Optical fiber bundle and optical connector

Abstract

Provided is an optical fiber bundle (50) in which, in a state in which a plurality of optical fibers (51) are bundled, the positional relationship of the optical fibers (51) is fixed, wherein the optical fibers (51) have a basic diameter part (63), a fine diameter part (61) that is thinner than the basic diameter part (63), and a tapered part (62) that is positioned between the basic diameter part (63) and the fine diameter part (61) and gradually increases in diameter from the fine diameter part (61) toward the basic diameter part (63), and adjacent tapered parts (62) are in contact with each other over the entire length thereof.

Description

Optical Fiber Bundles and Optical Connectors

[0001] The present invention relates to an optical fiber bundle, and more particularly to an optical fiber bundle for connecting multiple single-mode fibers to a multicore fiber. The present invention also relates to an optical connector using the optical fiber bundle. This application claims priority from Japanese Patent Application No. 2024-015067, filed February 2, 2024, the contents of which are incorporated herein by reference.

[0002] As communication networks become faster, the density of optical fibers used in data centers and the like is increasing, and the practical application of multicore fibers having multiple cores in one cladding is progressing. Multiple single-core fibers can be connected to each core of a multicore fiber. In this case, the cores of the multiple single-core fibers to be connected are bundled according to the arrangement of the cores of the multicore fiber, thereby enabling efficient connection between the multicore fiber and the single-core fiber.

[0003] Patent Document 1 discloses a bundle structure that can be used for such applications. In Patent Document 1, a portion of the cladding of the optical fiber is removed by etching or the like to reduce the diameter of the optical fiber, and multiple optical fibers are bundled and integrated with a capillary (ferrule). Generally, optical fibers reduced in diameter as described above have a reduced diameter portion on the tip side and a constant diameter portion on the base side.

[0004] Japanese Patent No. 6655448

[0005] An optical fiber passed through a ferrule is often fixed to the ferrule with an adhesive introduced through a through-hole formed in the ferrule. As will be described in detail later, the inventor discovered that the behavior of the adhesive introduced through the through-hole as it hardens is significantly related to the transmission loss of the fixed fiber, and completed the present invention.

[0006] The present invention aims to provide an optical fiber bundle that is less susceptible to transmission loss when fixed to a ferrule with an adhesive. Another object of the present invention is to provide an optical connector that reduces transmission loss inside the ferrule.

[0007] A first aspect of the present invention is an optical fiber bundle in which a plurality of optical fibers are bundled together and the relative positions of the optical fibers are fixed. The optical fibers have a base diameter portion, a small diameter portion that is smaller than the base diameter portion, and a tapered portion located between the base diameter portion and the small diameter portion and whose diameter gradually increases from the small diameter portion side toward the base diameter portion side. In this optical fiber bundle, adjacent tapered portions are in contact along their entire lengths.

[0008] A second aspect of the present invention is an optical connector including the optical fiber bundle according to the first aspect and a ferrule having a first hole, wherein the optical fiber bundle is fixed to the ferrule while being passed through the first hole.

[0009] According to the present invention, it is possible to provide an optical fiber bundle that is less likely to cause transmission loss when fixed to a ferrule with an adhesive.

[0010] Fig. 3 is a schematic cross-sectional view showing an optical connector according to a first embodiment of the present invention. Fig. 4 is a view showing an optical fiber bundle according to the first embodiment. Fig. 5 is a cross-sectional view taken along line II of Fig. 2. Fig. 6 is a cross-sectional view taken along line II-II of Fig. 2. Fig. 7 is a cross-sectional view taken along line III-III of Fig. 2. Fig. 8 is a schematic cross-sectional view showing a ferrule according to the first embodiment. Fig. 9 is a schematic cross-sectional view showing an optical connector according to a modified example of the first embodiment. Fig. 10 is a partially enlarged view showing an optical fiber bundle according to a second embodiment of the present invention.

[0011] A first embodiment of the present invention will be described with reference to Fig. 1 to Fig. 6. Fig. 1 is a schematic cross-sectional view showing an optical connector 1 according to this embodiment. The optical connector 1 includes a ferrule 10 and an optical fiber bundle (hereinafter, sometimes simply referred to as "bundle") 50 passed through the ferrule 10. The bundle 50 is an optical fiber bundle according to the present invention.

[0012] FIG. 2 shows a side view of the bundle 50. The bundle 50 is formed by bundling four optical fibers 51 and fixing their relative positions with adhesive, heat-shrink tubing, or the like. The optical fiber 51 has a basic structure including a core (described below), a cladding 52 surrounding the core, and a coating 53 covering the cladding. A portion of the coating 53 on the tip side, including the portion that passes through the ferrule 10, has been removed. In the following description, the portion from which the coating 53 has been removed will be referred to as the tip portion 60. The tip portion 60 has a thin diameter portion 61 closest to the tip, a basic diameter portion 63 closest to the coating 53 and having a diameter larger than that of the thin diameter portion 61, and a tapered portion 62 located between the thin diameter portion 61 and the basic diameter portion 63. The tapered portion 62 has the same diameter as the thin diameter portion 61 at the connection portion with the thin diameter portion 61 and the same diameter as the basic diameter portion at the connection portion with the basic diameter portion 63. The diameter of the tapered portion 62 is formed so as to gradually increase from the small diameter portion 61 side toward the base diameter portion 63 side. As a result, the outer shape of the tapered portion 62 is a truncated cone. The small diameter portion 61 and the tapered portion 62 can be produced by changing the thickness of the cladding 52 by etching, cutting, melting and stretching, or the like.

[0013] The bundle 50 according to this embodiment can be formed by bundling four optical fibers 51 and fixing their relative positions with adhesive, heat-shrink tubing, or the like. Figures 3, 4, and 5 are schematic cross-sectional views taken along lines I-I, II-II, and III-III in Figure 2, respectively, and are cross sections perpendicular to the longitudinal direction of the bundle 50. As shown in Figures 3 to 5, the thin-diameter portion 61, the tapered portion 62, and the basic diameter portion 63 differ only in the thickness of the cladding 52, and the cores 54 have the same diameter. As shown in Figures 3 to 5, the four optical fibers 51 constituting the bundle 50 are arranged in a generally square pattern in each of the thin-diameter portion 61, the tapered portion 62, and the basic diameter portion 63. The outer circumferential surfaces of adjacent optical fibers 51 are in direct contact along their entire lengths in each of the thin-diameter portion 61, the tapered portion 62, and the basic diameter portion 63, or are in close proximity with an adhesive or the like having a thickness of 100 μm or less sandwiched between them. In other words, in a cross section perpendicular to the longitudinal direction of the bundle 50, the distance between the central axes of adjacent optical fibers 51 is ensured to be equal to or less than the diameter of the optical fiber at that location plus 100 μm, and in the present invention, this state is defined as ``adjacent optical fibers are in contact.''

[0014] As shown in FIG. 6 , the ferrule 10 is a cylindrical member having a first hole 20 extending longitudinally and penetrating the ferrule 10. The first hole 20 has a small-diameter portion 21 located at one longitudinal end of the ferrule 10, a large-diameter portion 23 located at the end opposite the small-diameter portion 21, and an expanded-diameter portion 22 connecting the small-diameter portion 21 and the large-diameter portion 23. In this embodiment, the cross-sectional shape of the first hole 20 is generally circular throughout its entire length. The inner diameter of the expanded-diameter portion 22 is the same as that of the small-diameter portion 21 at the connection portion with the small-diameter portion 21 and the same as that of the large-diameter portion 23 at the connection portion with the large-diameter portion 23. The inner diameter of the expanded-diameter portion 22 is gradually increased from the small-diameter portion 21 side toward the large-diameter portion 23 side. A second hole 30 is formed in the outer peripheral surface of the ferrule 10, reaching the first hole 20. In this embodiment, the second hole 30 opens into the expanded-diameter portion 22 of the first hole 20. There are no particular restrictions on the material of the ferrule 10, but since it has a complex shape as described above, it is preferable to manufacture it by resin molding.

[0015] 1 , the bundle 50 passed through the ferrule 10 is passed through the first hole 20 so that the thin diameter portion 61 is located within the small diameter portion 21 and the basic diameter portion 63 is located within the large diameter portion 23. As a result, the tapered portion 62 is located approximately within the expanded diameter portion 22. The dimensions of the thin diameter portion 61, the tapered portion 62, and the basic diameter portion 63 in the longitudinal direction of the optical fiber 51 do not necessarily have to be the same as the dimensions of the small diameter portion 21, the expanded diameter portion 22, and the large diameter portion 23 in the longitudinal direction of the first hole 20. Thus, for example, the thin diameter portion 61 may be slightly longer than the small diameter portion 21, and the tapered portion 62 may be slightly longer or shorter than the expanded diameter portion 22.

[0016] While the bundle 50 and the ferrule 10 are maintained in the positional relationship described above, adhesive is introduced into the ferrule 10 through the second hole 30 and cured, whereby the bundle 50 and the ferrule 10 are joined together by the adhesive Ad, and the positional relationship between the bundle 50 and the ferrule 10 is fixed. After that, when the necessary processes for forming each part are completed, the optical connector 1 is completed.

[0017] In the above-described process of joining the bundle 50 and the ferrule 10, the adhesive introduced into the ferrule 10 shrinks as it hardens. It is known that this shrinkage causes stress to act on the optical fiber positioned inside the ferrule 10, and therefore an adhesive with as little shrinkage as possible is selected, but the inventor's investigation has revealed that even in such a case, the impact of the adhesive's hardening shrinkage is by no means small.

[0018] In particular, the stress generated by shrinkage is large in the area where the second hole 30 communicates with the first hole 20, since the adhesive is present in a larger volume there than in other areas. The inventors conducted a simulation study and found that the stress generated by the cure shrinkage of the adhesive in this area is likely to displace the part of the fiber in contact with the adhesive by about 15 μm in the radial direction.

[0019] In other words, it is quite possible that the bundle located within the ferrule 10 will be in a state in which the optical fibers are pseudo-bent at this location, which will inevitably result in transmission loss within the completed optical connector. Furthermore, in prototypes made by the inventors, it was confirmed that the square arrangement of the four optical fibers within the expanded diameter section 22 was disrupted, or that although a square arrangement was maintained in both the small diameter section 21 and the expanded diameter section 22, the relative positions of the four optical fibers changed between the small diameter section 21 and the expanded diameter section 22. In these prototypes, the four fibers were arranged as shown in FIG. 2 and passed through the first hole of the ferrule without being fixed, and a bundle was formed within the ferrule using adhesive introduced through the second hole. Therefore, it was also revealed that when forming a bundle within a ferrule, care should be taken to prevent the adhesive from shrinking during cure.

[0020] Based on the above findings, the inventors solved this problem by modifying the shape of the optical fibers that make up the bundle. That is, by forming tapered portion 62, located between thin-diameter portion 61 and basic-diameter portion 63, to be sufficiently long, bundle 50 can sufficiently maintain a square arrangement in which the outer peripheral surfaces of adjacent optical fibers are in contact with each other along the entire length of the portion where tapered portion 62 is bundled. Furthermore, because the central axes of the four optical fibers are sufficiently close at the boundary between tapered portion 62 and thin-diameter portion 61, it is easy to achieve a state in which adjacent optical fibers are in contact with each other along the entire length, even in thin-diameter portion 61.

[0021] As shown in FIG. 1 , the area where the tapered portions 62 are bundled overlaps the second hole 30 of the ferrule 10 in the longitudinal direction of the optical connector 1. Therefore, stress caused by the large volume of adhesive Ad located in the second hole 30 is borne by the area where the tapered portions 62 are bundled. Here, the area where the tapered portions 62 are bundled has a larger volume per unit length than the area where the narrow-diameter portions 61 are bundled, and adjacent optical fibers are arranged in a square shape with contact. As a result, when stress acts on one tapered portion 62, even a small displacement immediately brings it into contact with the adjacent tapered portion. The support provided by the other tapered portions suppresses excessive displacement. As a result of this behavior, the area where the tapered portions 62 are bundled functions roughly like a rigid body formed by four tapered portions, and is therefore able to fully withstand stress caused by the adhesive introduced into the second hole 30. As a result, an optical connector can be configured in which transmission loss within the ferrule due to the above-mentioned bending and other microbending (microbending) is significantly suppressed.

[0022] Although the length of the tapered portion 62 according to this embodiment is not particularly limited, it is preferably longer than the small diameter portion 61. Considering the typical dimensions of the ferrule 10 and the second hole 30, it is preferably approximately 0.5 mm to 12 mm. The taper angle, which is the angle between the outer circumferential surface of the tapered portion 62 and the central axis of the optical fiber 51 at the rear end of the optical fiber 51, is determined by the difference in diameter between the small diameter portion 61 and the basic diameter portion 63 and the length of the tapered portion 62. If the taper angle exceeds 10°, it becomes difficult to ensure that adjacent optical fibers are in contact with each other over the entire length of the portion where the tapered portion 62 is bundled. From this perspective, the taper angle of the tapered portion 62 is preferably approximately 0.1° to 10°, and more preferably approximately 0.1° to 5°. In the ferrule 10, the value of the taper angle (first value) of the enlarged diameter section 22, which is the angle between the inner surface of the enlarged diameter section 22 and the central axis of the first hole 20 on the rear end side of the optical fiber 51, needs to be equal to or greater than the value of the taper angle (second value) in the side view shown in Figure 2 at the part of the bundle 50 where the tapered sections 62 are bundled. However, if the difference between the first value and the second value is 1° or less, the thickness of the layer of adhesive disposed between the inner surface of the enlarged diameter section 22 and the outer surface of the tapered section 62 becomes closer to uniform. As a result, the variation in the distribution of stress caused by the cure shrinkage of the adhesive is reduced, and the occurrence of microbends and the like can be further reduced, which is preferable.

[0023] In the present embodiment, an example has been described in which only the tapered portion 62 is located at the position where the second hole 30 communicates with the first hole 20. This is a preferred embodiment, and from the perspective of realizing this, it is preferable that the length of the tapered portion 62 be equal to or greater than the dimension of the second hole 30 in the longitudinal direction of the ferrule 10, but this is not essential.

[0024] In the modified example shown in Figure 7, the shapes of the first hole of the ferrule and the optical fiber are different from those shown in Figure 1, resulting in a portion of the base diameter portion 63 being exposed in the second hole 30. However, even in this configuration, the small diameter portion 61 is not exposed in the second hole 30, thereby preventing stress from acting on the small diameter portion 61 due to the large volume of adhesive Ad in the second hole 30. This achieves the same effect. However, if too much of the base diameter portion is exposed in the second hole, it may be difficult to achieve a sufficient tapered portion length, or the ferrule may become too long to accommodate a tapered portion of the desired length. Therefore, it is preferable to carefully determine the dimensions and positional relationship of each part. From this perspective, it is preferable that the tapered portion 62 overlaps 25% or more of the second hole 30 in the longitudinal direction of the first hole 20.

[0025] A second embodiment of the present invention will be described with reference to Fig. 8. In the following description, components common to those already described will be assigned the same reference numerals and redundant description will be omitted.

[0026] 8 is an enlarged view of a portion of the bundle 150 according to this embodiment. The tip end 160 of the optical fiber 151 constituting the bundle has two tapered portions: a first tapered portion 1621 connected to the small diameter portion 61 and a second tapered portion 1622 connected to the basic diameter portion 63. Between the first tapered portion 1621 and the second tapered portion 1622, there is a medium diameter portion 164 having a diameter larger than that of the small diameter portion 61 and smaller than that of the basic diameter portion 63.

[0027] The bundle 150 of this embodiment, which is made up of four optical fibers 151, is arranged in a square shape with adjacent optical fibers in contact with each other in both the first tapered portion 1621 and the second tapered portion 1622, and has generally the same effect as the bundle 50 of the first embodiment.

[0028] Furthermore, because bundle 150 has medium diameter portion 164, there is an advantage in that it is easy to form a bundle by bundling optical fibers by attaching a heat shrink tube or the like to medium diameter portion 164. Normally, the difference in size between the inner diameter of the small diameter portion of a ferrule and the thin diameter portion is small, so it is not easy to bundle the thin diameter portion with a heat shrink tube, but if a heat shrink tube is attached to the tapered portion, the diameter differs between the front end and the rear end, and the amount of tightening force applied by the heat shrink tube differs between the front end and the rear end, which is one cause of microbends and the like.

[0029] For this reason, the heat shrink tube is often attached only to the basic diameter portion 63, and in such cases, the behavior of the thin diameter portion 61 is less regulated by the heat shrink tube, so that the four optical fibers may become unraveled in the thin diameter portion 61, making it difficult to pass them through the small diameter portion of the ferrule. However, in the bundle 150 of this embodiment, the heat shrink tube can be attached to the medium diameter portion 164, which is closer to the thin diameter portion, and as a result, the optical fibers can be prevented from becoming unraveled in the thin diameter portion 61.

[0030] When an optical connector is constructed using the bundle 150, second holes may be formed in the ferrule to be used at positions corresponding to the first tapered portion 1621 and the second tapered portion 1622. In this way, although the adhesive is introduced into two locations, the volume of the adhesive placed in each second hole can be made smaller than in the first embodiment, and the effect of stress caused by cure shrinkage can be further reduced.

[0031] Although each embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and configuration changes and combinations within the scope of the gist of the present invention are also included. Some examples of changes are shown below, but these are not all inclusive, and other changes are also possible. These changes can be applied to any embodiment, and two or more of them may be combined as appropriate.

[0032] The bundle according to the present invention is not limited to the one configured with the four optical fibers described above, but may be configured with, for example, nine or sixteen optical fibers arranged in a square.

[0033] The bundle according to the present invention may have a lead portion thinner than the thin portion, located distally of the thin portion. This makes it easier to pass the bundle through the thin portion of the ferrule, improving the efficiency of manufacturing the optical connector. After the bundle is passed through the thin portion of the ferrule, the lead portion is no longer needed, so it can be simply cut off by protruding it from the thin portion. The lead portion can be provided, for example, by making the cladding of the optical fiber thinner than the thin portion or by completely removing it.

[0034] In fabricating an optical connector using the bundle according to the present invention, the four optical fibers may be integrated in advance and then passed through the ferrule, or, as described above, the four optical fibers may be temporarily placed and passed through the ferrule, and then the four optical fibers may be integrated with adhesive introduced through the second hole. In this case, the bundle according to the present invention is completed within the ferrule, and the completed optical connector can be said to be formed using the bundle according to the present invention.

[0035] According to the present invention, it is possible to provide an optical fiber bundle that is less likely to cause transmission loss when fixed to a ferrule with an adhesive.

[0036] 1...optical connector, 10...ferrule, 20...first hole, 21...small diameter section, 22...expanded diameter section, 23...large diameter section, 30...second hole, 50, 150...optical fiber bundle, 51, 151...optical fiber, 61...small diameter section, 62...tapered section, 63...basic diameter section, 164...medium diameter section, 1621...first tapered section, 1622...second tapered section

Claims

1. An optical fiber bundle in which a plurality of optical fibers are bundled together and the positional relationship of the optical fibers is fixed, wherein the optical fibers have a basic diameter portion, a thin diameter portion that is thinner than the basic diameter portion, and a tapered portion that is located between the basic diameter portion and the thin diameter portion and whose diameter gradually increases from the thin diameter portion side toward the basic diameter portion side, and wherein adjacent tapered portions are in contact along their entire lengths.

2. The optical fiber bundle according to claim 1, wherein a plurality of said optical fibers are arranged in a square array in a cross section perpendicular to the longitudinal direction of said optical fiber bundle.

3. An optical fiber bundle according to claim 1 or 2, wherein the taper angle of the tapered portion is 0.1° or more and 5° or less.

4. An optical fiber bundle according to any one of claims 1 to 3, wherein the tapered portion of the optical fiber is longer than the small diameter portion.

5. An optical fiber bundle according to any one of claims 1 to 4, wherein the optical fiber has a first tapered portion and a second tapered portion as the tapered portion, and a medium diameter portion between the first tapered portion and the second tapered portion that is thicker than the small diameter portion and thinner than the basic diameter portion.

6. An optical connector comprising: an optical fiber bundle according to any one of claims 1 to 5; and a ferrule having a first hole, wherein the optical fiber bundle is fixed to the ferrule in a state where it is passed through the first hole.

7. An optical connector according to claim 6, wherein the ferrule has a second hole that opens on its outer peripheral surface and communicates with the first hole, and the optical fiber bundle is arranged so that the tapered portion overlaps 25% or more of the second hole in the longitudinal direction of the first hole.

8. The optical connector according to claim 7, wherein the optical fiber bundle is arranged so that the tapered portion overlaps the entire area of the second hole in the longitudinal direction of the first hole.

9. An optical connector as described in any one of claims 6 to 8, wherein the first hole has a small diameter section through which the thin diameter section is passed, a large diameter section through which the basic diameter section is passed, and an expanded diameter section located between the small diameter section and the large diameter section, and a first value of the taper angle of the expanded diameter section is equal to or greater than a second value of the taper angle of the portion of the optical fiber bundle where the tapered sections are bundled, and the difference between the first value and the second value is 1° or less.

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