Optical fiber

Abstract

This optical fiber comprises a core region and a cladding surrounding the core region. A dopant for changing the refractive index is added to at least one among the core region and the cladding. In a cross-section perpendicular to the longitudinal direction of the optical fiber, the concentration distribution of the dopant is rotationally asymmetric with respect to the center of the cladding. In the cross-section, the following formula (1) is satisfied.

Description

optical fiber

[0001] This application claims priority from Japanese Patent Application No. 2023-188622, filed on November 2, 2023, the contents of which are incorporated herein by reference.

[0002] Patent Document 1 discloses a multicore fiber including a plurality of cores and a cladding surrounding the plurality of cores.

[0003] For example, in such a multicore fiber, in order to identify each core, the concentration distribution of the dopant added to at least one of the core and the cladding may be made rotationally asymmetric with respect to the center of the cladding. Specifically, the cores may be arranged rotationally asymmetric with respect to the center of the cladding. Alternatively, multiple cores may be arranged to have rotational symmetry, and then a marker may be placed at a position that breaks this rotational symmetry.

[0004] Japanese Patent Application Publication No. 2022-66064

[0005] In general, the thermal expansion coefficient and viscosity of a material doped with a dopant change depending on the type and concentration of the dopant. Therefore, if the dopant concentration distribution in an optical fiber is rotationally asymmetric, the distribution of the thermal expansion coefficient and viscosity will also be rotationally asymmetric. Therefore, during the manufacturing (drawing) of an optical fiber, rotationally asymmetric stress is applied to the cross section of the optical fiber due to the tension and heat applied during manufacturing. Such stress can cause the optical fiber to warp, i.e., fiber curl.

[0006] The present invention has been made in consideration of the above circumstances, and has an object to provide an optical fiber capable of suppressing fiber curl.

[0007] In order to solve the above problems, an optical fiber according to a first aspect of the present invention is an optical fiber comprising a core region and a cladding surrounding the core region, wherein at least one of the core region and the cladding is doped with a dopant that changes the refractive index, and in a cross section perpendicular to the longitudinal direction of the optical fiber, the concentration distribution of the dopant is rotationally asymmetric with respect to the center of the cladding, and the following formula (1) is satisfied in the cross section:

[0008]

[0009] According to the first aspect of the present invention, an optical fiber in which fiber curling is suppressed can be realized.

[0010] A second aspect of the present invention is the optical fiber of the first aspect, wherein the following formula (2) is satisfied at the cross section:

[0011]

[0012] In addition, according to a third aspect of the present invention, the optical fiber of the first or second aspect includes a plurality of the core regions.

[0013] Furthermore, a fourth aspect of the present invention relates to the optical fiber of the third aspect, further comprising a marker having a refractive index different from that of the cladding, wherein the plurality of core regions are arranged at positions having two-fold or more rotational symmetry with respect to the center of the cladding in the cross section, and the marker is arranged at a position that allows the plurality of core regions to be identified.

[0014] Aspect 5 of the present invention is the optical fiber of aspect 4, wherein the marker includes a central portion and an annular portion surrounding the central portion, and the difference in composition between the central portion and the cladding is smaller than the difference in composition between the annular portion and the cladding.

[0015] A sixth aspect of the present invention is the optical fiber of the fourth or fifth aspect, wherein the following formula (3) is satisfied in the cross section:

[0016]

[0017] A seventh aspect of the present invention is the optical fiber of the sixth aspect, wherein the following formula (4) is satisfied at the cross section:

[0018]

[0019] According to the above aspect of the present invention, an optical fiber capable of suppressing fiber curl can be provided.

[0020] 1 is a cross-sectional view showing an optical fiber according to a first embodiment of the present invention; FIG. 2 is a cross-sectional view showing an optical fiber according to a second embodiment of the present invention; FIG. 3 is a cross-sectional view showing an optical fiber according to a third embodiment of the present invention; １ 1 is a graph showing the relationship between the curvature 1 / R of the optical fiber and the parameter p ２ 10 is a graph showing the relationship between

[0021] First Embodiment An optical fiber according to a first embodiment of the present invention will now be described with reference to the drawings. As shown in Fig. 1, an optical fiber 10A according to this embodiment includes a core region 1 and a cladding 2. The optical fiber 10A is made of silica glass.

[0022] In this specification, the longitudinal direction of the cladding 2 (optical fiber 10A) is simply referred to as the "longitudinal direction." The longitudinal direction is also the direction parallel to the central axis O of the cladding 2 (optical fiber 10A). The longitudinal direction is also referred to as the axial direction. Unless otherwise specified, the term "cross section" in this specification means a cross section perpendicular to the longitudinal direction. Viewing in a cross section is referred to as a "cross-sectional view." When viewed from the longitudinal direction, the direction perpendicular to the central axis O of the cladding 2 is referred to as the radial direction. The direction approaching the central axis O along the radial direction is referred to as the radially inner direction, and the direction away from the central axis O is referred to as the radially outer direction. When viewed from the longitudinal direction, the direction going around the central axis O is referred to as the circumferential direction.

[0023] The core region 1 includes at least a core through which light is guided. In addition to the core, the core region 1 may also include a subregion (not shown) having a refractive index different from that of the core. The subregion may be provided, for example, to adjust the characteristics of light guided through the core. In each embodiment in this specification, a case will be described in which the core region 1 does not include such a subregion, and includes only the core. In other words, in this embodiment, the core region 1 corresponds to the core. Therefore, in this specification, the core region 1 will be referred to as the "core 1" unless otherwise specified.

[0024] The cladding 2 surrounds the core 1 in a cross-sectional view. In other words, the cladding 2 covers the outer peripheral surface of the core 1. The core 1 according to this embodiment has a substantially circular shape in a cross-sectional view. The cladding 2 according to this embodiment has a substantially circular outer shape in a cross-sectional view. However, the term "substantially circular" in this specification also includes cases where the shape can be considered circular if manufacturing errors are removed. Specifically, for the core 1, an error of 2% or less in non-circularity in the cross section corresponds to a manufacturing error. Similarly, for the cladding 2, an error of 2% or less in non-circularity in the cross section corresponds to a manufacturing error.

[0025] A dopant is added to at least one of the core 1 and the cladding 2. The dopant is a material that is added to the optical fiber preform during the manufacture of the optical fiber 10A in order to make the refractive indexes of the core 1 and the cladding 2 different.

[0026] Specifically, the optical fiber 10A according to this embodiment is manufactured by drawing (spinning) an optical fiber preform made of silica glass. Here, the optical fiber preform includes a core and a cladding surrounding the core. The core is the portion that will become the core 1 of the optical fiber 10A after drawing, and the cladding is the portion that will become the cladding 2 of the optical fiber 10A after drawing.

[0027] At least one of the core and cladding is doped with a dopant that changes the refractive index of the silica glass, for example, during the manufacture of an optical fiber preform. More specifically, a dopant that changes the refractive index is added to the core (core 1), a dopant that changes the refractive index is added to the cladding (cladding 2), or both. This results in an optical fiber 10A in which the average refractive index of the cladding 2 is lower than the refractive index of the core 1. Because the average refractive index of the cladding 2 is lower than the refractive index of the core region 1 (core), the optical fiber 10A can confine light in the core 1. Examples of dopants include fluorine, boron, germanium, aluminum, phosphorus, erbium, ytterbium, and chlorine.

[0028] More specifically, a dopant is added to at least one of the core 1 and the cladding 2 so that the effective refractive index that influences the light guided through the core 1 is higher than the average refractive index of the cladding 2. The term "effective refractive index" refers to the effective value of the refractive index that takes into account the seepage into the cladding 2 of the electric field that constitutes the light guided through the core 1. In other words, a portion of the electric field that constitutes the light guided through the core 1 seeps into the cladding 2. Therefore, the value of the effective refractive index that actually influences the light is smaller than the value of the refractive index of the core 1 and larger than the value of the refractive index of the cladding 2. If this effective refractive index is larger than the refractive index of the cladding 2, the light can be guided through the core 1.

[0029] As shown in Fig. 1, in an optical fiber 10A according to this embodiment, the core 1 is disposed at a position shifted from the center O of the cladding 2 in a cross-sectional view. The cladding 2 has two regions 2a and 2b that are in contact with each other at a boundary line B. The two regions 2a and 2b have different refractive indices. The dopant concentration differs between the regions 2a and 2b. As a result, the dopant concentration distribution in the cross section of the optical fiber 10A is rotationally asymmetric with respect to the center O.

[0030] The cladding 2 may have three or more regions with different refractive indices. Alternatively, the cladding 2 may have a refractive index profile that varies continuously across its cross section. Alternatively, the cladding 2 may have a uniform refractive index profile across its entire cross section. The same applies to the core 1 (and the markers 3, which will be described later).

[0031] Second Embodiment Next, a second embodiment will be described, which has the same basic configuration as the first embodiment. Therefore, the same components are denoted by the same reference numerals, and the description thereof will be omitted, and only the differences will be described.

[0032] 2 , unlike the optical fiber 10A according to the first embodiment, the optical fiber 10B according to this embodiment includes a plurality of cores 1. Furthermore, the optical fiber 10B according to this embodiment further includes markers 3 in addition to the cores 1 and cladding 2.

[0033] The multiple cores 1 are arranged in positions in the cross section of the optical fiber 10B that have two-fold or more rotational symmetry with respect to the center O of the cladding 2. In the illustrated example, the optical fiber 10B has four cores 1, and the four cores 1 are arranged in positions that have four-fold rotational symmetry with respect to the center O.

[0034] The number of cores 1 included in the optical fiber 10B can be changed as appropriate, and may be three or less, or five or more. Similarly, the multiple cores 1 may be arranged so as to have three-fold or less rotational symmetry, or may be arranged so as to have five-fold or more rotational symmetry. Furthermore, the term "having rotational symmetry" in this specification does not only refer to cases where rotational symmetry is strictly present, but also includes cases where rotational symmetry can be considered to be present once manufacturing errors are removed.

[0035] The marker 3 according to this embodiment has a substantially circular shape in a cross-sectional view. The marker 3 is disposed in the cladding 2. The marker 3 has a refractive index different from that of the cladding 2. In order to make the refractive indexes of the cladding 2 and the marker 3 different, a dopant may be added to at least one of the cladding 2 and the marker 3.

[0036] The markers 3 are arranged at positions where the multiple cores 1 can be identified. For example, the markers 3 may be arranged at positions that break the rotational symmetry of the multiple cores 1. In the illustrated example, the markers 3 are arranged at positions that break the four-fold rotational symmetry of the optical fiber 10B. By providing such markers 3, it is possible to improve the identification of the cores 1. Note that the size, arrangement, shape, and number of the markers 3 can be changed as appropriate as long as the multiple cores 1 can be identified.

[0037] 2, the multiple cores 1 are arranged in positions that have two-fold or more rotational symmetry with respect to the center O of the cladding 2 in the cross section of the optical fiber 10B. However, the cores 1 may also be arranged so as to be rotationally asymmetric with respect to the center O of the cladding 2. Even in this case, markers 3 for identifying the cores 1 may be arranged.

[0038] By arranging the marker 3 at a position where multiple cores 1 can be identified, in this embodiment, as in the first embodiment, the dopant concentration distribution in the cross section of the optical fiber 10B is rotationally asymmetric with respect to the center O.

[0039] In the second embodiment, the structure in which the cores 1 arranged at positions with rotational symmetry are identified by the markers 3 is shown, but the structure for identifying the cores 1 is not limited to this. For example, the cores 1 may be identified by making the refractive index distribution, size, or shape of at least one of the four cores 1 different from the refractive index distribution, size, or shape of the other three cores 1. Even in this case, as in the first and second embodiments, the dopant concentration distribution in the cross section of the optical fiber 10 is rotationally asymmetric with respect to the center O.

[0040] Third Embodiment Next, a third embodiment will be described, which has the same basic configuration as the second embodiment. Therefore, the same components are denoted by the same reference numerals, and the description thereof will be omitted, and only the differences will be described.

[0041] 3, the optical fiber 10C according to this embodiment differs from the optical fiber 10B according to the second embodiment in the shape of the marker 3. Specifically, the marker 3 according to this embodiment has a double structure including a central portion 3 a and a ring portion 3 b.

[0042] The ring portion 3b surrounds the central portion 3a in a cross-sectional view. In other words, the ring portion 3b covers the outer peripheral surface of the central portion 3a. The central portion 3a according to this embodiment has a substantially circular shape in a cross-sectional view. The ring portion 3b has a substantially circular outer shape in a cross-sectional view. In other words, the ring portion 3b has an annular (e.g., annular) shape surrounding the central portion 3a in a cross-sectional view.

[0043] The composition of the central portion 3 a and the composition of the ring portion 3 b are different from each other. As a result, the refractive index of the central portion 3 a and the refractive index of the ring portion 3 b are different from each other. More specifically, the difference between the composition of the central portion 3 a and the composition of the cladding 2 (first composition difference) is smaller than the difference between the composition of the ring portion 3 b and the composition of the cladding 2 (second composition difference). As a result, the difference between the refractive index of the central portion 3 a and the refractive index of the cladding 2 (first refractive index difference) is smaller than the difference between the refractive index of the ring portion 3 b and the refractive index of the cladding 2 (second refractive index difference). In order to make the composition and refractive index different between the central portion 3 a and the ring portion 3 b, a dopant may be added to at least one of the central portion 3 a and the ring portion 3 b.

[0044] For example, the concentration of the dopant added to the central portion 3 a may be less than the concentration of the dopant added to the ring portion 3 b and may be equal to or greater than the concentration of the dopant added to the cladding 2. By making the dopant concentrations different between the central portion 3 a and the ring portion 3 b in this manner, it is possible to realize a configuration in which the first refractive index difference (first composition difference) is smaller than the second refractive index difference (second composition difference), as described above.

[0045] Furthermore, the central portion 3a may have the same composition as the cladding 2. That is, the first composition difference may not exist. In this case, the refractive index of the central portion 3a and the refractive index of the cladding 2 match. That is, the first refractive index difference becomes zero. When the composition (refractive index) of the central portion 3a and the composition (refractive index) of the cladding 2 match, the marker 3 can also be considered to have only the ring portion 3b. In other words, it can also be considered that an annular (circular ring) marker 3 (3a) is arranged in the cladding 2.

[0046] In this embodiment, as in the second embodiment, the markers 3 are arranged at positions where the multiple cores 1 can be identified (positions that break the rotational symmetry of the cores 1). Therefore, in the cross section of the optical fiber 10C, the dopant concentration distribution is rotationally asymmetric with respect to the center O.

[0047] (Conditions for Suppressing Fiber Curl) In general, the thermal expansion coefficient and viscosity of a material doped with a dopant change depending on the type and concentration of the dopant. Therefore, when the dopant concentration distribution is rotationally asymmetric, as in the optical fibers 10A to 10C according to the first to third embodiments, the distributions of the thermal expansion coefficient and viscosity also become rotationally asymmetric. Therefore, during the manufacturing (drawing) of the optical fibers 10A to 10C, there is a possibility that rotationally asymmetric stresses are applied to the cross sections of the optical fibers 10A to 10C due to the tension and heat applied during the manufacturing process. Such stresses can cause the optical fibers 10A to 10C to warp, i.e., fiber curl.

[0048] To address this issue, the inventors of the present application have discovered that fiber curl can be suppressed by making the optical fibers 10A to 10C satisfy certain conditions. These conditions will be explained below using specific test examples. Note that the present invention is not limited to the following test examples.

[0049] Optical fibers according to Comparative Examples 1 to 4 and Examples 1 to 10 were manufactured. Then, the fiber curl that occurred in each of the manufactured optical fibers was evaluated. Table 1 is a table summarizing the details of the manufacturing conditions of the optical fiber and the evaluation results of the fiber curl that occurred. Note that Example 11 shows the results of a simulation performed on a computer. Specifically, in Example 11, the stress acting on the cross section of the optical fiber was calculated by simulation, and the parameter p １ (described later) and calculate the obtained parameter p １ and the curvature is calculated from equation (7) (described later).

[0050]

[0051] In Table 1, the "Diameter D" row indicates the diameter of the manufactured optical fibers 10A to 10C. The "Arrangement" row indicates the arrangement of the cores and markers. That is, arrangement "A" corresponds to the structure of the optical fiber 10A according to the first embodiment (see FIG. 1), arrangement "B" corresponds to the structure of the optical fiber 10B according to the second embodiment (see FIG. 2), and arrangement "C" corresponds to the structure of the optical fiber 10C according to the third embodiment (see FIG. 3). The "Tension" row indicates the tension applied to the optical fiber when it was drawn. In the test examples in Table 1, the tension applied to the optical fiber during drawing was changed within a range of 30 gf to 195 gf.

[0052] The optical fibers of Comparative Examples 1 to 4 and Examples 1 and 2 were manufactured with different core diameters. Specifically, the core diameter was 3.6 μm in Comparative Example 2 and Comparative Example 4, 6.2 μm in Comparative Example 1 and Example 1, and 9.8 μm in Comparative Example 3 and Example 2. Furthermore, the marker diameter was 6.2 μm, which was common to all of Examples 3 to 11.

[0053] In Table 1, the "Curvature" column shows the results of measuring the curvature of the manufactured optical fibers 10A to 10C. The "Fiber Curl" column shows the results of evaluating the fiber curl using three levels: "poor," "good," and "excellent." "Poor" is the lowest rating, corresponding to a large fiber curl. "Excellent" is the highest rating, corresponding to a small fiber curl. "Good" is an intermediate rating between "poor" and "excellent." Specifically, a curvature of 0.25 or less was evaluated as "excellent," a curvature of more than 0.25 and less than 0.5 was evaluated as "good," and a curvature of more than 0.5 was evaluated as "poor." The curvatures (0.25 and 0.5) used as the criteria for judging "poor," "good," and "excellent" are based on the IEC 60793-2-50 standard.

[0054] In Table 1, "p １ " "p ２ " is a parameter introduced by the inventors of the present invention to consider the conditions under which fiber curl can be suppressed. １ and p ２ are defined by the following equations (5) and (6) at the cross sections of the optical fibers 10A to 10C, respectively.

[0055]

[0056]

[0057] In equation (5), M corresponds to the magnitude of the bending moment acting on the optical fibers 10A to 10C. The bending moment M is generated due to the longitudinal stress σ (tensile stress, compressive stress) remaining in the optical fibers 10A to 10C. The parameter p １ The definition is based on an analogy with structural mechanics concepts applied to a cylindrical member with one end fixed (a so-called cantilever beam).

[0058] That is, in terms of structural mechanics, the curvature 1 / R of the cantilever is proportional to the bending moment M and inversely proportional to the second moment of area I. Here, if the cantilever is cylindrical, the second moment of area I is proportional to the fourth power of the diameter D of the cylinder. By analogy applying this concept to the optical fibers 10A to 10C, the magnitude of the fiber curl (curvature 1 / R) is proportional to M / D ４ , i.e., the parameter p １ It is considered to be proportional to

[0059] FIG. 4 shows the relationship between the magnitude of the fiber curl (curvature 1 / R) and the parameter p １ 4 is a graph showing the relationship between the magnitude of the fiber curl and the parameter p. Each plot in FIG. 4 is based on Table 1. However, the data of Example 11 is not plotted in FIG. 4. As shown in FIG. 4, the relationship between the magnitude of the fiber curl and the parameter p １ A positive correlation was confirmed between the L １ is an approximate straight line obtained from the plot. １ is expressed by the following equation (7): From equation (7), the parameter p １ It can be derived that the curvature 1 / R can be made 0.5 or less (fiber curl "good") by making the parameter p １ It follows that by making the curvature 1 / R equal to or less than 0.85, the curvature 1 / R can be made equal to or less than 0.25 (fiber curl "excellent").

[0060]

[0061] Here, it is considered that the main cause of fiber curl in the optical fibers 10B and 10C is the stress σ acting on the markers 3 provided at asymmetric positions. ２ is a parameter that focuses on the difference between the stress σ acting on the region A where the marker 3 exists and the stress σ acting on the region A' that is located point-symmetrically to the region A with respect to the center O. ２ It is expected that a more accurate evaluation will be possible by using the formula (6). The right-hand side of the formula (6) can also be regarded as limiting the integral range of the bending moment M in the formula (*) (see formula 5) to only the areas A and A'.

[0062] FIG. 5 shows the relationship between the magnitude of the fiber curl (curvature 1 / R) and the parameter p ２ 5 is a graph showing the relationship between the magnitude of the fiber curl and the parameter p ２ A positive correlation was confirmed between the L ２ is an approximate straight line obtained from the plot. ２ is expressed by the following equation (8): From equation (8), the parameter p ２ It can be derived that the curvature 1 / R can be made 0.5 or less (fiber curl "good") by making the parameter p ２ It follows that by making the curvature 1 / R equal to or less than 0.41, the curvature 1 / R can be made equal to or less than 0.25 (fiber curl "excellent").

[0063]

[0064] In light of the above, this specification provides optical fibers 10A to 10C each including a core 1 (core region 1) and a clad 2 surrounding the core 1, wherein at least one of the core 1 and the clad 2 is doped with a dopant that changes the refractive index, and in a cross section perpendicular to the longitudinal direction of the optical fibers 10A to 10C, the concentration distribution of the dopant is rotationally asymmetric with respect to the center O of the clad 2, and the above-mentioned parameter p １ The present invention proposes optical fibers 10A to 10C having a curvature 1 / R of 1.70 or less. This configuration makes it possible to realize optical fibers 10A to 10C having a curvature 1 / R of 0.5 or less and suppressing fiber curl.

[0065] Also, the parameter p １ In this case, the curvature 1 / R is 0.25 or less, and the optical fibers 10A to 10C in which fiber curl is further suppressed can be realized.

[0066] Furthermore, the optical fibers 10B to 10C each include a plurality of cores 1 and further include a marker having a refractive index different from that of the cladding 2, the plurality of cores 1 are arranged at positions that have two or more rotational symmetries with respect to the center O of the cladding 2 in the cross section, the marker 3 is arranged at a position that allows the plurality of cores 1 to be identified, and the above-mentioned parameter p ２ may be 0.74 or less. According to this configuration, fiber curl can be more reliably suppressed in the optical fibers 10B and 10C in which the markers 3 are provided at asymmetric positions.

[0067] Also, the parameter p ２ In this case, fiber curl can be more reliably suppressed in the optical fibers 10B and 10C in which the markers 3 are provided at asymmetric positions.

[0068] Note that the parameter p １ , p ２ The value of parameter p can vary depending on various conditions, such as the size, arrangement, shape, and number of the portions 1, 2, and 3, the type and concentration of the dopant, the shape of the optical fiber preform, and the tension and drawing speed when drawing the optical fibers 10A to 10C. By appropriately controlling these conditions when manufacturing (drawing) the optical fibers 10A to 10C, the parameter p １ , p ２ By setting the value of θ to an appropriate value, optical fibers 10A to 10C can be manufactured in which fiber curling is suppressed.

[0069] Table 2 summarizes the results of tests to confirm the effectiveness of the marker 3 having a double structure. In Table 2, the rows labeled "Arrangement" and "Curvature" have the same meanings as the rows labeled "Arrangement" and "Curvature" in Table 1. The only difference between the two optical fibers 10B and 10C is the shape of the marker 3; the rest of the configuration can be considered to be identical. In addition, in the optical fiber 10C, the composition of the center portion 3a of the marker 3 was the same as the composition of the cladding 2.

[0070]

[0071] In Table 2, the "Entire cross section" row refers to the bending moment M calculated based on the above-mentioned formula (*) (see Equation 5). The "Marker only" row refers to the bending moment calculated by restricting the integral range in formula (*) to only regions A and A' (see also Figure 2). The "Marker and core only" row refers to the bending moment calculated by restricting the integral range in formula (*) to only the region where core 1 is present and regions A and A'.

[0072] As shown in Table 2, it was found that the optical fiber 10C having a marker 3 with a dual structure can have a smaller curvature than the optical fiber 10B having a marker 3 without a dual structure. In other words, it was found that the optical fiber 10C having a marker 3 with a dual structure can suppress fiber curling compared to the optical fiber 10B having a marker 3 without a dual structure. This is thought to be because by making the composition of the center portion 3a the same as the composition of the cladding 2 (i.e., by eliminating the first composition difference described above), the region to which asymmetric stress is applied can be limited to the annular portion 3b. It is believed that narrowing the region to which asymmetric stress is applied reduces the force that tends to warp the optical fiber 10C, thereby suppressing fiber curling. Referring to Table 2, it can be seen that the bending moment of the "marker only" of the optical fiber 10C is approximately 35% smaller than that of the optical fiber 10B.

[0073] Even when the first composition difference exists, if the first composition difference (first refractive index difference) is smaller than the above-described second composition difference (second refractive index difference), it is considered that fiber curl can be suppressed compared to when the marker 3 does not have a double structure. This is because the magnitude of the asymmetric stress applied to the central portion 3 a can be made smaller than the magnitude of the asymmetric stress applied to the portion of the marker 3 that does not have a double structure corresponding to the central portion 3 a.

[0074] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Furthermore, the components in the above-described embodiments can be replaced with well-known components as appropriate, and the above-described embodiments can be combined as appropriate, without departing from the spirit of the present invention.

[0075] 10A, 10B, 10C... Optical fiber 1... Core 2... Cladding 3... Marker

Claims

1. An optical fiber comprising a core region and a cladding surrounding the core region, wherein at least one of the core region and the cladding is doped with a dopant that changes the refractive index, and in a cross section perpendicular to the longitudinal direction of the optical fiber, the concentration distribution of the dopant is rotationally asymmetric with respect to the center of the cladding, and the following formula (1) is satisfied in the cross section.

2. The optical fiber according to claim 1, wherein the following formula (2) is satisfied at the cross section:

3. The optical fiber according to claim 1 or 2, comprising a plurality of said core regions.

4. The optical fiber according to claim 3, further comprising a marker having a refractive index different from the refractive index of the cladding, the multiple core regions being arranged at positions having two or more fold rotational symmetry with respect to the center of the cladding in the cross section, and the marker being arranged at a position that allows the multiple core regions to be identified.

5. The optical fiber according to claim 4, wherein the marker includes a center portion and an annulus surrounding the center portion, and a difference in composition between the center portion and the cladding is smaller than a difference in composition between the annulus and the cladding.

6. The optical fiber according to claim 4 or 5, wherein the following formula (3) is satisfied in the cross section:

7. The optical fiber according to claim 6, wherein the following formula (4) is satisfied at the cross section:

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