WO2014132793A1

WO2014132793A1 - Multi-core fiber

Info

Publication number: WO2014132793A1
Application number: PCT/JP2014/053141
Authority: WO
Inventors: 雄佑佐々木; 竹永　勝宏; 晋聖齊藤; 正則小柴
Original assignee: 株式会社フジクラ; 国立大学法人北海道大学
Priority date: 2013-02-27
Filing date: 2014-02-12
Publication date: 2014-09-04
Also published as: JP5808767B2; JP2014164269A

Abstract

A multi-core fiber (1) comprises: a cladding (20); a plurality of core elements (10) provided within the cladding (20), each of which has a core (11), an inner cladding layer (12) that surrounds the core (11), and an outer cladding layer (13) that surrounds the inner cladding layer (12) and that has a mean refractive index lower than that of the cladding (20) and the inner cladding layer (12); and a plurality of stress-adjusting parts (15) provided within the cladding (20) and having a mean refractive index lower than that of the cladding (20) and the inner cladding layers (12). The stress-adjusting parts (15) are arranged such that, in the light propagating through each core element (10), the effective refractive index for polarized waves belonging to the same LP mode is reduced.

Description

Multi-core fiber

The present invention relates to a multi-core fiber, and is suitable for reducing crosstalk.

An optical fiber used in a currently popular optical fiber communication system has a structure in which the outer periphery of one core is surrounded by a clad, and information is transmitted by propagation of an optical signal in the core. Is done. In recent years, with the spread of optical fiber communication systems, the amount of transmitted information has increased dramatically. As the amount of information increases, optical fiber communication systems use WDM (Wavelength Division Multiplexing), PDM (Polarization Division Multiplexing), or multilevel modulation to perform large-capacity long-distance optical communication. ing. On the other hand, it is said that there is a limit of transmission capacity in a communication system using an existing single mode fiber.

By the way, in order to further increase the transmission capacity per fiber, it is possible to transmit a plurality of signals by light propagating through each core using a multi-core fiber in which the outer periphery of the plurality of cores is surrounded by one clad. Are known.

However, in multi-core fibers, crosstalk between cores may occur. When the diameter of multi-core fibers is reduced, the distance between cores becomes smaller, and this crosstalk is more likely to occur. Accordingly, there is a need for a multi-core fiber that can reduce crosstalk between cores.

Non-Patent Document 1 below proposes to provide a low refractive index portion called a trench around each core in order to suppress crosstalk between cores, and crosstalk can be prevented by providing the low refractive index portion. It has been confirmed that it was suppressed.

However, it has been found that depending on the structure of the multi-core fiber, polarization mode dispersion (PMD) may deteriorate.

When the polarization mode dispersion deteriorates, the signal propagating through each core of the multi-core fiber affects the signal quality such as spreading of the pulse, and there arises a problem that the restriction on the communication distance and the bit rate is increased.

Therefore, an object of the present invention is to provide a multi-core fiber that can reduce polarization mode dispersion and perform large-capacity transmission over a long distance.

The multi-core fiber of the present invention is provided with a clad, an inner clad layer surrounding the core, the inner clad layer surrounding the core, and an average refractive index lower than the clad and the inner clad layer. A plurality of core elements having an outer cladding layer, and a stress adjusting section provided in the cladding and having an average refractive index lower than that of the cladding and the inner cladding layer. Are arranged so that the difference in effective refractive index of polarized waves belonging to the same LP mode is small.

It has been found that such a multi-core fiber can suppress birefringence, which is a difference in effective refractive index of a plurality of polarized waves propagating through a core element. Therefore, the polarization mode dispersion can be reduced as compared with the case where the stress adjusting unit is not provided.

As described above, according to the present invention, it is possible to provide a multi-core fiber that can reduce polarization mode dispersion and perform large-capacity communication over a long distance.

It is a figure which shows the cross section perpendicular | vertical to the length direction of the multi-core fiber in 1st Embodiment. It is a figure which shows the mode of a core element. It is a figure which shows the mode of a stress adjustment part. It is a figure which shows the cross section perpendicular | vertical to the length direction of the multi-core fiber in 2nd Embodiment. It is a figure which shows the cross section perpendicular | vertical to the length direction of the multi-core fiber in 3rd Embodiment. 3 is a graph showing the results of PMD measurement in Example 1. It is a figure which shows the allocation state of the core number in Example 1. FIG. 10 is a graph showing the results of PMD measurement in Comparative Example 1. It is a graph which shows the mode of the fluctuation | variation of birefringence at the time of changing the predetermined parameter in Example 1. FIG. 6 is a graph showing a PMD measurement result in Example 2. It is a figure which shows the allocation state of the core number in Example 2. FIG. 10 is a graph showing a PMD measurement result in Comparative Example 2. 10 is a graph showing PMD measurement results in Example 3. It is a figure which shows the allocation state of the core number in Example 3. FIG. 10 is a graph showing PMD measurement results in Comparative Example 3. It is a graph which shows the mode of the fluctuation | variation of birefringence when the predetermined parameter in Example 3 is changed.

(1) First Embodiment A first embodiment for carrying out the present invention will be described in detail with reference to the drawings.

FIG. 1 is a view showing a cross section perpendicular to the length direction of the multi-core fiber in the first embodiment. As shown in FIG. 1, the multi-core fiber 1 in the present embodiment includes a plurality of core elements 10, a plurality of stress adjusting portions 15, a clad 20, an inner protective layer 31 that covers the outer peripheral surface of the clad 20, and an inner protective layer. An outer protective layer 32 covering the outer peripheral surface of 31 is provided as a main component.

The plurality of core elements 10 are rod-shaped members arranged in the clad 20 and have the same structure. FIG. 2 is a diagram illustrating a state of the core element 10. Specifically, FIG. 2A is a diagram showing a cross section perpendicular to the length direction of the multi-core fiber 1 in the core element 10, and FIG. 2B is a diagram showing a refractive index distribution in the core element 10.

As shown in FIG. 2A, each core element 10 includes a core 11, an inner cladding layer 12 surrounding the outer peripheral surface of the core 11, and an outer cladding layer 13 surrounding the outer peripheral surface of the inner cladding layer 12. Yes. In the present embodiment, the diameter of the core 11, the outer diameter of the inner cladding layer 12, and the outer diameter of the outer cladding layer 13 are approximately the same for each of the plurality of core elements 10.

As shown in FIG. 2B, the average refractive index n ₂ of the inner cladding layer 12 and the average refractive index n ₄ of the cladding 20 are set lower than the average refractive index n ₁ of the core 11. Further, the average refractive index n ₃ of the outer cladding layer 13 is made lower than the average refractive index n ₂ of the inner cladding layer 12 and the average refractive index n ₄ of the cladding 20. Here, the average refractive index n ₄ of the average refractive index n ₂ and the cladding 20 of the inner cladding layer 12, in this embodiment, are approximately equal.

Thus, from the viewpoint of refractive index, the outer cladding layer 13 is formed as a groove between the inner cladding layer 12 in each core element 10 and the cladding 20 covering the core element 10. That is, by the average refractive index n ₃ of the outer cladding layer 13 of each core element 10 is smaller than the average refractive index n ₄ of the average refractive index n ₂ and the cladding 20 of the inner cladding layer 12, is the core element 10 It has a trench structure.

Therefore, in each core element 10, the light confinement effect on the core 11 becomes large, and it becomes difficult for light to leak from the core element 10. As a result, in the multi-core fiber 1 in the present embodiment, crosstalk between the core elements 10 adjacent to each other is significantly suppressed.

In FIG. 2B, Δ ₁ indicates a relative refractive index difference of the core 11 with respect to the clad 20, Δ ₂ indicates a relative refractive index difference of the outer cladding layer 13 with respect to the clad 20, and r ₁ indicates a radius of the core 11. shows, r ₂ represents the distance from the center of the core 11 to the outer peripheral surface of the inner cladding layer. The r ₃ represents the center of the core 11, one-half of the distance between the center of the core element 10 or the stress controlled part 15 closest to the core element 10 having the core 11. Further, W indicates the width (thickness) of the outer cladding layer 13.

As shown in FIG. 1, the number of core elements 10 in this embodiment is twelve, and six core elements 10 are arranged around the central axis of the clad 20, and further outside the six core elements 10. Six core elements 10 are arranged. The distance between the central axes of the core elements adjacent to each other (hereinafter referred to as the inter-core distance) Λ1 is the same, and the cross-sectional shape connecting the central axes of the three core elements adjacent to each other is an equilateral triangle.

In the cross section orthogonal to the length direction of the multi-core fiber 1, the apex of the first regular hexagon with respect to the center of the clad 20 and the centers of the six core elements 10 arranged as the inside are in a state of matching. . Further, the vertex of the second regular hexagon larger than the first regular hexagon with respect to the center of the clad 20 and the centers of the six core elements 10 arranged as the outside are in a state of matching. Furthermore, each center of the six core elements 10 arranged as the outside is located on a perpendicular line passing through the center of the side in the first regular hexagon. In addition, the center of the core element 10 and the center of the core 11 in the core element 10 coincide with each other.

The plurality of stress adjusting portions 15 are rod-shaped members arranged in the clad 20 in order to reduce PMD, and have the same structure. FIG. 3 is a diagram illustrating a state of the stress adjusting unit 15. Specifically, FIG. 3A is a diagram showing a cross section perpendicular to the length direction of the multi-core fiber 1 in the stress adjusting unit 15, and FIG. 3B is a diagram showing a refractive index distribution in the stress adjusting unit 15. is there.

As shown in FIG. 3A, each stress adjusting portion 15 has a single-layer structure unlike the core element 10 having a three-layer structure. In the case of this embodiment, the diameter of the stress adjusting portion 15 is approximately the same as the diameter of the inner cladding layer 12 in the core element 10.

As shown in FIG. 3B, the average refractive index n ₅ of the stress adjusting unit 15 is lower than the average refractive index n ₂ of the inner cladding layer 12 and the average refractive index n _{4 of the} cladding 20, and the outer cladding layer 13. It is higher than the average refractive index n ₃ of the.

Note that Δ in FIG. 3B indicates the relative refractive index difference of the stress adjusting unit 15 with respect to the clad 20, and d ₁ indicates the radius of the stress adjusting unit 15. D ₂ indicates a half of the distance between the center of the stress adjusting unit 15 and the center of the core element 10 or the stress adjusting unit 15 closest to the stress adjusting unit 15.

As shown in FIG. 1, the number of the stress adjusting portions 15 in the present embodiment is seven, one stress adjusting portion 15 is arranged at the center of the clad 20, and six stresses around the outer core element 10. An adjustment unit 15 is arranged. The distance Λ2 between the central axes of the stress adjusting portions 15 adjacent to each other (hereinafter referred to as the distance between the adjusting portions) Λ2 is the same, and the cross-sectional shape connecting the centers of the three stress adjusting portions 15 adjacent to each other is an equilateral triangle. . In other words, the stress adjusting unit 15 is disposed so as to be rotationally symmetric with respect to the center of the clad 20 and is disposed so as to sandwich the core element 10 therebetween.

In the cross section perpendicular to the length direction of the multi-core fiber 1, the apex of the regular hexagon with respect to the center of the clad 20 and the centers of the six stress adjusting portions 15 are in agreement. In addition, the regular hexagon which makes the center of the six stress adjustment parts 15 a vertex corresponds with the state which rotated the regular hexagon which makes the vertex the center of the six core elements 10 arrange | positioned as an outer side by 30 degrees.

Such a stress adjusting unit 15 is made of, for example, quartz to which a dopant that lowers the refractive index such as fluorine is added. The core 11 is made of, for example, quartz to which a dopant for increasing the refractive index such as germanium is added, and the inner cladding layer 12 and the cladding 20 are made of, for example, pure quartz to which no dopant is added. The outer cladding layer 13 is made of, for example, quartz to which a dopant that lowers the refractive index such as fluorine is added, and the inner protective layer 31 and the outer protective layer 32 are made of, for example, different types of ultraviolet curable resins. Is done.

As described above, in the present embodiment, the plurality of stress adjusting portions 15 are disposed so as to be rotationally symmetric with respect to the center of the clad 20 and are disposed so as to sandwich the core element 10 therebetween.

It was found that birefringence can be suppressed in regions other than the core element 10 when arranged in this way. Therefore, the polarization mode dispersion can be reduced as compared with the case where the stress adjusting unit 15 is not provided. In this way, the multi-core fiber 1 is provided that can reduce polarization mode dispersion and perform large-capacity transmission over a long distance.

(2) Second Embodiment Next, a second embodiment will be described in detail with reference to FIG. In addition, about the component which is the same as that of 1st Embodiment, or an equivalent component, the overlapping description is abbreviate | omitted except the case where it attaches | subjects the same referential mark and demonstrates especially.

FIG. 4 is a view showing a cross section perpendicular to the length direction of the multi-core fiber in the second embodiment. As shown in FIG. 4, the multi-core fiber 2 in the present embodiment is different from the multi-core fiber 1 in the first embodiment in that the arrangement positions of the core element 10 and the stress adjusting unit 15 are different.

Specifically, the number of core elements 10 is twelve, and twelve core elements 10 are arranged around the central axis of the clad 20 with the inter-core distances being approximately the same. That is, in the cross section orthogonal to the length direction of the multi-core fiber 1, the vertex of the regular hexagon with respect to the center of the clad 20, the midpoint of the side of the regular hexagon, and the centers of the 12 core elements 10 coincide. It is in a state to do.

On the other hand, the number of stress adjusting portions 15 is seven, and one stress adjusting portion 15 is disposed at the center of the clad 20. Further, six stress adjusting portions 15 are arranged between the one stress adjusting portion 15 and the core element 10 in a state in which the distance between the adjusting portions is approximately the same. Note that all of the stress adjusting portions 15 are arranged inside each of the plurality of core elements 10, and the cross-sectional shape connecting the centers of the three adjacent stress adjusting portions 15 is an equilateral triangle. In other words, the stress adjusting unit 15 is arranged so as to be rotationally symmetric with respect to the center of the clad 20.

It has been found that even when the stress adjusting unit 15 is arranged in this way, birefringence can be suppressed in a region other than the core element 10 as in the first embodiment. Therefore, the polarization mode dispersion can be reduced as compared with the case where the stress adjusting unit 15 is not provided. In this way, the multi-core fiber 2 that can reduce the polarization mode dispersion and transmit a large volume over a long distance is provided.

(3) Third Embodiment Next, a third embodiment will be described in detail with reference to FIG. In addition, about the component which is the same as that of 1st Embodiment, or an equivalent component, the overlapping description is abbreviate | omitted except the case where it attaches | subjects the same referential mark and demonstrates especially.

FIG. 5 is a view showing a cross section perpendicular to the length direction of the multi-core fiber in the third embodiment. As shown in FIG. 5, the multi-core fiber 3 in the present embodiment is different from the multi-core fiber 1 in the first embodiment in that the arrangement positions of the core element 10 and the stress adjusting unit 15 are different arrangement positions.

Specifically, twelve core elements 10 are provided, and twelve core elements 10 are arranged in an annular shape around the central axis of the clad 20 with the inter-core distances being approximately the same. That is, in the cross section orthogonal to the length direction of the multicore fiber 1, the centers of the twelve core elements 10 are positioned on the circumference with the center of the clad 20 as a reference.

On the other hand, the number of the stress adjusting portions 15 is one, and the center axis of the clad 20 and the central axis of the stress adjusting portion 15 coincide with each other, and the inside of each core element 10 is rotationally symmetric with respect to the center of the clad 20. It is arranged to become.

In addition, although the diameter of the stress adjustment part 15 in the said 1st Embodiment was made into the same grade as the diameter of the inner side cladding layer 12 in the core element 10, the diameter of the stress adjustment part 15 in this embodiment is from the diameter of the core element 10. Is also enlarged.

It has been found that even when the stress adjusting unit 15 is arranged in this way, birefringence can be suppressed in a region other than the core element 10 as in the first embodiment. Therefore, the polarization mode dispersion can be reduced as compared with the case where the stress adjusting unit 15 is not provided. In this way, the multi-core fiber 3 is provided that can reduce the polarization mode dispersion and enable large-capacity transmission over a long distance.

(4) Modifications Although the first to third embodiments have been described above as examples, the present invention is not limited to the above embodiments.

For example, in the above embodiment, the number of core elements 10 is twelve, but may be eleven or less or thirteen or more. In short, various numbers can be applied as long as the number is two or more. Further, the arrangement position of the core element 10 in the clad 20 can be applied to other than the above embodiment. The values of various parameters (n ₁ to n ₃ , r ₁ to r ₃ , Δ ₁ to Δ ₂ ) in the core element 10 can be changed as appropriate.

In the first embodiment and the second embodiment, the number of the stress adjusting portions 15 is seven. In the third embodiment, the number of the stress adjusting portions 15 is one. However, the number is not one or seven. Can be applied. Further, the arrangement position of the stress adjusting unit 15 in the clad 20 or the size of the stress adjusting unit 15 can be applied to other than the above embodiment. Further, the values of various parameters (n ₅ , d ₁ to d ₂ , Δ) in the stress adjusting unit 15 can be changed as appropriate.
In short, the stress adjusting unit 15 has an average refractive index lower than that of the clad 20 and the inner clad layer 12, and the difference in effective refractive index of polarized waves belonging to the same LP mode in the light propagating through each core element 10 is What is necessary is just to arrange | position in the clad 20 so that it may become small compared with the case where the said stress adjustment part 15 is not provided.
Specifically, for example, the stress adjusting unit 15 that reduces the difference from the effective refractive index of polarized light belonging to the LP01 mode in the light propagating through each core element 10 as compared with the case where the stress adjusting unit 15 is not provided. For example. As another example, the stress adjusting unit 15 is provided so that the difference between the light propagating through each core element 10 and the effective refractive index of the polarized light belonging to the LP11 mode is smaller than the case where the stress adjusting unit 15 is not provided. Is mentioned. As another example, a stress adjusting unit 15 is provided so that the difference between the light propagating through each core element 10 and the effective refractive index of the polarized light belonging to the LP21 mode becomes smaller than when the stress adjusting unit 15 is not provided. Can be mentioned.
In addition, it is more preferable to provide the stress adjusting unit 15 so that the difference in effective refractive index of polarized light belonging to the LP01 mode in the light propagating through each core element 10 becomes smaller than the case where the stress adjusting unit 15 is not provided. .

In the above embodiment, the stress adjuster 15 is made of quartz to which a dopant that lowers the refractive index such as fluorine is added. However, the stress adjuster 15 may be made of holes.

It should be noted that the components in the above-described multi-core fibers 1 to 3 and the relationship between the components are appropriately combined and omitted within the scope not departing from the purpose of the present application other than the contents shown in the embodiment and the modified examples. , Modification, addition of well-known technology, etc. can be made.

Hereinafter, the contents of the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these.

<1-1> Example 1
A multi-core fiber having the same structure as the multi-core fiber 1 of the first embodiment was prototyped. Various parameters in the core element 10 are shown in Table 1 below. Various parameters in one stress adjusting unit 15 arranged at the center of the clad 20 are shown in Table 2 below, and various parameters in the six stress adjusting units 15 arranged around the one stress adjusting unit 15 are shown in the following table. It was set to 3.

The symbols in Table 1 below are the same as the symbols shown in FIG. 2B as various parameters in the core element 10. The symbols in the following Table 2 and Table 3 are the same as those shown in FIG. 3B as various parameters in the stress adjusting unit 15.

FIG. 6 shows the PMD measurement results in the wavelength 1.55 μm band of each core 11 of the multi-core fiber manufactured in this way. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Further, the core numbers “1” to “12” on the horizontal axis in FIG. 6 are assigned to the cores 11 in the twelve core elements 10 as shown in FIG. The black circles in FIG. 7 are markers for connecting to other fibers.

<1-2> Comparative Example 1
The stress adjusting section 15 in the multi-core fiber 1 of the first embodiment was omitted, and a multi-core fiber that was the same as that of the above-described Example 1 was manufactured for the components other than the stress adjusting section 15.

FIG. 8 shows the measurement results of PMD in the wavelength 1.55 μm band at each core 11 of this multi-core fiber. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Also, the core numbers “1” to “12” on the horizontal axis in FIG. 8 are assigned as shown in FIG. 7 as in the first embodiment, and the marker positions are the same as those in FIG.

<1-3> Comparison From the comparison between FIG. 6 and FIG. 8, when the stress adjustment unit 15 is provided, PMD can be improved in any core 11 compared to the case where the stress adjustment unit 15 is not provided. I understood.

When birefringence of the LP01 mode propagating through the core element 10 in Example 1 was calculated, it was 8.9 × 10 ⁻⁷ when the core number was an odd number, and 4.2 × when the core number was an even number. 10 ⁻⁸ . Further, when the birefringence of the LP01 mode propagating through the core element 10 in the comparative example 1 is calculated, it is 2.0 × 10 ⁻⁶ when the core number is an odd number, and 2.6 × 10 ⁶ when the core number is an even number. It was ^-6 . From these calculation results, it was found that the birefringence can be significantly reduced when the stress adjusting unit 15 is provided, compared to the case where the stress adjusting unit 15 is not provided.

The birefringence is calculated as follows. That is, the thermal expansion coefficient of the stress adjusting unit 15 when the temperature was changed by 1000 ° C. was measured, and the stress change was calculated according to Reference 1 from the measurement result. Then, the birefringence in the 1.55 μm wavelength band was derived using the numerical analysis shown in Reference Document 2. Reference 1 is Konaoka Okamoto, “Basics of Optical Path” pp. 250-252 Corona, and Reference 2 is K. Okamoto, et al., “Stress analysis of optical fibers by a finite element method ”IEEE J. Quantum Electron, vol. QE-17, pp.2123-2129, 1981.

By the way, when various parameters in Table 1 in Example 1 and various parameters other than d ₂ / d ₁ and Δ in Table 2 are fixed, the parameters of d ₂ / d ₁ and Δ are changed. The calculation result of birefringence is shown in FIG.

In addition, when the horizontal axis in FIG. 9 is 0, this corresponds to the case where the stress adjusting unit 15 is not provided. Further, “inside” described on the right side of the graph in FIG. 9 means the core 11 (core 11 of core numbers 1 to 7) of the core element 10 arranged as the inside, and is described on the right side of the graph. The “outside” means the core 11 of the core element 10 (core 11 having core numbers 8 to 12) arranged as the outside.

As shown in FIG. 9, it was found that d ₂ / d ₁ is preferably 2 or more in order to lower the birefringence as compared with the case where the stress adjusting unit 15 is not provided. It was also found that Δ is preferably in the range of −0.2% to −0.4%.

<2-1> Example 2
A multi-core fiber having the same structure as the multi-core fiber 2 of the second embodiment was prototyped. Various parameters in the core element 10 are shown in Table 4 below. Various parameters in the stress adjusting unit 15 are shown in Table 5 below.

The symbols in Table 4 below are the same as the symbols shown in FIG. 2B as various parameters in the core element 10. Further, the symbols in Table 5 below are the same as those shown in FIG. 3B as various parameters in the stress adjusting unit 15.

FIG. 10 shows a wavelength of 1.55 μm band of each core 11 of the multi-core fiber manufactured in this way. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Also, the core numbers “1” to “12” on the horizontal axis in FIG. 10 are assigned to the cores 11 in the twelve core elements 10 as shown in FIG. The black circles in FIG. 10 are markers for connecting to other fibers.

<2-2> Comparative Example 2
The stress adjusting unit 15 in the multi-core fiber 2 of the second embodiment is omitted, and the multi-core fiber that is the same as that of the above-described Example 2 is manufactured for the components other than the stress adjusting unit 15.

FIG. 12 shows the measurement results of PMD in the wavelength 1.55 μm band at each core 11 of this multicore fiber. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Also, the core numbers “1” to “12” on the horizontal axis in FIG. 12 are assigned as shown in FIG. 11 as in the second embodiment, and the marker positions are the same as those in FIG.

<2-3> Comparison From the comparison between FIG. 10 and FIG. 12, when the stress adjustment unit 15 is provided, PMD can be improved in any core 11 compared to the case where the stress adjustment unit 15 is not provided. I understood.

When birefringence of the LP01 mode propagating through the core element 10 in Example 2 was calculated, the birefringence when the core number is odd is 1.44 × 10 ⁻⁶ , and 7 when the core number is even. 1 × 10 ⁻⁷ . Further, when the birefringence of the LP01 mode propagating through the core element 10 in Comparative Example 2 was calculated, it was 4.5 × 10 ⁻⁶ when the core number was an odd number, and 8.7 × when the core number was an even number. 10 ⁻⁶ . From these calculation results, it was found that the birefringence can be significantly reduced when the stress adjusting unit 15 is provided, compared to the case where the stress adjusting unit 15 is not provided.

<3-1> Example 3
A multi-core fiber having the same structure as the multi-core fiber 3 of the third embodiment was prototyped. Various parameters in the core element 10 are shown in Table 6 below. Various parameters in the stress adjusting unit 15 are shown in Table 7 below.

The symbols in Table 6 below are the same as the symbols shown in FIG. 2B as various parameters in the core element 10. Further, the symbols in the following Table 7 coincide with the symbols shown in FIG. 3B as various parameters in the stress adjusting unit 15.

FIG. 13 shows the PMD measurement results in the 1.55 μm wavelength band of each core 11 of the multi-core fiber thus prototyped. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Further, the core numbers “1” to “12” on the horizontal axis in FIG. 13 are assigned to the cores 11 in the twelve core elements 10 as shown in FIG. A black circle in FIG. 14 is a marker for connecting to another fiber.

<3-2> Comparative Example 3
The multi-core fiber which is the same as that of the above-mentioned Example 3 was made as a prototype for the components other than the stress adjusting unit 15 in the multi-core fiber 3 of the third embodiment. Various parameters in the stress adjusting unit 15 of this comparative example are shown in Table 8 below.

FIG. 15 shows the measurement results of PMD in the wavelength 1.55 μm band at each core 11 of this multi-core fiber. The PMD measurement result is specifically an average value when the wavelength is changed within a range of 1529 nm to 1625 nm. Also, the core numbers “1” to “12” on the horizontal axis in FIG. 15 are assigned as shown in FIG. 14 as in the third embodiment, and the marker positions are the same as those in FIG.

<3-3> Comparison From the comparison between FIG. 13 and FIG. 15, it was found that PMD can be improved in any core 11 when d ₂ / d ₁ and Δ in the stress adjusting unit 15 are larger.

When birefringence of the LP01 mode propagating through the core element 10 in Example 3 was calculated, the birefringence in all core numbers was 9.08.9 × 10 ⁻⁷ . Further, when birefringence of the LP01 mode propagating through the core element 10 in Comparative Example 3 was calculated, the birefringence in all the core numbers was 2.61 × 10 ⁻⁵ . From these calculation results, it was found that the larger the d ₂ / d ₁ and Δ in the stress adjusting portion 15, the more the birefringence can be reduced.

By the way, when various parameters in Table 6 in Example 3 and various parameters other than d ₂ / d ₁ and Δ in Table 7 are fixed, the parameters of d ₂ / d ₁ and Δ are changed. The calculation result of birefringence is shown in FIG. In addition, when the horizontal axis in FIG. 16 is 0, this corresponds to the case where the stress adjusting unit 15 is not provided.

As shown in FIG. 16, when d ₂ / d ₁ is smaller than 2, Δ is 0 to 0.1%, and when d ₂ / d ₁ is 2, Δ is 0 to −0.2%. It was found that birefringence can be suppressed by setting%. It was also found that when d ₂ / d ₁ is greater than 2, birefringence can be stably suppressed regardless of the value of Δ.

The multi-core fiber according to the present invention may be used in the field of handling optical fibers.

DESCRIPTION OF SYMBOLS 1-3 ... Multi-core fiber 10 ... Core element 11 ... Core 12 ... Inner cladding layer 13 ... Outer cladding layer 15 ... Stress adjustment part 20 ... Cladding 31 ... Inner Protective layer 32 ... Outer protective layer

Claims

Clad,
A plurality of core elements provided in the cladding and having a core, an inner cladding layer surrounding the core, and an outer cladding layer surrounding the inner cladding layer and having an average refractive index lower than that of the cladding and the inner cladding layer When,
A stress adjusting portion provided in the cladding, having a lower average refractive index than the cladding and the inner cladding layer;
The multi-core fiber, wherein the stress adjusting unit is arranged so that a difference in effective refractive index of polarized light belonging to the same LP mode in light propagating through each core element is small.
A plurality of the stress adjusting portions;
The multi-core fiber according to claim 1, wherein the multi-core fiber is arranged so as to be rotationally symmetric with respect to a center of the clad.
A plurality of the stress adjusting portions;
The multi-core fiber according to claim 1, wherein the multi-core fiber is arranged so as to sandwich the core element as an intermediate.
The multi-core fiber according to any one of claims 1 to 3, wherein the stress adjusting unit is made of quartz to which a fluorine dopant is added.
5. The relative refractive index difference of the stress adjusting portion with respect to the clad is in a range of −0.2% to −0.4%, according to any one of claims 1 to 4. Multi-core fiber.
The radius of the stress adjustment part is d 1, and a half of the distance between the center of the stress adjustment part and the core element or the center of the stress adjustment part closest to the stress adjustment part is d 2 6. The multi-core fiber according to claim 1, wherein d 2 / d 1 is set to be larger than 2 in such a case.
The multi-core fiber according to any one of claims 1 to 6, wherein the stress adjusting portion is a hole.