A genetic algorithm-based low-crosstalk few-mode fiber structure optimization design method

Abstract

The application relates to the technical field of optical fiber transmission, and particularly discloses a low-crosstalk few-mode optical fiber structure optimization design method based on a genetic algorithm, which comprises the following steps: determining fixed structure parameters of a few-mode optical fiber, and setting an optimization range of groove structure parameters to be optimized; in the optimization range, N groups of groove structure parameters are randomly generated to form an initial population; for each group of groove structure parameters in the initial population, a mode-to-mode crosstalk value corresponding to the group of groove structure parameters under a preset bending condition is calculated based on a coupled-mode theory; M groups of parameters with high fitness are selected from the current population as excellent individuals according to the crosstalk value; the excellent individuals are subjected to crossover and mutation operations to generate a new generation of population; and groove structure parameters corresponding to the lowest crosstalk value in the population of all generations are output; the application can not only quantify the mapping relationship between shape and crosstalk, but also guide the optical fiber structure to meet the mechanical reliability requirement of a single-mode optical fiber while keeping low crosstalk.

Description

Technical Field

[0001] This invention relates to the field of optical fiber transmission technology, specifically a low-crosstalk few-mode optical fiber structure optimization design method based on genetic algorithms. Background Technology

[0002] Few-mode fiber significantly increases the transmission capacity of a single fiber by transmitting several spatial modes within a single core. However, compared to mature single-mode fiber technology, few-mode fiber still faces a series of severe challenges in moving from the laboratory to large-scale engineering deployment. In engineering deployments, bending is unavoidable and severely impacts the performance of few-mode fiber. In practical deployments, especially in space-constrained scenarios such as metropolitan area networks and data center interconnects, fiber bending is inevitable. When the fiber bends, it disrupts the propagation field distribution of the modes, leading to phase matching between modes and causing strong mode coupling, manifested as a sharp increase in mode crosstalk.

[0003] Currently, the engineering deployment and performance evaluation of few-mode optical fibers still rely on traditional mechanical reliability testing methods designed for single-mode fibers, lacking effective means for the coordinated evaluation of inter-mode crosstalk and bending robustness in few-mode fibers. Existing testing procedures typically focus only on the total loss change caused by macrobending, failing to quantify the impact of bending or stress disturbances on inter-mode coupling and crosstalk. In actual laying or microbending environments, mode energy transfer caused by structural deformation in few-mode fibers is difficult to effectively monitor and predict, leading to system performance fluctuations and even link failures. This not only relies on extensive experimental trial and error to evaluate fiber adaptability, which is inefficient, but also easily overlooks critical mode instability mechanisms. After transmission anomalies occur, the cause can often only be indirectly inferred through post-event mode analysis or OTDR, making reliable pre-deployment prediction difficult.

[0004] Most existing few-mode fiber designs are designed to optimize mode characteristics in a straight-through state, failing to adequately consider performance at specific bending radii. Single-mode fiber has been developed for decades and has mature mechanical reliability testing standards (such as ITU-T G.652), which clearly specifies a 15 mm radius as a typical test condition for simulating the most stringent short-term bending conditions, such as during construction and fiber coiling. Currently, the few-mode fiber field lacks a widely recognized industry-recognized evaluation specification for bending resistance and intermodal crosstalk, corresponding to the mature standard system of single-mode fiber. This standard gap not only leads to a lack of reliable basis for equipment manufacturers and operators in the selection and deployment of few-mode fiber, but also seriously restricts its smooth integration with existing single-mode fiber infrastructure in terms of mechanical compatibility and fusion splicing upgrade paths. Most existing general-purpose few-mode fiber designs have not systematically optimized for the intermodal coupling and crosstalk mechanisms caused by stress-induced micro-bending or macro-bending deformation (scales on the micrometer to millimeter scale) in actual engineering projects, thus resulting in uncertainty in their performance stability and long-term reliability under complex operating conditions.

[0005] Despite the rapid development of space multiplexing and few-mode transmission technologies, few-mode fiber still faces challenges in achieving seamless integration with existing single-mode fiber infrastructure at the engineering level. The core bottleneck lies in the fact that current mechanical performance testing standards do not consider the mode dimension, while existing crosstalk assessment methods are detached from actual mechanical disturbance conditions. Particularly under micro-bending or macro-bending conditions, the critical physical link of "stress-deformation-mode coupling" is not included in a unified evaluation framework. Similarly, current indicators used to evaluate fiber bending resistance (such as a 15mm short-term bending radius) are only applicable to loss criteria for single-mode fiber and cannot reflect the inter-mode crosstalk degradation caused by bending in few-mode fiber. Furthermore, existing crosstalk simulation or laboratory testing methods typically assume ideal static fiber, ignoring the unavoidable micrometer- to millimeter-level random deformations in real-world deployments. Summary of the Invention

[0006] The purpose of this invention is to provide a low-crosstalk few-mode fiber structure optimization design method based on genetic algorithm to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A low-crosstalk few-mode fiber structure optimization design method based on genetic algorithm, the method comprising:

[0009] The fixed structural parameters of the few-mode fiber are determined, and the optimization range of the trench structure parameters to be optimized is set. The fixed structural parameters include the core radius, cladding radius, core refractive index, cladding refractive index, and operating wavelength. The trench structure parameters to be optimized include at least the width and refractive index of the first trench and the width and refractive index of the second trench.

[0010] Within the optimization range, N sets of trench structure parameters are randomly generated to form an initial population;

[0011] For each set of trench structure parameters in the initial population, the inter-mode crosstalk value corresponding to it under the preset bending conditions is calculated based on coupled-mode theory.

[0012] Based on the crosstalk value, select the M group parameters with high fitness from the current population as excellent individuals;

[0013] Crossover and mutation operations are performed on the aforementioned superior individuals to generate a new generation of population;

[0014] Repeat the screening process until the preset convergence condition is met, and output the trench structure parameters corresponding to the lowest crosstalk value in each generation of the population as the optimal fiber structure design parameters.

[0015] As a further embodiment of the present invention, the groove structure is a double groove structure arranged around the fiber core, including a first groove and a second groove arranged sequentially from the inside to the outside.

[0016] The refractive index of the first trench is higher than that of the fiber core, and the refractive index of the second trench is between that of the cladding and the fiber core.

[0017] As a further embodiment of the present invention, the refractive index of the first trench is 2.75% higher than that of the fiber core.

[0018] As a further embodiment of the present invention, the refractive index of the second trench is 0.12% higher than that of the cladding.

[0019] As a further aspect of the present invention, the step of calculating the inter-mode crosstalk value corresponding to each group of trench structure parameters in the population under preset bending conditions based on coupled-mode theory specifically includes:

[0020] For each set of trench structure parameters, the effective refractive index and transverse electric field distribution of each guided mode in the few-mode fiber are calculated by numerical simulation.

[0021] The propagation constant difference between modes is calculated based on the effective refractive index, and the coupling coefficient between modes is calculated based on the transverse electric field distribution.

[0022] Based on the propagation constant difference and the coupling coefficient, the maximum crosstalk value is calculated using a formula and used as the objective function value for evaluating fitness.

[0023] As a further embodiment of the present invention, the maximum crosstalk value is:

[0024] ;

[0025] in, The coupling coefficient is... This represents the difference in transmission constants between the two modes.

[0026] As a further embodiment of the present invention, the fiber core, the first trench, and the second trench are made of a mixture of silicon dioxide matrix and germanium dioxide.

[0027] As a further embodiment of the present invention, the preset bending condition is: the optical fiber is wound around a bending axis with a radius of 15 mm for evaluation.

[0028] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention proposes a mechanical disturbance-intermode crosstalk coupling evaluation method and optimization design framework for few-mode optical fibers, which can effectively characterize and suppress unintended mode coupling induced by fixed structural deformation (such as periodic microbending, local macrobending) under typical engineering bending or stress conditions; it can not only quantify the "deformation-crosstalk" mapping relationship, but also guide the optical fiber structure to meet the mechanical reliability requirements of single-mode optical fibers (such as 15mm bending radius) while maintaining low crosstalk. It is mainly used for the deployment compatibility verification and standardization promotion of few-mode optical fibers in actual communication networks. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention.

[0030] Figure 1 A cross-sectional view of the optical fiber provided in an embodiment of the present invention.

[0031] Figure 2 The refractive index histograms of various parts of the optical fiber provided in the embodiments of the present invention.

[0032] Figure 3 Examples of light spot changes corresponding to different bending directions provided in embodiments of the present invention are shown in (a) for bending along the positive X-axis, (b) for bending along the negative X-axis, (c) for bending along the positive Y-axis, and (d) for bending along the negative Y-axis.

[0033] Figure 4 The following are 15mm bending spot patterns of a common few-mode optical fiber provided in the embodiments of the present invention, wherein (a) is the LP01 mode spot pattern, (b) is the LP11b mode spot pattern, (c) is the LP11a mode spot pattern, (d) is the LP21a mode spot pattern, (e) is the LP21b mode spot pattern, and (f) is the LP02 mode spot pattern.

[0034] Figure 5 The optimized few-mode fiber 15mm bending spot pattern provided in the embodiments of the present invention includes: (a) LP01 mode spot pattern, (b) LP11b mode spot pattern, (c) LP11a mode spot pattern, (d) LP21a mode spot pattern, (e) LP21b mode spot pattern, and (f) LP02 mode spot pattern. Detailed Implementation

[0035] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0036] like Figures 1 to 5 As shown in the embodiment of the present invention, a method for optimizing the design of low crosstalk few-mode fiber structures based on genetic algorithms is provided, the method comprising:

[0037] The fixed structural parameters of the few-mode fiber are determined, and the optimization range of the trench structure parameters to be optimized is set. The fixed structural parameters include the core radius, cladding radius, core refractive index, cladding refractive index, and operating wavelength. The trench structure parameters to be optimized include at least the width and refractive index of the first trench and the width and refractive index of the second trench.

[0038] Within the optimization range, N sets of trench structure parameters are randomly generated to form an initial population;

[0039] For each set of trench structure parameters in the initial population, the inter-mode crosstalk value corresponding to it under the preset bending conditions is calculated based on coupled-mode theory.

[0040] Based on the crosstalk value, select the M group parameters with high fitness from the current population as excellent individuals;

[0041] Crossover and mutation operations are performed on the aforementioned superior individuals to generate a new generation of population;

[0042] Repeat the screening process until the preset convergence condition is met, and output the trench structure parameters corresponding to the lowest crosstalk value in each generation of the population as the optimal fiber structure design parameters.

[0043] In this embodiment, within the two-dimensional parameter space defined by the groove width and refractive index, and within the feasible region allowed by the manufacturing process and physical principles, several sets of parameter combinations are obtained through significant correction of the light spot. A certain number of parameter combinations are then randomly generated according to the characteristics of these parameters to form an initial population. This step aims to provide a sufficiently diverse starting point for the optimization process, ensuring a global exploration of the solution space.

[0044] For each individual in the population, i.e., each set of groove parameters, mode analysis is performed using the finite element method or finite difference method to calculate the effective refractive index of each corresponding guided mode. Then, the propagation constant difference is derived based on mode coupling theory. With coupling coefficient Substituting the above parameters into the crosstalk power ratio formula, the crosstalk value XT at a specified fiber length is calculated. This crosstalk value serves as the objective function value for quantifying the performance of individual components.

[0045] Based on the crosstalk values ​​calculated above, all individuals in the current population are sorted and selected. A fitness-based selection strategy is adopted, prioritizing the retention of individuals with the lowest crosstalk values ​​and best performance, while eliminating individuals with poor performance. This process ensures that excellent genetic traits are preserved to the next generation, guiding the population to evolve towards better performance.

[0046] The evolutionary process is then executed. This process includes two genetic operations: crossover and mutation. The crossover operation pairs and partially swaps the parameters of selected superior individuals to generate new individuals that possess characteristics of their parents. The mutation operation randomly perturbs a parameter of an individual with a certain probability to introduce new genetic traits, maintain population diversity, and prevent the algorithm from getting trapped in local optima prematurely.

[0047] The algorithm iterates and determines convergence. The evaluation, selection, and evolution steps described above are repeated cyclically, constituting a complete iterative generation. As the iterations proceed, the average fitness of the population and the performance of the best individual will continuously improve. When the improvement of the optimal solution across multiple consecutive generations falls below a preset threshold, or when the maximum number of iterations is reached, the algorithm is considered converged, and the groove parameters corresponding to the best-performing individual in each generation are output as the final optimization result.

[0048] To address the issue of optical cable bending and beam axis deviation leading to beam offset, a trench structure is employed to mitigate the degree of offset. Crosstalk is calculated based on the effective refractive index of different modes after trench insertion and the structural parameters of the optical fiber. Crosstalk is calculated using coupled-mode theory. To avoid local optima and find the global optimum in the optimization of trench parameter combinations, a genetic algorithm is used for optimization. This algorithm first randomly generates an initial population within the feasible region, then iteratively performs the following steps: evaluating the crosstalk value of each individual as fitness; selecting superior individuals based on fitness; and generating a new population through crossover and mutation operations to maintain diversity and explore new solutions. After iteration until the convergence condition is met, the optimal trench parameter combinations from each generation are output as the final result.

[0049] In a preferred embodiment of the present invention, the groove structure is a double groove structure arranged around the fiber core, including a first groove and a second groove arranged sequentially from the inside to the outside.

[0050] The refractive index of the first trench is higher than that of the fiber core, and the refractive index of the second trench is between that of the cladding and the fiber core.

[0051] The refractive index of the first groove is 2.75% higher than that of the fiber core.

[0052] The refractive index of the second trench is 0.12% higher than that of the cladding.

[0053] The fiber core, the first trench, and the second trench are made of a mixture of silicon dioxide matrix and germanium dioxide.

[0054] In this embodiment, the fiber core is made by doping germanium dioxide (GeO2) into a silica matrix, thereby increasing its refractive index from the baseline value of 1.444 for pure quartz to 1.45365. This increase in refractive index relative to pure quartz is approximately 0.67%.

[0055] First trench (high refractive index ring): In the first trench region, a ring structure with a refractive index of 1.49365 is formed by doping silicon dioxide with a higher concentration of germanium dioxide than that in the fiber core. This refractive index is not only significantly higher than that of the outer cladding, but also 2.75% higher than that of the adjacent fiber core, forming a strong anti-resonant structure to enhance the confinement of the fiber core's guided mode.

[0056] Second trench (shallow barrier layer): In the second trench region, the refractive index of silicon dioxide is precisely controlled to 1.44582 by doping it with trace amounts of germanium dioxide. This value is only about 0.12% higher than that of pure quartz, forming a weak refractive index barrier to help adjust the mode field distribution.

[0057] Outer cladding: The outer cladding material is pure silicon dioxide, or silicon dioxide doped with trace amounts of germanium dioxide to compensate for process fluctuations. Its refractive index is stabilized at 1.44482, serving as a reference for the entire fiber refractive index structure.

[0058] The core radius of the optical fiber is r1=r core =9.2um, cladding radius r4=r clad =62.5um, fiber core refractive index n2=n core =1.45365, cladding refractive index n4=n clad =1.44482.

[0059] The width of the first trench is w1 = 3.5 μm; the width of the second trench is w2 = 3.5 μm.

[0060] The refractive index of the first trench is n1 = n2 + trench_d1 = 1.45356 + 0.04, where trench_d1 is the difference in refractive index between the first trench and the fiber core.

[0061] The refractive index of the second trench is n3 = n4 + trench_d2 = 1.44482 + 0.001, where trench_d2 is the difference in refractive index between the second trench and the cladding.

[0062] As a preferred embodiment of the present invention, the step of calculating the inter-mode crosstalk value corresponding to each group of trench structure parameters in the population under preset bending conditions based on coupled-mode theory specifically includes:

[0063] For each set of trench structure parameters, the effective refractive index and transverse electric field distribution of each guided mode in the few-mode fiber are calculated by numerical simulation.

[0064] The propagation constant difference between modes is calculated based on the effective refractive index, and the coupling coefficient between modes is calculated based on the transverse electric field distribution.

[0065] Based on the propagation constant difference and the coupling coefficient, the maximum crosstalk value is calculated using a formula and used as the objective function value for evaluating fitness.

[0066] In this embodiment, the derivation process of calculating Pcross using coupled-mode theory crosstalk is as follows:

[0067] ;

[0068] ;

[0069] In the formula For pattern Pattern The intensity of the impact, Describes the pattern For pattern The intensity of the impact, and Let be the propagation constants for modes i and j.

[0070] From the power conservation condition, we can obtain ,in and These represent the power carried by the two modes, respectively.

[0071] Solving the mode coupling equations yields:

[0072] ;

[0073] ;

[0074] when When z is small, A(z) is maximum and B(z) is very small at the coupling length L. When the phase ratio is 0, the optical power is completely switched from mode B to mode A, at which point the phase matching condition is met. In the calculation, it is assumed that the transmission length L is 10 km.

[0075] Based on coupled-mode theory, the general formula for crosstalk is:

[0076] ;

[0077] in, Coupling coefficient:

[0078] The unit is m -1 .

[0079] In the formula, This represents the phase change per unit length experienced by a wave propagating in a vacuum (or free space). The few-mode light in this design propagates at a wavelength of 1550 nm, therefore the wavelength in a vacuum is used. .

[0080] Relative refractive index difference:

[0081] ;

[0082] S is the overlap integral factor, a function related to the power spectral density of the bending perturbation, expressed as:

[0083] ;

[0084] in, and The transverse electric field distributions for modes i and j are shown below. and Here are the propagation constants for modes i and j:

[0085] , ;

[0086] in, and Let be the effective refractive index corresponding to mode i and mode j.

[0087] ;

[0088] in, The difference between the transmission constants of the two modes is called phase mismatch. The larger the mismatch, the more difficult it is for coupling to occur.

[0089] In a preferred embodiment of the present invention, the maximum crosstalk is taken in subsequent calculations, that is, when the sine term is 1, the maximum crosstalk value is:

[0090] ;

[0091] ;

[0092] in, The coupling coefficient is... This represents the difference in transmission constants between the two modes.

[0093] When optical fibers are bent, there are multiple directions. In three-dimensional space, let the Z-axis be the transmission direction. In this design, the bending direction is defined as four directions: positive X-axis, negative X-axis, positive Y-axis, and negative Y-axis.

[0094] Figure 4 To achieve the desired result in the software COMSOL according to the aforementioned parameter r1=r core =9.2um, r4=r clad =62.5um, n2=n core =1.45365, n4=n clad =1.44482 The six-mode spot pattern of a common few-mode fiber simulated by, from Figure 4 It can be seen that when the few-mode fiber is bent by 15mm, the central axis of the light spot will deviate significantly, and the energy of the light spot will overflow from the fiber core.

[0095] The crosstalk matrix calculated according to coupled-mode theory in the software MATLAB is shown in Table 1.

[0096] Table 1. Crosstalk parameters of the original few-mode fiber after micro-bending disturbance.

[0097]

[0098] In few-mode fiber (FMF), different spatial modes (such as LP01, LP11, etc.) should propagate independently. However, due to factors such as bending, manufacturing defects, and stress, coupling can occur between modes, causing some of the energy from one mode to be transferred to other modes—this is intermode crosstalk. The larger the negative value (such as -30dB), the smaller the crosstalk and the better the isolation.

[0099] The crosstalk matrix clearly shows that the few-mode fiber is bent, resulting in large crosstalk, which does not meet the requirements for metropolitan area network transmission (≤20dBm).

[0100] Figure 5 To obtain six pattern spot images from the simulation of the optimized structure obtained by adding the groove parameter combination structure obtained by the algorithm based on the aforementioned parameters in the software COMSOL: the width of the first groove is 3.5um and the refractive index is 1.45756; the width of the second groove is 3.5um and the refractive index is 1.44492.

[0101] In the software MATLAB, the crosstalk matrix of the optimized structure calculated according to the coupled mode theory is shown in Table 2.

[0102] Table 2 Crosstalk parameters of optimized few-mode fiber after micro-bending disturbance

[0103]

[0104] Based on the optimized crosstalk matrix and spot pattern, it is easy to see that the central axis of the optimized fiber spot offset is corrected and the crosstalk is significantly reduced.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-crosstalk few-mode fiber structure optimization design method based on genetic algorithm, characterized in that, The method includes: The fixed structural parameters of the few-mode fiber are determined, and the optimization range of the trench structure parameters to be optimized is set. The fixed structural parameters include the core radius, cladding radius, core refractive index, cladding refractive index, and operating wavelength. The trench structure parameters to be optimized include at least the width and refractive index of the first trench and the width and refractive index of the second trench. Within the optimization range, N sets of trench structure parameters are randomly generated to form an initial population; For each set of trench structure parameters in the initial population, based on coupled-mode theory, the inter-mode crosstalk value corresponding to it under a preset bending condition is calculated. The preset bending condition is: the optical fiber is wound around a bending axis with a radius of 15 mm for evaluation. Based on the crosstalk value, select the M group parameters with high fitness from the current population as excellent individuals; Crossover and mutation operations are performed on the aforementioned superior individuals to generate a new generation of population; Repeat the screening steps until the preset convergence condition is met, and output the trench structure parameters corresponding to the lowest crosstalk value in each generation of the population as the optimal fiber structure design parameters. The groove structure is a double groove structure arranged around the fiber core, including a first groove and a second groove arranged sequentially from the inside to the outside. Wherein, the refractive index of the first trench is higher than that of the fiber core, and the refractive index of the second trench is between that of the cladding and the fiber core. The step of calculating the inter-mode crosstalk value under preset bending conditions for each group of trench structure parameters in the population, based on coupled-mode theory, specifically includes: For each set of trench structure parameters, the effective refractive index and transverse electric field distribution of each guided mode in the few-mode fiber are calculated by numerical simulation. The propagation constant difference between modes is calculated based on the effective refractive index, and the coupling coefficient between modes is calculated based on the transverse electric field distribution. Based on the propagation constant difference and the coupling coefficient, the maximum crosstalk value is calculated by a formula and used as the objective function value for evaluating fitness. The maximum crosstalk value is: ； in, The coupling coefficient is... This represents the difference in transmission constants between the two modes.

2. The method for optimizing the design of low crosstalk few-mode fiber structures based on genetic algorithms according to claim 1, characterized in that, The refractive index of the first groove is 2.75% higher than that of the fiber core.

3. The method for optimizing the design of low-crosstalk few-mode fiber structures based on genetic algorithms according to claim 2, characterized in that, The refractive index of the second trench is 0.12% higher than that of the cladding.

4. The method for optimizing the design of low-crosstalk few-mode fiber structures based on genetic algorithms according to claim 1, characterized in that, The fiber core, the first trench, and the second trench are made of a mixture of silicon dioxide matrix and germanium dioxide.

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