WO2025004909A1 - Polarization-maintaining multicore optical fiber and method for manufacturing polarization-maintaining multicore optical fiber - Google Patents

Abstract

A polarization-maintaining multicore optical fiber (1) comprises: a plurality of cores (2) disposed side by side in a first direction (X) orthogonal to an axial direction (Z); a plurality of optical claddings (3) respectively provided around the plurality of cores (2); a plurality of low refractive index parts (4) provided so as to sandwich each of the plurality of cores (2) therebetween in the first direction (X); and a common physical cladding (5) having a lower refractive index than the refractive index of the plurality of cores (2) and surrounding the plurality of cores (2), the plurality of optical claddings (3), and the plurality of low refractive index parts (4). The plurality of low refractive index parts (4) are disposed side by side alternately with the plurality of cores (2) in the first direction (X), at least a part of the outer periphery of each of the plurality of low refractive index parts (4) is provided in contact with the core (2), and the distance between the low refractive index parts (4) adjacent to each other in the first direction (X) is shorter than the length of each of the plurality of cores (2) in a second direction (Y) orthogonal to the axial direction (Z) and the first direction (X).

Description

Polarization-maintaining multi-core optical fiber and method for manufacturing the same

The present disclosure relates to a polarization-maintaining multi-core optical fiber and a method for manufacturing a polarization-maintaining multi-core optical fiber.
This application claims priority based on Japanese Application No. 2023-106126 filed on June 28, 2023, and incorporates by reference all of the contents of the above-mentioned Japanese application.

Patent Document 1 describes a polarization-maintaining multicore optical fiber (hereinafter, polarization-maintaining MCF) that includes a polarization-maintaining core having a core and a pair of low refractive index sections, an optical cladding, and a common physical cladding, and a method for manufacturing the polarization-maintaining MCF.

International Publication No. 2022/172910

A polarization-maintaining MCF according to one embodiment of the present disclosure includes a plurality of cores arranged side by side in a first direction perpendicular to the axial direction, a plurality of optical claddings having a refractive index lower than that of the plurality of cores and arranged around each of the plurality of cores, a plurality of low refractive index sections having a refractive index lower than that of the plurality of cores and arranged to sandwich each of the plurality of cores in the first direction, and a common physical cladding having a refractive index lower than that of the plurality of cores and surrounding the plurality of cores, the plurality of optical claddings, and the plurality of low refractive index sections, wherein the plurality of low refractive index sections are arranged alternately with the plurality of cores in the first direction, at least a portion of the outer periphery of each of the plurality of low refractive index sections is arranged in contact with the core, and the distance between adjacent low refractive index sections in the first direction is shorter than the length of each of the plurality of cores in a second direction perpendicular to the axial direction and the first direction.

FIG. 1 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to a first embodiment. FIG. 2 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to the second embodiment. FIG. 3 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to the third embodiment. FIG. 4 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to a fourth embodiment. 5A to 5C are process diagrams showing a method for manufacturing a polarization-maintaining MCF according to the second embodiment. 6A to 6C are cross-sectional views for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 7 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 8 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 9 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 10 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 11 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 12 is a cross-sectional view for explaining a manufacturing method of the polarization-maintaining MCF according to the second embodiment. FIG. 13 is a cross-sectional view for explaining a manufacturing method of the polarization-maintaining MCF according to the second embodiment. FIG. 14 is a cross-sectional view for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment.

[Problem to be solved by this disclosure]
In the polarization-maintaining MCF described in Patent Document 1, two low refractive index sections are provided for each core, so a sufficient inter-core distance is required, which places a large restriction on the core density.

The present disclosure aims to provide a polarization-maintaining MCF that allows for improved core density and a method for manufacturing the polarization-maintaining MCF.

[Effects of the present disclosure]
According to the present disclosure, a polarization-maintaining MCF capable of increasing core density and a method for manufacturing the polarization-maintaining MCF are provided.

Description of the embodiments of the present disclosure
First, the contents of the embodiments of the present disclosure will be listed and described.
(1) A polarization-maintaining MCF according to one embodiment of the present disclosure comprises a plurality of cores arranged side by side in a first direction perpendicular to an axial direction, a plurality of optical claddings having a refractive index lower than that of the plurality of cores and arranged around each of the plurality of cores, a plurality of low refractive index sections having a refractive index lower than that of the plurality of cores and arranged to sandwich each of the plurality of cores in the first direction, and a common physical cladding having a refractive index lower than that of the plurality of cores and surrounding the plurality of cores, the plurality of optical claddings, and the plurality of low refractive index sections, wherein the plurality of low refractive index sections are arranged alternately with the plurality of cores in the first direction, at least a portion of the outer periphery of each of the plurality of low refractive index sections is arranged in contact with the core, and the distance between adjacent low refractive index sections in the first direction is shorter than the length of each of the plurality of cores in a second direction perpendicular to the axial direction and the first direction.
In this polarization-maintaining MCF, one low refractive index section provided between two adjacent cores is used for maintaining the polarization of both cores. Therefore, compared to a configuration in which two dedicated low refractive index sections are provided for each core, it is possible to improve the core density. The low refractive index section is in contact with the core, so that the core density can be improved. The core density can also be improved because the distance between the low refractive index sections is shorter than the length of the core in the second direction. In addition, because the distance between the low refractive index sections is shorter than the length of the core in the second direction, the polarization maintenance of the core is realized due to the shape of the core.

(2) In the above (1), the multiple low refractive index sections include a first low refractive index section arranged between adjacent cores in the first direction and two second low refractive index sections arranged outside the multiple cores in the first direction, and the size of the second low refractive index section may be smaller than the size of the first low refractive index section. In this case, it is possible to further improve the core density.

(3) In the above (1), the multiple low refractive index sections may include a first low refractive index section arranged between adjacent cores in the first direction and two second low refractive index sections arranged outside the multiple cores in the first direction, and the size of the second low refractive index section may be equal to the size of the first low refractive index section. In this case, since no tool change is required when drilling holes in the first low refractive index section and the second low refractive index section, manufacturing is easier than in a configuration with different sizes.

(4) In any one of (1) to (3) above, the multiple low refractive index sections include a first low refractive index section arranged between adjacent cores in the first direction, and two second low refractive index sections arranged outside the multiple cores in the first direction, and the refractive index of the second low refractive index section may be lower than the refractive index of the first low refractive index section. In this case, it is possible to further improve the core density.

(5) In any one of (1) to (3) above, the multiple low refractive index sections may include a first low refractive index section arranged between adjacent cores in the first direction and two second low refractive index sections arranged outside the multiple cores in the first direction, and the refractive index of the second low refractive index section may be equal to the refractive index of the first low refractive index section. In this case, since there is no need to change the composition of the base material that becomes the first low refractive index section and the second low refractive index section, manufacturing is easier than in a configuration with different refractive indices.

(6) A method for manufacturing a polarization-maintaining MCF according to one aspect of the present disclosure includes the steps of: forming a plurality of holes in a base material including a plurality of core portions arranged side by side in a first direction perpendicular to an axial direction; a plurality of optical cladding portions having a refractive index lower than that of the plurality of core portions and provided around each of the plurality of core portions; and a common physical cladding portion having a refractive index lower than that of the plurality of core portions and surrounding the plurality of core portions and the plurality of optical cladding portions, the step of forming a plurality of holes so as to sandwich each of the central axes of the plurality of core portions in the first direction; inserting a plurality of low refractive index portion base materials having a refractive index lower than that of the plurality of core portions, one by one, into each of the plurality of holes; and drawing the base material and the plurality of low refractive index portion base materials after integrating them by heating or while integrating them by heating, wherein the plurality of holes may be formed in positions that are alternately aligned with the central axes of the plurality of core portions in the first direction and overlap with the plurality of core portions and the plurality of optical cladding portions.
In this method for manufacturing a polarization-maintaining MCF, one low refractive index section is formed between two adjacent cores, so that one low refractive index section can be used for polarization maintenance of both cores. This allows for improved core density compared to a configuration in which two dedicated low refractive index sections are formed for each core. The core density can also be improved because the holes are formed at positions that overlap with the core sections. In addition, because the holes are formed at positions that overlap with the core sections, the polarization maintenance of the cores is achieved due to the shape of the cores.

[Details of the embodiment of the present disclosure]
Specific examples of the polarization-maintaining MCF and the manufacturing method of the polarization-maintaining MCF according to the present embodiment will be described with reference to the drawings as necessary. Note that the present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and duplicate descriptions will be omitted.

(Polarization-Maintaining MCF)
Fig. 1 is a diagram showing a cross section perpendicular to the axial direction of the polarization-maintaining MCF according to the first embodiment. As shown in Fig. 1, the polarization-maintaining MCF 1 according to the first embodiment includes a plurality of cores 2, a plurality of optical claddings 3, a plurality of low refractive index sections 4, and one common physical cladding 5. The axial direction (or longitudinal direction) of the polarization-maintaining MCF 1 is defined as the Z-axis direction. In this embodiment, the number of cores 2 is three, the number of optical claddings 3 is three, and the number of low refractive index sections is four.

The multiple cores 2 are arranged side by side in the X-axis direction perpendicular to the Z-axis direction. In a cross section perpendicular to the axial direction of the polarization-maintaining MCF1 (hereinafter also simply referred to as the "cross section"), the multiple cores 2 are arranged on a straight line m that passes through the central axis C of the polarization-maintaining MCF1 and extends in the X-axis direction. The multiple cores 2 are spaced apart from one another.

The core 2 has a shape that is linearly symmetrical with respect to the line m in the cross section. The multiple cores 2 have the same shape in the cross section. In the cross section, the length (maximum length) of the core 2 in the X-axis direction is shorter than the length (maximum length) of the core 2 in the Z-axis direction and the Y-axis direction perpendicular to the X-axis direction. Due to such a shape of the core 2, polarization maintenance of the core 2 is achieved. The core 2 has polarization maintaining axes in the X-axis direction and the Y-axis direction. The core 2 is glass. The multiple cores 2 have the same composition. The multiple cores 2 have the same refractive index.

Multiple optical claddings 3 are provided around each of the multiple cores 2. One optical cladding 3 is provided for each core 2. The number of optical claddings 3 is equal to the number of cores 2. Each optical cladding 3 is divided into two regions in the Y-axis direction by multiple low refractive index portions 4. In other words, each optical cladding 3 includes two regions arranged to sandwich the core 2 in the Y-axis direction.

The two regions of each optical cladding 3 are configured to be symmetrical with respect to the line m in the cross section. The optical claddings 3 are spaced apart from each other. The optical claddings 3 are provided in contact with both ends of the core 2 in the Y-axis direction. The optical claddings 3 are glass having a different composition from the core 2. The optical claddings 3 have the same composition. The optical claddings 3 have the same refractive index. The optical claddings 3 have a refractive index lower than that of the cores 2.

The multiple low refractive index sections 4 are arranged to sandwich each of the multiple cores 2 in the X-axis direction. The multiple low refractive index sections 4 are arranged alternately with the multiple cores 2 in the X-axis direction. The number of low refractive index sections 4 is the number of cores 2 + 1. The multiple low refractive index sections 4 are arranged on a straight line m in the cross section. The multiple low refractive index sections 4 are spaced apart from each other. The multiple low refractive index sections 4 have the same shape in the cross section. The multiple low refractive index sections 4 have a circular shape in the cross section.

The multiple low refractive index sections 4 include a first low refractive index section 4A and two second low refractive index sections 4B. In this embodiment, the number of first low refractive index sections 4A is three. The first low refractive index section 4A is disposed between adjacent cores 2 in the X-axis direction. The two second low refractive index sections 4B are disposed outside the multiple cores 2 in the X-axis direction. The two second low refractive index sections 4B are located at both ends of the multiple low refractive index sections 4 in the X-axis direction. The first low refractive index section 4A is disposed between the two second low refractive index sections 4B.

The low refractive index sections 4 are provided in contact with the cores 2. The low refractive index sections 4 are provided in contact with at least one core 2 adjacent in the X-axis direction. The first low refractive index section 4A is provided in contact with two cores 2 adjacent in the X-axis direction. The second low refractive index section 4B is provided in contact with one core 2 adjacent in the X-axis direction. In the cross section, the outer periphery of the core 2 excluding the portion in contact with the low refractive index section 4 forms a part (arc) of a first virtual circle, and the low refractive index section 4 is disposed inside the first virtual circle. In the cross section, the outer periphery of the optical cladding 3 excluding the portion in contact with the low refractive index section 4 and the core 2 forms a part (arc) of a second virtual circle with a larger diameter than the first virtual circle, and the low refractive index section 4 is also disposed inside the second virtual circle.

The distance (shortest distance) between adjacent low refractive index sections 4 in the X-axis direction is shorter than the length (maximum length) of each of the multiple cores 2 in the Y-axis direction. The low refractive index section 4 covers the entire circumference of the core 2 together with the optical cladding 3. The low refractive index section 4 is a glass with a different composition from the core 2. The multiple low refractive index sections 4 have the same composition. The multiple low refractive index sections 4 have the same refractive index. The low refractive index section 4 has a refractive index lower than the refractive index of the multiple cores 2. Considering the refractive index fluctuation in the radial direction, even if the multiple low refractive index sections 4 have a refractive index variation of less than 0.05%, the multiple low refractive index sections 4 can be considered to have the same refractive index. When the difference between the refractive index of the first low refractive index section 4A and the refractive index of the second low refractive index section 4B is less than 0.05%, the refractive index of the first low refractive index section 4A and the refractive index of the second low refractive index section 4B are equivalent.

The common physical cladding 5 collectively surrounds the multiple cores 2, the multiple optical claddings 3, and the multiple low refractive index sections 4. The common physical cladding 5, together with the multiple optical claddings 3, constitutes the cladding of the polarization-maintaining MCF 1. The common physical cladding 5 is provided in contact with each of the multiple optical claddings 3 and the multiple low refractive index sections 4. The common physical cladding 5 is not in contact with the multiple cores 2. The common physical cladding 5 is glass of a different composition from the cores 2. The common physical cladding 5 has a refractive index lower than that of the multiple cores 2.

The core 2, optical cladding 3, low refractive index section 4, and common physical cladding 5 are composed of, for example, silica glass. The core 2, optical cladding 3, low refractive index section 4, and common physical cladding 5 are composed, for example, such that the refractive index of the core 2 > the refractive index of the common physical cladding 5 > the refractive index of the optical cladding 3 ≥ the refractive index of the low refractive index section 4. If this refractive index relationship is satisfied, the silica glass that composes the core 2, optical cladding 3, low refractive index section 4, and common physical cladding 5 may contain a trace amount of germanium (Ge), chlorine, or fluorine.

As described above, in the polarization-maintaining MCF 1, one first low refractive index section 4A provided between two adjacent cores 2 is used for maintaining the polarization of both cores 2. In other words, one common first low refractive index section 4A is provided for two adjacent cores 2. Therefore, the inter-core distance can be shortened compared to a configuration in which two dedicated low refractive index sections are provided for each core. As a result, it is possible to improve the core density. In addition, an increase in the base material size (base material diameter) can be avoided.

The hole drilling process performed during the manufacture of the polarization-maintaining MCF1 has limitations in the diameter of the hole drilling tool and the distance between holes that can be drilled. For this reason, the polarization-maintaining MCF1 allows the inter-core distance to be shorter than in a configuration in which two dedicated low refractive index sections are provided for each core. The multiple low refractive index sections 4 are provided in contact with the multiple cores 2. This allows for a further improvement in core density.

In this embodiment, the multiple low refractive index sections 4 have the same refractive index, but the refractive index of the second low refractive index section 4B may be lower than the refractive index of the first low refractive index section 4A. In this case, the leakage loss of the core 2 is further reduced by the second low refractive index section 4B arranged on the outside. Therefore, the core 2 can be arranged further outward, that is, in a region closer to the outer peripheral surface 5a of the common physical cladding 5. As a result, it is possible to further improve the core density.

FIG. 2 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to the second embodiment. As shown in FIG. 2, the polarization-maintaining MCF 1A according to the second embodiment differs from the polarization-maintaining MCF 1 in that the size of the second low refractive index section 4B is smaller than the size of the first low refractive index section 4A in the cross section. In the polarization-maintaining MCF 1A, the number of cores 2 is four, the number of optical claddings 3 is four, and the number of low refractive index sections 4 is five. Both the first low refractive index section 4A and the second low refractive index section 4B are circular in cross section.

The size of the second low refractive index portion 4B is the diameter of the second low refractive index portion 4B, and the size of the first low refractive index portion 4A is the diameter of the first low refractive index portion 4A. The diameter of the second low refractive index portion 4B is shorter than the diameter of the first low refractive index portion 4A. In the cross section, the size of the low refractive index portion 4 is defined as the maximum length of the low refractive index portion 4. For example, in the cross section, if the low refractive index portion 4 is elliptical, the size of the low refractive index portion 4 is the length of the major axis of the low refractive index portion 4.

In the polarization-maintaining MCF 1A, the small size of the second low-refractive index portion 4B makes it easier to maintain the distance between the second low-refractive index portion 4B and the outer peripheral surface 5a of the common physical cladding 5. This allows the core 2 to be positioned further outward, that is, in an area closer to the outer peripheral surface 5a of the common physical cladding 5. As a result, it is possible to further improve the core density.

FIG. 3 is a diagram showing a cross section perpendicular to the axial direction of the polarization-maintaining MCF according to the third embodiment. As shown in FIG. 3, the polarization-maintaining MCF 1B according to the third embodiment differs from the polarization-maintaining MCF 1A in that a plurality of cores 2 are arranged in two rows on lines m1 and m2 parallel to line m. In the cross section, lines m1 and m2 are symmetrical with respect to line m. The number of cores 2 arranged on lines m1 and m2 is equal to each other. Four cores 2, four optical claddings 3, and five low refractive index sections 4 are arranged on lines m1 and m2, respectively. In the polarization-maintaining MCF 1B, since a plurality of cores 2 are arranged in two rows, it is possible to further improve the core density. In addition, in the cross section, the size of the second low refractive index section 4B may be equivalent to the size of the first low refractive index section 4A. Equivalent includes manufacturing variations within 1 μm. The diameter of the hole-drilling tool used when forming the polarization-maintaining MCF base material varies from lot to lot, but the same hole-drilling tool can be used when forming holes of the same size. This reduces the manufacturing variation in hole size, and as a result, reduces the manufacturing variation in the size of the first low-refractive index portion 4A and the second low-refractive index portion 4B after fiberization. In addition, the refractive index of the second low-refractive index portion 4B may be lower than the refractive index of the first low-refractive index portion 4A.

FIG. 4 is a diagram showing a cross section perpendicular to the axial direction of the polarization-maintaining MCF according to the fourth embodiment. As shown in FIG. 4, the polarization-maintaining MCF 1C according to the fourth embodiment differs from the polarization-maintaining MCF 1 in that the cores 2 are arranged in four rows at equal intervals on lines m1, m2, m3, and m4 that are parallel to line m. In the cross section, the lines m1 and m2 are line-symmetrical with respect to line m, and the lines m3 and m4 are line-symmetrical with respect to line m. Line m1 is located between lines m and m3. Line m2 is located between lines m and m4. Two cores 2, two optical claddings 3, and three low refractive index sections 4 are arranged on lines m1, m2, m3, and m4, respectively. In the polarization-maintaining MCF 1C, the cores 2 are arranged in four rows, so that the core density can be further improved.

The size of the second low refractive index portion 4B on the straight lines m3 and m4 is smaller than the size of the first low refractive index portion 4A. The first low refractive index portion 4A and the second low refractive index portion 4B on the straight lines m3 and m4 are both circular in cross section, and the diameter of the second low refractive index portion 4B is smaller than the diameter of the first low refractive index portion 4A. This makes it easier to maintain the distance between the second low refractive index portion 4B on the straight lines m3 and m4 and the outer peripheral surface 5a of the common physical clad 5. Therefore, the core 2 can be arranged further outward, that is, in an area closer to the outer peripheral surface 5a of the common physical clad 5. As a result, it is possible to further improve the core density. Note that in the cross section, the size of the second low refractive index portion 4B on the straight lines m3 and m4 may be equal to the size of the first low refractive index portion 4A. Equivalent includes manufacturing variations within 1 μm. The diameter of the hole drilling tool used when forming the polarization-maintaining MCF base material varies from lot to lot, but the same hole drilling tool can be used when forming holes of the same size. This reduces manufacturing variation in the size of the holes, which reduces manufacturing variation in the size of the first low refractive index portion 4A and the second low refractive index portion 4B after fiberization. The refractive index of the second low refractive index portion 4B may be lower than the refractive index of the first low refractive index portion 4A.

(Method of Manufacturing Polarization-Maintaining MCF)
5 is a process diagram showing a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIGS. 6 to 14 are cross-sectional views for explaining a method for manufacturing a polarization-maintaining MCF according to the second embodiment. The method for manufacturing a polarization-maintaining MCF 1A includes a preparation step S1, a first hole-making step S2, a first insertion step S3, a first heating and integrating step S4, a second hole-making step S5, a second insertion step S6, a second heating and integrating step S7, a third hole-making step S8, a third insertion step S9, a third heating and integrating step S10, and a drawing step S11. The polarization-maintaining MCF 1A is manufactured by performing these steps in the following order, for example, but the order of the steps may be changed as appropriate. Each step will be described below.

The preparation process S1 is a process of preparing the core base material 12 (see FIG. 7) that will become the core 2, the first cladding base material 10 (see FIG. 6) that will become the optical cladding 3, the second cladding base material 14 (see FIG. 9) that will become the common physical cladding 5, and the low refractive index portion base materials 19, 20 (see FIG. 13) that will become the low refractive index portion 4. Each base material is cylindrical and made of silica glass. Each base material is configured so that the refractive index of the core base material 12>the refractive index of the second cladding base material 14>the refractive index of the first cladding base material 10≧the refractive index of the low refractive index base materials 19, 20. If this refractive index relationship is met, the silica glass that constitutes each base material may contain a small amount of germanium (Ge), chlorine, or fluorine.

The first hole drilling process S2 is a process for forming a hole 11 for inserting a core preform 12 (see FIG. 7) into the first clad preform 10, as shown in FIG. 6. The hole 11 has a circular cross section. The hole 11 has a central axis parallel to a predetermined axis (e.g., the central axis) of the first clad preform 10. The hole 11 does not protrude from the first clad preform 10, and the entire inner surface of the hole 11 is made up of the first clad preform 10. The hole 11 is formed so as to be coaxial with the first clad preform 10.

The first insertion step S3 is a step of inserting the core base material 12 into the hole 11 of the first clad base material 10, as shown in FIG. 7. The core base material 12 is inserted with the outer diameter of the core base material 12 appropriately adjusted to match the inner diameter of the hole 11. The outer diameter of the core base material 12 is adjusted so that the core base material 12 can be inserted into the hole 11. A predetermined axis (e.g., a central axis) of the first clad base material 10 and the central axis of the core base material 12 are parallel to each other.

The first heating integration process S4 is a process for integrating the first clad base material 10 and the core base material 12 by heating, as shown in FIG. 8. This results in the first base material 13. The first base material 13 has a core portion formed by the core base material 12 and an optical clad portion formed by the first clad base material 10.

The second hole drilling process S5 is a process for forming a plurality of holes 15 for inserting the first base material 13 (see FIG. 8) into the second clad base material 14, as shown in FIG. 9. The plurality of holes 15 are formed side by side in the X-axis direction perpendicular to the axial direction (Z-axis direction) of the second clad base material 14. The holes 15 have a circular cross section. The holes 15 have a central axis parallel to a predetermined axis (e.g., the central axis) of the second clad base material 14. The holes 15 do not protrude from the second clad base material 14, and the entire inner surface of the holes 15 is made up of the second clad base material 14.

The second insertion step S6 is a step of inserting the first base material 13 one by one into the holes 15 of the second clad base material 14, as shown in FIG. 10. The first base material 13 is inserted in a state where the outer diameter of the first base material 13 is appropriately adjusted to match the inner diameter of the holes 15. The outer diameter of the first base material 13 is adjusted so that the first base material 13 can be inserted into the holes 15. A predetermined axis (e.g., a central axis) of the second clad base material 14 and the central axis of the first base material 13 are parallel to each other.

The second heating integration process S7 is a process of integrating the second clad base material 14 and the first base material 13 by heating, as shown in FIG. 11. This results in a second base material 16. The second base material 16 has a core portion formed by the core base material 12, an optical clad portion formed by the first clad base material 10, and a common physical clad portion formed by the second clad base material 14. In the second base material 16, the multiple core portions are arranged side by side in the X-axis direction perpendicular to the axial direction (Z-axis direction) of the second base material 16. The optical clad portion has a refractive index lower than that of the multiple core portions, and is provided around each of the multiple core portions. The common physical clad portion surrounds the multiple core portions and the multiple optical clad portions. The second base material 16 has a clad portion formed by the optical clad portion and the common physical clad portion.

The third hole drilling process S8 is a process of forming multiple holes 17, 18 for inserting low refractive index portion base materials 19, 20 (see FIG. 13) in the second base material 16, as shown in FIG. 12. The multiple holes 17, 18 are formed so as to sandwich the central axes 12C of the multiple core base materials 12 in the X-axis direction. The multiple holes 17, 18 are formed alternately with the central axes 12C of the multiple core base materials 12 in the X-axis direction. The multiple holes 17, 18 are formed at positions overlapping the multiple core base materials 12 and the multiple first clad base materials 10. In other words, the holes 17, 18 are formed so as to partially remove the core base material 12 and the first clad base material 10.

The holes 17 and 18 have a circular cross section. The holes 17 and 18 have a central axis parallel to a predetermined axis (e.g., central axis 16C) of the second base material 16. The holes 17 and 18 do not protrude from the second base material 16, and the entire inner surface of the holes 17 and 18 is formed by the second base material 16.

Hole 17 is formed so that its central axis is located midway between the central axes 12C of two adjacent core preforms 12 in the X-axis direction. Hole 18 is formed at a position sandwiching the multiple core preforms 12 in the X-axis direction. Holes 17, 18 are arranged alternately with the multiple core preforms 12. The size (diameter) of hole 18 is smaller than the size (diameter) of hole 17. Hole 18 is formed so that the distance from the central axis 12C of the core preform 12 to hole 18 in the X-axis direction is equal to the distance from the central axis 12C of the core preform 12 to hole 17 in the X-axis direction. When two holes 17 are adjacent to each other, the two holes 17 are formed so that they are point-symmetrical with respect to the central axis 12C of the core 2. The minimum lengths in the X-axis direction of the multiple core preforms 12 are equal to each other.

The third insertion step S9 is a step of inserting the low refractive index portion base materials 19, 20 one by one into the holes 17, 18 of the second base material 16, as shown in FIG. 13. The low refractive index portion base material 19 is inserted into the hole 17, and the low refractive index portion base material 20 is inserted into the hole 18. The low refractive index portion base materials 19, 20 are inserted in a state where the outer diameter of the second base material 16 is appropriately adjusted to match the inner diameter of the holes 17, 18. The outer diameters of the low refractive index portion base materials 19, 20 are adjusted so that they can be inserted into the holes 17, 18. A predetermined axis (for example, the central axis 16C) of the second base material 16 and the central axis of the low refractive index portion base materials 19, 20 are parallel to each other.

The third heating integration process S10 is a process for integrating the second base material 16 and the low refractive index portion base materials 19, 20 by heating, as shown in FIG. 14. This results in a polarization-maintaining MCF base material 21. The polarization-maintaining MCF base material 21 comprises a core portion formed by the core base material 12, an optical cladding portion formed by the first cladding base material 10, a common physical cladding portion formed by the second cladding base material 14, and a low refractive index portion formed by the low refractive index portion base materials 19, 20.

The drawing process S11 is a process for drawing the polarization-maintaining MCF 1A from the polarization-maintaining MCF base material 21. In the drawing process, the polarization-maintaining MCF base material 21 is melted by heating and stretched to produce the polarization-maintaining MCF 1A.

As described above, in this manufacturing method, only one hole 17 is formed between two adjacent core preforms 12. Since one common hole 17 is formed for the two core preforms 12, the number of steps for drilling holes can be reduced. This makes manufacturing easier and reduces costs. According to this manufacturing method, one low refractive index section 4 is formed between two adjacent cores 2 of the polarization-maintaining MCF 1A, so one low refractive index section 4 can be used to maintain the polarization of both cores 2. Therefore, compared to a configuration in which two dedicated low refractive index sections 4 are formed for each core 2, it is possible to improve the core density. In the third hole drilling process S8, the multiple holes 17, 18 are formed at positions that overlap the multiple core preforms 12 and the multiple first cladding preforms 10. Therefore, due to the shape of the core 2 whose length in the X-axis direction is shorter than its length in the Y-axis direction, the polarization maintenance of the core 2 is realized.

In addition, in the third insertion step S9, the first ends of the holes 17, 18 of the second base material 16 are sealed, and the low refractive index portion base materials 19, 20 are inserted from the open second ends of the holes 17, 18. Instead of the third heating and integration step S10 and the drawing step S11, a drawing step may be performed while integrating the second base material 16 and the low refractive index portion base materials 19, 20 by heating.

Here, we have explained the manufacturing method for polarization-maintaining MCF 1A, but polarization-maintaining MCFs 1, 1B, and 1C can also be manufactured in a similar manner by adjusting the positions and diameters of the holes to be drilled.

Although the embodiments and modifications have been described above, the present disclosure is not necessarily limited to the above-mentioned embodiments and modifications, and various modifications are possible without departing from the gist of the disclosure. Furthermore, the above-mentioned embodiments and modifications may be combined as appropriate.

1, 1A, 1B, 1C...Polarization maintaining MCF
2... Core 3... Optical cladding 4... Low refractive index section 4A... First low refractive index section 4B... Second low refractive index section 5... Common physical cladding 5a... Outer circumferential surface 10... First cladding base material 11... Hole 12... Core base material 12C... Center axis 13... First base material 14... Second cladding base material 15... Hole 16... Second base material 17, 18... Hole 19... Low refractive index portion base material 20... Low refractive index portion base material 21... Polarization-maintaining MCF base material C... central axis m, m1, m2, m3, m4... straight lines

Claims (6)

A plurality of cores arranged side by side in a first direction perpendicular to the axial direction;
a plurality of optical claddings each having a refractive index lower than the refractive index of the plurality of cores and disposed around each of the plurality of cores;
a plurality of low refractive index portions having a refractive index lower than the refractive indexes of the plurality of cores and provided so as to sandwich each of the plurality of cores in the first direction;
a common physical cladding having a refractive index lower than a refractive index of the cores and surrounding the cores, the optical claddings, and the low refractive index portions;
the low refractive index portions are arranged alternately with the cores in the first direction,
At least a part of an outer periphery of each of the plurality of low refractive index portions is provided in contact with the core,
a distance between adjacent low refractive index portions in the first direction is shorter than a length of each of the plurality of cores in the axial direction and a second direction perpendicular to the first direction;
Polarization-maintaining multicore optical fiber. the plurality of low refractive index portions include a first low refractive index portion disposed between the cores adjacent to each other in the first direction, and two second low refractive index portions disposed outside the plurality of cores in the first direction,
The size of the second low refractive index portion is smaller than the size of the first low refractive index portion.
The polarization-maintaining multi-core optical fiber according to claim 1 . the plurality of low refractive index portions include a first low refractive index portion disposed between the cores adjacent to each other in the first direction, and two second low refractive index portions disposed outside the plurality of cores in the first direction,
The size of the second low refractive index portion is equal to the size of the first low refractive index portion.
The polarization-maintaining multi-core optical fiber according to claim 1 . the plurality of low refractive index portions include a first low refractive index portion disposed between the cores adjacent to each other in the first direction, and two second low refractive index portions disposed outside the plurality of cores in the first direction,
The refractive index of the second low refractive index portion is lower than the refractive index of the first low refractive index portion.
The polarization-maintaining multi-core optical fiber according to claim 1 . the plurality of low refractive index portions include a first low refractive index portion disposed between the cores adjacent to each other in the first direction, and two second low refractive index portions disposed outside the plurality of cores in the first direction,
The refractive index of the second low refractive index portion is equal to the refractive index of the first low refractive index portion.
The polarization-maintaining multi-core optical fiber according to claim 1 . a plurality of core portions arranged side by side in a first direction perpendicular to an axial direction; a plurality of optical cladding portions having a refractive index lower than that of the plurality of core portions and provided around each of the plurality of core portions; and a common physical cladding portion having a refractive index lower than that of the plurality of core portions and surrounding the plurality of core portions and the plurality of optical cladding portions, the method comprising the steps of: forming a plurality of holes so as to sandwich each of central axes of the plurality of core portions in the first direction;
inserting a plurality of low refractive index portion base materials, each having a refractive index lower than the refractive index of the core portions, into each of the plurality of holes, one by one;
and drawing the base material and the plurality of low refractive index portion base materials after integrating them by heating or while integrating them by heating,
the plurality of holes are arranged alternately with central axes of the plurality of core portions in the first direction and are formed at positions overlapping the plurality of core portions and the plurality of optical cladding portions;
A method for manufacturing a polarization-maintaining multi-core optical fiber.

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