WO2024013876A1 - Multi-core optical fiber, and design method - Google Patents

Abstract

The objective of the present invention is to provide a coupled multi-core optical fiber and a design method that take into consideration leakage loss, bending loss, and a cutoff wavelength.　A multi-core optical fiber design method according to the present invention is characterized by: finding a core structure range with which a v-value is less than 2.405; finding a core separation range with which a coefficient of coupling κn with an adjacent core is at least equal to 5.6×10-5; and calculating a cross-sectional structure of the multi-core optical fiber with which the core structure range and the core separation range are satisfied, and a relationship between a leakage loss, a bending loss and a cutoff wavelength of a single-core optical fiber provided with only one core of the core structure is satisfied.

Description

Multi-core optical fiber and design method

The present disclosure relates to a coupled multi-core optical fiber and a method for designing the same.

In order to overcome the transmission capacity limitations of single mode fibers (SMF), space division multiplexing (Space Division Multiplexing) using multi-core fibers (Multi-Core Fibers) and few-mode fibers (Few-Mode Fibers) iplexing:SDM ) technology is attracting attention (for example, see Non-Patent Document 1).

In particular, coupled MCF (C-MCF) compensates for inter-core crosstalk (XT) through MIMO (Multiple-Input/Multiple-Output) signal processing, which reduces the core spacing compared to non-coupled MCF and FMF. can be reduced, and space utilization efficiency can be significantly improved (see, for example, Non-Patent Document 2). Further, in the C-MCF, group delay spread (GDS) is proportional to the square root of the distance due to random mode coupling occurring during fiber propagation. Since GDS is directly connected to the load of MIMO signal processing, C-MCF, which can achieve lower GDS characteristics than FMF, is promising as an SDM fiber for long-distance transmission.

Japanese Patent Application Publication No. 2017-167196

For example, Patent Document 1 and Non-Patent Document 3 disclose C-MCF design methods to obtain sufficient mode coupling. However, conventional C-MCF design methods do not mention the basic optical properties of leakage loss, bending loss, and cutoff wavelength. Therefore, there is a problem in that it is difficult to design a C-MCF that ensures these optical properties as a communication fiber.

Therefore, in order to solve the above problems, the present invention aims to provide a coupled multi-core optical fiber and a design method that take leakage loss, bending loss, and cutoff wavelength into consideration.

In order to achieve the above object, the multi-core optical fiber design method according to the present invention determines target values for bending loss, leakage loss, and cutoff wavelength, creates a core structure that allows single mode propagation, a core pitch that provides sufficient coupling, The core arrangement was determined based on the relationship between the bending loss, leakage loss, and cutoff wavelength of a single-core optical fiber having a core with the core structure.

Specifically, the present invention provides a method for designing a multi-core optical fiber comprising a plurality of cores each having a core structure with a core radius a and a relative refractive index difference Δ,
Setting a target leakage loss α _L ^(MC) , a target bending loss α _L ^(MC) , and a target cutoff wavelength λ _c ^(MC) of the multi-core optical fiber;
finding a range of the core structure in which the v value is less than 2.405;
Finding a range of core spacing in which the coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and satisfying the range of the core structure and the range of the core spacing, and having one core of the core structure. Calculate the cross-sectional structure of the multi-core optical fiber that satisfies the relationship between the leakage loss α _L ^(SC) , the bending loss α _b ^(SC) , and the cutoff wavelength λ _c ^(SC) of the single-core optical fiber having only It is characterized by

This design method uses the relationship between the optical properties of a single-core optical fiber and the optical properties of a multi-core optical fiber to specify the cross-sectional structure including the core arrangement. Therefore, it is possible to design a coupled multi-core optical fiber that ensures optical properties as a communication fiber. Therefore, the present invention can provide a method for designing a coupled multi-core optical fiber in consideration of leakage loss, bending loss, and cutoff wavelength.

Here, regarding leakage loss, perform the following work.
When the shortest distance from the center of the core to the outer circumference of the cladding is d, find the shortest distance d that satisfies the relationship of the number C1, and calculate the diameter of the cladding from the shortest distance d and the arrangement of the cores. Calculate D.

Regarding leakage loss, the following work may be performed.
When the cladding diameter D is fixed,
calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
finding the leakage loss α _L ^(SC) of a single-core optical fiber that satisfies the relationship of the number C2; and finding the single-core optical fiber that satisfies the leakage loss α _L ^(SC) from within the range of the core structure. The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

In addition, regarding bending loss, perform the following work.
When the cladding diameter D is fixed,
calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
finding the bending loss α _b ^(SC) of a single-core optical fiber that satisfies the relationship of the number C3; and finding the single-core optical fiber that satisfies the bending loss α _b ^(SC) from within the range of the core structure; The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

For the cutoff wavelength, do the following:
finding a cut-off wavelength λ _c ^{(SC) of} a single-core optical fiber that _satisfies the relationship of the number C4; The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

The coupled multicore optical fiber designed by the above design method includes a plurality of cores each having a core radius a and a relative refractive index difference Δ, and has a cross-sectional structure that satisfies the following conditions.
(1) The v value is less than 2.405,
(2) The coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and (3) The leakage loss α _L ^(MC) , bending loss α _L ^(MC) , and cutoff of the multi-core optical fiber are Each of the wavelengths λ _c ^(MC) is a number C1, which is the relationship between the bending loss α _b (SC) and the leakage loss α _L ^(SC) of a single-core optical fiber having only one core of the core ^structure. and the number C4, which is the relationship with the cutoff wavelength λ _c ^(SC) .

Further, the coupled multi-core optical fiber may include a plurality of cores having a core structure with a core radius a and a relative refractive index difference Δ, and the cross-sectional structure may satisfy the following conditions.
(1) The v value is less than 2.405,
(2) The coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and (3) The leakage loss α _L ^(MC) , bending loss α _L ^(MC) , and cutoff of the multi-core optical fiber are Each of the wavelengths λ _c ^(MC) is a number C2, which is the relationship between the bending loss α _b (SC) and the leakage loss α _L ^(SC) of a single-core optical fiber comprising only one core of the core ^structure. and the number C4, which is the relationship with the cutoff wavelength λ _c ^(SC) .

Note that the above inventions can be combined as much as possible.

The present invention can provide a coupled multi-core optical fiber and a design method that take leakage loss, bending loss, and cutoff wavelength into consideration.

FIG. 2 is a diagram illustrating a cross-sectional structure of a multi-core optical fiber. FIG. 2 is a diagram illustrating the relationship between the number of cores and bending loss of a multi-core optical fiber. FIG. 2 is a diagram illustrating a method for designing a multi-core optical fiber according to the present invention. It is a figure explaining the design method (step S02) of the multi-core optical fiber based on this invention. FIG. 2 is a diagram illustrating the core structure of a multi-core optical fiber. It is a figure explaining the design method (step S03) of the multi-core optical fiber based on this invention. It is a figure explaining the design method (step S04) of the multi-core optical fiber based on this invention. It is a figure explaining the shortest distance d from the center of a core to the outer periphery of a cladding. FIG. 2 is a diagram illustrating the relationship between the number of cores of a multi-core optical fiber and the shortest distance d from the center of the core to the outer periphery of the cladding. It is a figure explaining the design method (step S04) of the multi-core optical fiber based on this invention. It is a figure explaining the design method (step S04) of the multi-core optical fiber based on this invention. FIG. 1 is a diagram illustrating an example of a multi-core optical fiber according to the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and the drawings indicate the same components.

[Key points of the invention]
FIG. 1 is a diagram illustrating a cross-sectional structure of a multi-core optical fiber (a core 12 disposed within a cladding 11). (A) shows a typical core arrangement with 2 cores, (B) with 4 cores, (C) with 8 cores, and (D) with 12 cores. FIG. 2 is a diagram illustrating the dependence of bending loss on the number of cores. It can be seen from FIG. 2 that the bending loss tends to increase as the number of cores increases. From FIG. 2, even if the core structure is optimally designed for one core (SMF), if the number of cores changes, the bending loss characteristics may not meet the requirements. Therefore, it is necessary to perform optimal core design according to the number of cores.

Furthermore, since the cutoff wavelength depends on the bending loss, the cutoff wavelength also changes depending on the number of cores. Furthermore, leakage losses vary as well depending on the number of cores. Therefore, an optical fiber design method is required that takes into consideration the three characteristics of bending loss, leakage loss, and cutoff wavelength depending on the number of cores.

Therefore, in this embodiment, a method of designing a coupled multi-core optical fiber with sufficiently small loss for an arbitrary number of cores will be described, taking into account bending loss, leakage loss, and cutoff wavelength that change depending on the number of cores.

FIG. 3 is a flowchart illustrating the design method of this embodiment. This design method is a design method for a multi-core optical fiber comprising a plurality of cores having a core structure with a core radius a and a relative refractive index difference Δ,
Setting a target leakage loss α _L ^(MC) , a target bending loss α _L ^(MC) , and a target cutoff wavelength λ _c ^{(MC) of} the multi-core optical fiber (step S01);
finding a range of the core structure in which the v value is less than 2.405 (step S02);
finding a core spacing range in which the coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more (step S03); and satisfying the core structure range and the core spacing range, and A cross section of the multi-core optical fiber that satisfies the relationship between the leakage loss α _L ^(SC) , the bending loss α _b ^(SC) , and the cutoff wavelength λ _c ^(SC) of a single-core optical fiber having only one core. Calculating the structure (step S04)
It is characterized by

FIG. 4 is a diagram explaining step S02. FIG. 5 is a diagram illustrating the core structure. In this embodiment, a step index type core structure will be considered. FIG. 4 is a diagram illustrating the relationship between the core radius a and the relative refractive index difference Δ so that the v value of the step index optical fiber satisfies v=2.405. It also shows the electric field distribution of the guided mode. In the region below the solid line, only the LP01 mode propagates. On the other hand, higher-order modes propagate in the region above the solid line. Therefore, in order to perform single mode operation, it is necessary to set the core structure to a region (range of the core structure) below the solid line where the v value is 2.405.

From the above, in order for each core of a multi-core optical fiber to operate in a single mode, it is necessary to have a core structure in which the v value is less than 2.405.

In the following explanation, unless otherwise specified, the core radius a = 4.2 μm, and the relative refractive index difference Δ of the core to the cladding is 0.35% (however, the plot in FIG. 4 is a = 4.5 μm). .

FIG. 6 is a diagram illustrating step S03. Specifically, FIG. 6 is a diagram illustrating the relationship between the distance between the centers of two adjacent cores (core spacing) and the coupling coefficient κ _n between the cores. Note that the core structure in this figure is a step index type having parameters as shown in the table in the figure, and the number of cores is two. As the core spacing becomes smaller, the value of the coupling coefficient κ _n increases. According to Patent Document 2 and the like, the lower limit of the coupling coefficient for obtaining sufficient coupling is approximately 5.6×10 ⁻⁵ .

In other words, since the range of the core structure of the core radius a and the relative refractive index difference Δ is obtained in step S02, the range of the core spacing can be found from there using FIG.

(Work 1 of step S04)
FIG. 7 is a diagram illustrating the design for leakage loss in step S04. FIG. 8 is a diagram illustrating the "shortest distance d from the center of the core to the outer periphery of the clad" discussed in step S04, that is, the distance d from the center of the outermost core to the outer periphery of the clad. Further, FIG. 9 is a diagram illustrating the relationship between the number of cores and the distance d. The distance d decreases as the number of cores increases.

Specifically, FIG. 7 is an example of calculating the dependence of leakage loss on distance d. The vertical axis is normalized by dividing the value of leakage loss [dB] for each core number n by the loss value [dB] of SMF of single core (SC) (n = 1) (plot of n = 1). value (leakage loss to SC ratio). As shown in FIG. 7, the leakage loss increases exponentially as the number of cores increases, that is, as the distance d decreases, compared to the case of a single-core structure. Here, an increase in leakage loss is expressed as "deterioration." The deterioration rate _αL can be approximated by the number 0.
[Number 0]
α _L =10 ²⁷ ×10 ^-0.43d

In other words, in order to achieve a leakage loss equal to or less than the target leakage loss α _L ^(MC) at the used wavelength, in step S04,
The shortest distance d that satisfies the relationship of the number C1 is found, and the diameter D of the cladding 11 is calculated from the shortest distance d and the arrangement of the cores 12.

Specifically, considering the leakage loss α _L ^(SC) of a single-core optical fiber and the deterioration rate α _L for a desired number of cores, the distance d is set so that the deterioration rate α _L is less than or equal to the value of the following formula. , that is, the cladding diameter D is set.

(Work 2 of step S04)
Note that if the diameter D of the cladding has already been determined, in order to achieve a leakage loss equal to or less than the target leakage loss α _L ^(MC) at the wavelength used, in step S04,
calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
finding the leakage loss α _L ^(SC) of a single-core optical fiber that satisfies the relationship of the number C2; and finding the single-core optical fiber that satisfies the leakage loss α _L ^(SC) from within the range of the core structure. The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

Specifically, considering the target leakage loss α _L ^(MC) of the multi-core optical fiber and the deterioration rate α _L depending on the number of cores, the range of the core structure found in step S02 is calculated as the leakage loss α _L in the single-core structure. It is limited to a core structure in which ^(SC) is less than or equal to the value of the following formula.

Note that the leakage loss decreases toward the upper right region in the graph of core radius and refractive index difference shown in FIG. However, in order to operate in single mode, the wavelength must be below the solid line representing the cutoff wavelength.

(Work 3 of step S04)
FIG. 10 is a diagram illustrating the design regarding bending loss in step S04. Specifically, FIG. 10 is an example of calculation of the dependence of bending loss on distance d. Since the distance d changes depending on the number of cores as shown in FIG. 9, FIG. 10 can also be said to be an example of calculation of the dependence of bending loss on the number of cores n.

The bending loss is the loss when uniform bending is applied with a bending radius of 30 mm and 100 windings. In Fig. 10, the horizontal axis is the distance d [μm], and the vertical axis is the value of bending loss [dB] for each core number n divided by the value of bending loss [dB] of SMF of single core (SC) (n = 1). (bending loss to SC ratio α _b ). Although the bending loss to SC ratio α _b varies depending on the number of cores n, the bending loss in the multi-core structure is several tens of times larger than that in the case of the single-core structure. Here, an increase in bending loss is expressed as "deterioration." The maximum amount of deterioration of the bending loss versus SC ratio α _b can be approximately approximated by a broken line. At this time, the broken line can be approximated by equation 3A.
[Number 3A]
α _b =450×10 ^-0.025d

That is, in the case where the diameter D of the cladding has already been determined, in order to achieve a bending loss equal to or less than the target bending loss α _b ^(MC) at the wavelength used, in step S04,
calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
finding the bending loss α _b ^(SC) of a single-core optical fiber that satisfies the relationship of the number C3; and finding the single-core optical fiber that satisfies the bending loss α _b ^(SC) from within the range of the core structure; The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

Specifically, considering the target bending loss α _b ^(MC) of the multi-core optical fiber and the maximum deterioration rate α _b for the number of cores, the range of the core structure found in step S02 is calculated based on the bending loss α in the single-core structure. _b Limited to core structures in which ^(SC) is less than or equal to the value of the following formula.

Note that the bending loss decreases toward the upper right region in the graph of core radius and refractive index difference shown in FIG. However, in order to operate in single mode, the wavelength must be below the solid line representing the cutoff wavelength.
In addition, since the maximum deterioration amount α _b has a relatively small slope with respect to the distance d, if an attempt is made to increase the distance d to achieve the desired α _b , the diameter D of the cladding will become extremely large and the bending rupture strength will deteriorate. Undesirable.

(Work 4 of step S04)
FIG. 11 is a diagram illustrating the design of the cutoff wavelength in step S04. Specifically, FIG. 11 shows an example of calculation of the dependence of the cutoff wavelength on the distance d. Since the distance d changes depending on the number of cores as shown in FIG. 9, FIG. 11 can also be said to be an example of calculating the dependence of the cutoff wavelength on the number of cores n.

The horizontal axis in FIG. 11 is the distance d [μm], and the vertical axis is the increment λc [nm] from the cutoff wavelength of the single-core (SC) (n=1) SMF for the cutoff wavelength for each core number n. As shown in FIG. 11, the dependence of the cutoff wavelength on the number of cores n is very small. Although the cutoff wavelength increment λc varies depending on the number of cores, the cutoff wavelength of the multi-core structure is approximately 30 to 40 nm longer than the cutoff wavelength of the single-core structure.

That is, in order to realize the target cutoff wavelength λc ^(MC) , in step S04,
finding a cut-off wavelength λ _c ^{(SC) of} a single-core optical fiber that _satisfies the relationship of the number C4; The core structure is specified by the core radius a of the fiber and the relative refractive index difference Δ with respect to the cladding.

Specifically, considering that the maximum shift amount due to multi-core is 40 nm, the range of the core structure found in step S02 is set to a cut-off wavelength λ c with the target cut-off wavelength λ _c ^(MC) set to a short wavelength of 40 _nm. ^(SC) is limited to the core structure of a single-core optical fiber.

Note that the cutoff wavelength becomes shorter toward the lower left region in the graph of core radius and refractive index difference shown in FIG. Since the dependence of the cutoff wavelength increment λc on the distance d is almost zero, attempting to achieve the desired λc by increasing the distance d is not preferable because the diameter D of the cladding becomes extremely large and the bending rupture strength deteriorates.

In step S04, the above-mentioned operations 1, 3, and 4 or operations 2, 3, and 4 are performed to find a cross-sectional structure (core structure and distance d) that satisfies each requirement.

(Example)
In this example, the structure of the multi-core optical fiber (12 cores) shown in FIG. 1(D) was calculated using the design method described above.
The goal of step S01 was set as follows.
・Target leakage loss α _L ^(MC) ≦10 ^-1 [dB/km]
・Target bending loss α _L ^(MC) ≦2 [dB], (bending radius 30 mm, 100 turns)
・Target cutoff wavelength λ _c ^(MC) <1530 [nm]

Figure 12 shows the calculation results. Based on the optical characteristics of a single-core optical fiber, we were able to calculate the structure of a multi-core optical fiber (12 cores) that satisfies the above goals. Note that the relative refractive index difference Δ between the core and the cladding is 0.35%.

11: Core 12: Clad

Claims (7)

1. A method for designing a multi-core optical fiber comprising a plurality of cores having a core structure with a core radius a and a relative refractive index difference Δ,
   Setting a target leakage loss α _L ^(MC) , a target bending loss α _L ^(MC) , and a target cutoff wavelength λ _c ^(MC) of the multi-core optical fiber;
   finding a range of the core structure in which the v value is less than 2.405;
   Finding a range of core spacing in which the coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and satisfying the range of the core structure and the range of the core spacing, and having one core of the core structure. Calculate the cross-sectional structure of the multi-core optical fiber that satisfies the relationship between the leakage loss α _L ^(SC) , the bending loss α _b ^(SC) , and the cutoff wavelength λ _c ^(SC) of the single-core optical fiber having only A method for designing a multi-core optical fiber characterized by:
2. A method for designing a multi-core optical fiber according to claim 1, comprising:
   When the shortest distance from the center of the core to the outer circumference of the cladding is d, find the shortest distance d that satisfies the relationship of the number C1, and calculate the diameter of the cladding from the shortest distance d and the arrangement of the cores. A method for designing a multi-core optical fiber, the method comprising calculating D.
   

   
3. A method for designing a multi-core optical fiber according to claim 1, comprising:
   When the cladding diameter D is fixed,
   calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
   finding the leakage loss α _L ^(SC) of a single-core optical fiber that satisfies the relationship of the number C2; and finding the single-core optical fiber that satisfies the leakage loss α _L ^(SC) from within the range of the core structure. A method for designing a multi-core optical fiber, characterized in that the core structure is specified by a core radius a of the fiber and a relative refractive index difference Δ with respect to the cladding.
   

   
4. A method for designing a multi-core optical fiber according to claim 1, comprising:
   When the cladding diameter D is fixed,
   calculating the shortest distance d from the center of the core to the outer circumference of the cladding from the core arrangement and the diameter D of the cladding;
   finding the bending loss α _b ^(SC) of a single-core optical fiber that satisfies the relationship of the number C3; and finding the single-core optical fiber that satisfies the bending loss α _b ^(SC) from within the range of the core structure; A method for designing a multi-core optical fiber, characterized in that the core structure is specified by a core radius a of the fiber and a relative refractive index difference Δ with respect to the cladding.
   

   
5. A method for designing a multi-core optical fiber according to claim 1, comprising:
   finding a cut-off wavelength λ _c ^{(SC) of} a single-core optical fiber that _satisfies the relationship of the number C4; A method for designing a multi-core optical fiber, characterized in that the core structure is specified by a core radius a of the fiber and a relative refractive index difference Δ with respect to a cladding.
   

   
6. A multi-core optical fiber comprising a plurality of cores each having a core radius a and a relative refractive index difference Δ, and whose cross-sectional structure satisfies the following conditions.
   (1) The v value is less than 2.405,
   (2) The coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and (3) The multi-core optical fiber has leakage loss α _L ^(MC) , bending loss α _L ^(MC) , and cutoff. Each of the wavelengths λ _c ^(MC) is a number C1, which is the relationship between the bending loss α _b (SC) and the leakage loss α _L ^(SC) of a single-core optical fiber having only one core of the core ^structure. and the number C4, which is the relationship with the cutoff wavelength λ _c ^(SC) .
   

   

   However, d is the shortest distance from the center of the core to the outer periphery of the cladding.
   

   

   
7. A multi-core optical fiber comprising a plurality of cores each having a core radius a and a relative refractive index difference Δ, and whose cross-sectional structure satisfies the following conditions.
   (1) The v value is less than 2.405,
   (2) The coupling coefficient κ _n of adjacent cores is 5.6×10 ⁻⁵ or more, and (3) The leakage loss α _L ^(MC) , bending loss α _L ^(MC) , and cutoff of the multi-core optical fiber are Each of the wavelengths λ _c ^(MC) is a number C2, which is the relationship between the bending loss α _b (SC) and the leakage loss α _L ^(SC) of a single-core optical fiber comprising only one core of the core ^structure. and the number C4, which is the relationship with the cutoff wavelength λ _c ^(SC) .
   

   

   d is the shortest distance from the center of the core to the outer periphery of the cladding.
   

   

   

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