,1 , EH 1,1 , HE 14,1 , EH 12,1 Dispersion and dispersion slope of the modes at 1550 nm.

[0059] Figure 7 It is a schematic diagram of the cross section and refractive index distribution of the seven-ring core graded-index optical fiber, which adopts a parabolic refractive index distribution, where Λ is the core spacing, n1 is the refractive index at the maximum concentration position of germanium doping in the ring core region, which is 1.4590 at a wavelength of 1550nm, n2 is the refractive index at the maximum concentration position of fluorine doping in the ring core region, which is 1.4415 at a wavelength of 1550nm, and n0 is the refractive index of pure silica in the cladding region, which is 1.4440 at a wavelength of 1550nm.

[0060] Figure 8 When the structural parameters are r1=10μm, r2=12.4μm, r3=13.6μm, r4=15.2μm, r5=62.5μm, mf_GeO2=10mol%, and mf_CaF2=20mol%, the optical fiber corresponds to HE 2,1 , HE 3,1 , HE 4,1 , HE 5,1 , EH 1,1 , EH 2,1 , EH 3,1 The dispersion of the mode varies with wavelength.

[0061] Fig. 9 In the invented optical fiber structure, when the transmission distance L = 100km and the core spacing Λ = 46μm, the OAM synthesized by the HE mode at different wavelengths 1,1 、OAM 2,1 、OAM 3,1 、OAM 4,1 Mode crosstalk.

[0062] The specific embodiments of the present invention are further described below in conjunction with the accompanying drawings:

[0063] Embodiment 1:

[0064] The present invention is based on a groove-assisted gradient refractive index non-zero dispersion shifted ring core optical fiber. Figure 1As shown, it includes a central cylinder, an annular core region, a core trench interlayer, an annular trench region, and an outer fiber cladding from the inside out, with refractive indices of n0, n Ge , n0, n F , n0.

[0065] The annular core region is a graded-index ring with a refractive index greater than that of other regions, and its refractive index distribution satisfies n0 < n1; the annular trench region is a graded-index ring with a refractive index less than that of other regions, and its refractive index distribution satisfies n2 < n0.

[0066] In this embodiment, the materials of the central cylinder, the core trench interlayer, and the outer fiber cladding are silica, the material of the annular core region is germanium-doped silica with a gradually changing germanium doping concentration, and the material of the annular trench region is calcium fluoride-doped silica with a gradually changing fluoride doping concentration.

[0067] The above cross-sectional structure remains unchanged along the length direction of the optical fiber.

[0068] Figure 2 (a) shows that when the structural parameters are r1 = 51.5 μm, r2 = 53.7 μm, r3 = 55.5 μm, r4 = 60 μm, r5 = 62.5 μm, α = 4, mf_GeO2 = 11 mol%, mf_CaF2 = 80 mol%, and the communication wavelength λ = 1550 nm, n1 = 1.4605, n2 = 1.4303, n3 = 1.4440. The ring-core optical fiber can well match the annular optical field distribution of the OAM mode. The OAM 1,1 mode synthesized by the EH 2,1 mode has the lowest dispersion of 2.42 ps / nm / km, and the OAM 14,1 mode synthesized by the HE 13,1 mode has the highest dispersion of 8.71 ps / nm / km;

[0069] The optical fiber corresponds to the OAM 1,1 , OAM 2,1 , OAM 3,1 , OAM 4,1 , OAM 5,1 , OAM 6,1 , OAM 7,1 , OAM 8,1 , OAM 9,1 , OAM 10,1 , OAM 11,1 , OAM 12,1 , OAM 13,1The dispersion of the mode varies with wavelength. The dispersion of 13 orbital angular momentum modes in the C band (1530nm-1565nm) is maintained between 1ps / nm / km and 10ps / nm / km. The optical fiber can support 50 OAM mode transmissions to meet the ITU-T G.655.C standard. 1,1 OAM of Mode Synthesis 2,1 mode has a minimum of 982.4μm 2 The effective mode field area is given by HE 14,1 OAM of Mode Synthesis 13,1 mode has the highest 1369.0μm 2 The effective mode field area.

[0070] Among them, mf_GeO2 and mf_CaF2 are the maximum concentrations of germanium doping and fluorine doping.

[0071] Figure 2 (b) OAM synthesized from the EH mode 2,1 、OAM 3,1 、OAM 4,1 、OAM 5,1 、OAM 6,1 、OAM 7,1 、OAM 8,1 、OAM 9,1 、OAM 10,1 、OAM 11,1 、OAM 12,1 、OAM 13,1 The dispersion of the mode varies with wavelength. From the figure, it can be seen that the dispersion of the 13 orbital angular momentum modes in the optical fiber of the present invention in the C band is maintained between 1ps / nm / km and 10ps / nm / km. 1,1 To OAM 13,1 mode, and the dispersion characteristics can meet the ITU-T G.655.C standard for non-zero dispersion-shifted optical fiber.

[0072] Figure 3 The different refractive index distribution coefficients α correspond to HE 2,1 and HE 5,1 The dispersion and dispersion slope of the mode. It can be seen from the figure that when α = 4, HE 2,1 and HE 5,1 mode has the lowest dispersion. As α increases, HE 2,1 and HE 5,1 The dispersion slope of the mode decreases, which means that the greater the refractive index contrast between the annular core and the fiber cladding, and between the annular groove and the fiber cladding, the smaller the variation of the OAM mode dispersion with wavelength.

[0073] Figures 4 to 6The inner diameter of the annular core, the width of the annular core, and the maximum concentration of germanium doping in the annular core of the optical fiber of the present invention are adjusted respectively, corresponding to the dispersion or dispersion slope of different modes at 1550nm. From the figure, it can be seen that the various parameter combinations of the optical fiber of the present invention can provide lower dispersion characteristics that meet the suppression of four-wave mixing for multiple orbital angular momentum modes at 1550nm.

[0074] Figure 7 It is a seven-core optical fiber developed on the basis of single-ring-core graded-index optical fiber. The seven cores are distributed symmetrically around the center, with six outer cores surrounding a central core, and the core spacing between two adjacent cores is the same. This structure can increase the number of OAM modes supported by the optical fiber and maintain low nonlinear effects.

[0075] like Figure 8 As shown, when the structural parameters of the graded refractive index seven-core optical fiber are the inner diameter of the ring core r1 = 10 μm, the outer diameter of the ring core r2 = 12.4 μm, the inner diameter of the groove r3 = 13.6 μm, the outer diameter of the groove r4 = 15.2 μm, the fiber radius r5 = 62.5 μm, the annular core region is doped with mf_GeO2 = 10 mol%, the annular groove region is doped with mf_CaF2 = 20 mol%, the transmission distance L = 100 km, the core spacing Λ = 46 μm, and the communication wavelength λ = 1550 nm, n1 = 1.4590, n2 = 1.4415, n0 = 1.4440, and HE 21 OAM of Mode Synthesis 1,1 The mode has the lowest dispersion of 2.79ps / nm / km, which is determined by HE 51 OAM of Mode Synthesis 4,1 The mode has the highest dispersion, or OAM, of 8.56 ps / nm / km. 1,1 , OAM 2,1 , OAM 3,1 , OAM 4,1 The dispersion of the four orbital angular momentum modes in the C band is maintained between 1ps / nm / km and 10ps / nm / km. The seven-core fiber can support a total of 98 OAM mode transmissions to meet the ITU-TG.655.C standard. 1,1 OAM of Mode Synthesis 2,1 mode has a minimum of 342.3μm 2 The effective mode area is given by EH 3,1 OAM of Mode Synthesis 4,1 mode has a maximum of 349.2μm 2 In addition, when the communication wavelength λ = 1550nm, the HE 5,1 OAM of Mode Synthesis 4,1 The crosstalk of the mode is -41.5dB, which is provided by HE2 ,1 OAM of Mode Synthesis1,1 The crosstalk of the modes is -57.6dB. In the C band, as the wavelength increases, the crosstalk of each mode increases slightly, but can still be maintained below -40dB.

[0076] r1=10μm, r2=12.4μm, r3=13.6μm, r4=15.2μm, r5=62.5μm, mf_GeO2=10

[0077] mol%, when mf_CaF2=20mol%, the optical fiber corresponds to HE 2,1 , HE 3,1 , HE 4,1 , HE 5,1 , EH 1,1 , EH 2,1 , EH 3,1 The dispersion of the mode varies with wavelength.

[0078] It can be seen from the figure that the dispersion of the four orbital angular momentum modes in the optical fiber of the present invention in the C band (1530nm-1565nm) is maintained between 1ps / nm / km and 10ps / nm / km. 1,1 To OAM 4,1 mode. The dispersion characteristics can meet the ITU-T G.655.C standard for non-zero dispersion-shifted optical fiber. The seven-core optical fiber can support a total of 98 OAM mode transmissions.

[0079] Fig. 9 It is the OAM synthesized by HE mode at different wavelengths when the transmission distance L = 100km and the core spacing Λ = 46μm 1,1 、OAM 2,1 、OAM 3,1 、OAM 4,1 Mode crosstalk. Since there are multiple fiber cores in the same cladding, two fiber cores with close distances will interfere with each other due to lateral coupling. When the crosstalk is lower than -30dB, the communication system has high stability. From the figure, we can see that with the increase of wavelength, the crosstalk of each mode increases slightly, but it can still be kept below -40dB.

Claims

1. A groove-assisted non-zero dispersion-shifted ring-core optical fiber, characterized in that: The invention is a graded refractive index ring core optical fiber, comprising a ring core, a ring groove and an optical fiber cladding; the refractive index of the ring core is greater than the refractive index of the optical fiber cladding, and the refractive index of the optical fiber cladding is greater than the refractive index of the ring groove; The optical fiber cladding includes a central cylinder, an interlayer and an outer optical fiber cladding, each layer having the same refractive index; The annular fiber core is sleeved on the central cylinder, the interlayer is sleeved on the annular fiber core, the annular groove is sleeved on the interlayer, and the outer optical fiber cladding is sleeved on the annular groove.

2. The trench-assisted non-zero dispersion-shifted ring-core optical fiber according to claim 1, characterized in that: The annular core and the annular groove use the same refractive index distribution coefficient α. The refractive index distribution expression is given by the annular core refractive index n Ge、 The refractive index of the annular groove is n F The formula determines that when α=2, it is a parabolic refractive index, and when α=∞, it is a step refractive index. The refractive index of the annular groove area is lower than that of the optical fiber cladding, which has the effect of limiting the range of the light field and suppressing crosstalk.

3. The trench-assisted non-zero dispersion-shifted ring-core optical fiber according to claim 1, characterized in that: The width of the ring core is d = r2-r1, where r1 is the inner diameter of the ring core region, and r2 is the outer diameter of the ring core region. The width range of the ring core is: Wherein, n1 is the refractive index at the position of the maximum concentration of germanium doping in the annular core region, n2 is the refractive index at the position of the maximum concentration of fluorine doping in the annular core region, and λ is the communication wavelength.

4. The trench-assisted non-zero dispersion-shifted ring-core optical fiber according to claim 3, characterized in that: The materials of the optical fiber cladding, the annular core, and the annular groove are a combination of silica, germanium-doped silica, and fluorine-doped silica materials under the condition of satisfying the above-mentioned refractive index distribution; The materials of the central cylinder, interlayer and outer optical fiber cladding are silicon dioxide, the material of the annular core is germanium-doped silicon dioxide with a gradient doping concentration of germanium dioxide, and the material of the annular groove region is fluorine-doped silicon dioxide with a gradient doping concentration of calcium fluoride; the gradient rule is determined by the refractive index distribution expression; Among them, the maximum value range of germanium doping concentration in the annular core area is 8-16mol%, corresponding to the refractive index n1 range of 1.456-1.468; the fluorine doping concentration range of the annular groove is 0-100mol%, and the fluorine doping concentration of the gradient refractive index single ring core optical fiber is uniformly taken as mf_CaF2=80mol%, corresponding to the refractive index n2=1.4303.

5. The trench-assisted non-zero dispersion-shifted ring-core optical fiber according to claim 4, characterized in that: The refractive index distribution expressions in the annular core and the annular groove region of the graded-refractive-index single-ring-core optical fiber are respectively: Among them, r is the distance from the center of the gradient refractive index single ring core optical fiber, r1 and r2 are the inner diameter and outer diameter of the ring core respectively, r3 and r4 are the inner diameter and outer diameter of the ring groove respectively, α is the refractive index distribution parameter, n1 is the refractive index at the position of the maximum concentration of germanium doping in the ring core region, n2 is the refractive index at the position of the maximum concentration of fluorine doping in the ring core region, and n0 is the refractive index of pure silica in the optical fiber cladding region.

6. A multi-ring core gradient refractive index optical fiber, comprising seven gradient refractive index single ring core optical fibers as described in any one of claims 1 to 5, wherein the structure is centrally symmetrically distributed, with six outer ring cores surrounding a central ring core, and the core spacings of two adjacent ring cores are the same.

7. The multi-ring core graded-refractive-index optical fiber according to claim 6, characterized in that: The core spacing ranges from 40 to 48 μm.

8. A method for adjusting a groove-assisted non-zero dispersion-shifted ring-core fiber according to any one of claims 1 to 5, characterized in that: The following steps are involved: Determine the refractive index distribution coefficient, the refractive index distribution coefficient ranges from 2 to 10, and the refractive index distribution coefficient that meets the low dispersion requirement is selected according to the optical fiber geometry; Determine the radius of the central cylinder of the optical fiber cladding. The radius of the shaped area is in the range of 45-53μm. The high-order OAM mode is greatly affected by the radius of the central cylinder. The radius of the central cylinder can be appropriately increased while meeting the ITU-T G.655.C standard. Determine the width of the annular core region, which ranges from 1.2 to 2.5 μm. Select the width of the annular core region that meets the low dispersion requirements based on the radius of the central cylinder, the single-ring core and multi-ring core structures. When radial high-order modes appear, the width of the annular core region should be reduced to ensure that it is less than d determined by the radial single-mode empirical formula; Determine the maximum concentration of germanium doping in the annular core region, the maximum concentration of germanium doping in the annular core region is in the range of 8-16 mol%, corresponding to the refractive index n1 range of 1.456-1.468; High germanium doping concentration can support more OAM mode transmission. When the low-order mode dispersion is less than 1ps / nm / km, the germanium doping concentration should be reduced to ensure that it meets the ITU-T G.655.C standard.

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