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

Abstract

One embodiment of the present disclosure provides a polarization maintaining optical fiber having an easy-to-manufacture structure for making it possible to reduce a bending loss when bent into a small radius. A polarization maintaining optical fiber of the present disclosure comprises: a core that extends along a fiber axis; a pair of stress application portions; one or more low refractive index portions; and a common cladding that surrounds the core, the stress application portions, and the low refractive index portions. On the cross-section of the polarization maintaining optical fiber, the common cladding is disposed between the stress application portions and the core, the stress application portions are disposed on both sides of the core in a state of being away from the core, the common cladding is disposed between the low refractive index portions, the low refractive index portions are disposed away from each other, the common cladding is disposed between the low refractive index portions and the core, the common cladding is disposed between the low refractive index portions and the stress application portions, and the low refractive index portions are disposed around the core in a state of being away from both of the core and the stress application portions.

Description

Polarization-maintaining optical fiber and method of manufacturing the same

[0001] This disclosure relates to a polarization-maintaining optical fiber and a method for manufacturing the polarization-maintaining optical fiber. This application claims priority from Japanese Patent Application No. 2022-142279, filed on September 7, 2022, the contents of which are relied upon and incorporated herein by reference in their entirety.

[0002] Polarization-maintaining optical fibers are used to connect polarization-dependent optical devices in optical transmission and reception systems. In recent years, there has been an increasing need for miniaturization of modules containing optical devices, and the use of polarization-maintaining optical fibers with small bending radii is required.

[0003] For example, the polarization-maintaining optical fiber disclosed in Patent Document 1 includes a core, a first cladding coat surrounding the core, a second cladding coat with a low refractive index that surrounds the first cladding coat and functions as a trench, a third cladding coat surrounding the second cladding coat, and a pair of stress-applying portions arranged to sandwich the core. The radius ratio r2 / r1 of the first cladding coat to the second cladding coat is 2.5 to 4.5. The refractive index volume V of the second cladding coat is 25 μm ２ ・% or more 110μm ２ % or less. The pair of stress-applying portions are arranged so that each physically separates the second cladding coat. In the polarization-maintaining optical fiber of Patent Document 1, the second cladding coat is doped with F (fluorine) to function as a trench, which increases the difference between the refractive index of the second cladding coat and the refractive index at the outer edge of the mode field diameter (hereinafter referred to as "MFD"). Therefore, the polarization-maintaining optical fiber of Patent Document 1 can reduce bending loss without reducing the MFD when bent to a small radius, compared to a polarization-maintaining optical fiber that does not have a trench corresponding to the second cladding coat.

[0004] US2015 / 0268413A1 publication

[0005] The polarization-maintaining optical fiber of the present disclosure comprises a core extending along the fiber axis, a pair of stress-applying portions, one or more low-refractive-index portions, and a common cladding surrounding the core, the pair of stress-applying portions, and the one or more low-refractive-index portions. In particular, on a cross section of the polarization-maintaining optical fiber perpendicular to the fiber axis, the common cladding is disposed between the pair of stress-applying portions and the core. The pair of stress-applying portions are disposed on both sides of the core at a distance from the core. The common cladding is disposed between each of the plurality of low-refractive-index portions. The plurality of low-refractive-index portions are disposed at a distance from one another. The common cladding is disposed between the one or more low-refractive-index portions and the core. The common cladding is disposed between the one or more low-refractive-index portions and the pair of stress-applying portions. The one or more low-refractive-index portions are disposed around the core at a distance from both the core and the pair of stress-applying portions.

[0006] FIG. 1 is a diagram illustrating the structure of a polarization-maintaining optical fiber according to the present disclosure. FIG. 2 is a diagram illustrating various arrangement patterns of low refractive index portions on a cross section of a polarization-maintaining optical fiber according to the present disclosure. FIG. 3 is a diagram illustrating the arrangement conditions of low refractive index portions on a cross section of a polarization-maintaining optical fiber according to the present disclosure. FIG. 4 is a diagram illustrating a method for manufacturing an optical fiber preform for obtaining a polarization-maintaining optical fiber according to the present disclosure. FIG. 5 is a diagram illustrating the configuration of a drawing apparatus for obtaining a polarization-maintaining optical fiber according to the present disclosure. FIG. 6 is a graph showing the dependence of the cutoff wavelength λcc and bending loss on the refractive index volume V of a polarization-maintaining optical fiber according to the present disclosure. FIG. 7 is a graph showing the dependence of the bending loss and cutoff wavelength of a polarization-maintaining optical fiber according to the present disclosure on various MFDs at a wavelength of 1.31 μm. FIG. 8 is a graph showing the dependence of the core radius and relative refractive index difference of a polarization-maintaining optical fiber according to the present disclosure on various MFDs at a wavelength of 1.31 μm.

[0007] [Problem to be Solved by the Present Disclosure] As a result of studying the above-mentioned conventional techniques, the inventors discovered the following problem. Specifically, the polarization-maintaining optical fiber of Patent Document 1 includes a core, a first cladding coat, an F-doped second cladding coat, a third cladding coat, and a pair of stress-applying portions. In this polarization-maintaining optical fiber, the pair of stress-applying portions are arranged so as to physically separate the second cladding coat. In the preform manufacturing process, a through hole is first formed in the center of an F-doped rod that will become the second cladding coat after drawing. A core rod composed of a core portion and a first cladding portion that will become the core and the first cladding coat after drawing is inserted into the through hole of the F-doped rod, and these are then integrated to obtain a first intermediate preform composed of a core portion, a first cladding portion, and a second cladding portion. Next, a through hole is formed in the center of a third cladding portion, which serves as a jacket material that will become the third cladding coat after drawing. A first intermediate preform is inserted into the through hole of the third cladding portion, and these are then integrated to obtain a second intermediate preform comprising a core portion and first to third cladding portions. A pair of through holes is formed in the obtained second intermediate preform at predetermined locations, and a pair of stress-applying rods that will become a pair of stress-applying portions after drawing are inserted into the pair of through holes, respectively. These are then integrated to obtain an optical fiber preform for obtaining a polarization-maintaining optical fiber. Thus, to obtain the polarization-maintaining optical fiber of Patent Document 1, the optical fiber preform requires many complex steps to complete. Furthermore, most of the expensive F-doped rosso is discarded during the process of manufacturing the first intermediate preform and the process of inserting the pair of stress-applying rods, which hinders effective material utilization. This could lead to increased manufacturing costs and reduced production efficiency.

[0008] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a polarization-maintaining optical fiber having an easily manufacturable structure that enables reduction of bending loss when bent to a small radius, and a method for manufacturing the same.

[0009] Effect of the Present Disclosure According to the present disclosure, a polarization-maintaining optical fiber having a structure that enables reduction of bending loss when bent to a small radius can be obtained.

[0010] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0011] The polarization-maintaining optical fiber of the present disclosure comprises: (1) a core extending along the fiber axis, a pair of stress-applying portions, one or more low-refractive-index portions, and a common cladding surrounding each of the core, the pair of stress-applying portions, and the one or more low-refractive-index portions. In particular, on a cross section of the polarization-maintaining optical fiber perpendicular to the fiber axis, the common cladding is disposed between the pair of stress-applying portions and the core. The pair of stress-applying portions are disposed on both sides of the core at a distance from the core. The common cladding is disposed between each of the plurality of low-refractive-index portions. The plurality of low-refractive-index portions are disposed at a distance from each other. The common cladding is disposed between the one or more low-refractive-index portions and the core. The common cladding is disposed between the one or more low-refractive-index portions and the pair of stress-applying portions. The one or more low-refractive-index portions are disposed around the core at a distance from both the core and the pair of stress-applying portions.

[0012] As described above, one or more low-refractive-index sections arranged at intervals around the core and spaced apart from each other from a pair of stress-applying sections function as trench layers arranged around the core. This allows a polarization-maintaining optical fiber provided with low-refractive-index sections to achieve reduced bending loss when bent to a small radius. Furthermore, because each low-refractive-index section is not in contact with a pair of stress-applying sections, a complex arrangement of elements in the fiber cross section is avoided, reducing the number of manufacturing steps and enabling lower manufacturing costs.

[0013] (2) In (1) above, the outline of each of the one or more low-refractive-index portions on the cross section of the polarization-maintaining optical fiber may have a shape consisting of only straight lines, only curved lines, or a combination of straight lines and curved lines. Specifically, the outline of each low-refractive-index portion may be any shape consisting of at least one of straight lines and curved lines, such as a circle, ellipse, rectangle, or triangle. Thus, in the polarization-maintaining optical fiber of the present disclosure, there are no restrictions on the shape of the cross section of each low-refractive-index portion as long as it is not in contact with a stress-applying portion.

[0014] (3) In (1) or (2), the polarization-maintaining optical fiber may have two to six low-refractive-index sections as the one or more low-refractive-index sections. In this case, the two to six low-refractive-index sections may be arranged at positions on the cross section where the center-to-center distance from the center of the core is equal and so as not to overlap with either of a pair of stress-applying sections. Here, when the cross section of each low-refractive-index section is circular, a preform can be manufactured by collapsing a common clad rod inserted into a through hole with a cylindrical low-refractive-index rod. This enables the polarization-maintaining optical fiber of the present disclosure to be manufactured at low cost. Note that the low-refractive-index rod is a component that will become a low-refractive-index section after drawing, and the common clad rod is a component that will become a common clad after drawing. While a small number of low-refractive-index sections reduces the manufacturing cost associated with forming the through holes into which the low-refractive-index rods are inserted, this weakens the confinement of the fundamental mode, which contributes to increased bending loss. Therefore, according to the polarization-maintaining optical fiber of the present disclosure, it is important to adjust the number of low refractive index portions so as to obtain the desired cutoff wavelength and bending loss characteristics.

[0015] The latest version of ITU-T standard G.657.B3 was published in November 2016. G657.B3 is described on page 9 of this latest version, and the MFD at a wavelength of 1310 nm (=1.31 μm) is 8.6 μm or more and 9.2 μm or less. However, because the tolerance is ±0.4 μm, the MFD at a wavelength of 1310 nm is actually 8.2 μm or more and 9.6 μm or less. The cutoff wavelength is 1260 nm (=1.26 μm) or less. The bending loss when bending once with a bending radius of 5 mm is 0.15 dB or less at a wavelength of 1550 nm (=1.55 μm).

[0016] (4) In any of (1) to (3) above, the pair of stress-applying portions may each have an outer diameter of 30 μm or more and 40 μm or less on the cross section of the polarization-maintaining optical fiber. The relative refractive index difference of each of the pair of stress-applying portions with respect to the common cladding may be 0.0% or less. That is, the refractive index of each of the pair of stress-applying portions may be equal to or less than the refractive index of the common cladding. Furthermore, the ratio a / b_SAP (hereinafter referred to as "Ra_SAP") of the core radius a to the shortest distance b_SAP from the center of the core to the outline of each of the pair of stress-applying portions may be 0.4 or more and 0.6 or less. If the outer diameter of each stress-applying portion is less than 30 μm or more than 40 μm, the birefringence in the core decreases, resulting in deterioration of the polarization-maintaining characteristics. Similarly, if Ra_SAP is less than 0.4 and the shortest distance from the center of the core to the outline of each stress-applying portion is large, the birefringence in the core also decreases. On the other hand, if Ra_SAP is greater than 0.6, the through-holes for inserting the stress-applying rods and the through-holes for inserting the core rods, which are provided in the common clad rod that is to become the common clad during preform manufacturing, become close to each other, making manufacturing difficult. The stress-applying rod is a component that will become one of a pair of stress-applying portions after fiber drawing, and the core rod is a component that will become the core after fiber drawing. Furthermore, each stress-applying portion also contributes to reducing bending loss. In particular, when the polarization-maintaining optical fiber is bent so that the centers of each stress-applying portion are oriented in the 0° direction, i.e., so that each stress-applying portion is parallel to the bending plane, bending loss is significantly reduced. Here, the bending plane is a plane that indicates the bending direction of the polarization-maintaining optical fiber and includes the central axis of the polarization-maintaining optical fiber in the bent state.

[0017] (5) In any of the above (1) to (4), the refractive index volume V, defined as the product of the total area of ​​one or more low refractive index portions on the cross section of the polarization-maintaining optical fiber and the absolute value of the average value of the relative refractive index differences of the one or more low refractive index portions, is 20 μm ２ ・% or more 120 μm ２ By appropriately selecting the refractive index volume V, a polarization-maintaining optical fiber conforming to ITU-T standard G.657.B3 can be obtained.

[0018] (6) In any of (1) to (5) above, the polarization-maintaining optical fiber may have an MFD of 8.2 μm or more and 10.2 μm or less at a wavelength of 1.31 μm, a bending loss of less than 0.15 dB at a wavelength of 1.55 μm, and a cutoff wavelength of less than 1.26 μm. Here, the bending loss is measured in a state where the polarization-maintaining optical fiber is bent once with a bending radius of 5 mm so that each of a pair of stress-applying portions is parallel to the bending plane. The refractive index volume V is 20 μm. ２ %, the bending resistance is low and the bending loss at a wavelength of 1.55 μm exceeds 0.15 dB. ２ If the MFD exceeds 0.05%, the distance between the low refractive index sections will be too narrow when six low refractive index sections are arranged, making it difficult to form through holes in the preform manufacturing process. Note that there is a trade-off between the cutoff wavelength and the bending loss, and the tolerance for a structure that satisfies ITU-T standard G.657.B3 depends on the MFD.

[0019] (7) In either of (1) to (6) above, the core may have a radius of 3 μm or more and 5 μm or less on the cross section of the polarization-maintaining optical fiber, and the relative refractive index difference of the core with respect to the common cladding may be 0.2% or more and 0.5% or less. The relative refractive index difference of one or more low refractive index portions with respect to the common cladding may be -1.0% or more and -0.5% or less. By appropriately selecting the core radius and the relative refractive index difference of the core from within these ranges, the MFD described in (6) above is realized.

[0020] The present disclosure also provides a method for manufacturing a polarization-maintaining optical fiber, as described in any one of (1) to (7). Specifically, the method includes a preform manufacturing step for manufacturing an optical fiber preform for obtaining the polarization-maintaining optical fiber, and a drawing step for drawing the optical fiber preform manufactured in the preform manufacturing step. The preform manufacturing step includes first to fourth sub-steps. In the first sub-step, a common clad rod that will become part of the common clad after drawing is prepared. In the second sub-step, a core rod including a portion that will become the core after drawing, one or more low-refractive-index rods that will become one or more low-refractive-index portions after drawing, and a pair of stress-applying rods that will become a pair of stress-applying portions after drawing are individually prepared. That is, the second sub-step for the core rod, the second sub-step for the one or more low-refractive-index rods, and the second sub-step for the pair of stress-applying rods do not need to be performed simultaneously. In the third substep, the formation of a first through hole into which a core rod is inserted, the formation of one or more second through holes into which one or more low-refractive-index rods are individually inserted, and the formation of a pair of third through holes into which a pair of stress-applying rods are individually performed in the common clad rod. That is, the third substep for the first through hole, the third substep for the second through hole, and the third substep for the third through hole do not need to be performed simultaneously. In the fourth substep, the integration of the common clad rod and the core rod, the integration of the common clad rod and one or more low-refractive-index rods, and the integration of the common clad rod and a pair of stress-applying rods are performed separately. That is, the fourth substep for integrating the core rod, the fourth substep for integrating the low-refractive-index rods, and the fourth substep for integrating the stress-applying rods do not need to be performed simultaneously.

[0021] In the optical fiber preform obtained through the first to fourth sub-steps, a common cladding rod is disposed between each of the core rod, one or more low-refractive-index rods, and each of the pair of stress-applying rods. The manufacturing method of Patent Document 1 requires the removal of a portion of the previously formed trench layer to provide a pair of stress-applying portions. On the other hand, the manufacturing method of the present disclosure simplifies the manufacturing process compared to the manufacturing method of Patent Document 1 and does not require the removal of the trench layer once formed, which is expected to reduce costs from the perspective of the manufacturing process. Furthermore, the manufacturing method of the present disclosure can provide a low-bend-loss polarization-maintaining optical fiber that complies with ITU-T Standard G.657.B3.

[0022] As described above, each aspect listed in the [Description of Embodiments of the Present Disclosure] section can be applied to all of the remaining aspects individually or to all combinations of these remaining aspects.

[0023] [Details of the embodiments of the present disclosure] Specific structures of the polarization-maintaining optical fiber and the method for manufacturing the polarization-maintaining optical fiber of the present disclosure will be described in detail below with reference to the accompanying drawings. 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 of the claims. Furthermore, in the description of the drawings, identical elements are given the same reference numerals, and redundant explanations will be omitted.

[0024] FIG. 1 is a diagram illustrating the structure of a polarization-maintaining optical fiber according to the present disclosure (denoted as "fiber structure" in FIG. 1). The top row of FIG. 1 (denoted as "cross-sectional structure" in FIG. 1) shows an example of the cross-sectional structure of a polarization-maintaining optical fiber 10 according to the present disclosure. The second row of FIG. 1 (denoted as "refractive index profile (on line L1)" in FIG. 1) shows the refractive index profile along line L1 in the cross section of the polarization-maintaining optical fiber 10 at the top row. The third row of FIG. 1 (denoted as "refractive index profile (on line L2)" in FIG. 1) shows the refractive index profile along line L2 in the cross section of the polarization-maintaining optical fiber 10 at the top row. The bottom row of FIG. 1 (denoted as "refractive index profile (on line L3)" in FIG. 1) shows the refractive index profile along line L3 in the cross section of the polarization-maintaining optical fiber 10 at the top row.

[0025] 1 , a polarization-maintaining optical fiber 10 of the present disclosure comprises a glass optical fiber 20 containing silica glass as a main component, and a resin coating 30 provided on the outer peripheral surface of the glass optical fiber 20. The glass optical fiber 20 comprises a core 40 extending along a fiber axis AX, a common cladding 50, a pair of stress-applying portions 60A, 60B spaced apart from the core 40 and disposed so as to sandwich the core 40, and one or more low-refractive-index portions 70A surrounding the core 40 and spaced apart from both the core 40 and the pair of stress-applying portions 60A, 60B.

[0026] The refractive index profile shown in the second row of FIG. 1 is given by the relative refractive index difference at each portion along a line L1 passing through the center of the core 40 and the center of each of the pair of stress-applying portions 60A, 60B in a cross section of the polarization-maintaining optical fiber 10 perpendicular to the fiber axis AX, as shown in the top row of FIG. 1. The core 40 has a radius of 3 μm or more and 5 μm or less. The relative refractive index difference of the core 40 with respect to the common cladding 50 is 0.2% or more and 0.5% or less. Each of the pair of stress-applying portions 60A, 60B has an outer diameter of 30 μm or more and 40 μm or less. The relative refractive index difference of each of the pair of stress-applying portions 60A, 60B with respect to the common cladding 50 is 0.0% or less. Note that each of the pair of stress-applying portions 60A, 60B may have a positive relative refractive index difference with respect to the common cladding 50, as indicated by the dashed line.

[0027] The refractive index profile shown in the third row of Fig. 1 is given by the relative refractive index difference at each portion along line L2 passing through the center of the core 40 and the center of any one of the low refractive index portions 70A in the cross section of the polarization-maintaining optical fiber 10, as shown in the top row of Fig. 1. The relative refractive index difference of each low refractive index portion 70A with respect to the common cladding 50 is -1.0% or more and -0.5% or less. In this way, one or more low refractive index portions 70A surrounding the core 40 function as a trench layer as a whole. With this structure, a polarization-maintaining optical fiber 10 provided with one or more low refractive index portions 70A achieves reduced bending loss when bent to a small radius.

[0028] The refractive index profile shown in the bottom row of Figure 1 is given by the relative refractive index difference at each portion along line L3 that passes through the center of core 40 and the midpoint between two adjacent low refractive index portions 70A in the cross section of polarization-maintaining optical fiber 10, as shown in the top row of Figure 1.

[0029] As described above, in the polarization-maintaining optical fiber 10 of the present disclosure, the core 40, the pair of stress-applying portions 60A, 60B, and one or more low-refractive-index portions 70A are arranged in physically separate positions across a portion of the common cladding 50. Therefore, as shown in the second to bottom rows of Fig. 1 , the refractive index profile of the polarization-maintaining optical fiber 10 has different shapes in its cross section when lines passing through the center of the core 40 are rotated in the circumferential direction of the core 40. Furthermore, because each low-refractive-index portion 70A is not in contact with the pair of stress-applying portions 60A, 60B, a complex arrangement of elements in the cross section is avoided, which reduces the number of manufacturing steps and enables lower manufacturing costs.

[0030] Furthermore, in the polarization-maintaining optical fiber 10 having the above-described structure, the provision of a low-refractive-index portion increases the difference between the refractive index of each low-refractive-index portion 70A and the effective refractive index obtained at the outer edge of the MFD, as with the trench layer in Patent Document 1. This reduces bending loss when bent to a small radius compared to polarization-maintaining optical fibers without low-refractive-index portions. Additionally, the MFD, bending loss, and cutoff wavelength can be easily designed to satisfy ITU-T Standard G.657.B3. By appropriately selecting the diameter of the core 40, the positions and sizes of each low-refractive-index portion 70A, and the pair of stress-applying portions 60A and 60B, it is possible to reduce manufacturing costs while still complying with ITU-T Standard G.657.B3.

[0031] 2 is a diagram showing various arrangement patterns of low refractive index portions on a cross section of a polarization-maintaining optical fiber according to the present disclosure (denoted as "arrangement patterns of low refractive index portions" in FIG. 2). Patterns A to H shown in FIG. 2 are all arrangement patterns on a cross section of a glass optical fiber 20 perpendicular to the fiber axis AX.

[0032] On the cross section of the polarization-maintaining optical fiber 10 of the present disclosure, the outline of each low-refractive-index portion may have a shape composed of only straight lines, only curved lines, or a combination of straight lines and curved lines. Note that the cross-sectional shape of the low-refractive-index portion applied to the polarization-maintaining optical fiber 10 of the present disclosure, defined by the outline on the cross section of the polarization-maintaining optical fiber 10, is not limited to the shape defined by the outline, as long as it is not in contact with a stress-applying portion, as exemplified in FIG. 2 .

[0033] Specifically, in pattern A shown in FIG. 2, two groups of low refractive index portions are arranged across the core 40 so as not to contact the pair of stress-applying portions 60A, 60B. Each group of low refractive index portions is composed of three low refractive index portions 70A, each having a circular outline. Pattern B shown in FIG. 2, like pattern A, also has two groups of low refractive index portions arranged. However, each group of low refractive index portions is composed of two low refractive index portions 70B, each having a circular outline. Pattern C shown in FIG. 2, like patterns A and B described above, also has two groups of low refractive index portions arranged. However, each group of low refractive index portions is composed of one low refractive index portion 70C, each having a circular outline.

[0034] Pattern D shown in FIG. 2 has two groups of low refractive index portions arranged similarly to pattern A, but the outline of each low refractive index portion 70D is noncircular. That is, each group of low refractive index portions is composed of three low refractive index portions 70D, each with an elliptical outline. Furthermore, the two groups of low refractive index portions in pattern E shown in FIG. 2 are each composed of three low refractive index portions 70E. The outline of each low refractive index portion 70E is rectangular. The two groups of low refractive index portions in pattern F shown in FIG. 2 are each composed of one low refractive index portion 70F. The outline of each low refractive index portion 70F is triangular. The two groups of low refractive index portions in pattern G shown in FIG. 2 are each composed of one low refractive index portion 70G. The outline of each low refractive index portion 70G is trapezoidal. Furthermore, the two groups of low refractive index portions in pattern H shown in FIG. 2 are each composed of two low refractive index portions 70H. The outline of each low refractive index portion 70H is star-shaped.

[0035] Fig. 3 is a diagram for explaining the arrangement conditions of low-refractive-index portions on the cross section of the polarization-maintaining optical fiber 10 of the present disclosure. In the following explanation, the low-refractive-index portion 70A of pattern A shown in Fig. 2 will be described as a representative example of a low-refractive-index portion. Furthermore, as shown in Fig. 3, on the cross section of the polarization-maintaining optical fiber 10 of the present disclosure, the arrangement of the core 40, each low-refractive-index portion 70A, and the pair of stress-applying portions 60A, 60B is defined by an orthogonal coordinate system (x-y coordinate system) with the center of the core 40 as the origin. Note that in Fig. 3, the x-axis coincides with line L1 shown in the top row of Fig. 1, and the y-axis coincides with line L2.

[0036] The radius of the core 40 is a. The shortest distance from the center of the core 40 to the outline of each low-refractive-index portion 70A is b_A. The longest distance from the center of the core 40 to the outline of each low-refractive-index portion 70A is c_A. The shortest distance from the center of the core 40 to the outline of each of the pair of stress-applying portions 60A, 60B is b_SAP. In the polarization-maintaining optical fiber 10 of the present disclosure, the ratio Ra_SAP (= a / b_SAP) of the radius a of the core 40 to the shortest distance b_SAP is 0.4 or more and 0.6 or less. If the outer diameter of each of the pair of stress-applying portions 60A, 60B is less than 30 μm or more than 40 μm, the birefringence in the core 40 decreases, and the polarization-maintaining characteristics deteriorate. Similarly, if Ra_SAP is less than 0.4 and the shortest distance from the center of the core 40 to the outline of the pair of stress-applying portions 60A, 60B is large, the birefringence in the core 40 also decreases. On the other hand, if Ra_SAP is greater than 0.6, the through-holes for inserting the stress-applying rods and the through-holes for inserting the core rods, which are provided in the common clad rod that is to become the common clad 50 during preform manufacturing, will be close to each other, making manufacturing difficult. The stress-applying rod is a component that will become one of the stress-applying portions 60A, 60B after fiber drawing, and the core rod is a component that will become the core 40 after fiber drawing. Furthermore, the pair of stress-applying portions 60A, 60B also contribute to reducing bending loss. In particular, when the polarization-maintaining optical fiber 10 is bent so that the centers of the pair of stress-applying portions 60A, 60B are oriented in the 0-degree direction, i.e., so that each of the pair of stress-applying portions 60A, 60B is parallel to the bending plane, a line L1 passing through the centers of the pair of stress-applying portions 60A, 60B at each position along the longitudinal direction of the polarization-maintaining optical fiber 10 becomes perpendicular to the bending plane, significantly reducing bending loss.

[0037] Additionally, to satisfy ITU-T Standard G.657.B3, the polarization-maintaining optical fiber 10 of the present disclosure has two to six low-refractive-index portions, such as low-refractive-index portions 70A to 70H shown in FIG. 2. In this case, the two to six low-refractive-index portions are arranged at equal center-to-center distances from the center of the core 40 on the cross section and are arranged so as not to overlap any of the pair of stress-applying portions 60A and 60B. Note that when the cross-sectional shape of each low-refractive-index portion is circular, such as low-refractive-index portions 70A to 70C, the polarization-maintaining optical fiber 10 can be manufactured at low cost. Furthermore, during the manufacturing process of the optical fiber preform, manufacturing costs associated with forming through holes in the common cladding rod that will become the common cladding 50 after drawing are reduced. On the other hand, a small number of low-refractive-index portions weakens the confinement of the fundamental mode, which contributes to increased bending loss. Therefore, with the polarization-maintaining optical fiber 10 of the present disclosure, it is important to adjust the number of low-refractive-index portions so as to obtain the desired cutoff wavelength and bending loss characteristics.

[0038] FIG. 4 is a diagram illustrating a method for manufacturing an optical fiber preform 100C for obtaining a polarization-maintaining optical fiber 10 according to the present disclosure (denoted as "Preform Manufacturing" in FIG. 4). FIG. 5 is a diagram illustrating the configuration of a drawing apparatus for obtaining a polarization-maintaining optical fiber 10 according to the present disclosure. The upper part of FIG. 4 (denoted as "Step ST1" in FIG. 4) illustrates a preparation step for preparing components constituting the intermediate preform 100B. The middle part of FIG. 4 (denoted as "Step ST2" in FIG. 4) illustrates an intermediate preform manufacturing step for obtaining the intermediate preform 100B, in which hole-opening, insertion, and sintering are performed. The lower part of FIG. 4 (denoted as "Step ST3" in FIG. 4) illustrates an optical fiber preform manufacturing step for obtaining the optical fiber preform 100C from the intermediate preform 100B, in which component preparation, hole-opening, insertion, and sintering are performed. In the following description, as with the example of FIG. 3, the low-refractive-index portion 70A of pattern A shown in FIG. 2 will be described as a representative example of a low-refractive-index portion.

[0039] The manufacturing method of the polarization-maintaining optical fiber 10 of the present disclosure includes a preform manufacturing process illustrated in FIG. 4 and a drawing process illustrated in FIG. 5. The preform manufacturing process includes first to fourth sub-processes to obtain an optical fiber preform 100C. In the first sub-process, a common clad rod 100A is prepared. The common clad rod 100A is processed to leave a clad outer portion 403 that will become the physical clad layer of the common clad 50 after drawing. This physical clad layer is called a jacket layer.

[0040] In the second sub-process, the core rod 400A, one or more low-refractive-index rods 700A, and a pair of stress-applying rods 600A and 600B are prepared separately. The core rod 400A is a pure silica rod having a central core portion 401 that will become the core 40 after drawing, and is composed of the core portion 401 and an inner cladding portion 402 that surrounds the core portion 401. The inner cladding portion 402 is the portion that will become the optical cladding layer of the common cladding 50 after drawing. The one or more low-refractive-index rods 700A are components that will become one or more low-refractive-index portions 70A after drawing. The pair of stress-applying rods 600A and 600B are components that will become a pair of stress-applying portions 60A and 60B after drawing. It should be noted that the second sub-step for the core rod 400A, the second sub-step for one or more low refractive index rods 700A, and the second sub-step for the pair of stress-applying rods 600A, 600B do not need to be performed simultaneously.

[0041] In the third sub-step, the following steps are individually performed on the common clad rod 100A: forming a first through hole 400B into which a core rod 400A is inserted; forming one or more second through holes 700B into which one or more low-refractive-index rods 700A are individually inserted; and forming a pair of third through holes 610A, 610B into which a pair of stress-applying rods 600A, 600B are individually inserted. Note that the common clad rod 100A in which the first through hole 400B is formed corresponds to the clad outer portion 403. The third sub-step for the first through hole 400B, the third sub-step for the second through hole 700B, and the third sub-step for the third through holes 610A, 610B do not need to be performed simultaneously.

[0042] In the fourth substep, the integration of the common clad rod 100A with the core rod 400A, the integration of the common clad rod 100A with one or more low-refractive-index rods 700A, and the integration of the common clad rod 100A with the pair of stress-applying rods 600A and 600B are performed separately. The fourth substep for integrating the core rods, the fourth substep for integrating the low-refractive-index rods, and the fourth substep for integrating the stress-applying rods do not need to be performed simultaneously. The integration of the common clad rod 100A with the core rod 400A is achieved by collapsing the common clad rod 100A while the core rod 400A is inserted into the first through-hole 400B. Furthermore, the integration results in the inner clad portion 402 of the core rod 400A and the outer clad portion 403 of the common clad rod 100A forming the common clad 50 after drawing. The integration of the common clad rod 100A and one or more low-refractive-index rods 700A is achieved by collapsing the common clad rod 100A with one or more low-refractive-index rods 700A inserted into one or more second through-holes 700B, respectively. Furthermore, the integration of the common clad rod 100A and the pair of stress-applying rods 600A, 600B is achieved by collapsing the common clad rod 100A with the pair of stress-applying rods 600A, 600B inserted into the pair of third through-holes 610A, 610B.

[0043] After the first sub-step is performed, the second to fourth sub-steps for the core rod 400A, the second to fourth sub-steps for the low-refractive-index rod 700A, and the second to fourth sub-steps for the pair of stress-applying rods 600A, 600B may be performed in this order. As an example, in the preform manufacturing process of Fig. 4, the first sub-step is followed by the second to fourth sub-steps for the core rod 400A, and then the second to fourth sub-steps for both the low-refractive-index rod 700A and the pair of stress-applying rods 600A, 600B are performed.

[0044] Specifically, in step ST1 shown in the upper part of Fig. 4, a first sub-step for the common clad rod 100A and a second sub-step for the core rod 400A are performed. That is, in step ST1, the common clad rod 100A and the core rod 400A composed of a core portion 401 and an inner clad portion 402 are prepared. Subsequently, in step ST2 shown in the middle part of Fig. 4, a third sub-step and a fourth sub-step are performed for the core rod 400A, and an intermediate preform 100B is finally obtained. This intermediate preform 100B is the common clad rod 100A integrated with the core rod 400A, and is composed of the inner clad portion 402 and the outer clad portion 403 after the third sub-step.

[0045] In step ST3 shown in the lower part of Figure 4, one or more low-refractive-index rods 700A and a pair of stress-applying rods 600A, 600B are further prepared as a second sub-step. Subsequently, a third sub-step and a fourth sub-step for the one or more low-refractive-index rods 700A and a third sub-step and a fourth sub-step for the pair of stress-applying rods 600A, 600B are simultaneously performed in parallel to finally obtain an optical fiber preform 100C. This optical fiber preform 100C is a common clad rod 100A in which, in addition to a core rod 400A, one or more low-refractive-index rods 700A and a pair of stress-applying rods 600A, 600B are integrated. In the optical fiber preform 100C obtained through the first to fourth sub-steps, the common clad rod 100A is disposed between the core rod 400A, the one or more low-refractive-index rods 700A, and the pair of stress-applying rods 600A, 600B.

[0046] The optical fiber preform 100C manufactured as described above is set in the drawing apparatus shown in FIG. 5 to obtain the polarization-maintaining optical fiber 10 of the present disclosure. The drawing apparatus includes a heater 200, a resin applicator 300, a roller 410, and a winding device 420. When the winding device 420 rotates in the direction indicated by arrow S, a glass optical fiber is drawn from one end of the optical fiber preform 100C heated by the heater 200. The outer peripheral surface of this glass optical fiber is coated with resin by the resin applicator 300, and finally, the resin-coated polarization-maintaining optical fiber 10 is wound around the drum of the winding device 420 via the roller 410. The cross-sectional structure of the polarization-maintaining optical fiber 10 taken along line I-I shown in FIG. 5 corresponds to the cross-sectional structure shown in the upper part of FIG. 1.

[0047] Next, the optical characteristics of a polarization-maintaining optical fiber 10 having the cross-sectional structure of pattern A shown in FIG. 2 will be described with reference to FIGS. 6 to 8 . FIG. 6 is a graph showing the dependence of the cutoff wavelength λcc and bending loss on the refractive index volume V of a polarization-maintaining optical fiber of the present disclosure. FIG. 7 is a graph showing the dependence of the bending loss and cutoff wavelength of a polarization-maintaining optical fiber of the present disclosure on various MFDs at a wavelength of 1.31 μm (labeled "MFD Dependence" in FIG. 7 ). The upper part of FIG. 7 (labeled "Bending Loss-λcc Characteristics with Varying MFD" in FIG. 7 ) shows the change in bending loss versus λcc for various MFDs. The lower part of FIG. 7 (labeled "λcc_min-MFD Characteristics" in FIG. 7 ) shows the change in cutoff wavelength λcc_min versus MFD at a wavelength of 1.31 μm. FIG. 8 is a graph showing the dependence of the core radius and relative refractive index difference of the polarization-maintaining optical fiber of the present disclosure on the MFD.

[0048] First, in FIG. 6, the horizontal axis represents the refractive index volume V (μm ２%. This refractive index volume V is the product of the total cross-sectional area of ​​the six low refractive index portions 70A shown as pattern A among the various low refractive index portions 70A to 70H shown in FIG. 2 and the absolute value of the average relative refractive index difference of the six low refractive index portions 70A. The relative refractive index difference of each low refractive index portion 70A with respect to the common cladding 50 is set to be greater than or equal to -1.0% and less than or equal to -0.5%. The upper column on the vertical axis represents the cutoff wavelength λcc (μm). The lower column on the vertical axis represents the bending loss (dB). This bending loss is the transmission loss measured when light of a wavelength of 1.55 μm is input to a polarization-maintaining optical fiber to be measured, bent once with a bending radius of 5 mm so that each of a pair of stress-applying portions is parallel to the bending plane.

[0049] As can be seen from FIG. 6, the cutoff wavelength λcc and the bending loss at a wavelength of 1.55 μm show dependence on the refractive index volume V. Each low refractive index portion 70A functions as a trench layer, thereby contributing to the reduction of bending loss. In addition, as the refractive index volume V increases, the bending loss decreases while the cutoff wavelength λcc becomes longer. Therefore, in order to limit the increase in bending loss, the refractive index volume V should be set to 20 μm. ２ % or more. On the other hand, the refractive index volume V is 120 μm ２ %, it becomes difficult to form the second through-holes 700B for the low refractive index rods 700A during the manufacture of the base material. Therefore, the refractive index volume V is set to 120 μm ２ % or less. By appropriately selecting the refractive index volume V in this way, a polarization-maintaining optical fiber 10 that complies with ITU-T standard G.657.B3 can be obtained.

[0050] Next, in the upper part of FIG. 7, the horizontal axis is the cutoff wavelength λcc (dB). The vertical axis is the bending loss (dB), the same as the lower vertical axis in FIG. 6. In FIG. 7, graph G710 shows the bending loss-λcc characteristics of a sample with an MFD of 10.2 μm at a wavelength of 1.31 μm. Graph G720 shows the bending loss-λcc characteristics when the MFD is 9.6 μm. Graph G730 shows the bending loss-λcc characteristics of a sample with the MFD of 8.8 μm. Graph G740 shows the bending loss-λcc characteristics of a sample with the MFD of 8.2 μm. The dashed lines in the upper part of FIG. 7 indicate bending loss = 0.15 dB and λcc = 1.26 μm, respectively. In the lower part of FIG. 7, the horizontal axis is the MFD (μm) at a wavelength of 1.31 μm. The vertical axis represents the cutoff wavelength λcc_min (μm) when the bending loss at a wavelength of 1.55 μm is 0.15 dB. The dashed line shown in the lower part of Fig. 7 represents the cutoff wavelength λcc_min = 1.26 μm.

[0051] As can be seen from the upper and lower panels of Figure 7, the cutoff wavelength λcc and bending loss at a wavelength of 1.55 μm show dependence on the MFD at a wavelength of 1.31 μm. That is, the smaller the MFD, the stronger the optical confinement and the greater the tolerance, and therefore the smaller the MFD, the greater the tolerance. In particular, as shown in the upper panel of Figure 7, if the MCF is 8.2 μm or greater and 10.2 μm or less, a bending loss of less than 0.15 dB at a wavelength of 1.55 μm and a cutoff wavelength λcc of less than 1.26 μm can be achieved. Furthermore, as shown in the lower panel of Figure 7, if the cutoff wavelength λcc_min (μm) when the bending loss at a wavelength of 1.55 μm is 0.15 dB is less than 1.26 μm, the ITU-T standard G.657.B3 is satisfied. In this way, if the MFD at a wavelength of 1.31 μm is between 8.2 μm and 10.2 μm, it is possible to achieve a cutoff wavelength λcc and bending loss that satisfy the ITU-T standard G.657.B3. However, this means that, because there is a trade-off between the cutoff wavelength λcc and the bending loss, the tolerance that satisfies the ITU-T standard G.657.B3 depends on the MFD.

[0052] In FIG. 8, the horizontal axis represents the relative refractive index difference of the core 40 with respect to the common cladding 50 (denoted as "core Δ" in FIG. 8). The vertical axis represents the radius a of the core 40. In FIG. 8, graph G810 shows the relationship between the core Δ and the core radius when the MFD is 7 μm at a wavelength of 1.31 μm. Graph G820 shows the relationship between the core Δ and the core radius when the MFD is 8 μm. Graph G830 shows the relationship between the core Δ and the core radius when the MFD is 9 μm. Graph G840 shows the relationship between the core Δ and the core radius when the MFD is 10 μm. Graph G850 shows the relationship between the core Δ and the core radius when the MFD is 11 μm. Graph G860 shows the relationship between the core Δ and the core radius when the MFD is 12 μm. Graph G870 shows the relationship between the core Δ and the core radius when the MFD is 13 μm.

[0053] 8, when selecting the allowable range of the core Δ and core radius for graph G830 as the reference line among graphs G810 to G870, the radius of the core 40 on the cross section of the polarization-maintaining optical fiber 10 should be within the range of 3 μm to 5 μm. Also, the relative refractive index difference of the core 40 with respect to the common cladding 50 should be within the range of 0.2% to 0.5%.

[0054] As can be seen from the description of the above-mentioned embodiments, this specification also discloses the following aspects: [Supplementary Note 1] A polarization-maintaining optical fiber comprising: a core extending along a fiber axis, a pair of stress-applying portions, one or more low-refractive-index portions, and a common cladding surrounding the core, the pair of stress-applying portions, and the one or more low-refractive-index portions, wherein, on a cross section of the polarization-maintaining optical fiber perpendicular to the fiber axis, the pair of stress-applying portions are disposed on both sides of the core, spaced apart from the core with a portion of the common cladding therebetween, and the one or more low-refractive-index portions are disposed around the core, spaced apart from each other with a portion of the common cladding therebetween, and spaced apart from both the core and the pair of stress-applying portions with a portion of the common cladding therebetween.[Supplementary Note 2] A method for manufacturing a polarization-maintaining optical fiber according to any one of claims 1 to 7, comprising: a preform manufacturing step for manufacturing an optical fiber preform for obtaining the polarization-maintaining optical fiber; and a drawing step for drawing the optical fiber preform manufactured by the preform manufacturing step, wherein the preform manufacturing step comprises: a first sub-step for preparing a common clad rod that is to become a part of the common clad after drawing; a second sub-step for separately preparing a core rod including a portion that is to become the core after drawing, preparing one or more low refractive index rods that are to become the one or more low refractive index portions after drawing, and preparing a pair of stress-applying rods that are to become the pair of stress-applying portions after drawing; and a third sub-step for separately forming, in the common clad rod, a first through hole into which the core rod is to be inserted, one or more second through holes into which the one or more low refractive index rods are to be individually inserted, and a pair of third through holes into which the pair of stress-applying rods are to be individually inserted, a fourth sub-step of separately integrating the common clad rod with the core rod, integrating the common clad rod with the one or more low refractive index rods, and integrating the common clad with the pair of stress-applying rods, wherein in the optical fiber preform, the core rod, each of the one or more low refractive index rods, and each of the pair of stress-applying rods are arranged physically separated from each other with a portion of the common clad rod sandwiched therebetween.

[0055] DESCRIPTION OF SYMBOLS 10... Polarization-maintaining optical fiber 20... Glass optical fiber 30... Resin coating 40... Core 50... Common cladding 60A, 60B... Stress-applying portion 70A to 70H... Low refractive index portion 100A... Common cladding rod 100B... Intermediate base material 100C... Optical fiber base material 200... Heater 300... Resin application device 400A... Core rod 400B... First through hole 401... Core portion 402... Inner cladding portion 403... Outer cladding portion 410... Roller 420... Winding device 600A, 600B... Stress-applying rod 610A, 610B... Third through hole 700A... Low refractive index rod 700B... Second through hole S... Arrow

Claims

1. a core extending along the fiber axis; A pair of stress applying portions; One or more low refractive index sections; a common clad surrounding the core, the pair of stress-applying portions, and each of the one or more low refractive index portions; A polarization-maintaining optical fiber comprising: On a cross section of the polarization-maintaining optical fiber perpendicular to the fiber axis, the common clad is disposed between the pair of stress-applying portions and the core, the pair of stress-applying portions are disposed on both sides of the core in a state spaced apart from the core, the common clad is disposed between each of the plurality of low refractive index portions; The plurality of low refractive index portions are arranged apart from each other, the common clad is disposed between the one or more low refractive index sections and the core; the common clad is disposed between the one or more low refractive index portions and the pair of stress-applying portions, The one or more low refractive index portions are arranged around the core while being spaced apart from both the core and the pair of stress-applying portions. Polarization-maintaining optical fiber.

2. On the cross section, the outline of each of the one or more low refractive index portions is The shape is composed of only straight lines, only curved lines, or a combination of straight lines and curved lines.

2. The polarization-maintaining optical fiber of claim 1.

3. As the one or more low refractive index portions, the polarization-maintaining optical fiber has 2 to 6 low refractive index portions, On the cross section, the two to six low refractive index portions are arranged at positions that are equally spaced from the center of the core, and are arranged so as not to overlap with any of the pair of stress-applying portions.

2. The polarization-maintaining optical fiber of claim 1.

4. On the cross section, each of the pair of stress applying portions has an outer diameter of 30 μm or more and 40 μm or less, a relative refractive index difference of each of the pair of stress-applying portions with respect to the common cladding is 0.0% or less; a ratio a / b_SAP of a radius a of the core to a shortest distance b_SAP from the center of the core to each of the contours of the pair of stress applying portions is 0.4 or more and 0.6 or less; The polarization-maintaining optical fiber according to any one of claims 1 to 3.

5. A refractive index volume V defined by the product of the total area of ​​the one or more low refractive index portions and the absolute value of the average value of the relative refractive index differences of the one or more low refractive index portions on the cross section is 20 μm ２ ・% or more 120 μm ２ ・% or less, The polarization-maintaining optical fiber according to any one of claims 1 to 3.

6. A mode field diameter of 8.2 μm or more and 10.2 μm or less at a wavelength of 1.31 μm; a bending loss measured in a state where the polarization-maintaining optical fiber is bent once with a bending radius of 5 mm so that each of the pair of stress-applying portions is parallel to a bending plane, the bending loss being less than 0.15 dB at a wavelength of 1.55 μm; and a cutoff wavelength of less than 1.26 μm. The polarization-maintaining optical fiber according to any one of claims 1 to 3.

7. On the cross section, the core has a radius of 3 μm or more and 5 μm or less; The relative refractive index difference of the core with respect to the common cladding is 0.2% or more and 0.5% or less. The polarization-maintaining optical fiber according to any one of claims 1 to 3.

8. A method for producing a polarization-maintaining optical fiber according to any one of claims 1 to 3, comprising the steps of: a preform manufacturing process for manufacturing an optical fiber preform for obtaining the polarization-maintaining optical fiber; a drawing step of drawing the optical fiber preform manufactured by the preform manufacturing step; Equipped with The base material manufacturing process includes: a first sub-step of preparing a common clad rod to become part of the common clad after drawing; a second sub-step of separately carrying out a core rod including a portion to become the core after drawing, a low refractive index rod or rods to become the one or more low refractive index portions after drawing, and a pair of stress-applying rods to become the pair of stress-applying portions after drawing; a third sub-step of forming, in the common clad rod, a first through hole into which the core rod is inserted, one or more second through holes into which the one or more low refractive index rods are individually inserted, and a pair of third through holes into which the pair of stress-applying rods are individually inserted; a fourth sub-step of integrating the common clad rod with the core rod, integrating the common clad rod with the one or more low index rods, and integrating the common clad rod with the pair of stress-applying rods, separately; Including, In the optical fiber preform, the common clad rod is disposed between the core rod, each of the one or more low refractive index rods, and each of the pair of stress-applying rods. A method for manufacturing a polarization-maintaining optical fiber.