WO2024055360A1

WO2024055360A1 - Multi-core few-mode fiber inter-mode crosstalk measurement method and apparatus

Info

Publication number: WO2024055360A1
Application number: PCT/CN2022/122044
Authority: WO
Inventors: 向练; 芮昊宇
Original assignee: 苏州大学
Priority date: 2022-09-16
Filing date: 2022-09-28
Publication date: 2024-03-21
Also published as: CN115455355A; CN115455355B

Abstract

The present invention relates to the technical field of optical fibers, in particular to a multi-core few-mode fiber inter-mode crosstalk measurement method and apparatus, and a computer storage medium. The multi-core few-mode fiber inter-mode crosstalk measurement method comprises: first calculating the electric field distribution in each mode according to optical fiber parameters; then calculating an inter-mode coupling coefficient according to the electric field distribution and injection power; and finally re-deriving a calculation way of an inter-supermode crosstalk quantity on the basis of the coupled mode theory. Compared with the Monte Carlo model and the ISMXT analysis model, the method has the most prominent advantage of taking into account the mode coupling coefficient varying with the distance, which further ensures the calculation accuracy and eliminates the redundancy caused by polarization mode coupling, thereby getting closer to actual optical fiber laying conditions and achieving a wider application range.

Description

A method and device for detecting inter-mode crosstalk in multi-core, few-mode optical fiber

Technical field

The invention relates to the field of optical fiber technology, and in particular, to a method, device and computer storage medium for inter-mode crosstalk detection of multi-core, few-mode optical fiber.

Background technique

As Internet data traffic continues to grow exponentially, the information backbone network based on single-mode fiber (Single-Mode Fiber, SMF) transmission is rapidly approaching its capacity limit. In the past, capacity increases in SMF transmission systems have been achieved by leveraging various dimensions, including polarization and wavelength division multiplexing, as well as advanced modulation formats and coherent transmission techniques. However, the coming capacity crunch means we need new transmission technologies to support society's capacity needs. To alleviate the consequential costs and increased energy requirements associated with linear capacity expansion using additional SMFs, Space Division Multiplexing (SDM) within a single fiber can provide a solution. By introducing additional orthogonal multiplexing dimensions, capacity, spectrum and energy efficiency can be reduced, thereby reducing cost per bit, which is critical to sustaining the business models of key network stakeholders. To realize the promise of sustainable development management, a new paradigm is envisioned, which increases the capacity of SMFs by two orders of magnitude. SDM is implemented through Multiple-input Multiple-output (MIMO) transmission, using the spatial mode of Multi-mode Fiber (MMF) or multiple single-mode fiber cores as channels. In recent years, research has developed a unique MMF, namely few-mode fiber (Few-mode Fiber, FMF), which is used to transmit a total of three or six linear polarization (LP) modes. Driven by the rapid enhancement of high-speed electronic technology, Digital Signal Processing (DSP) MIMO technology can quickly restore mixed transmission channels, thereby improving spectral efficiency when spatial channels occupy the same wavelength. State-of-the-art single-carrier FMF transmission experiments show that by utilizing six spatial modes, the capacity in a single fiber is increased to a spectral efficiency of 32bit/(s·Hz). If multi-core transmission is used, some research has used 12 single-mode fiber cores to achieve a transmission with a spectral efficiency of 109 bit/(s·Hz). In another work, there is research that combines the characteristics of multi-core and few-mode, using 7-core 3-mode few-mode multi-core fiber (Few-mode Multicore Fiber, FM-MCF), and achieving 255Tbit in 1km transmission /s maximum rate.

Therefore, FM-MCF has become a hot topic of research in recent years. At the same time, although FM-MCF can bring ultra-high transmission capacity, it still has two important problems that will affect transmission performance. One is that different modes have different transmission paths in the fiber core, which will lead to time differences in each signal received at the receiving end, eventually resulting in bit errors. This is called modal dispersion. The second reason is that coupling between modes occurs between different paths, thereby reducing transmission performance. There are currently a large number of studies showing that modal dispersion can be alleviated through DSP technology or the introduction of dispersion-compensating optical fibers, but the evaluation of mode coupling is not yet complete. Therefore, accurate evaluation of mode coupling facilitates the preparation of multi-core few-mode fibers.

In the Monte Carlo model, the author uses a mode electric field with polarization mode coupling effects, which further increases the computational complexity. And when dealing with the coupling coefficient, it is regarded as a constant that has nothing to do with the transmission distance, which is not consistent with the actual transmission situation. In addition, in the theory proposed by the ISMXT model, it is proved that the crosstalk between supermodes caused by polarization mode coupling can be ignored. Therefore, when calculating the crosstalk between supermodes, the theory in the Monte Carlo model can be further simplified. . In the ISMXT analytical model, the author only considered the impact of longitudinal perturbation, eliminated some redundant terms, and calculated inter-mode crosstalk directly from the mode power. From the final result, the polarization mode coupling band in the Monte Carlo model The impact is indeed negligible. However, the ISMXT analytical model approximates the most important mode coupling coefficient, and the inter-mode crosstalk calculated from general empirical values is obviously not rigorous enough.

Contents of the invention

To this end, the technical problem to be solved by the present invention is to overcome the problem in the prior art that the processing of mode coupling coefficients is inconsistent with the actual situation.

In order to solve the above technical problems, the present invention provides a multi-core few-mode optical fiber inter-mode crosstalk detection method, which includes:

Obtain fiber parameters and injection power;

Calculate the electric field intensity within the core range and the electric field intensity within the cladding range according to the optical fiber parameters;

Calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range and the injected power;

The inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory is replaced with the inter-mode coupling mode coefficient, and the optical fiber inter-mode crosstalk value is calculated.

Preferably, the fiber parameters include:

Core refractive index, cladding refractive index, core radius, core propagation constant, core spacing and optical wavelength.

Preferably, the calculation of the electric field intensity within the core range and the electric field intensity within the cladding range based on the optical fiber parameters includes:

Use the Helmholtz equation to calculate the amplitude of the longitudinal electric field within the fiber core and within the cladding:

Among them, E _z1 and E _z2 respectively represent the amplitude of the longitudinal electric field within the fiber core and the amplitude of the longitudinal electric field within the cladding. A=jUC/βa, j is an imaginary unit, and C is a system constant.

n ₁ is the core refractive index, β is the core propagation constant, a is the core radius, k = 2π/λ is the wave number, λ represents the light wavelength, J _f (x) represents the first kind of Bessel function of order f , K _f (x) represents the second kind of modified Bessel function of order f,

n ₂ is the refractive index of the cladding, r represents the radial direction in the cylindrical coordinate system, and θ represents the circumferential direction in the cylindrical coordinate system;

Substituting the amplitude of the longitudinal electric field within the core range and the cladding range into Maxwell's equation, the amplitude of the transverse electric field within the core range and the cladding range is obtained:

Among them, E _r1 and E _r2 respectively represent the amplitude of the transverse electric field within the core range and the amplitude of the transverse electric field within the cladding range, and J' _f (x) represents the first derivative of J _f (x);

According to the amplitude of the longitudinal electric field within the core and the amplitude of the transverse electric field within the core, the electric field strength within the core is calculated:

Calculate the electric field strength within the cladding range based on the amplitude of the longitudinal electric field within the cladding range and the amplitude of the transverse electric field within the cladding range:

Among them, _Em and _En represent the electric field intensity within the core range and the cladding range respectively, and the amplitude of the electric field in the circumferential direction

e _r represents the unit vector in the r direction,

express

The unit vector in the direction, e _z represents the unit vector in the z direction.

Preferably, before calculating the inter-mode coupling mode coefficient according to the electric field intensity within the core range, the electric field intensity within the cladding range and the injection power, the method includes:

In the electric field intensity within the cladding range, the single-core transverse polar coordinate system (r, θ) is expanded to the dual-core transverse polar coordinate system (R, Θ),

D represents the core distance.

Preferably, the inter-mode coupling mode coefficient is calculated based on the electric field intensity within the core range, the electric field intensity within the cladding range and the injection power:

Calculate the double integral of the electric field strength within the fiber core, the electric field strength within the cladding and the difference between the core refractive index and the cladding refractive index, the angular frequency and the vacuum dielectric constant, and then divide by 4 times Injection power, the inter-mode coupling mode coefficient is obtained:

Among them, ω is the angular frequency, ε ₀ is the vacuum dielectric constant, E _m and H _m represent the electric field intensity and magnetic field intensity in the core range respectively, and E _n represents the electric field intensity in the cladding range, and e _z represents The unit vector in the z direction, n ₁ and n ₂ represent the refractive index of the fiber core and the refractive index of the cladding respectively, the symbol * represents the conjugate operation of the matrix, P represents the injected power,

Preferably, the inter-core crosstalk calculation equation based on coupled mode theory is:

Among them, N is the number of segments after segmenting the optical fiber, d is the segment length after segmenting the optical fiber, Δβ _eq,mn,i is the equivalent phase mismatch of the i-th segment,

is the modified coupling mode coefficient between fiber cores in the i-th section, k _i (d) is the coupling mode coefficient between fiber cores.

Preferably, the inter-core coupling mode coefficient k _i (d) is replaced by the inter-mode coupling mode coefficient k _mn,i , and the optical fiber inter-mode crosstalk value is calculated:

Among them, g _mn,i is the modified inter-mode coupling mode coefficient of the i-th section.

The invention also provides a multi-core few-mode optical fiber inter-mode crosstalk detection device, which includes:

Fiber parameter and injection power acquisition module, used to obtain fiber parameters and injection power;

An electric field strength calculation module, used to calculate the electric field strength within the core range and the electric field strength within the cladding range based on the optical fiber parameters;

An inter-mode coupling mode coefficient calculation module, used to calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range, and the injection power;

An optical fiber inter-mode crosstalk value calculation module is used to replace the inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory with the inter-mode coupling mode coefficient, and calculate the optical fiber inter-mode crosstalk value.

Preferably, the multi-core few-mode optical fiber inter-mode crosstalk detection device is applied to heterogeneous or homogeneous optical fibers.

The present invention also provides a computer-readable storage medium. A computer program is stored on the computer-readable storage medium. When the computer program is executed by a processor, the above-mentioned method for detecting inter-mode crosstalk in multi-core, low-mode optical fiber is implemented. step.

The above technical solution of the present invention has the following advantages compared with the existing technology:

The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to the present invention first calculates the electric field distribution of each mode according to the fiber parameters, then calculates the inter-mode coupling coefficient according to the electric field distribution and injection power, and finally re-derives based on the coupled mode theory Compared with the Monte Carlo model and the ISMXT analytical model, the most prominent advantage of this method for calculating the amount of crosstalk between supermodes is that it takes into account the mode coupling coefficient that changes with distance, which further ensures the accuracy of the calculation and eliminates The redundancy brought by polarization mode coupling is more in line with the actual fiber laying situation and has a wider application range.

Description of drawings

In order to make the content of the present invention easier to understand clearly, the present invention will be further described in detail below based on specific embodiments of the present invention and in conjunction with the accompanying drawings, wherein:

Figure 1 is a flow chart for the implementation of the inter-mode crosstalk detection method for multi-core few-mode optical fibers according to the present invention;

Figure 2 is a schematic diagram of the longitudinal change of inter-mode crosstalk;

Figure 3 is a schematic diagram of the dual-core polar coordinate system;

Figure 4 is a schematic diagram of the crosstalk estimation model based on coupled mode theory;

Figure 5 is a crosstalk comparison flow chart;

Figure 6 shows the lateral electric field distribution of three modes: LP01, LP11a and LP11b;

Figure 7 is a schematic diagram of the transverse structure of a dual-core optical fiber;

Figure 8 is a schematic diagram of nonlinear crosstalk changing with power;

Figure 9 is a schematic diagram of the change of mode coupling coefficient with core spacing;

Figure 10 is a schematic diagram of the inter-mode coupling coefficient changing with the core diameter ratio;

Figure 11 is a schematic diagram of the inter-mode coupling coefficient changing with the refractive index difference;

Figure 12 is a structural block diagram of a multi-core few-mode optical fiber inter-mode crosstalk detection device provided by an embodiment of the present invention.

Detailed ways

The core of the present invention is to provide a method, device and computer storage medium for inter-mode crosstalk detection of multi-core, few-mode optical fibers, which take into account the mode coupling coefficient changing with distance and are more in line with the actual optical fiber laying situation.

In order to enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

Please refer to Figure 1, which is a flow chart of the implementation of the inter-mode crosstalk detection method for multi-core few-mode optical fibers provided by the present invention; the specific operation steps are as follows:

The principle of step-type multi-core few-mode optical fiber is to change the angle of light incident on the fiber core, so that each mode is transmitted along a different fold line trajectory in a single fiber core. The specific schematic diagram is shown in Figure 2;

Since crosstalk between modes will occur very frequently, we can adopt the segmentation idea, assuming that crosstalk will only occur once in a small segment, and the mode coupling coefficient can be regarded as a constant in each small segment.

S101: Obtain fiber parameters and injection power;

The optical fiber parameters include core refractive index, cladding refractive index, core radius, core propagation constant, core spacing and optical wavelength.

S102: Calculate the electric field intensity within the core range and the electric field intensity within the cladding range according to the optical fiber parameters;

Since the optical fiber is a cylindrical structure, the mode electric field can be generated into a cylindrical polar coordinate form, so that the electric field vector can be expressed as a superposition of three dimensions:

Calculate the electric field strength within the fiber core according to the amplitude of the longitudinal electric field within the fiber core and the amplitude of the transverse electric field within the fiber core:

e _r represents the unit vector in the r direction,

express

S103: Calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range, and the injection power;

The mode coupling coefficient is closely related to the electric field distribution of each mode;

As shown in Figure 3, in order to consider the mode coupling between the two cores, the single-core transverse polar coordinate system (r, θ) in the electric field intensity within the cladding range is expanded to the dual-core transverse polar coordinate system (R ,Θ),

D represents the core spacing;

The final result is obtained by numerical solution.

S104: Replace the inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory with the inter-mode coupling mode coefficient, and calculate the optical fiber inter-mode crosstalk value.

The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to the present invention first calculates the electric field distribution of each mode according to the fiber parameters, then calculates the inter-mode coupling coefficient according to the electric field distribution and injection power, and finally re-derives based on the coupled mode theory Compared with the Monte Carlo model and the ISMXT analytical model, the most prominent advantage of this method for calculating the amount of crosstalk between supermodes is that it takes into account the mode coupling coefficient that changes with distance, which further ensures the accuracy of the calculation and eliminates The redundancy brought by polarization mode coupling is more in line with the actual fiber laying situation and has a wider application range. On this basis, we can study the crosstalk characteristics in different communication systems. Based on the transmission characteristics of crosstalk in different working areas, the physical geometric characteristics of the fiber itself and the relationship between crosstalk and fiber parameters, we can then study ways to reduce crosstalk between different transmission modes. Theoretical methods.

Based on the above embodiment, this embodiment further explains step S104:

This invention calculates inter-mode crosstalk based on coupled mode theory. First, it is necessary to introduce the principles of coupled mode theory and important solution methods:

The coupled mode equation is:

Among them, A _n and A _m are the slowly changing electric field complex amplitudes of core n and core m respectively, N is the number of fiber cores, and K _mn is the coupling coefficient from core n to core m. β _m and β _n are the propagation constants of m and n cores respectively, j is the imaginary unit, and z represents the longitudinal propagation direction. In order to solve linear crosstalk based on coupled mode theory, a general semi-analytic mathematical model is proposed, which accurately solves the linear coupled mode equation according to the distribution of phase matching points (Phase Machining Points, PMP). The principle is shown in Figure 4, where d is the segment length after segmenting the optical fiber, P ₀ represents the injection power, P _n and P _m represent the output power of n core and m core respectively. Based on the above segmented model, we can regard equation (1) as two component forms:

where Δβ _eq,mn,i is the equivalent phase mismatch of the i-th segment, which can be expressed as Δβ _eq,mn,i =β _eq,m,i -β _eq,n,i ; K _mn,i (z) is The coupling mode coefficient of the i-th section between core m and core n. When the segment length is small enough, K _mn,i (z) can be regarded as a constant K _mn,i in the i-th section. When the optical power is transmitted in the fiber core n, the analytical solution of the electric field of the fiber core m and n at the end of each section can be obtained by solving equation (2) as:

where T is the coefficient matrix of the solution, which can be expressed as:

in the formula

is the modified coupling mode coefficient of the i-th section, and it is assumed that the coupling mode coefficient between fiber cores k _i (d) = K _mn,i (d) = K _nm,i (d). In this way, the electric field amplitude of each segment can be obtained from the value of the previous segment, and the change of crosstalk in each segment can also be obtained. By superimposing them, the total crosstalk value can be calculated:

We only need to replace k _i (d) with the coupling coefficient k _mn,i between modes, and then equation (5) can be changed into the crosstalk value formula between modes:

The invention can provide a fast and accurate crosstalk estimation calculation method for multi-core and few-mode optical fiber communication systems, and its application range is wider. This method is not only applicable to heterogeneous optical fibers, but also to real homogeneous optical fibers and ideal completely homogeneous optical fibers. This method is based on the coupled mode theory that reflects the physical characteristics of optical fiber transmission, and re-derives the inter-mode coupling coefficient based on the random disturbances present in the fiber core. Therefore, it can more accurately reflect the changing process of inter-mode crosstalk than previous models. Based on this model, we can have a more forward-looking understanding of inter-mode crosstalk in few-mode multi-core optical fibers.

As shown in Figure 5, based on the above embodiment, this embodiment verifies the accuracy of the above method through simulation, as follows:

The transverse electric field distributions of the three modes LP01, LP11a and LP11b are shown in Figure 6, where LP01 is the transmission fundamental mode, and LP11a and LP11b are two mutually orthogonal polarization modes. Figure 7 shows the lateral arrangement of the two fiber cores;

Consider a dual-core three-mode fiber with core radius a = 2.5 μm, cladding refractive index n ₂ = 1.45, core refractive index approximately n ₁ = 1.4545, core spacing D _nm = 30 μm, and optical pulse wavelength 1550 nm. First, compare the differences between the three crosstalk calculation methods when the transmission distance is z=10m. The comparison results are shown in Figure 8. Figure 8 shows the changes in ISMXT as the transmission distance increases. From the overall change trend, the present invention is basically consistent with the MonteCarlo model and the ISMXT analytical model. As the transmission distance gradually becomes longer, the present invention gradually maintains a gap of about 3dB with the other two. This is mainly due to random disturbance factors. The influence of the model begins to be reflected, and the mode coupling coefficient calculated by the other two methods is an approximate empirical value and cannot accurately estimate the impact of random disturbances. The inter-mode crosstalk results of the present invention have increased volatility. This is because the phase matching points between each mode are randomly distributed with distance, and the point with fluctuation indicates the occurrence of a phase matching point.

Figure 9 shows the process as the core spacing changes. The core radius a = 2.5 μm. As the core spacing gradually increases, the inter-mode coupling coefficient gradually decreases, which shows that appropriately increasing the core spacing is one of the methods to effectively reduce inter-mode crosstalk. In addition, Figure 9 considers three cases where the intrinsic effective refractive index difference is 0.21%, 0.31% and 0.41% respectively. It can be found that as the refractive index difference becomes larger, the inter-mode coupling coefficient will gradually decrease, and in the core When the spacing is small, the reduction is more obvious.

We also performed simulations of other physical properties based on the derived model. Figures 10 and 11 show the changes with core diameter ratio (ratio of core spacing to core radius) and intrinsic effective refractive index difference, respectively. As shown in Figure 10, overall, an increase in the core diameter ratio will make the inter-mode coupling coefficient smaller. However, when the refractive index difference is large, this effect is not obvious. On the contrary, when the refractive index difference is small, The effect is more obvious in this case, which shows that the refractive index difference plays a more important role. In Figure 11, we set the abscissa to the refractive index difference, and finally found that no matter whether the value of the core diameter ratio is large or small, the increase in the refractive index difference will significantly reduce the mode coupling coefficient. Based on the above conclusion, we found that physical parameters such as core spacing, core diameter ratio and effective refractive index difference will affect the mode coupling coefficient, thereby affecting the changes in inter-mode crosstalk. Based on the above conclusions, we can further reduce inter-mode crosstalk by making multi-core few-mode optical fibers with suitable physical properties.

Please refer to Figure 12. Figure 12 is a structural block diagram of a multi-core few-mode optical fiber inter-mode crosstalk detection device provided by an embodiment of the present invention; the specific device may include:

Optical fiber parameter and injection power acquisition module 100, used to acquire optical fiber parameters and injection power;

The electric field strength calculation module 200 is used to calculate the electric field strength within the core range and the electric field strength within the cladding range according to the optical fiber parameters;

The inter-mode coupling mode coefficient calculation module 300 is used to calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range, and the injection power;

The optical fiber inter-mode crosstalk value calculation module 400 is used to replace the inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory with the inter-mode coupling mode coefficient, and calculate the optical fiber inter-mode crosstalk value.

The inter-mode crosstalk detection device for multi-core and few-mode optical fiber in this embodiment is used to implement the aforementioned inter-mode crosstalk detection method for multi-core and few-mode optical fiber. Therefore, the specific implementation of the multi-core and few-mode optical fiber inter-mode crosstalk detection device can be found in the multi-core and few-mode optical fiber mentioned above. The embodiment part of the few-mode optical fiber inter-mode crosstalk detection method, for example, the optical fiber parameter and injection power acquisition module 100, the electric field strength calculation module 200, the inter-mode coupling mode coefficient calculation module 300, and the optical fiber inter-mode crosstalk value calculation module 400 are respectively used. In order to implement steps S101, S102, S103, and S104 in the inter-mode crosstalk detection method for multi-core few-mode optical fibers, the specific implementation methods can be referred to the descriptions of the corresponding embodiments, and will not be described again here.

Specific embodiments of the present invention also provide a computer-readable storage medium. A computer program is stored on the computer-readable storage medium. When the computer program is executed by a processor, the above-mentioned multi-core few-mode optical fiber inter-mode crosstalk is realized. Steps of the detection method.

Those skilled in the art will understand that embodiments of the present application may be provided as methods, systems, or computer program products. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment that combines software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk memory, CD-ROM, optical storage, etc.) having computer-usable program code embodied therein.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each process and/or block in the flowchart illustrations and/or block diagrams, and combinations of processes and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing device to produce a machine, such that the instructions executed by the processor of the computer or other programmable data processing device produce a use A device for realizing the functions specified in one process or multiple processes of the flowchart and/or one block or multiple blocks of the block diagram.

These computer program instructions may also be stored in a computer-readable memory that causes a computer or other programmable data processing apparatus to operate in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including the instruction means, the instructions The device implements the functions specified in a process or processes of the flowchart and/or a block or blocks of the block diagram.

These computer program instructions may also be loaded onto a computer or other programmable data processing device, causing a series of operating steps to be performed on the computer or other programmable device to produce computer-implemented processing, thereby executing on the computer or other programmable device. Instructions provide steps for implementing the functions specified in a process or processes of a flowchart diagram and/or a block or blocks of a block diagram.

Obviously, the above-mentioned embodiments are only examples for clear explanation and are not intended to limit the implementation. For those of ordinary skill in the art, other changes or modifications may be made based on the above description. An exhaustive list of all implementations is neither necessary nor possible. The obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

A method for detecting inter-mode crosstalk in multi-core, few-mode optical fiber, which is characterized by including:

Obtain fiber parameters and injection power;

Calculate the electric field intensity within the core range and the electric field intensity within the cladding range according to the optical fiber parameters;

Calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range and the injected power;

The inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory is replaced with the inter-mode coupling mode coefficient, and the optical fiber inter-mode crosstalk value is calculated.
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 1, characterized in that the optical fiber parameters include:

Core refractive index, cladding refractive index, core radius, core propagation constant, core spacing, and wavelength of light.
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 1, wherein the calculation of the electric field intensity within the fiber core range and the electric field intensity within the cladding range based on the fiber parameters includes:

Use the Helmholtz equation to calculate the amplitude of the longitudinal electric field within the fiber core and within the cladding:

Among them, E z1 and E z2 respectively represent the amplitude of the longitudinal electric field within the fiber core and the amplitude of the longitudinal electric field within the cladding. A=jUC/βa, j is an imaginary unit, and C is a system constant.
n 1 is the core refractive index, β is the core propagation constant, a is the core radius, k = 2π/λ is the wave number, λ represents the light wavelength, J f (x) represents the first kind of Bessel function of order f , K f (x) represents the second kind of modified Bessel function of order f,
n 2 is the refractive index of the cladding, r represents the radial direction in the cylindrical coordinate system, and θ represents the circumferential direction in the cylindrical coordinate system;

Substituting the amplitude of the longitudinal electric field within the core range and the cladding range into Maxwell's equation, the amplitude of the transverse electric field within the core range and the cladding range is obtained:

Among them, E r1 and E r2 respectively represent the amplitude of the transverse electric field within the core range and the amplitude of the transverse electric field within the cladding range, and J′ f (x) represents the first derivative of J f (x);

Calculate the electric field strength within the fiber core according to the amplitude of the longitudinal electric field within the fiber core and the amplitude of the transverse electric field within the fiber core:

Calculate the electric field strength within the cladding range based on the amplitude of the longitudinal electric field within the cladding range and the amplitude of the transverse electric field within the cladding range:

Among them, Em and En represent the electric field intensity within the core range and the cladding range respectively, and the amplitude of the electric field in the circumferential direction
e r represents the unit vector in the r direction,
express
The unit vector in the direction, e z represents the unit vector in the z direction.
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 3, characterized in that the calculation is based on the electric field intensity within the fiber core range, the electric field intensity within the cladding range and the injection power. The inter-mode coupling mode coefficients include:

In the electric field intensity within the cladding range, the single-core transverse polar coordinate system (r, θ) is expanded to the dual-core transverse polar coordinate system (R, Θ),
D represents the core distance.
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 4, wherein the inter-mode crosstalk is calculated based on the electric field intensity within the fiber core range, the electric field intensity within the cladding range and the injection power. Coupling mode coefficients include:

Calculate the double integral of the electric field strength within the fiber core, the electric field strength within the cladding and the difference between the core refractive index and the cladding refractive index, the angular frequency and the vacuum dielectric constant, and then divide by 4 times Injection power, the inter-mode coupling mode coefficient is obtained:

Among them, ω is the angular frequency, ε 0 is the vacuum dielectric constant, E m and H m represent the electric field intensity and magnetic field intensity in the core range respectively, and E n represents the electric field intensity in the cladding range, and e z represents The unit vector in the z direction, n 1 and n 2 represent the refractive index of the fiber core and the refractive index of the cladding respectively, the symbol * represents the conjugate operation of the matrix, P represents the injected power,
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 1, characterized in that the inter-core crosstalk calculation equation based on coupled mode theory is:

Among them, N is the number of segments after segmenting the optical fiber, d is the segment length after segmenting the optical fiber, Δβ eq,mn,i is the equivalent phase mismatch of the i-th segment,
is the modified coupling mode coefficient between fiber cores in the i-th section, k i (d) is the coupling mode coefficient between fiber cores.
The inter-mode crosstalk detection method of multi-core few-mode optical fiber according to claim 6, characterized in that the inter-core coupling mode coefficient k i (d) is replaced by the inter-mode coupling mode coefficient k mn,i , Calculate the crosstalk value between optical fiber modes:

Among them, g mn,i is the modified inter-mode coupling mode coefficient of the i-th section.
A multi-core few-mode optical fiber inter-mode crosstalk detection device, which is characterized by including:

Fiber parameter and injection power acquisition module, used to obtain fiber parameters and injection power;

An electric field strength calculation module, used to calculate the electric field strength within the core range and the electric field strength within the cladding range based on the optical fiber parameters;

An inter-mode coupling mode coefficient calculation module, used to calculate the inter-mode coupling mode coefficient according to the electric field intensity within the fiber core range, the electric field intensity within the cladding range, and the injection power;

An optical fiber inter-mode crosstalk value calculation module is used to replace the inter-core coupling mode coefficient in the inter-core crosstalk calculation equation based on coupled mode theory with the inter-mode coupling mode coefficient, and calculate the optical fiber inter-mode crosstalk value.
A multi-core few-mode optical fiber inter-mode crosstalk detection device according to claim 8, characterized in that it is applied to heterogeneous or homogeneous optical fibers.
A computer-readable storage medium, characterized in that a computer program is stored on the computer-readable storage medium, and when the computer program is executed by a processor, the multi-core system according to any one of claims 1 to 7 is implemented. Steps of the inter-mode crosstalk detection method for few-mode optical fiber.