Polarization-maintaining multicore optical fiber and method for manufacturing polarization-maintaining multicore optical fiber

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

[0001] This disclosure relates to a polarization-maintaining multi-core optical fiber and a method for manufacturing the polarization-maintaining multi-core optical fiber. This application claims priority to Japanese Application No. 2023-106126, filed on June 28, 2023, and incorporates the entire contents of said Japanese application by reference.

[0002] Patent Document 1 describes a polarization-maintaining multi-core optical fiber (hereinafter referred to as polarization-maintaining MCF) having a polarization-maintaining core having a core and a pair of low refractive index portions, an optical cladding, and a common physical cladding, and a method for manufacturing the polarization-maintaining MCF.

[0003] International Publication No. 2022 / 172910

[0004] A polarization-maintaining MCF according to one aspect 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.

[0005] FIG. 1 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to the 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 the fourth embodiment. FIG. 5 is a process diagram showing a method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 6 is a cross-sectional view 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 method for manufacturing a polarization-maintaining MCF according to the second embodiment. FIG. 13 is a cross-sectional view for explaining a method for manufacturing a 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.

[0006] [Problem to be Solved by the Present Disclosure] In the polarization-maintaining MCF described in Patent Document 1, two low refractive index sections are provided for each core, which requires a sufficient inter-core distance, which places a large restriction on core density.

[0007] An object of the present disclosure is to provide a polarization-maintaining MCF that allows for an improvement in core density and a method for manufacturing the polarization-maintaining MCF.

[0008] Effect of the Present Disclosure The present disclosure provides a polarization-maintaining MCF that allows for an improvement in core density and a method for manufacturing the polarization-maintaining MCF.

[0009] [Description of 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 aspect of the present disclosure includes: 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 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 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 arranged between two adjacent cores serves to maintain the polarization of both cores. Therefore, compared to a configuration in which two dedicated low refractive index sections are provided for each core, the core density can be improved. The fact that the low refractive index sections are in contact with the core also makes it possible to improve the core density. 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. Furthermore, because the distance between the low refractive index sections is shorter than the length of the core in the second direction, the shape of the core allows for polarization maintenance of the core.

[0010] (2) In the above (1), the plurality of low refractive index sections may include a first low refractive index section disposed between adjacent cores in the first direction and two second low refractive index sections disposed outside the plurality of 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.

[0011] (3) In the above (1), the plurality of low-refractive-index portions may include a first low-refractive-index portion disposed between adjacent cores in the first direction and two second low-refractive-index portions disposed outside the plurality of cores in the first direction, and the size of the second low-refractive-index portion may be equal to the size of the first low-refractive-index portion. In this case, since no tool change is required when drilling holes in the first low-refractive-index portion and the second low-refractive-index portion, manufacturing is easier than in a configuration in which the sizes are different.

[0012] (4) In any one of (1) to (3) above, the plurality of low refractive index sections may include a first low refractive index section disposed between adjacent cores in the first direction and two second low refractive index sections disposed outside the plurality of 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.

[0013] (5) In any one of (1) to (3) above, the plurality of low-refractive-index portions may include a first low-refractive-index portion disposed between adjacent cores in the first direction and two second low-refractive-index portions disposed outside the plurality of cores in the first direction, and the refractive index of the second low-refractive-index portion may be equal to the refractive index of the first low-refractive-index portion. In this case, it is not necessary to change the composition of the base material that becomes the first low-refractive-index portion and the second low-refractive-index portion, making manufacturing easier than in a configuration in which the refractive indexes are different.

[0014] (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 the axial direction; a plurality of optical cladding portions each having a refractive index lower than that of the core portions and provided around each of the core portions; and a common physical cladding portion having a refractive index lower than that of the core portions and surrounding the plurality of core portions and the plurality of optical cladding portions; inserting a plurality of low refractive index portion base materials each having a refractive index lower than that of the core portions 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 of manufacturing a polarization-maintaining MCF, one low-refractive-index section is formed between two adjacent cores, allowing one low-refractive-index section to serve as the polarization-maintaining section for 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 holes are also formed at positions overlapping the core sections, allowing for improved core density. Furthermore, because the holes are formed at positions overlapping the core sections, the core's polarization is maintained due to its shape.

[0015] [Details of the Embodiments of the Present Disclosure] Specific examples of polarization-maintaining MCFs and methods for manufacturing polarization-maintaining MCFs according to the present embodiments will be described with reference to the drawings as necessary. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, identical elements in the drawings will be assigned the same reference numerals, and duplicate explanations will be omitted.

[0016] (Polarization-Maintaining MCF) Fig. 1 is a diagram showing a cross section perpendicular to the axial direction of a polarization-maintaining MCF according to the first embodiment. As shown in Fig. 1, the polarization-maintaining MCF 1 according to the first embodiment includes multiple cores 2, multiple optical claddings 3, multiple low-refractive-index portions 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 portions is four.

[0017] The multiple cores 2 are arranged side by side in the X-axis direction, which is perpendicular to the Z-axis direction. In a cross section (hereinafter simply referred to as "cross section") perpendicular to the axial direction of the polarization-maintaining MCF 1, the multiple cores 2 are arranged on a straight line m that passes through the central axis C of the polarization-maintaining MCF 1 and extends in the X-axis direction. The multiple cores 2 are spaced apart from one another.

[0018] The core 2 has a shape that is line-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 that is perpendicular to the X-axis direction. Due to this 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.

[0019] A plurality of optical claddings 3 are provided around each of the plurality of 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 a plurality of low refractive index portions 4. That is, each optical cladding 3 includes two regions provided so as to sandwich the core 2 in the Y-axis direction.

[0020] The two regions of each optical cladding 3 are configured to be symmetrical with respect to the line m in the cross section. The multiple optical claddings 3 are spaced apart from each other. The optical claddings 3 are provided in contact with both ends of the cores 2 in the Y-axis direction. The optical claddings 3 are made of glass with a different composition from that of the cores 2. The multiple optical claddings 3 have the same composition. The multiple optical claddings 3 have the same refractive index. The optical claddings 3 have a refractive index lower than that of the multiple cores 2.

[0021] The multiple low refractive index portions 4 are arranged so as to sandwich each of the multiple cores 2 in the X-axis direction. The multiple low refractive index portions 4 are arranged alternately with the multiple cores 2 in the X-axis direction. The number of low refractive index portions 4 is the number of cores 2 + 1. The multiple low refractive index portions 4 are arranged on a straight line m in the cross section. The multiple low refractive index portions 4 are spaced apart from each other. The multiple low refractive index portions 4 have the same shape in the cross section. The multiple low refractive index portions 4 have a circular shape in the cross section.

[0022] The multiple low refractive index portions 4 include a first low refractive index portion 4A and two second low refractive index portions 4B. In this embodiment, the number of first low refractive index portions 4A is three. The first low refractive index portion 4A is disposed between adjacent cores 2 in the X-axis direction. The two second low refractive index portions 4B are disposed outside the multiple cores 2 in the X-axis direction. The two second low refractive index portions 4B are located at both ends of the multiple low refractive index portions 4 in the X-axis direction. The first low refractive index portion 4A is disposed between the two second low refractive index portions 4B.

[0023] The multiple low-refractive-index portions 4 are provided in contact with the multiple cores 2. Each low-refractive-index portion 4 is provided in contact with at least one core 2 adjacent to it in the X-axis direction. The first low-refractive-index portion 4A is provided in contact with two cores 2 adjacent to it in the X-axis direction. The second low-refractive-index portion 4B is provided in contact with one core 2 adjacent to it in the X-axis direction. In cross section, the outer periphery of the core 2 excluding the portion in contact with the low-refractive-index portions 4 forms a part (arc) of a first imaginary circle, and the low-refractive-index portions 4 are located inside the first imaginary circle. In cross section, the outer periphery of the optical cladding 3 excluding the portion in contact with the low-refractive-index portions 4 and the core 2 forms a part (arc) of a second imaginary circle with a larger diameter than the first imaginary circle, and the low-refractive-index portions 4 are also located inside the second imaginary circle.

[0024] The distance (shortest distance) between adjacent low-refractive-index portions 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 portion 4, together with the optical cladding 3, covers the entire circumference of the core 2. The low-refractive-index portion 4 is made of glass with a different composition from that of the core 2. The multiple low-refractive-index portions 4 have the same composition. The multiple low-refractive-index portions 4 have the same refractive index. The low-refractive-index portion 4 has a refractive index lower than that of the multiple cores 2. Considering refractive index fluctuations in the radial direction, even if the multiple low-refractive-index portions 4 have a refractive index variation of less than 0.05%, the multiple low-refractive-index portions 4 can be considered to have the same refractive index. When the difference between the refractive index of the first low-refractive-index portion 4A and the refractive index of the second low-refractive-index portion 4B is less than 0.05%, the refractive index of the first low-refractive-index portion 4A and the refractive index of the second low-refractive-index portion 4B are equivalent.

[0025] The common physical cladding 5 collectively surrounds the cores 2, the optical claddings 3, and the low refractive index portions 4. The common physical cladding 5, together with the 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 optical claddings 3 and the low refractive index portions 4. The common physical cladding 5 is not in contact with the cores 2. The common physical cladding 5 is a glass having a different composition from that of the cores 2. The common physical cladding 5 has a refractive index lower than that of the cores 2.

[0026] The core 2, the optical cladding 3, the low refractive index portion 4, and the common physical cladding 5 are made of, for example, silica glass. The core 2, the optical cladding 3, the low refractive index portion 4, and the common physical cladding 5 are configured, for example, so 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 portion 4. If this refractive index relationship is satisfied, the silica glass that makes up the core 2, the optical cladding 3, the low refractive index portion 4, and the common physical cladding 5 may contain a small amount of germanium (Ge), chlorine, or fluorine.

[0027] 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, the core density can be improved. Furthermore, an increase in the preform size (preform diameter) can be avoided.

[0028] In the drilling process performed during the manufacture of the polarization-maintaining MCF 1, there are limitations on the diameter of the drilling tool and the distance between holes that can be drilled. From this perspective, the polarization-maintaining MCF 1 allows for a shorter inter-core distance compared to 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.

[0029] In this embodiment, the plurality of low-refractive-index portions 4 have the same refractive index, but 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. In this case, the second low-refractive-index portion 4B disposed on the outside further reduces the leakage loss of the core 2. Therefore, the core 2 can be disposed further outward, i.e., in a region closer to the outer peripheral surface 5a of the common physical cladding 5. As a result, the core density can be further improved.

[0030] 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 portions 4B is smaller than the size of the first low-refractive-index portions 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 portions 4 is five. Both the first low-refractive-index portions 4A and the second low-refractive-index portions 4B are circular in cross section.

[0031] 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 has an elliptical shape, the size of the low refractive index portion 4 is the length of the major axis of the low refractive index portion 4.

[0032] In the polarization-maintaining MCF 1A, the small size of the second low-refractive-index portion 4B makes it easy to maintain the distance between the second low-refractive-index portion 4B and the outer peripheral surface 5 a of the common physical cladding 5. This allows the core 2 to be disposed further outward, i.e., in a region closer to the outer peripheral surface 5 a of the common physical cladding 5. As a result, the core density can be further improved.

[0033] FIG. 3 is a diagram illustrating a cross section perpendicular to the axial direction of a 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 multiple cores 2 are arranged in two rows on lines m1 and m2, which are parallel to line m. In the cross section, lines m1 and m2 are symmetrical with respect to line m. The numbers of cores 2 arranged on lines m1 and m2 are equal. Four cores 2, four optical claddings 3, and five low-refractive-index portions 4 are arranged side by side on lines m1 and m2, respectively. In the polarization-maintaining MCF 1B, the multiple cores 2 are arranged in two rows, which allows for further improvement in core density. Note that, in the cross section, the size of the second low-refractive-index portions 4B may be equivalent to the size of the first low-refractive-index portions 4A. "Equal" includes manufacturing variations of within 1 μm. Although the diameter of the drilling tool used to form the polarization-maintaining MCF preform varies from lot to lot, the same drilling tool can be used to form holes of the same size. This reduces manufacturing variations in hole size, thereby reducing manufacturing variations in the sizes of the first low-refractive-index portion 4A and the second low-refractive-index portion 4B after fiberization. Furthermore, 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.

[0034] 4 is a diagram showing a cross section perpendicular to the axial direction of a 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 a plurality of cores 2 are arranged at equal intervals in four rows along lines m1, m2, m3, and m4, which are parallel to line m. In the cross section, lines m1 and m2 are line-symmetric with respect to line m, and lines m3 and m4 are line-symmetric 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 portions 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, which enables further improvement in core density.

[0035] The size of the second low refractive index portion 4B on the 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 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 lines m3 and m4 and the outer peripheral surface 5a of the common physical cladding 5. This allows the core 2 to be positioned further outward, i.e., in a region closer to the outer peripheral surface 5a of the common physical cladding 5. As a result, the core density can be further improved. Note that the size of the second low refractive index portion 4B on the lines m3 and m4 in cross section may be equal to the size of the first low refractive index portion 4A. "Equal" includes manufacturing variations within 1 μm. Although the diameter of the drilling tool used to form the polarization-maintaining MCF preform varies from lot to lot, the same drilling tool can be used to form holes of the same size. Therefore, it is possible to reduce manufacturing variations in the hole size, and as a result, it is possible to reduce manufacturing variations in the sizes of the first low refractive index portion 4A and the second low refractive index portion 4B after fiberization. Furthermore, 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.

[0036] (Method of Manufacturing a Polarization-Maintaining MCF) FIG. 5 is a process diagram showing the method of manufacturing a polarization-maintaining MCF according to the second embodiment. FIGS. 6 to 14 are cross-sectional views illustrating the method of manufacturing a polarization-maintaining MCF according to the second embodiment. The method of manufacturing a polarization-maintaining MCF 1A includes a preparation step S1, a first hole-drilling step S2, a first insertion step S3, a first heating and integrating step S4, a second hole-drilling step S5, a second insertion step S6, a second heating and integrating step S7, a third hole-drilling 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, although the order of the steps may be changed as appropriate. Each step will be described below.

[0037] The preparation step S1 is a step of preparing a core preform 12 (see FIG. 7) that will become the core 2, a first cladding preform 10 (see FIG. 6) that will become the optical cladding 3, a second cladding preform 14 (see FIG. 9) that will become the common physical cladding 5, and low-refractive-index portion preforms 19, 20 (see FIG. 13) that will become the low-refractive-index portion 4. Each preform is cylindrical and made of silica glass. Each preform is configured so that the refractive index of the core preform 12 > the refractive index of the second cladding preform 14 > the refractive index of the first cladding preform 10 ≥ the refractive index of the low-refractive-index preforms 19, 20. As long as this refractive index relationship is met, the silica glass that constitutes each preform may contain a trace amount of germanium (Ge), chlorine, or fluorine.

[0038] As shown in Fig. 6 , the first hole drilling step S2 is a step of forming a hole 11 in the first clad preform 10 for inserting a core preform 12 (see Fig. 7 ). 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 circumferential surface of the hole 11 is made of the first clad preform 10. The hole 11 is formed so as to be coaxial with the first clad preform 10.

[0039] 7, the first insertion step S3 is a step of inserting the core preform 12 into the hole 11 of the first clad preform 10. The core preform 12 is inserted with the outer diameter of the core preform 12 appropriately adjusted to match the inner diameter of the hole 11. The outer diameter of the core preform 12 is adjusted so that the core preform 12 can be inserted into the hole 11. A predetermined axis (for example, a central axis) of the first clad preform 10 and the central axis of the core preform 12 are parallel to each other.

[0040] 8, the first heating and integrating step S4 is a step of integrating the first cladding preform 10 and the core preform 12 by heating, thereby obtaining the first preform 13. The first preform 13 includes a core portion formed by the core preform 12 and an optical cladding portion formed by the first cladding preform 10.

[0041] As shown in Fig. 9 , the second hole drilling step S5 is a step of forming a plurality of holes 15 in the second clad base material 14 for inserting the first base material 13 (see Fig. 8 ). The plurality of holes 15 are formed side by side in the X-axis direction, which is 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 circumferential surface of the holes 15 is made of the second clad base material 14.

[0042] 10 , the second insertion step S6 is a step of inserting the first base materials 13 one by one into the holes 15 of the second cladding base material 14. The first base materials 13 are 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 (for example, a central axis) of the second cladding base material 14 and the central axis of the first base material 13 are parallel to each other.

[0043] As shown in FIG. 11 , the second heating and integrating step S7 is a step of integrating the second cladding preform 14 and the first preform 13 by heating. This results in a second preform 16. The second preform 16 includes a core portion formed by the core preform 12, an optical cladding portion formed by the first cladding preform 10, and a common physical cladding portion formed by the second cladding preform 14. In the second preform 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 preform 16. The optical cladding 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 cladding portion surrounds the multiple core portions and the multiple optical cladding portions. The second preform 16 includes a cladding portion formed by the optical cladding portion and the common physical cladding portion.

[0044] As shown in Fig. 12 , the third hole drilling step S8 is a step of forming a plurality of holes 17, 18 in the second preform 16 for inserting low refractive index portion preforms 19, 20 (see Fig. 13 ). The plurality of holes 17, 18 are formed so as to sandwich the central axes 12C of the plurality of core preforms 12 in the X-axis direction. The plurality of holes 17, 18 are formed so as to alternate with the central axes 12C of the plurality of core preforms 12 in the X-axis direction. The plurality of holes 17, 18 are formed at positions overlapping with the plurality of core preforms 12 and the plurality of first cladding preforms 10. In other words, the holes 17, 18 are formed so as to partially remove the core preform 12 and the first cladding preform 10.

[0045] The holes 17, 18 have a circular cross section. The holes 17, 18 have central axes parallel to a predetermined axis (e.g., central axis 16C) of the second base material 16. The holes 17, 18 do not protrude from the second base material 16, and the entire inner circumferential surfaces of the holes 17, 18 are formed by the second base material 16.

[0046] The 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. The hole 18 is formed at a position sandwiching the multiple core preforms 12 in the X-axis direction. The holes 17, 18 are arranged alternately with the multiple core preforms 12. The size (diameter) of the hole 18 is smaller than the size (diameter) of the hole 17. The hole 18 is formed so that the distance from the central axis 12C of the core preform 12 to the hole 18 in the X-axis direction is equal to the distance from the central axis 12C of the core preform 12 to the hole 17. When two holes 17 are adjacent to each other, the two holes 17 are formed so that they are point-symmetric with respect to the central axis 12C of the core 2. The minimum lengths of the multiple core preforms 12 in the X-axis direction are equal to each other.

[0047] 13, the third insertion step S9 is a step of inserting low refractive index portion base materials 19, 20 one by one into holes 17, 18 of second base material 16. Low refractive index portion base material 19 is inserted into hole 17, and low refractive index portion base material 20 is inserted into hole 18. The low refractive index portion base materials 19, 20 are inserted in a state where the outer diameter of second base material 16 is appropriately adjusted to match the inner diameters of holes 17, 18. The outer diameters of low refractive index portion base materials 19, 20 are adjusted so that they can be inserted into holes 17, 18. A predetermined axis (e.g., central axis 16C) of second base material 16 and the central axes of low refractive index portion base materials 19, 20 are parallel to each other.

[0048] 14 , the third heating and integration step S10 is a step of integrating the second base material 16 and the low-refractive-index portion base materials 19 and 20 by heating. This results in a polarization-maintaining MCF base material 21. The polarization-maintaining MCF base material 21 includes 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 and 20.

[0049] The drawing step S11 is a step of drawing a polarization-maintaining MCF 1A from a polarization-maintaining MCF base material 21. In the drawing step, the polarization-maintaining MCF base material 21 is melted by heating and stretched, thereby producing the polarization-maintaining MCF 1A.

[0050] As described above, in this manufacturing method, only one hole 17 is formed between two adjacent core preforms 12. Because one common hole 17 is formed for the two core preforms 12, the number of steps required for drilling can be reduced. This facilitates manufacturing 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, allowing one low-refractive-index section 4 to serve as the polarization-maintaining section for both cores 2. This improves core density compared to a configuration in which two dedicated low-refractive-index sections 4 are formed for each core 2. In the third hole-drilling step S8, multiple holes 17, 18 are formed at positions overlapping with multiple core preforms 12 and multiple first cladding preforms 10. Therefore, polarization maintenance of the core 2 is achieved 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.

[0051] In addition, in the third insertion step S9, the first ends of the holes 17 and 18 of the second base material 16 are sealed, and then the low refractive index portion base materials 19 and 20 are inserted from the open second ends of the holes 17 and 18. In place of the third heating and integration step S10 and the drawing step S11, a drawing step may be performed in which the second base material 16 and the low refractive index portion base materials 19 and 20 are integrated by heating.

[0052] Although the manufacturing method for the polarization-maintaining MCF 1A has been described above, the polarization-maintaining MCFs 1, 1B, and 1C can also be manufactured in the same manner by adjusting the positions and diameters of the holes to be drilled.

[0053] Although the embodiments and modifications have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure. In addition, the above-described embodiments and modifications may be combined as appropriate.

[0054] 1, 1A, 1B, 1C...Polarization-maintaining MCF 2...Cord 3...Optical club 4...Low-refractive-index section 4A...First low-refractive-index section 4B...Second low-refractive-index section 5...Common physics club 5a...Outer surface 10...First club base material 11...Hole 12...Cord base material 12C...Center axis 13...First base material 14...Second club base material 15...Hole 16...Second base material 17, 18...Hole 19...Low-refractive-index section base material 20...Low-refractive-index section base material 21...Polarization-maintaining MCF base material C...Center axis m, m1, m2, m3, m4...Straight line

Claims

1. A polarization-maintaining multi-core optical fiber comprising: 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 a distance between adjacent low refractive index sections in the first direction is shorter than a length of each of the plurality of cores in a second direction perpendicular to the axial direction and the first direction.

2. The polarization-maintaining multi-core optical fiber according to claim 1, wherein the plurality of 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 plurality of cores in the first direction, and a size of the second low refractive index section is smaller than a size of the first low refractive index section.

3. The polarization-maintaining multi-core optical fiber according to claim 1, wherein the plurality of 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 plurality of cores in the first direction, and a size of the second low refractive index section is equal to a size of the first low refractive index section.

4. A polarization-maintaining multi-core optical fiber according to any one of claims 1 to 3, wherein the plurality of 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 plurality of cores in the first direction, and the refractive index of the second low refractive index section is lower than the refractive index of the first low refractive index section.

5. A polarization-maintaining multi-core optical fiber according to any one of claims 1 to 3, wherein the plurality of 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 plurality of cores in the first direction, and the refractive index of the second low refractive index section is equivalent to the refractive index of the first low refractive index section.

6. A method for manufacturing a polarization-maintaining multi-core optical fiber, comprising: a preform 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 method comprising: forming a plurality of holes to sandwich each of central axes of the plurality of core portions in the first direction; inserting a plurality of low refractive index portion preforms 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 preform and the plurality of low refractive index portion preforms after integrating them by heating or while integrating them by heating, wherein the plurality of holes are formed at positions that are alternately arranged 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.

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