WO2024053252A1 - Polarization maintaining optical fiber and method for manufacturing polarization maintaining optical fiber - Google Patents

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 for manufacturing polarization-maintaining optical fiber

The present disclosure relates to a polarization-maintaining optical fiber and a method of manufacturing the polarization-maintaining optical fiber.
This application claims priority from Japanese Patent Application No. 2022-142279 filed on September 7, 2022, relies on the contents thereof, and is incorporated herein by reference in its entirety.

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 including optical devices, and the use of polarization-maintaining optical fibers with small bending radii is required.

For example, the polarization-maintaining optical fiber disclosed in Patent Document 1 includes a core, a first clad coat surrounding the core, and a second clad coat with a low refractive index surrounding the first clad coat and functioning as a trench. , a third clad coat surrounding the second clad coat, and a pair of stress applying parts arranged to sandwich the core. The radius ratio r2/r1 of the first clad coat and the second clad coat is 2.5 or more and 4.5 or less. The refractive index volume V of the second cladding coat is 25 μm ² ·% or more and 110 μm ² ·% or less. The pair of stress applying parts are arranged so as to physically separate the second clad coat. In the polarization-maintaining optical fiber of Patent Document 1, a second cladding coat doped with F (fluorine) is provided to function as a trench, so that the refractive index of the second cladding coat and the mode field diameter (hereinafter referred to as " The difference between the refractive index at the outer edge of the refractive index (hereinafter referred to as "MFD") becomes large. Therefore, the polarization-maintaining optical fiber of Patent Document 1 does not reduce the MFD when bent to a small radius, compared to a polarization-maintaining optical fiber to which a trench corresponding to the second cladding coat is not applied. Bending loss can be reduced.

US2015/0268413A1 publication

The polarization-maintaining optical fiber of the present disclosure includes a core extending along the fiber axis, a pair of stress applying parts, one or more low refractive index parts, a core, a pair of stress applying parts, and one or more low refractive index parts. a common cladding surrounding each of the refractive index sections. In particular, on a cross section of the polarization maintaining optical fiber perpendicular to the fiber axis, a common cladding is disposed between the pair of stress applying parts and the core. The pair of stress applying parts are arranged on both sides of the core and apart from the core. A common cladding is arranged between each of the plurality of low refractive index parts. The plurality of low refractive index parts are arranged apart from each other. A common cladding is disposed between the one or more low refractive index portions and the core. A common cladding is disposed between the one or more low refractive index parts and the pair of stress applying parts. One or more low refractive index sections are arranged around the core and spaced apart from both the core and the pair of stress applying sections.

FIG. 1 is a diagram for explaining the structure of the polarization-maintaining optical fiber of the present disclosure. FIG. 2 is a diagram showing various arrangement patterns of low refractive index portions on a cross section of the polarization maintaining optical fiber of the present disclosure. FIG. 3 is a diagram for explaining the arrangement conditions of the low refractive index portion on the cross section of the polarization maintaining optical fiber of the present disclosure. FIG. 4 is a diagram for explaining a method for manufacturing an optical fiber preform for obtaining a polarization-maintaining optical fiber of the present disclosure. FIG. 5 is a diagram showing the configuration of a drawing apparatus for obtaining the polarization-maintaining optical fiber of 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 in the polarization maintaining optical fiber of the present disclosure. FIG. 7 is a graph showing the dependence of bending loss and cutoff wavelength of the polarization maintaining optical fiber of 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 the polarization-maintaining optical fiber of the present disclosure on various MFDs at a wavelength of 1.31 μm.

[Problems that this disclosure seeks to solve]
As a result of studying the above-mentioned prior art, the inventors discovered the following problem. That is, the polarization-maintaining optical fiber of Patent Document 1 includes a core, a first clad coat, a second clad coat doped with F, a third clad coat, and a pair of stress applying parts. In this polarization-maintaining optical fiber, each of the pair of stress applying parts is arranged so as to physically divide the second cladding coat. In the base material manufacturing process, first, a through hole is formed in the center of the F-added rod that will become the second clad coat after drawing, and the rod is made up of a core part and a first clad part that will become the core and first clad coat after drawing. The resulting core rod is inserted into the through hole of the F-added rod, and these are further integrated to obtain a first intermediate base material composed of a core portion, a first cladding portion, and a second cladding portion. . Next, a through hole is formed in the center of the third cladding part as a jacket material that will become the third cladding coat after drawing, and the first intermediate base material is inserted into the through hole of the third cladding part, and further, By integrating these, a second intermediate base material composed of the core portion and the first to third cladding portions is obtained. A pair of through-holes are formed at predetermined locations in the second intermediate base material obtained, and a pair of stress-applying rods that will become a pair of stress-applying parts after wire drawing are respectively inserted into the pair of through-holes, and further, By integrating these, an optical fiber preform for obtaining a polarization maintaining optical fiber is obtained. In this way, in order to obtain the polarization-maintaining optical fiber of Patent Document 1, the number of steps required to complete the optical fiber preform is large and complicated, and most of the expensive F-doped Rosso is used in the first intermediate preform. The material is discarded during the manufacturing process and the process of inserting the pair of stress-applying rods, which hinders the effective use of the material. This could lead to increased manufacturing costs and decreased production efficiency.

The present disclosure has been made in order to solve the above-mentioned problems, and provides a polarization-maintaining optical fiber and its polarization-maintaining optical fiber that has an easy-to-manufacture structure that makes it possible to reduce bending loss when bent to a small radius. The purpose is to provide a manufacturing method.

[Effects of this disclosure]
According to the present disclosure, a polarization-maintaining optical fiber can be obtained that has a structure that enables reduction of bending loss when bent to a small radius.

[Description of embodiments of the present disclosure]
First, the contents of the embodiments of the present disclosure will be individually listed and explained.

The polarization maintaining optical fiber of the present disclosure includes:
(1) A core extending along the fiber axis, a pair of stress applying parts, and one or more low refractive index parts, and surrounding each of the core, the pair of stress applying parts, and one or more low refractive index parts A common cladding. In particular, on a cross section of the polarization maintaining optical fiber perpendicular to the fiber axis, a common cladding is disposed between the pair of stress applying parts and the core. The pair of stress applying parts are arranged on both sides of the core and apart from the core. A common cladding is arranged between each of the plurality of low refractive index parts. The plurality of low refractive index parts are arranged apart from each other. A common cladding is disposed between the one or more low refractive index portions and the core. A common cladding is disposed between the one or more low refractive index parts and the pair of stress applying parts. One or more low refractive index sections are arranged around the core and spaced apart from both the core and the pair of stress applying sections.

As described above, the one or more low refractive index parts arranged at intervals around the core and also arranged at intervals from each of the pair of stress applying parts serve as a trench layer arranged around the core. Function. As a result, the polarization-maintaining optical fiber provided with the low refractive index portion achieves a reduction in bending loss when bent to a small radius. Furthermore, since each low refractive index section does not contact the pair of stress applying sections, complication of element arrangement in the fiber cross section can be avoided, the number of manufacturing steps can be reduced, and manufacturing costs can be reduced.

(2) In (1) above, the contour of each of the one or more low refractive index portions on the cross section of the polarization-maintaining optical fiber is composed of only a straight line, only a curved line, or a combination of a straight line and a curved line. It may have a different shape. Specifically, the outline of each low refractive index portion is composed of all shapes, such as circles, ellipses, quadrangles, and triangles, which are composed of at least one of straight lines and curved lines. In this manner, in the polarization-maintaining optical fiber of the present disclosure, there is no restriction on the shape of the cross section of each low refractive index section as long as it does not contact the stress applying section.

(3) In (1) or (2) above, the polarization maintaining optical fiber may have 2 or more and 6 or less low refractive index parts as one or more low refractive index parts. In this case, on the cross section, the two to six low refractive index parts are arranged at positions where the center-to-center distances from the center of the core are equal, and so that they do not overlap with either of the pair of stress applying parts. may be placed. Here, when the cross section of each low refractive index portion is circular, the base material can be manufactured by collapsing a common clad rod inserted into a through hole with a cylindrical low refractive index rod. Therefore, the polarization maintaining optical fiber of the present disclosure can be manufactured at low cost. Note that the low refractive index rod is a member that becomes a low refractive index section after drawing, and the common cladding rod is a member that becomes a common cladding after drawing. When the number of low refractive index parts is small, manufacturing costs associated with forming through holes into which each low refractive index rod is inserted are reduced, but confinement of the fundamental mode becomes weaker, which is a factor that increases bending loss. becomes. 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 a desired cutoff wavelength and bending loss characteristics.

In addition, ITU-T standard G. 657. The latest version of 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, since the tolerance is ±0.4 μm, the MFD at a wavelength of 1310 nm is substantially 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).

(4) In any one of (1) to (3) above, each of the pair of stress applying portions may 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 between each of the pair of stress applying parts 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 parts may be lower than or equal to the refractive index of the common cladding. Further, the ratio a/b_SAP (hereinafter referred to as "Ra_SAP") of the radius a of the core to the shortest distance b_SAP from the center of the core to the contour of each of the pair of stress applying parts is 0.4 or more and 0.6 or less. There may be. When the outer diameter of each stress-applying portion is less than 30 μm or larger than 40 μm, birefringence in the core becomes small and polarization maintaining characteristics deteriorate. Furthermore, when Ra_SAP is less than 0.4 and the shortest distance from the center of the core to the contour of each stress applying portion is large, the birefringence in the core similarly decreases. On the other hand, if Ra_SAP is larger than 0.6, the through-hole for inserting the stress-applying rod provided in the common clad rod that should become the common clad during base material manufacturing is close to the through-hole for inserting the core rod, making it difficult to manufacture. become. Note that the stress applying rod is a member that becomes one of a pair of stress applying parts after wire drawing, and the core rod is a member that becomes a core after wire drawing. Furthermore, each stress applying part also contributes to reducing bending loss, and in particular, the polarization maintaining optical fiber is arranged so that the center of each stress applying part is in the 0 degree direction, that is, each stress applying part is parallel to the bending plane. When bent, bending losses are greatly reduced. Here, the bending plane is a plane for indicating the bending direction of the polarization-maintaining optical fiber, and includes the central axis of the polarization-maintaining optical fiber in the bent state.

(5) In any of (1) to (4) above, the total area of one or more low refractive index parts on the cross section of the polarization maintaining optical fiber and the total area of the one or more low refractive index parts The refractive index volume V defined by the product of the absolute value of the average value of the relative refractive index difference may be 20 μm ² ·% or more and 120 μm ² ·% or less. By appropriately selecting the refractive index volume V, the ITU-T standard G. 657. A polarization maintaining optical fiber conforming to B3 is obtained.

(6) In any of (1) to (5) above, the polarization maintaining optical fiber has an MFD of 8.2 μm or more and 10.2 μm or less at a wavelength of 1.31 μm and a MFD of 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 with the polarization maintaining optical fiber being bent once with a bending radius of 5 mm so that each of the pair of stress applying parts is parallel to the bending plane. When the refractive index volume V is less than 20 μm ² ·%, the bending resistance is low, and the bending loss at a wavelength of 1.55 μm exceeds 0.15 dB. On the other hand, when the refractive index volume V exceeds 120 μm ^2. %, the spacing between the low refractive index parts when six low refractive index parts are arranged is narrow, making it difficult to form through holes in the base material manufacturing process. . Note that there is a trade-off relationship between the cutoff wavelength and bending loss, and the ITU-T standard G. 657. The allowable width (tolerance) of the structure satisfying B3 depends on the MFD.

(7) In either (1) or (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 core with respect to the common cladding The relative refractive index difference may be 0.2% or more and 0.5% or less. Note that the relative refractive index difference of one or more low refractive index parts with respect to the common cladding may be -1.0% or more and -0.5% or less. By appropriately selecting the core radius and core relative refractive index difference within these ranges, the MFD described in (6) above can be realized.

The method for manufacturing a polarization-maintaining optical fiber of the present disclosure includes:
(8) Producing the polarization-maintaining optical fiber according to any one of (1) to (7) above. Specifically, the manufacturing method includes a base material manufacturing step of manufacturing an optical fiber preform to obtain the polarization maintaining optical fiber, and a drawing step of drawing the optical fiber preform manufactured by the preform manufacturing step. and. The base material manufacturing process includes a first sub-process to a fourth sub-process. In the first sub-step, a common cladding rod is prepared which is to become part of the common cladding after drawing. In the second sub-step, preparation of a core rod including a portion to become a core after drawing, preparation of one or more low refractive index rods to become one or more low refractive index parts after drawing, and applying a pair of stresses after drawing. The preparation of a pair of stress-applying rods to become a part is carried out separately. 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 in parallel. do not have. In the third sub-step, forming a first through hole into which the core rod is inserted, and forming one or more second through holes into which the one or more low refractive index rods are individually inserted, in the common clad rod. , and a pair of third through-holes into which the pair of stress applying rods are individually inserted are individually performed. That is, the third sub-step for the first through-hole, the third sub-step for the second through-hole, and the third sub-step for the third through-hole do not need to be performed in parallel. In the fourth sub-step, the integration of the common cladding rod and the core rod, the integration of the common cladding rod and one or more low refractive index rods, and the integration of the common cladding rod and the pair of stress applying rods are individually performed. . That is, the fourth sub-process for integrating the core rod, the fourth sub-process for integrating the low refractive index rod, and the fourth sub-process for integrating the stress applying rod do not need to be performed simultaneously. do not have.

In addition, in the optical fiber preform obtained through the above-mentioned first sub-step to fourth sub-step, between the core rod, each of the one or more low refractive index rods, and each of the pair of stress-applying rods, Common clad rods are located. In the manufacturing method of Patent Document 1, in order to provide a pair of stress applying portions, it is necessary to remove a portion of the previously formed trench layer. On the other hand, according to the manufacturing method of the present disclosure, compared to the manufacturing method of Patent Document 1, the manufacturing process is simplified, there is no need to remove the trench layer once formed, and the cost is reduced from the viewpoint of the manufacturing process. There is expected. Furthermore, according to the manufacturing method of the present disclosure, ITU-T standard G. 657. A low bending loss polarization maintaining optical fiber that complies with B3 can be obtained.

Above, each aspect listed in this [Description of embodiments of the present disclosure] column is applicable to each of the remaining aspects or to all combinations of these remaining aspects. .

[Details of embodiments of the present disclosure]
Hereinafter, the specific structure 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 with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all changes within the meaning and scope equivalent to the scope of the claims. In addition, in the description of the drawings, the same elements are given the same reference numerals and redundant description will be omitted.

FIG. 1 is a diagram for explaining the structure of the polarization-maintaining optical fiber of the present disclosure (denoted as "fiber structure" in FIG. 1). At the top of FIG. 1 (denoted as "cross-sectional structure" in FIG. 1), an example of the cross-sectional structure of the polarization-maintaining optical fiber 10 of the present disclosure is shown. The second row of FIG. 1 (indicated 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 in the top row. has been done. The third row of FIG. 1 (indicated 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 in the top row. has been done. The bottom row of FIG. 1 (denoted as "refractive index profile (above 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. ing.

As shown in the top row of FIG. 1, the polarization-maintaining optical fiber 10 of the present disclosure includes a glass optical fiber 20 mainly composed of silica glass, and a resin provided on the outer peripheral surface of the glass optical fiber 20. A covering 30 is provided. The glass optical fiber 20 includes a core 40 extending along the fiber axis AX, a common cladding 50, and a pair of stress applying portions 60A and 60B arranged to sandwich the core 40 at a distance from the core 40. It includes one or more low refractive index parts 70A that surround the core 40 and are spaced apart from both the core 40 and the pair of stress applying parts 60A and 60B.

1, the refractive index profile shown in the second row of FIG. The stress is given to each part along the line L1 passing through the center of each of the stress applying parts 60A and 60B by the relative refractive index difference. The core 40 has a radius of 3 μm or more and 5 μm or less. Further, the relative refractive index difference between the core 40 and the common cladding 50 is 0.2% or more and 0.5% or less. Each of the pair of stress applying parts 60A and 60B has an outer diameter of 30 μm or more and 40 μm or less. The relative refractive index difference between the pair of stress applying parts 60A and 60B with respect to the common cladding 50 is 0.0% or less. Note that each of the pair of stress applying portions 60A and 60B may have a positive relative refractive index difference with respect to the common cladding 50, as indicated by the broken line.

In the cross section of the polarization maintaining optical fiber 10, the refractive index profile shown in the third row of FIG. It is given by the relative refractive index difference to each part along the line L2 passing through the center of . 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, the one or more low refractive index portions 70A surrounding the core 40 function as a trench layer in the entire low refractive index portion 70A. With such a structure, the polarization-maintaining optical fiber 10 provided with one or more low refractive index portions 70A achieves a reduction in bending loss when bent to a small radius.

The refractive index profile shown in the bottom row of FIG. 1 shows two low refractive index parts adjacent to the center of the core 40 in the cross section of the polarization-maintaining optical fiber 10, as shown in the top row of FIG. The relative refractive index difference is given to each part along the line L3 passing through the intermediate position of 70A.

As described above, in the polarization maintaining optical fiber 10 of the present disclosure, in its cross section, each of the core 40, the pair of stress applying parts 60A, 60B, and the one or more low refractive index parts 70A is connected to the common cladding 50. They are located at physically separate locations with some parts in between. Therefore, as shown from the second row to the bottom row of FIG. It has a different shape when rotated in the direction. Furthermore, since each low refractive index portion 70A is not in contact with the pair of stress applying portions 60A and 60B, complication of the element arrangement in the cross section is avoided, reducing the number of manufacturing steps and making it possible to reduce manufacturing costs. .

Furthermore, according to the polarization-maintaining optical fiber 10 having the above-described structure, by providing the low refractive index portions, the refractive index of each low refractive index portion 70A and , the difference between the effective refractive index obtained at the outer edge of the MFD and the effective refractive index obtained at the outer edge of the MFD becomes large, and bending loss when bent to a small radius is reduced compared to a polarization-maintaining optical fiber that does not have a low refractive index section. Can be done. In addition, MFD, bending loss, and cutoff wavelength are determined according to ITU-T standard G. 657. It becomes easy to design to satisfy B3. If the diameter of the core 40, the position and size of each low refractive index portion 70A and the pair of stress applying portions 60A and 60B are appropriately selected, the ITU-T standard G. 657. It becomes possible to lower manufacturing costs while complying with B3.

FIG. 2 is a diagram showing various arrangement patterns of low refractive index portions on a cross section of the polarization-maintaining optical fiber of the present disclosure (in FIG. 2, it is written as “arrangement pattern of low refractive index portions”). Note that patterns A to H shown in FIG. 2 are all arrangement patterns on the cross section of the glass optical fiber 20 orthogonal to the fiber axis AX.

On the cross section of the polarization-maintaining optical fiber 10 of the present disclosure, the contour of each low refractive index portion may have a shape consisting of only a straight line, only a curved line, or a combination of a straight line and a curved line. 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 contour on the cross-section of the polarization-maintaining optical fiber 10 is as illustrated in FIG. There is no limit to the shape defined by the contour as long as it is not in contact with a stress-applying part.

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

Similar to pattern A, pattern D shown in FIG. 2 has two groups of low refractive index portions arranged, but the outline shape of each low refractive index portion 70D is non-circular. That is, each group of low refractive index portions is composed of three low refractive index portions 70D each having an elliptical outline. Further, each of the two groups of low refractive index portions of pattern E shown in FIG. 2 is composed of three low refractive index portions 70E. The outline shape of each low refractive index portion 70E is rectangular. Each of the two low refractive index section groups of pattern F shown in FIG. 2 is constituted by one low refractive index section 70F. The outline shape of each low refractive index portion 70F is triangular. Each of the two groups of low refractive index portions of pattern G shown in FIG. 2 is constituted by one low refractive index portion 70G. The outline shape of each low refractive index portion 70G is trapezoidal. Further, each of the two low refractive index section groups of pattern H shown in FIG. 2 is composed of two low refractive index sections 70H. The outline shape of each low refractive index portion 70H is star-shaped.

FIG. 3 is a diagram for explaining the arrangement conditions of the low refractive index portion on the cross section of the polarization maintaining optical fiber 10 of the present disclosure. In the following description, the low refractive index portion 70A of pattern A shown in FIG. 2 will be described as a representative example of the low refractive index portion. Further, 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 section 70A, and the pair of stress applying sections 60A and 60B is as follows. It is defined by an orthogonal coordinate system (xy coordinate system) with the origin at the center of . Note that in FIG. 3, the x-axis coincides with the line L1 shown at the top of FIG. 1, and the y-axis coincides with the line L2.

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 contour of each low refractive index portion 70A is c_A. Further, the shortest distance from the center of the core 40 to the contours of each of the pair of stress applying parts 60A and 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 and 60B is less than 30 μm or larger than 40 μm, birefringence in the core 40 becomes small and polarization maintaining characteristics deteriorate. Similarly, when Ra_SAP is less than 0.4 and the shortest distance from the center of the core 40 to the contours of the pair of stress applying parts 60A and 60B is large, the birefringence in the core 40 is similarly reduced. On the other hand, when Ra_SAP is larger than 0.6, the through-hole for inserting the stress applying rod provided in the common clad rod which is to become the common clad 50 during the manufacture of the base material and the through-hole for inserting the core rod become close to each other, resulting in a manufacturing problem. It becomes difficult. Note that the stress applying rod is a member that becomes either the stress applying portion 60A or 60B after wire drawing, and the core rod is a member that becomes the core 40 after wire drawing. Furthermore, the pair of stress applying parts 60A and 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 parts 60A and 60B are in the 0 degree direction, that is, each of the pair of stress applying parts 60A and 60B is parallel to the bending plane. The line L1 passing through the center of the pair of stress applying parts 60A and 60B at each location along the longitudinal direction of the polarization-maintaining optical fiber 10 is perpendicular to the bending plane, and at this time, the bending loss is greatly reduced.

In addition, the polarization maintaining optical fiber 10 of the present disclosure complies with ITU-T standard G. 657. In order to satisfy B3, two or more and six or less low refractive index parts are provided, such as the low refractive index parts 70A to 70H shown in FIG. In this case, the two to six low refractive index parts on the cross section are arranged at positions where the center-to-center distances from the center of the core 40 are the same, and both of the pair of stress applying parts 60A and 60B are arranged. They are arranged so that they do not overlap. Note that when the cross-sectional shape of each of the low refractive index portions 70A to 70C is circular, the polarization-maintaining optical fiber 10 can be manufactured at low cost. Further, in the process of manufacturing the optical fiber preform, manufacturing costs associated with forming through holes in the common clad rod that will become the common clad 50 after drawing are reduced. On the other hand, when the number of low refractive index portions is small, confinement of the fundamental mode becomes weaker, which becomes a factor in increasing bending loss. Therefore, according to the polarization-maintaining optical fiber 10 of the present disclosure, it is important to adjust the number of low refractive index portions so that the desired cutoff wavelength and bending loss characteristics can be obtained.

FIG. 4 is a diagram for explaining a method for manufacturing an optical fiber preform 100C for obtaining the polarization-maintaining optical fiber 10 of the present disclosure (denoted as "preform production" in FIG. 4). Moreover, FIG. 5 is a diagram showing the configuration of a drawing apparatus for obtaining the polarization-maintaining optical fiber 10 of the present disclosure. In the upper part of FIG. 4 (denoted as "step ST1" in FIG. 4), a preparation process for preparing members constituting the intermediate base material 100B is shown. The middle part of FIG. 4 (denoted as "Step ST2" in FIG. 4) shows an intermediate base material manufacturing process in which hole-opening, insertion, and sintering are performed to obtain the intermediate base material 100B. . The lower part of FIG. 4 (indicated as "Step ST3" in FIG. 4) shows a light beam in which member preparation, hole opening, insertion, and sintering are performed to obtain the optical fiber preform 100C from the intermediate preform 100B. The fiber preform manufacturing process is shown. In addition, in the following description, similarly to 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 the low refractive index portion.

The method for manufacturing the polarization-maintaining optical fiber 10 of the present disclosure includes a base material manufacturing process illustrated in FIG. 4 and a wire drawing process illustrated in FIG. 5. The preform manufacturing process includes a first sub-process to a fourth sub-process in order to obtain the optical fiber preform 100C. In the first sub-step, a common clad rod 100A is prepared. The common clad rod 100A is processed so as to leave a clad outer portion 403 that will become the physical clad layer of the common clad 50 after drawing. This physical cladding layer is called a jacket layer.

In the second sub-step, preparation of the core rod 400A, preparation of one or more low refractive index rods 700A, and preparation of the pair of stress applying rods 600A and 600B are performed individually. The core rod 400A includes a core portion 401 that will become the core 40 after drawing in the center of the pure silica rod, and is constituted by the core portion 401 and a clad inner portion 402 surrounding the core portion 401. The inner cladding portion 402 is a portion that becomes the optical cladding layer of the common cladding 50 after drawing. One or more low refractive index rods 700A are members that should become one or more low refractive index parts 70A after drawing. The pair of stress applying rods 600A and 600B are members that are to become a pair of stress applying parts 60A and 60B after wire drawing. Note that the second sub-process for the core rod 400A, the second sub-process for one or more low refractive index rods 700A, and the second sub-process for the pair of stress applying rods 600A and 600B are performed in parallel. It does not need to be implemented.

In the third sub-step, forming a first through hole 400B into which the core rod 400A is inserted, and forming one or more second through holes into which one or more low refractive index rods 700A are individually inserted into the common clad rod 100A. The formation of the through hole 700B and the formation of the pair of third through holes 610A, 610B into which the pair of stress applying rods 600A, 600B are individually inserted are performed individually. 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 and 610B do not need to be performed simultaneously. do not have.

In the fourth sub-step, the common clad rod 100A and the core rod 400A are integrated, the common clad rod 100A is integrated with one or more low refractive index rods 700A, and the common clad rod 100A and the pair of stress applying rods 600A, 600B are integrated. Integration is performed separately. The fourth sub-step for integrating the core rod, the fourth sub-step for integrating the low refractive index rod, and the fourth sub-step for integrating the stress applying rod do not need to be performed simultaneously. Note that the common clad rod 100A and the core rod 400A are integrated into one body by collapsing the common clad rod 100A while the core rod 400A is inserted into the first through hole 400B. Further, due to the integration, the clad inner part 402 of the core rod 400A and the clad outer part 403 of the common clad rod 100A constitute a part that will become the common clad 50 after drawing. The common clad rod 100A and one or more low refractive index rods 700A are integrated by inserting one or more low refractive index rods 700A into one or more second through holes 700B, and then inserting the common clad rod 100A into one or more low refractive index rods 700A. This is achieved by collapsing. Further, the common clad rod 100A and the pair of stress applying rods 600A, 600B are produced 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. Realized.

Note that after the first sub-step is carried out, 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 a pair of stress application steps are performed. The second to fourth sub-steps for the rods 600A and 600B may be performed in order. As an example, in the base material manufacturing process of FIG. 4, first, following the first sub-process, the second to fourth sub-processes for the core rod 400A are performed, and then the low refractive index rod 700A and the pair of stress-applying The second to fourth sub-steps are performed for both rods 600A and 600B.

Specifically, in step ST1 shown in the upper part of FIG. 4, a first sub-process for the common clad rod 100A and a second sub-process for the core rod 400A are performed. That is, in step ST1, a common clad rod 100A and a core rod 400A composed of a core part 401 and a clad inner part 402 are prepared. Subsequently, in step ST2 shown in the middle part of FIG. 4, a third sub-step and a fourth sub-step for the core rod 400A are performed, and finally an intermediate base material 100B is obtained. This intermediate base material 100B is a common clad rod 100A into which a core rod 400A is integrated, and is composed of a clad inner part 402 and a clad outer part 403 after the third sub-step.

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

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 wire drawing device includes a heater 200, a resin coating device 300, a roller 410, and a winding device 420. When the winding device 420 rotates in the direction indicated by the arrow S, the glass optical fiber is pulled out from one end of the optical fiber preform 100C heated by the heater 200. This glass optical fiber is coated with resin on its outer peripheral surface by a resin coating device 300, and finally, the polarization maintaining optical fiber 10 coated with resin is transferred to a drum of a winding device 420 via a roller 410. It is wound up. The cross-sectional structure of the polarization-maintaining optical fiber 10 along line II shown in FIG. 5 corresponds to the cross-sectional structure shown in the upper part of FIG.

Next, as the optical characteristics of the polarization maintaining optical fiber of the present disclosure, the optical characteristics of the polarization maintaining optical fiber 10 having the cross-sectional structure of pattern A shown in FIG. 2 will be explained using FIGS. 6 to 8. . Note that FIG. 6 is a graph showing the dependence of the cutoff wavelength λcc and bending loss on the refractive index volume V in the polarization maintaining optical fiber of the present disclosure. FIG. 7 is a graph showing the dependence of the bending loss and cutoff wavelength of the polarization-maintaining optical fiber of the present disclosure on various MFDs at a wavelength of 1.31 μm (denoted as “MFD dependence” in FIG. 7). . In the upper part of FIG. 7 (denoted as "bending loss-λcc characteristics when changing MFD" in FIG. 7), changes in bending loss with respect to λcc are shown for various MFDs. The lower part of FIG. 7 (denoted as "λcc_min-MFD characteristics" in FIG. 7) shows the change in the cutoff wavelength λcc_min for the MFD at a wavelength of 1.31 μm. Further, FIG. 8 is a graph showing the dependence of the core radius and relative refractive index difference on the MFD of the polarization maintaining optical fiber of the present disclosure.

First, in FIG. 6, the horizontal axis is the refractive index volume V (μm ² ·%). This refractive index volume V is the total cross-sectional area of the six low refractive index parts 70A shown as pattern A among the various low refractive index parts 70A to 70H shown in FIG. This is the product of the absolute value of the average relative refractive index difference of the refractive index portion 70A. Note that the relative refractive index difference of each low refractive index section 70A with respect to the common cladding 50 is set to -1.0% or more and -0.5% or less. The upper part of the vertical axis is the cutoff wavelength λcc (μm). The lower part of the vertical axis is the bending loss (dB). This bending loss was measured by inputting light with a wavelength of 1.55 μm while the polarization-maintaining optical fiber to be measured was bent once with a bending radius of 5 mm so that each of the pair of stress applying parts was parallel to the bending plane. Transmission loss is sometimes measured.

As can be seen from FIG. 6, the bending loss at the cutoff wavelength λcc and the wavelength of 1.55 μm shows dependence on the refractive index volume V. Each low refractive index portion 70A functions as a trench layer and thus contributes to reducing bending loss. In addition, as the refractive index volume V increases, the bending loss decreases while the cutoff wavelength λcc increases. Therefore, in order to limit the increase in bending loss, the refractive index volume V should be 20 μm ² ·% or more. On the other hand, if the refractive index volume V exceeds 120 μm ^2. %, it becomes difficult to form the second through hole 700B for the low refractive index rod 700A during base material manufacturing, so the refractive index volume V exceeds 120 μm ^2. % The following is sufficient. In this way, by appropriately selecting the refractive index volume V, the ITU-T standard G. 657. A polarization-maintaining optical fiber 10 compliant with B3 is obtained.

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

As can be seen from the upper and lower parts of FIG. 7, the bending loss at the cutoff wavelength λcc and the wavelength of 1.55 μm shows dependence on the MFD at the wavelength of 1.31 μm. That is, the smaller the MFD, the stronger the light confinement and the greater the tolerance, so the smaller the MFD, the greater the tolerance. In particular, as shown in the upper part of FIG. 7, if the MCF is 8.2 μm or more and 10.2 μm or less, the bending loss is less than 0.15 dB at a wavelength of 1.55 μm and the cutoff wavelength is less than 1.26 μm. It is possible to realize λcc. Furthermore, as shown in the lower part of FIG. 7, if the cutoff wavelength λcc_min (μm) is less than 1.26 μm when the bending loss at a wavelength of 1.55 μm is 0.15 dB, then the ITU-T standard G. 657. Satisfy B3. In this way, if the MFD at a wavelength of 1.31 μm is 8.2 μm or more and 10.2 μm or less, it complies with the ITU-T standard G. 657. A cutoff wavelength λcc and bending loss satisfying B3 can be achieved. However, since there is a trade-off relationship between the cutoff wavelength λcc and bending loss, this applies to ITU-T standard G. 657. This means that the tolerance that satisfies B3 depends on the MFD.

In FIG. 8, the horizontal axis is 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 is the radius a of the core 40. In FIG. 8, graph G810 shows the relationship between core Δ and core radius when the MFD is 7 μm at a wavelength of 1.31 μm. Graph G820 shows the relationship between core Δ and core radius when the MFD is 8 μm. Graph G830 shows the relationship between core Δ and core radius when the MFD is 9 μm. Graph G840 shows the relationship between core Δ and core radius when the MFD is 10 μm. Graph G850 shows the relationship between core Δ and core radius when the MFD is 11 μm. Graph G860 shows the relationship between core Δ and core radius when the MFD is 12 μm. Graph G870 shows the relationship between core Δ and core radius when the MFD is 13 μm.

When selecting the allowable range of core Δ and core radius for graph G830 as a reference line among graphs G810 to G870, as can be seen from FIG. 8, the radius of core 40 on the cross section of polarization-maintaining optical fiber 10 is It is sufficient if the thickness falls within the range of 3 μm or more and 5 μm or less. Further, the relative refractive index difference between the core 40 and the common cladding 50 may be within a range of 0.2% or more and 0.5% or less.

As understood from the description of the embodiments described above, this specification also includes disclosure of the following aspects.
[Additional note 1]
a core extending along the fiber axis;
a pair of stress applying parts;
one or more low refractive index portions;
a common cladding surrounding the core, the pair of stress applying parts, and the one or more low refractive index parts;
A polarization-maintaining optical fiber comprising:
On a cross section of the polarization maintaining optical fiber perpendicular to the fiber axis,
The pair of stress applying parts are arranged on both sides of the core in a state separated from the core with a part of the common cladding in between,
The one or more low refractive index parts are separated from each other with a part of the common cladding in between, and are separated from both the core and the pair of stress applying parts with a part of the common cladding in between. spaced apart around the core;
Polarization maintaining optical fiber.
[Additional 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 of 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;
Preparation of a core rod including a portion to become the core after drawing, preparation of one or more low refractive index rods to become the one or more low refractive index portions after drawing, and the pair of stress applying portions after drawing. a second sub-step of separately preparing a pair of stress-applying rods to be used;
Forming a first through hole into which the core rod is inserted into the common clad rod; forming one or more second through holes into which the one or more low refractive index rods are individually inserted; and a third sub-step of individually forming a pair of third through holes into which the pair of stress applying rods are individually inserted,
a step of individually performing the integration of the common cladding rod and the core rod, the integration of the common cladding rod and the one or more low refractive index rods, and the integration of the common cladding and the pair of stress applying rods; four sub-processes,
including;
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 physically separated with a part of the common cladding rod in between. located in the state,
A method for manufacturing polarization-maintaining optical fiber.

10...Polarization maintaining optical fiber 20...Glass optical fiber 30...Resin coating 40...Core 50... Common cladding 60A, 60B...Stress applying parts 70A to 70H...Low refractive index part 100A...Common cladding rod 100B...Intermediate base material 100C... Optical fiber base material 200... Heater 300... Resin coating device 400A... Core rod 400B... First through hole 401... Core part 402... Clad inner part 403... Clad outer part 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 (8)

1. a core extending along the fiber axis;
   a pair of stress applying parts;
   one or more low refractive index parts,
   a common cladding surrounding each of the core, the pair of stress applying parts, and the one or more low refractive index parts;
   A polarization-maintaining optical fiber comprising:
   On a cross section of the polarization maintaining optical fiber perpendicular to the fiber axis,
   The common cladding is arranged between the pair of stress applying parts and the core,
   The pair of stress applying parts are arranged on both sides of the core in a state apart from the core,
   The common cladding is arranged between each of the plurality of low refractive index parts,
   The plurality of low refractive index parts are arranged apart from each other,
   The common cladding is arranged between the one or more low refractive index parts and the core,
   The common cladding is arranged between the one or more low refractive index parts and the pair of stress applying parts,
   The one or more low refractive index parts are arranged around the core in a state separated from both the core and the pair of stress applying parts.
   Polarization maintaining optical fiber.
2. On the cross section, the outline of each of the one or more low refractive index parts is:
   Having a shape composed of only straight lines, only curves, or a combination of straight lines and curves,
   The polarization maintaining optical fiber according to claim 1.
3. As the one or more low refractive index portions, the polarization maintaining optical fiber has 2 or more and 6 or less low refractive index portions,
   On the cross section, the two or more and six or less low refractive index parts are arranged at positions where the center-to-center distances from the center of the core are equal, and do not overlap with any of the pair of stress applying parts. It is arranged as follows.
   The polarization maintaining optical fiber according to claim 1 or claim 2.
4. On the cross section, each of the pair of stress applying parts 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 parts with respect to the common cladding is 0.0% or less,
   The ratio a/b_SAP of the radius a of the core to the shortest distance b_SAP from the center of the core to the contour of each of the pair of stress applying parts 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. On the cross section, refraction defined as the product of the total area of the one or more low refractive index parts and the absolute value of the average value of the relative refractive index difference of the one or more low refractive index parts The rate volume V is 20 μm ² ·% or more and 120 μm ² ·% or less,
   The polarization-maintaining optical fiber according to any one of claims 1 to 4.
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 when the polarization-maintaining optical fiber is bent once with a bending radius of 5 mm so that each of the pair of stress applying parts is parallel to the bending plane, and is 0.15 dB at a wavelength of 1.55 μm. with bending losses of less than
   and a cutoff wavelength of less than 1.26 μm.
   The polarization-maintaining optical fiber according to any one of claims 1 to 5.
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,
   A polarization-maintaining optical fiber according to any one of claims 1 to 6.
8. A method for manufacturing a polarization-maintaining optical fiber for manufacturing the polarization-maintaining optical fiber according to any one of claims 1 to 7, comprising:
   a preform manufacturing step of 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;
   Preparation of a core rod including a portion that will become the core after drawing, preparation of one or more low refractive index rods that will become the one or more low refractive index portions after drawing, and the pair of stress applying portions after drawing. a second sub-step of individually preparing a pair of stress applying rods;
   Forming a first through hole into which the core rod is inserted into the common clad rod, forming one or more second through holes into which the one or more low refractive index rods are individually inserted, and a third sub-step of individually forming a pair of third through holes into which the pair of stress applying rods are individually inserted;
   Integrating the common clad rod and the core rod, integrating the common clad rod and the one or more low refractive index rods, and integrating the common clad rod and the pair of stress applying rods are performed individually. A fourth sub-step;
   including;
   In the optical fiber preform, the common cladding 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 polarization-maintaining optical fiber.

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