A trench-assisted random-coupling multi-core optical fiber

Abstract

This invention discloses a trench-assisted random-coupled multi-core optical fiber. The random-coupled multi-core optical fiber includes an outer cladding and n identical trench-assisted cores disposed within the outer cladding, where 1 / 2 ≥ n ≥ 2. Each trench-assisted core, from the inside out, comprises a low-doped core, a high-doped core, an inner cladding, and a low-refractive-index trench-assisted layer. Each trench-assisted core supports only a single mode, namely HE. 11 If polarization state is considered, it is divided into x-polarized HE. 11 y-polarized HE 11 Therefore, the excited supermode order is the same as the number of cores in the trench-assisted type, while other modes of higher order are cut off. By taking different combinations of values ​​for the inner cladding and the low-refractive-index trench-assisted layer, electromagnetic field overlap can not only be suppressed to reduce the core spacing and increase the core density, but also the group velocity difference between supermodes caused by process fabrication errors can be reduced, thus enhancing the tolerance to process fabrication errors.

Description

A trench-assisted random-coupled multi-core optical fiber Technical Field

[0001] This invention belongs to the field of optical fiber communication, and specifically relates to a slot-assisted random-coupled multi-core optical fiber. Background Technology

[0002] With the rise of emerging technologies such as artificial intelligence, the Internet of Things, and cloud computing, humanity is entering an era of intelligent big data, leading to a rapid increase in communication traffic and placing higher demands on optical fiber transmission capacity. However, due to the limited maximum input power of optical fibers and the nonlinear Shannon limit, the maximum transmission capacity of communication systems based on traditional standard single-mode fiber (SMF) is limited to approximately 100 Tb / s. How to upgrade and expand these systems is a pressing issue. To overcome the transmission capacity limitations of SMF, Space Division Multiplexing (SDM) technology, proposed in 2010, increases the communication capacity of optical fiber systems without increasing the area of ​​optical fiber cabling. SDM-based optical fiber communication transmission systems utilize multi-core fiber (MCF) and few-mode fiber (FMF) as two effective methods, and have rapidly become a research hotspot in academia since their inception. MCF (Multi-Cell Fiber Coefficient) provides multiple spatial paths to transmit various signals simultaneously by arranging multiple independent fiber cores in a common cladding region; while FMF (Fiber-Mechanical Fiber Coefficient) utilizes multiple different orthogonal modes in the fiber core to transmit signals, making the transmission capacity of the fiber increase exponentially with the increase in the number of modes.

[0003] MCFs used in SDM can be divided into two categories. The first type is the weakly-coupled MCF, in which each core acts as an independent waveguide. Therefore, a large core spacing is required to ensure sufficiently low inter-core coupling and crosstalk (XT) between adjacent cores. However, continuously increasing the number of cores in an MCF will cause the fiber to lose its flexibility due to an excessively large cladding diameter. Therefore, the challenge in designing an MCF is to rationally select the core spacing and core arrangement within the limited cladding space, accommodating as many cores as possible while maintaining low inter-core crosstalk. The second type is the randomly-coupled MCF. Because the definition of coupling strength for multi-core fibers was not clear in the early stages of SDM technology development, randomly-coupled multi-core fibers were initially called strongly-coupled MCFs or coupled-core MCFs. This type of MCF deliberately introduces crosstalk by reducing the core-to-core distance to create strong coupling between cores, while simultaneously increasing core density. In particular, random coupling MCF is an effective method for achieving lower Group Delay Spread (GDS) caused by mode dispersion in long-distance transmission. In SDM few-mode transmission systems, the number of modes increases compared to single-mode transmission, leading to stronger inter-mode interference. This means that multiplexing and demultiplexing of transmission modes becomes more complex. Simultaneously, differences in group velocities between different modes cause mode dispersion. GDS caused by mode dispersion is one of the main problems limiting long-distance transmission because the magnitude of GDS determines the complexity of the receiver and the coverage range of the MIMO system. If the coupling between different fiber core modes is weak (weak coupling state), GDS is proportional to the transmission distance. If the mode coupling is strong (strong coupling state), thus exciting supermodes, these supermodes are defined as first-order supermodes, second-order supermodes, and so on, in descending order of neff. Random mixing of modes between supermodes with different group velocities due to perturbations such as random bending and twisting helps reduce GDS and makes it proportional to the square root of the transmission distance. The formula is expressed as follows:

[0004]

[0005] The phenomenon of light pulse broadening caused by the velocity difference of spatial mode groups is defined as spatial mode dispersion (SMD), and therefore k is called the spatial mode dispersion coefficient (SMD coefficient), with units of kJ / kJ. L represents the fiber transmission distance in km. The literature (Journal of Lightwave Technology, 29(21):3119-3128, 2011) indicates that introducing strong mode coupling between the cores of the MCF can reduce GDS and evolve according to the square root of the transmission distance during transmission. This coupling is advantageous; in direct detection systems, it reduces inter-symbol interference, while in systems using coherent detection, it reduces the time memory required for digital compensation of mode dispersion. The literature (Optical Fiber Communication Conference, 2019: Th4B.3.) reports a transmission experiment using a 7-core randomly coupled multi-core fiber to transmit a 12100km PDM-QPSK signal. The fiber cladding diameter is 125μm, the cores are arranged in a hexagonal grid, the core spacing is 23.5±0.2μm, the fiber attenuation at 1550nm is 0.172dB / km, and the dispersion is 21.1-21.4ps / nm / km. Experimental results show that its transmission capacity is superior to that achieved by seven single-mode fibers under the same conditions. A 2020 paper (Optical Fiber Communications Conference, IEEE, 2020: 1-3) reported reducing the spatial mode dispersion coefficient to a lower level by controlling parameters such as bending and twisting during fiber cabling. The literature verifies the feasibility of achieving a low GDS with random coupling MCF, which can effectively reduce the computational complexity of the digital signal processing at the receiver of the MIMO system after long-distance transmission and the number of taps in the recovery algorithm. It has good application prospects for long-distance transmission systems and repeaterless transmission systems.

[0006] In MCF (Multi-Fluorescent Fiber), it is generally desirable for all cores to have completely identical group velocities to reduce GDS (Gross Displacement). However, due to manufacturing process errors, even in homogeneous MCFs, slight fluctuations in parameters between different cores after fiber fabrication can cause changes in refractive index, resulting in differences in group velocities and thus pulse broadening. This phenomenon is called inter-core skew. To reduce pulse broadening caused by group velocity differences, the inter-core spacing can be reduced to introduce a certain strength of inter-core coupling, thereby exciting supermodes. Because the group refractive index difference between supermodes is small under appropriate core spacing, the group velocity difference is also small. When the fiber is subjected to perturbations such as bending or twisting, random mode aliasing between supermodes will further reduce the group velocity difference, thereby reducing GDS. This type of randomly coupled multi-core fiber has also been reported in the literature (multi-core fiber, multi-core fiber cable, and multi-core fiber transmission system), but its refractive index profile is step-distributed. In order to achieve sufficient aliasing, the large core spacing makes the core density not high enough. At the same time, due to the manufacturing process, the GDS achieved is not optimal. On the other hand, this invention introduces a trench auxiliary layer to regulate the group refractive index difference, enhances the tolerance to manufacturing errors, and minimizes GDS. The introduction of the trench auxiliary layer also suppresses the electric field overlap between adjacent cores, allowing the core spacing to be further reduced and the core density to be further increased. At the same time, the presence of the low-doped core structure increases the mode field area and further suppresses nonlinear effects. Summary of the Invention

[0007] The present invention aims to achieve a lower GDS, while enhancing the fiber core's tolerance to process fabrication errors, increasing the fiber core density, and further suppressing the nonlinear effects of the optical fiber. The objective is to propose a trench-assisted random-coupled multi-core optical fiber based on space division multiplexing technology.

[0008] This invention provides a trench-assisted random-coupled multi-core optical fiber, in which each trench-assisted core consists of four parts: a low-doped core, a high-doped core, an inner cladding, and a low-refractive-index trench-assisted layer. The trench-assisted core is placed within the outer cladding. Strong coupling is formed between the trench-assisted cores through appropriate core spacing to excite supermodes. By adjusting the relative refractive index difference and relative size of the high-doped core, the number of supermodes can be adjusted; by adjusting the relative refractive index difference and relative size of the low-doped core, the Aeff enhancement can be adjusted to suppress nonlinear effects; by adjusting the inner cladding and the low-refractive-index trench-assisted layer through different value combinations, electromagnetic field overlap can be suppressed, thereby reducing the core spacing to increase core density and simultaneously reducing the group velocity difference between core modes caused by fabrication errors, thus enhancing tolerance to fabrication errors.

[0009] The present invention is achieved by at least one of the following technical solutions.

[0010] A trench-assisted random-coupled multi-core optical fiber, characterized in that it includes an outer cladding and n identical trench-assisted fiber cores placed within the outer cladding, where 12≥n≥2;

[0011] Each trench-assisted fiber core consists of, from the inside out, a low-doped fiber core, a high-doped fiber core, an inner cladding, and a low-refractive-index trench auxiliary layer.

[0012] Each trench-assisted core supports only a single mode, namely HE. 11 If polarization state is considered, it is divided into x-polarized HE. 11 y-polarized HE 11 Therefore, the number of supermode orders excited is the same as that of the groove-assisted fiber core, while other modes of higher order are cut off.

[0013] Furthermore, when the number of cores n≤6, or n=8 or 10, all grooved cores are arranged in a single layer, and all grooved cores are evenly distributed on a circle with the geometric center of the outer cladding section as the center and the core spacing between adjacent grooved cores as Dco.

[0014] When the number of cores n = 12, the grooved cores are arranged in two layers. The first layer of grooved cores are evenly distributed on a circle with the geometric center of the outer cladding section as the center and the core spacing between adjacent grooved cores as Dco. The second layer of grooved cores are staggered at positions with a distance of Dco from the adjacent core spacing of the first layer of grooved cores.

[0015] When the number of cores n = 7, 9 or 11, one core is set at the geometric center of the outer cladding section, and the remaining grooved cores are evenly arranged on a circle with the geometric center of the outer cladding section as the center and the spacing between adjacent grooved cores as Dco.

[0016] Furthermore, both the outer and inner cladding layers are made of fused silica.

[0017] Furthermore, the refractive index of the low-doped and high-doped fiber cores is higher than that of the cladding material after doping.

[0018] Furthermore, the refractive index of the trench auxiliary layer is lower than that of the outer cladding layer after doping. In addition to further confining the mode field and suppressing the overlap of electromagnetic fields, it can also adjust different parameters to reduce the group velocity difference of the supermode caused by process preparation errors, that is, reduce ΔNg and enhance the tolerance to process preparation errors.

[0019] Furthermore, the radius of the highly doped fiber core is r1 = 4.5 ± 0.1 μm, the radius of the low-doped fiber core is a = r1 / 2, the radius of the inner cladding is r2, the width of the trench auxiliary layer is W, and the radius of the trench auxiliary layer is r3 = r2 + W.

[0020] By combining different values ​​of radius r3 and trench auxiliary layer width W, the overlap of electromagnetic fields can be suppressed to varying degrees. Simultaneously, the group velocity difference between core modes caused by fabrication errors is reduced, thus enhancing tolerance to fabrication errors. (Group velocity n) g It can be represented as:

[0021]

[0022] Where c is the speed of light in vacuum, β is the propagation constant, and ω is the angular frequency; the group velocity difference of the core modes is expressed as the difference in group velocities between two core modes, denoted as ΔNg, i.e., ΔNg = n g1 -n g2 n g1 and n g2 These are the group velocities of the two core modes, respectively;

[0023] Let the radius r2 of the inner cladding and the width W of the grooved auxiliary layer be set, and let W = 0, i.e., the absolute value of the core group velocity difference without grooved assistance, be |ΔNg0|; changing the values ​​of r2 and W (W≠0) will change the group velocity n of the grooved-assisted core. g The absolute value of the difference between r2 and W is |ΔNg|. The range of values ​​for r2 and W is the range of parameters that makes |ΔNg| < |ΔNg0|. The best parameter selection for r2 and W is the parameter point in this parameter range that makes |ΔNg| = 0.

[0024] Under different manufacturing errors, the |ΔNg| between core modes is relatively large. By combining the parameters of r2 and W, the |ΔNg| can be controlled to reduce the group velocity difference between modes, thereby reducing the group time delay spread (GDS).

[0025] Furthermore, low-doped and high-doped fiber cores control the normalized frequency of the optical fiber by changing the refractive index of the material through doping, thereby regulating the number of modes that the fiber core can support. At the same time, the presence of low-doped fiber cores can increase the mode field area, thereby enhancing the suppression of nonlinear effects in the optical fiber.

[0026] The relative refractive index difference between the low-doped core and the cladding is Δ2; the relative refractive index difference between the high-doped core and the cladding is Δ1; the relative refractive index difference between the trench auxiliary layer and the cladding is Δ tr ;where Δ1=0.35±0.01%, Δ2=Δ1 / 2, Δ tr= -0.7 ± 0.01%;

[0027] By increasing (decreasing) the values of r1 and Δ1, the number of supermode patterns can be increased (decreased), and the single-mode condition of the trench-assisted core needs to be maintained; by increasing a and decreasing the value of Δ2, the mode field area Aeff can be increased based on the original Aeff value when a = 0 and Δ2 = Δ1, so as to enhance the suppression effect on the nonlinear effect; by decreasing r2, increasing W, and Δ tr the value of can enhance the confinement effect on the electromagnetic field.

[0028] Further, similar to the method of using a twisting device to reduce polarization mode dispersion in single-mode fibers, twisting with a peak twisting rate TW = 3 turn / m is used to make the random aliasing degree sufficient to optimize the group time delay broadening GDS; there is an optimal selection range [D1, D2] for the core pitch Dco of adjacent trench-assisted cores at the peak twisting rate TW, and the core pitch Dco ∈ [D1, D2], D2 > D1 > 2*r3; when the core pitch Dco is very small, that is, Dco < D1, there is a large difference in the effective refractive indices between the supermodes of each order, so they can propagate stably and independently without energy aliasing. Similar to a few-mode transmission system, the group time delay broadening value of the supermode increases linearly with the transmission distance;

[0029] When Dco is very large, that is, Dco > D2, the energy aliasing between the cores is weak, each core is regarded as an independent waveguide, and the supermode is not excited either, belonging to the category of weakly coupled multi-core fibers, and the group time delay broadening value of the supermode increases linearly with the transmission distance;

[0030] Only when the core pitch is moderate, that is, Dco ∈ [D1, D2], can it not only keep a certain intensity of coupling between the cores to excite the supermode, but also make the difference in the effective refractive indices between the supermodes decrease and be able to fully randomly alias, so that the group time delay broadening value of the supermode increases with the square root of the distance;

[0031] The optimal selection range [D1, D2] of the core pitch is determined according to the coupled mode theory and using the transfer matrix method; that is, the initial state A(0) of each mode is transformed by the transfer matrix T(ω), and the state at L becomes A(L), that is, A(L) = T(ω)·A(0), where ω is the angular frequency of the electromagnetic wave; therefore, the group delay operator GDO(ω) is defined by T(ω) as:

[0032]

[0033] The calculation formula for the group time delay broadening GDS is:

[0034]

[0035] Where N is the number of supermodes containing polarization states, τ i It is the i-th eigenvalue of the group delay operator GDO(ω); use formula (3) to evaluate the GDS for each core spacing, and the core spacing corresponding to the minimum GDS is the required core spacing Dco.

[0036] Furthermore, a larger cladding radius r4 would reduce the mechanical reliability of the optical fiber, so consistent with standard single-mode fiber, r4 = 62.5 μm.

[0037] Compared with existing technologies, the beneficial effects of this invention are:

[0038] This paper proposes a trench-assisted random-coupled multi-core fiber. By adjusting the relative refractive index difference and relative size of the highly doped core, the relative position, relative refractive index difference, and relative size of the low-refractive-index trench auxiliary layer, and the relative size of the cladding, the GDS caused by mode dispersion can be optimized. This reduces the complexity of the receiver MIMO algorithm and decreases the time memory required for digital compensation of mode dispersion in coherent detection systems. Simultaneously, the presence of the low-doped core increases Aeff, thereby enhancing the suppression of nonlinear effects. Different combinations of values ​​for the cladding and the low-refractive-index trench auxiliary layer can not only suppress electromagnetic field overlap to reduce core spacing and increase core density, but also reduce group velocity differences between supermodes caused by fabrication errors, thus enhancing tolerance to fabrication errors. Attached Figure Description

[0039] Figure 1 is a cross-section of the grooved assisted fiber core proposed according to the present invention and a diagram of its horizontal material refractive index distribution;

[0040] Figure 2 is a cross-sectional view of a two-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention.

[0041] Figure 3 is a cross-sectional view of a 3-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0042] Figure 4 is a cross-sectional view of a 4-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0043] Figure 5 is a cross-sectional view of a 6-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0044] Figure 6 is a cross-sectional view of a 7-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0045] Figure 7 is a cross-sectional view of an 8-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0046] Figure 8 is a cross-sectional view of a 9-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention;

[0047] Figures 9a, 9b and 9c are schematic diagrams of three forms of cross-sectional views of a 12-core randomly coupled multi-core optical fiber in an embodiment of the trench-assisted fiber core deployment proposed according to the present invention.

[0048] Figures 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i, 10j, and 10k are the dependence of the group refractive index difference of the trench-assisted fiber core proposed in this invention on the parameters of the inner cladding radius and the width of the trench-assisted layer under different parameters.

[0049] Figure 11 is a graph showing the dependence of GDS on core spacing of a trench-assisted 7-core randomly coupled multi-core optical fiber according to an embodiment of the present invention.

[0050] Figure 12 is a graph showing the dependence of the GDS of a trench-assisted 7-core randomly coupled multi-core optical fiber on the transmission distance according to an embodiment of the present invention.

[0051] Figure 13 is a graph showing the dependence of the GDS of a trench-assisted 7-core randomly coupled multi-core optical fiber on the bending radius according to an embodiment of the present invention.

[0052] Figure 14 is a graph showing the dependence of the twist rate on the fiber length during the twisting process of the optical fiber using the sinusoidal modulation bidirectional twisting device employed in this invention. Detailed Implementation

[0053] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0054] Taking the trench-assisted 7-core randomly coupled multi-core optical fiber shown in Figure 6 as an example, the same reference numerals in the figures refer to the same components, and will not be described again.

[0055] Example:

[0056] A trench-assisted random-coupled multi-core optical fiber, as shown in Figure 6, includes an outer cladding 5 and seven identical trench-assisted fiber cores placed within the outer cladding 5.

[0057] As shown in Figure 1, each trench-assisted fiber core includes, from the inside out, a low-doped fiber core 1, a high-doped fiber core 2, an inner cladding layer 3, and a low-refractive-index trench auxiliary layer 4.

[0058] Low-doped fiber core 1 and high-doped fiber core 2 control the normalized frequency of the optical fiber by changing the refractive index of the material through doping, thereby regulating the number of modes that the fiber core can support. At the same time, the presence of low-doped fiber core 1 can increase the mode field area Aeff, thereby enhancing the suppression of nonlinear effects in the optical fiber.

[0059] Each trench-assisted core supports only a single mode, namely HE. 11 If polarization state is considered, it is divided into x-polarized HE. 11 y-polarized HE 11 Therefore, the number of supermode orders excited is the same as that of the trench-assisted fiber core, while other modes of higher order are cut off.

[0060] The inner cladding layer 3 and the low-refractive-index trench auxiliary layer 4, by using different combinations of radius and trench width values, achieve varying degrees of suppression of electromagnetic field overlap, while simultaneously reducing the group velocity differences between core modes caused by fabrication errors, thus enhancing tolerance to fabrication errors; group velocity n g It can be represented as:

[0061]

[0062] Where C is the speed of light in vacuum, β is the propagation constant, and ω is the angular frequency; the group velocity difference of the core modes is expressed as the difference in group velocities between two core modes, denoted as ΔNg, i.e., ΔNg = n g1 -n g2 n g1 and n g2 These are the group velocities for the two core modes, respectively.

[0063] As shown in Figures 6 and 8, when the number of fiber cores n = 7, 9 or 11, one fiber core is set at the geometric center of the cross section of the outer cladding 5, and the remaining grooved auxiliary fiber cores are evenly arranged on a circle with the geometric center of the cross section of the outer cladding 5 as the center and the spacing between adjacent grooved auxiliary fiber cores as Dco.

[0064] In this embodiment, there are 7 grooved auxiliary fiber cores, one of which is located at the geometric center of the cross section of the outer cladding layer 5, and the other six fiber cores are evenly arranged on a circle with an adjacent core spacing of Dco. The distance between the other six fiber cores and the fiber core located at the geometric center of the cross section of the outer cladding layer 5 is Dco.

[0065] In this embodiment, both the outer cladding layer 5 and the inner cladding layer 3 are made of fused silica. The refractive indices of the low-doped core 1 and the high-doped core 2 are higher than those of the outer cladding layer 5 after doping. The refractive index of the trench auxiliary layer 4 is lower than that of the outer cladding layer 5 after doping. This not only further confines the mode field and suppresses the overlap of electromagnetic fields, but also allows for the adjustment of different parameters to reduce the group velocity difference of the supermode caused by process fabrication errors, i.e., to reduce ΔNg and enhance tolerance to process fabrication errors.

[0066] The radius of the highly doped fiber core 2 is r1 = 4.5 ± 0.1 μm, the radius of the low-doped fiber core 1 is a = 2.25 ± 0.1 μm, the radius of the inner cladding layer 3 is r2, the width of the trench auxiliary layer 4 is W, and the radius of the trench auxiliary layer 4 is r3.

[0067] In this embodiment, Figures 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i, 10j, and 10k show the dependence of the group refractive index difference of a trench-assisted randomly coupled multi-core fiber on the parameters of the inner cladding and trench-assisted layer. Where r1 = 4.5 μm, Δ1 = 0.35 ± 0.01%, Δ2 = 0.175 ± 0.01%, Δ... tr = -0.70±0.01%. The black dotted lines, dashed lines, and solid lines in Figures 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i, 10j, and 10k represent the group refractive index difference ΔNg between supermodes caused by process fabrication errors of 0.5%, 1.0%, and 1.5%, respectively. It can be seen that ΔNg changes with different values ​​of r2 / r1, altering the ratio of W / r1. When W / r1 = 0, it indicates that the fiber core has no groove assistance, and the group refractive index difference between the supermodes at this time is denoted as ΔNg0. As shown in Figures 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i, 10j, and 10k, a smaller ΔNg than ΔNg0 can be obtained by changing the values ​​of r2 / r1 and W / r1. The design point in this embodiment is r2 / r1 = 1.5 and W / r1 = 0.38 in Figure 10d, that is, (r2, W) = (6.75μm, 1.71μm).

[0068] In this embodiment, the GDS for each core spacing is calculated using formulas (2) and (3), resulting in the GDS dependence diagram of the trench-assisted random-coupled multi-core fiber on the core spacing shown in Figure 11. The considered torsion rates are shown in Figure 14, where TW = 1, 3, and 5 turns / m, and σ = 0.3 turns / m. Figure 11 shows that based on the given fiber core structure parameters, a suitable core spacing @ wavelength λ = 1550 nm, transmission distance L = 1 km, and bending radius Rb = 140 mm can be obtained. Within this range, the GDS can be proportional to the square root of the transmission distance, thus achieving a lower GDS. The design point in this embodiment is Dco = 22 μm.

[0069]

[0070]

[0071] In this embodiment, Figure 12 shows the dependence of GDS on transmission distance for trench-assisted randomly coupled multi-core optical fibers. As can be seen from Figure 12, the core structure parameter design points given in Figures 10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i, 10j, 10k, and 11 can maintain the spatial mode dispersion coefficient in the C+L band. Rb=140mm, TW=3turn / m, σ=0.3turn / m.

[0072] In this embodiment, it is assumed that the optical fiber is wound on a fixed fiber optic spool, i.e., the bending radius Rb = 140 mm. However, under actual usage conditions or with different spool sizes, the performance of the fiber will vary due to changes in the strong bending (small bending) or weak bending (large bending, approximately straight). Therefore, Figure 14 shows the dependence of GDS on Rb for trench-assisted random-coupled multi-core optical fiber, where the simulated transmission distance L = 1 km, λ = 1550 nm, TW = 3 turns / m, and σ = 0.3 turns / m. As can be seen from Figure 14, GDS does not decrease unidirectionally with increasing Rb, but rather there exists an optimal waveguide bending condition. Therefore, to further optimize GDS, it is necessary to consider not only the fiber structure parameters but also the waveguide bending condition.

[0073] In the above embodiment, the number of grooved auxiliary fiber cores placed in the outer cladding is 7, but the number of fiber cores can be any integer of 2 or more.

[0074] Example 2:

[0075] In this embodiment, as shown in Figures 2, 3, 4, 5, and 7, when the number of fiber cores n≤6, or n=8 or 10, all groove-assisted fiber cores are arranged in a single layer, and all groove-assisted fiber cores are evenly arranged on a circle with the geometric center of the outer cladding layer 5 as the center and the spacing between adjacent groove-assisted fiber cores as Dco.

[0076] Example 3:

[0077] In this embodiment, as shown in Figures 9a, 9b and 9c, when the number of fiber cores n = 12, the grooved auxiliary fiber cores are arranged in two layers. The first layer of grooved auxiliary fiber cores is evenly distributed on a circle with the geometric center of the outer cladding layer 5 as the center and the core spacing between adjacent grooved auxiliary fiber cores as Dco. The second layer of grooved auxiliary fiber cores are arranged alternately at positions Dco away from the adjacent core spacing of the first layer of grooved auxiliary fiber cores.

[0078] In Figure 9a, the first layer has three fiber cores arranged on a circle centered at the geometric center of the outer cladding section, with an adjacent core spacing of Dco. The second layer has nine fiber cores arranged on a circle with a core spacing of Dco from the first layer and an adjacent core spacing of Dco between them. The circles of the first and second layers are concentric. Any fiber core in the first layer forms an equilateral triangle with its nearest two adjacent fiber cores in the second layer. In Figure 9b, the first layer has six fiber cores arranged on a circle centered at the geometric center of the outer cladding section, with an adjacent core spacing of Dco. The second layer has six fiber cores arranged on a circle centered at the geometric center of the outer cladding section, with an adjacent core spacing of Dco. The cores are arranged on a circle with a core spacing of Dco from the first layer. The circles of the first and second layers are concentric circles, and any core of the second layer forms an equilateral triangle with the two nearest adjacent cores of the first layer. In Figure 9c, the three cores of the first layer are arranged on a circle with the geometric center of the outer cladding section as the center and the adjacent core spacing of Dco. The second layer consists of three sides, and three cores are arranged on each side. The adjacent core spacing on each side is Dco. Any core of the first layer forms an equilateral triangle with the two nearest adjacent cores of the second layer.

[0079] As shown in Figure 13, similarly, the design point r2 / r1 = 1.5 and W / r1 = 0.38 are only special cases. Their values ​​can be any other values ​​in the figure that make |ΔNg| < |ΔNg0|. Different fiber core numbers and different design point values ​​correspond to different core spacing ranges [D1, D2], and the resulting GDS's dependence on transmission distance L and bending radius Rb also differs. However, the values ​​and calculation process are only a simple repetition of the method in the above embodiments and have no essential difference, so they will not be repeated.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A trench-assisted random-coupled multi-core optical fiber, characterized in that: The fiber includes an outer cladding layer (5) and n identical trench-assisted cores placed within the outer cladding layer (5), where 12 ≥ n ≥ 2; each trench-assisted core consists of a low-doped core (1), a high-doped core (2), an inner cladding layer (3), and a low-refractive-index trench-assisted layer (4) from the inside out; each trench-assisted core supports only a single mode, namely HE. 11 If polarization state is considered, it is divided into x-polarized HE. 11 y-polarized HE 11 Therefore, the excited supermode order is the same as the number of grooved cores, and other modes with higher orders are cut off; when the number of cores n≤6, or n=8 or 10, all grooved cores are arranged in a single layer, and all grooved cores are uniformly arranged on a circle with the geometric center of the outer cladding (5) as the center and the spacing between adjacent grooved cores as Dco; when the number of cores n= At 12 o'clock, the trench-assisted fiber cores are arranged in a double layer. The first layer of trench-assisted fiber cores are evenly distributed on a circle with the geometric center of the outer cladding (5) as the center and the core spacing between adjacent trench-assisted fiber cores is Dco. The second layer of trench-assisted fiber cores are staggered at positions with a distance of Dco from the adjacent core spacing of the first layer of trench-assisted fiber cores. When the number of fiber cores n=7, 9 or 11, one fiber core is set at the geometric center of the outer cladding (5), and the remaining trench-assisted fiber cores are evenly distributed on a circle with the geometric center of the outer cladding (5) as the center and the core spacing between adjacent trench-assisted fiber cores is Dco. The radius of the highly doped fiber core (2) is r1, the radius of the low-doped fiber core (1) is a=r1 / 2, the radius of the inner cladding (3) is r2, the width of the trench-assisted layer (4) is W, and the radius of the trench-assisted layer (4) is r3=r2+ W; Set the radius r2 of the inner cladding (3) and the width W of the grooved auxiliary layer (4), and denote W = 0, that is, the absolute value of the core group velocity difference without grooved assistance is |ΔNg0|; When W≠0, changing the values ​​of r2 and W will change the group velocity n of the grooved assisted core. g The absolute value of the difference |ΔNg|, the range of values ​​for r2 and W is the parameter range where |ΔNg| < |ΔNg0|, and the parameter selection for r2 and W is the parameter point within this parameter range where |ΔNg| = 0; under different manufacturing errors, |ΔNg| between core modes is relatively large. By combining the parameters of r2 and W, |ΔNg| can be controlled to reduce the group velocity difference between modes, thereby reducing the group time spread width (GDS); the core spacing corresponding to the minimum GDS is the required core spacing Dco.

2. The trench-assisted random-coupled multi-core optical fiber according to claim 1, characterized in that: The outer cladding (5) and the inner cladding (3) are both made of fused silica.

3. The trench-assisted random-coupled multi-core optical fiber according to claim 1, characterized in that: The refractive index of the low-doped fiber core (1) and the high-doped fiber core (2) is higher than that of the outer cladding layer (5) after doping.

4. The trench-assisted random-coupled multi-core optical fiber according to claim 1, characterized in that: The refractive index of the trench auxiliary layer (4) is lower than that of the outer cladding layer (5) after doping.

5. The trench-assisted random-coupled multi-core optical fiber according to claim 1, characterized in that: The radius r1 of the highly doped fiber core (2) is 4.

5. 0.1 μm; By taking different combinations of radius r3 and width W of the trench auxiliary layer (4), the inner cladding layer (3) and the trench auxiliary layer (4) suppress electromagnetic field overlap to different degrees, while reducing the group velocity difference between core modes caused by process fabrication errors, that is, enhancing the tolerance to process fabrication errors; group velocity n g Represented as: Where c is the speed of light in vacuum, β is the propagation constant, and ω is the angular frequency; the group velocity difference of the core modes is expressed as the difference in group velocities between two core modes, denoted as ΔNg, i.e., ΔNg = n g1 -n g2 n g1 and n g2 These are the group velocities for the two core modes, respectively.

6. The trench-assisted random-coupled multi-core optical fiber according to claim 1, characterized in that: The low-doped fiber core (1) and the high-doped fiber core (2) control the normalized frequency of the optical fiber by changing the refractive index of the material through doping, thereby regulating the number of modes that the fiber core can support. At the same time, the presence of the low-doped fiber core (1) can increase the mode field area, thereby enhancing the suppression of nonlinear effects in the optical fiber. The relative refractive index difference between the low-doped fiber core (1) and the outer cladding (5) is Δ2, and the relative refractive index difference between the high-doped fiber core (2) and the outer cladding (5) is Δ1. The relative refractive index difference between the trench auxiliary layer (4) and the outer cladding (5) is Δ tr Where Δ1 = 0.35 0.01%, Δ2=Δ1 / 2, Δ tr = 0.7 0.01%; The number of supermode modes is changed by altering the values ​​of r1 and Δ1, while maintaining the single-mode condition of the grooved core; The mode field area Aeff is increased by increasing a and decreasing Δ2, based on the original Aeff value (a = 0, Δ2 = Δ1), to enhance the suppression of nonlinear effects; The suppression of nonlinear effects is enhanced by decreasing r2 and Δ... tr Increasing the value of W enhances the binding effect on the electromagnetic field.

7. The trench-assisted random-coupled multi-core optical fiber according to claim 6, characterized in that: A peak torsional rate TW = 3 turn / m is used to ensure sufficient random aliasing to optimize the group time delay width (GDS). The core spacing Dco of the adjacent trench-assisted mode has an optimal selection range [D1, D2] at the peak torsional rate TW, where Dco ∈ [D1, D2], D2 > D1 > 2*r3. The optimal selection range [D1, D2] is determined based on coupled-mode theory and using the transfer matrix method. That is, the initial state A(0) of each mode is transformed by the transfer matrix T(ω) and becomes A(L) at position L, i.e., A(L) = T(ω)·A(0), where ω is the angular frequency of the electromagnetic wave. Therefore, the group delay operator GDO(ω) is defined using T(ω): The formula for calculating the group time-span width (GDS) is: Where N is the number of supermodes including polarization states, τ i It is the i-th eigenvalue of the group delay operator GDO(ω); use formula (3) to evaluate the GDS under each core spacing, and the core spacing corresponding to the minimum GDS is the required core spacing Dco; j is the imaginary number symbol.

8. The trench-assisted random-coupled multi-core optical fiber according to any one of claims 1 to 7, characterized in that: The radius of the outer cladding layer (5) is r4 = 62.5 μm.

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