JP2016151651A - Optical fiber and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber capable of reducing the cost of an optical transmission system of a relatively short section, and an optical transmission system including this optical fiber.SOLUTION: An optical fiber 1A comprises a coated optical fiber 10, a ferrule 20, and an enlarged core part 30. The coated optical fiber 10 includes a core 11 for propagating light having a wavelength of not more than 1100 nm in single mode, and a clad 12 arranged around the core 11 and having a lower refractive index than the refractive index of the core 11. The coated optical fiber 10 includes a step refractive index profile, and a diameter of the core 11 is not more than 7 μm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical fiber and an optical transmission system.

  Patent Document 1 describes a technique related to a multimode optical fiber. This document describes an example of an optical transmission system that includes a vertical cavity surface emitting laser (VCSEL) as a light source, a photodiode as a light receiving unit, and a multimode optical fiber that connects them. It is disclosed.

US Patent Application Publication No. 2013/0084048

  Multimode optical fiber is not suitable for long-distance signal transmission due to multiple propagation modes, but it has a large numerical aperture at the fiber end compared to single-mode optical fiber, making it easy to connect to other equipment. It has the characteristic of being. Therefore, the multimode optical fiber is preferably used for signal transmission in a relatively short section such as several hundred meters. However, a multimode optical fiber is generally more expensive than a single mode optical fiber, which is a factor that hinders cost reduction of an optical transmission system.

  The present invention has been made in view of such problems, and an object thereof is to provide an optical fiber capable of reducing the cost of an optical transmission system in a relatively short section, and an optical transmission system including the optical fiber. And

  In order to solve the above-described problems, an optical fiber according to the present invention has a core for propagating light having a wavelength of 1100 nm or less in a single mode, a periphery of the core, and a refractive index lower than the refractive index of the core. A step-type refractive index profile, and a core diameter of 7 μm or less.

  ADVANTAGE OF THE INVENTION According to this invention, the optical fiber which can reduce the cost of the optical transmission system of a comparatively short area, and an optical transmission system provided with this optical fiber can be provided.

FIG. 1 is a cross-sectional view showing the structure near the tip of the optical fiber according to the first embodiment. FIG. 2 is a perspective view showing the appearance of the vicinity of the tip of the optical fiber core line provided in the optical fiber. FIG. 3 is a cross-sectional view showing a modification of the enlarged core portion. FIG. 4 is a diagram showing a method for manufacturing an optical fiber. FIG. 5 is a cross-sectional view showing the structure near the tip of the optical fiber according to the second embodiment. FIG. 6 is a diagram showing a method for manufacturing an optical fiber. FIG. 7 is a diagram illustrating another method of manufacturing an optical fiber. FIG. 8 is a cross-sectional view showing the structure near the tip of the optical fiber according to the third embodiment. FIG. 9 is a diagram showing a method for manufacturing an optical fiber. FIG. 10 is a diagram conceptually showing the configuration of the optical transmission system as the fourth embodiment. FIG. 11 is a chart showing measurement results of bending loss (dB / m) with respect to light having a wavelength of 850 nm. FIG. 12 is a chart showing measurement results of bending loss (dB / m) with respect to light having a wavelength of 1100 nm.

[Description of Embodiment of Present Invention]
First, the contents of the embodiment of the present invention will be listed and described. (1) An optical fiber according to the present invention includes a core that propagates light having a wavelength of 1100 nm or less in a single mode, and a clad disposed around the core and having a refractive index lower than the refractive index of the core. It has a refractive index profile, and the core diameter is 7 μm or less. This optical fiber has a core diameter of 7 μm or less in order to propagate light having a wavelength of 1100 nm or less in a single mode. The refractive index profile has a relatively simple structure called a step type. Due to the characteristics of the single mode optical fiber as described above, the optical fiber can be provided at a lower cost than the multi-core optical fiber. Therefore, the cost of the optical transmission system can be reduced by adopting the optical fiber instead of the multi-core optical fiber that has been used for optical communication with a wavelength of 1100 nm or less in a relatively short section.

  (2) In the above optical fiber, the product of the relative refractive index difference between the core and the cladding and the diameter of the core may be 2.9% · μm or less. Thereby, light with a wavelength of 1100 nm or less can be suitably propagated in a single mode.

  (3) In the above optical fiber, the bending loss for light having a wavelength of 1100 nm or less may be 1 db / m or less when the optical fiber is wound only once in an annular shape having a radius of 7.5 mm. Thereby, the optical loss of the propagated light can be suppressed within a practical range.

  (4) In the optical fiber, the core may propagate light having a wavelength of 850 nm or less in a single mode, and the core may have a diameter of 6 μm or less. As a result, the optical fiber can be employed in place of the multi-core optical fiber that has been used for optical communication with a wavelength of 850 nm or less in a relatively short section, and the cost of such an optical transmission system can be reduced.

  (5) In the optical fiber, a product of a relative refractive index difference between the core and the clad and a diameter of the core may be 2% · μm or less. Thereby, light with a wavelength of 850 nm or less can be suitably propagated in a single mode.

  (6) The optical fiber may have a bending loss of 1 db / m or less for light having a wavelength of 850 nm or less when the optical fiber is wound only once in an annular shape having a radius of 7.5 mm. Thereby, the optical loss of the propagated light can be suppressed within a practical range.

  (7) The optical fiber is provided in contact with the tip of the core, has a refractive index equal to or higher than the refractive index of the core, and has a diameter in a cross section perpendicular to the first direction which is the longitudinal direction of the optical fiber. You may further provide the expansion core part currently expanded toward one direction outward. Thereby, even light from a light source having a wide radiation angle, such as VCSEL, can be efficiently incident on the optical fiber having a relatively small core diameter (in other words, a small numerical aperture).

  (8) The optical fiber is further provided with a ferrule attached to the tip of the optical fiber and having an end face located outside the tip of the core in the first direction, and a hole is formed from the tip of the core to the end face of the ferrule. The inner diameter of the hole in the cross section perpendicular to the first direction is enlarged from the tip of the core toward the end face of the ferrule, and the enlarged core portion is made of a resin having a refractive index equal to the refractive index of the core. It may be filled with. Thereby, an expansion core part can be realized suitably. In this case, the hole may be formed in the ferrule, or may be formed in the core and the clad.

  (9) The optical fiber further includes a ferrule attached to the tip of the optical fiber and having an end face located outside the tip of the core in the first direction, and a hole is formed from the tip of the core to the end face of the ferrule. The inner diameter of the hole in the cross section perpendicular to the first direction is constant from the tip of the core to the end face of the ferrule, the hole is filled with resin, and the enlarged core part is constituted by a part of the resin, The refractive index of the other part except a part in the resin may be smaller than the refractive index of the core. Thereby, an expansion core part can be realized suitably.

  (10) A first optical transmission system according to the present invention includes any one of the above optical fibers (1) to (3) and a semiconductor laser that outputs light having a wavelength of 1100 nm or less, and one end of the optical fiber; A light source that is optically coupled, and a light receiving unit that includes a photodiode and is optically coupled to the other end of the optical fiber. The first optical transmission system is preferably used for optical communication with a wavelength of 1100 nm or less in a relatively short section, for example, and any one of the above optical fibers (1) to (3) is used instead of the multi-core optical fiber. By providing, cost reduction can be achieved.

  (11) A second optical transmission system according to the present invention includes any one of the above optical fibers (4) to (6) and a semiconductor laser that outputs light having a wavelength of 850 nm or less, and one end of the optical fiber; A light source that is optically coupled, and a light receiving unit that includes a photodiode and is optically coupled to the other end of the optical fiber. This second optical transmission system is preferably used for optical communication with a wavelength of 850 nm or less in a relatively short section, for example, and any one of the above optical fibers (4) to (6) is used instead of the multi-core optical fiber. By providing, cost reduction can be achieved.

[Details of the embodiment of the present invention]
Specific examples of the optical fiber and the optical transmission system according to the embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to the claim are included. In the following description, the same reference numerals are given to the same elements in the description of the drawings, and redundant descriptions are omitted. In the following description, the direction from the end face of the optical fiber to the outside may be referred to as the front, and the direction from the end face to the inward may be referred to as the rear.

(First embodiment)
FIG. 1 is a cross-sectional view showing the structure near the tip of the optical fiber 1A according to the first embodiment, and shows a cross section along the first direction (that is, the longitudinal direction of the optical fiber 1A) A1 that is the light propagation direction. Show. FIG. 2 is a perspective view showing the appearance of the vicinity of the tip of the optical fiber core wire 10 provided in the optical fiber 1A.

The optical fiber 1 </ b> A of the present embodiment includes an optical fiber core wire 10, a ferrule 20, and an enlarged core portion 30. The optical fiber core wire 10 has a core 11 and a clad 12. The core 11 has a configuration for propagating light having a wavelength of 1100 nm or less (more preferably, a wavelength of 850 nm or less) in a single mode. Specifically, the core 11 is made of, for example, a material such as silica glass to which germania (GeO ₂ ) is added. ). The clad 12 is disposed around the core 11 and has a refractive index lower than that of the core 11. The clad 12 is made of a material such as silica glass. A suitable outer diameter of the clad 12 is, for example, 125 μm. The core 11 and the cladding 12 have a so-called step type (step index type) refractive index profile, and the refractive index distribution of the core 11 and the refractive index distribution of the cladding 12 are respectively constant in the radial direction.

  When the core 11 propagates light having a wavelength of 1100 nm or less, the product of the relative refractive index difference between the core 11 and the clad 12 and the diameter of the core 11 is preferably 2.9% · μm or less. Thereby, light with a wavelength of 1100 nm or less can be suitably propagated in a single mode. Moreover, it is preferable that the bending loss with respect to light with a wavelength of 1100 nm or less when it is wound only once in an annular shape with a radius of 7.5 mm is 1 db / m or less. Thereby, the optical loss of the propagated light can be suppressed within a practical range. In consideration of bending loss, when the diameter of the core 11 is 4 μm, it is preferable that the relative refractive index difference between the core and the clad is 0.7% or more.

  When the core 11 propagates light having a wavelength of 850 nm or less, the product of the relative refractive index difference between the core 11 and the cladding 12 and the diameter of the core 11 is preferably 2% · μm or less. Thereby, light with a wavelength of 850 nm or less can be suitably propagated in a single mode. Moreover, it is preferable that the bending loss with respect to light with a wavelength of 850 nm or less when it is wound only once in an annular shape with a radius of 7.5 mm is 1 db / m or less. Thereby, the optical loss of the propagated light can be suppressed within a practical range. In consideration of bending loss, when the diameter of the core 11 is 4 μm, it is preferable that the relative refractive index difference between the core and the clad is 0.4% or more.

  The ferrule 20 is a substantially cylindrical member attached to the tip of the optical fiber core wire 10. The ferrule 20 is made of a material such as polyetherimide. The ferrule 20 extends along the direction A <b> 1, and its central axis coincides with the central axis of the optical fiber core wire 10. The ferrule 20 has a pair of end surfaces 20a and 20b in the direction A1, and a fiber insertion hole 21 having a circular cross section penetrating from the end surface 20a to the end surface 20b. The end portion of the optical fiber core wire 10 is inserted into the fiber insertion hole 21, and the inner surface of the fiber insertion hole 21 and the outer peripheral surface of the optical fiber core wire 10 are fixed to each other.

  Here, a hole 13 is formed in the optical fiber 1A. In the present embodiment, the hole 13 is formed in the core 11 and the clad 12 of the optical fiber core wire 10, but the hole may be formed in a ferrule as in each embodiment described later.

  The hole 13 has a rotationally symmetric shape around the central axis of the optical fiber core 10, and is recessed from the end face 10a of the optical fiber core 10 toward the inside of the optical fiber core 10 along the direction A1. . Thereby, the front end 11 a of the core 11 is located behind the end surface 10 a of the optical fiber core wire 10. Furthermore, since the end surface 20a of the ferrule 20 is provided flush with the end surface 10a of the optical fiber core wire 10, the end surface 20a of the ferrule 20 is located outside (front) of the tip 11a of the core 11 in the direction A1. ing. Therefore, the hole 13 is formed from the tip 11 a of the core 11 to the end surface 20 a of the ferrule 20.

  An inner diameter L1 of the hole 13 in a cross section perpendicular to the direction A1 is enlarged from the tip 11a of the core 11 toward the end faces 10a and 20a. In one example, the hole 13 expands in a tapered shape from the tip end 11 a toward the end faces 10 a and 20 a, and the inner surface thereof has a conical shape centered on the central axis of the optical fiber core wire 10. The opening diameter of the hole 13 in the end face 10a is, for example, not less than 50 μm and not more than 100 μm. When the opening diameter of the hole 13 is 50 μm or more, optical coupling with another multimode optical fiber constituting the optical transmission system can be easily and efficiently performed. Further, the depth of the hole 13 in the direction A1 is, for example, 250 μm or more.

  The enlarged core portion 30 is provided inside the hole 13 and is in contact with the tip 11 a of the core 11. The enlarged core portion 30 is made of a transparent resin filled in the holes 13. The refractive index of the resin constituting the enlarged core portion 30 is equal to the refractive index of the core 11 or larger than the refractive index of the core 11. Since the shape of the resin follows the shape of the inner surface of the hole 13, the diameter of the enlarged core portion 30 in the cross section perpendicular to the direction A 1 is expanded from the tip 11 a toward the outside (front) in the direction A 1, similarly to the hole 13. It will be. In addition, the front surface 30a of the enlarged core portion 30 has a substantially spherical shape due to the surface tension of the resin, and incident light is condensed toward the tip 11a by the front surface 30a.

  As the resin constituting the expanded core portion 30, in addition to silicone resin, crosslinked nylon-based engineering plastic or crosslinked PBT-based engineering plastic (for example, Terralink (registered trademark) manufactured by Sumitomo Electric Fine Polymer), amorphous thermoplastic poly An etherimide resin (for example, ULTEM (registered trademark) manufactured by Savik) is suitable.

  FIG. 3 is a cross-sectional view showing various modified examples of the enlarged core portion 30. In the example shown in FIG. 3A, the contact surface between the resin constituting the enlarged core portion 30 and the clad 12 is accommodated in the hole 13. The top part of the front surface 30a of the enlarged core part 30 protrudes slightly forward from the end faces 10a and 20a. In the example shown in FIG. 3B, the amount of resin to be filled increases, and the contact surface between the resin constituting the enlarged core portion 30 and the clad 12 extends from the hole 13 to the end face 10a. Thereby, the front surface 30a of the expansion core part 30 is wider than the example of Fig.3 (a). Further, in the example shown in FIG. 3C, the amount of resin to be filled is further increased, and the contact surface between the resin constituting the enlarged core portion 30 and the clad 12 is further expanded to reach the end surface 20a. Yes. Thereby, the front surface 30a of the expansion core part 30 becomes still wider than the example of FIG.3 (b). Thus, the magnitude | size of the front surface 30a of the expansion core part 30 can be suitably adjusted with the quantity of resin with which it is filled.

  FIG. 4A to FIG. 4E are diagrams showing each step of the method of manufacturing the optical fiber 1A of the present embodiment. First, as shown in FIG. 4A, the fiber insertion hole 21 is formed by processing the ferrule 20. Next, as shown in FIG. 4B, the optical fiber core wire 10 is inserted into the fiber insertion hole 21, and the inner surface of the fiber insertion hole 21 and the outer surface of the optical fiber core wire 10 are bonded to each other with an adhesive or the like. Fix it. Subsequently, as shown in FIG. 4C, the end face 10a, 20a that is flush with each other is formed by polishing the end face of the ferrule 20 and the end face of the optical fiber core wire 10 together.

  Subsequently, as shown in FIG. 4 (d), a hole 13 is formed in the core 11 and the cladding 12 by cutting the end face 10 a of the optical fiber core wire 10. At this time, for example, the optical fiber core wire 10 may be cut using a needle having a conical tip shape. Thereafter, as shown in FIG. 4E, the enlarged core portion 30 is formed by filling the hole 13 with resin and curing the resin. Through the above steps, the optical fiber 1A of the present embodiment is completed.

  The effect obtained by the optical fiber 1A of the present embodiment having the above configuration will be described. In the optical fiber 1A, the diameter of the core 11 is 7 μm or less in order to propagate light having a wavelength of 1100 nm or less in a single mode. The refractive index profile has a relatively simple structure called a step type. Due to the characteristics of the single mode optical fiber as described above, the optical fiber can be provided at a lower cost than the multi-core optical fiber. Therefore, the cost of the optical transmission system can be reduced by adopting the optical fiber instead of the multi-core optical fiber that has been used for optical communication with a wavelength of 1100 nm or less in a relatively short section.

  The core 11 may propagate light having a wavelength of 850 nm or less in a single mode, and the diameter of the core 11 may be 6 μm or less. As a result, the optical fiber can be employed in place of the multi-core optical fiber that has been used for optical communication with a wavelength of 850 nm or less in a relatively short section, and the cost of such an optical transmission system can be reduced.

  Further, as in the present embodiment, the optical fiber 1A is provided in contact with the tip 11a of the core 11, is made of a resin having a refractive index equal to the refractive index of the core 11, and has a diameter in a cross section perpendicular to the direction A1. You may provide the expansion core part 30 which expands toward the direction A1 outward from 11a. Thereby, even light from a light source having a wide radiation angle such as VCSEL can be efficiently incident on the optical fiber 1A having a relatively small diameter of the core 11 (in other words, a small numerical aperture). The transmission loss in the transmission system can be made equal to the case where a multimode optical fiber is used.

  Further, as in the present embodiment, the hole 13 is formed from the tip 11a of the core 11 to the end face 20a of the ferrule 20, and the inner diameter L1 of the hole 13 is enlarged from the tip 11a toward the end face 20a. The portion 30 may be formed by filling the hole 13 with a resin having a refractive index equal to that of the core 11. Thereby, said expansion core part 30 is suitably realizable.

(Second Embodiment)
FIG. 5 is a cross-sectional view showing the structure near the tip of the optical fiber 1B according to the second embodiment, and shows a cross section along the first direction (that is, the longitudinal direction of the optical fiber 1B) A2 that is the light propagation direction. Show.

  The optical fiber 1B of the present embodiment includes a ferrule 40A, an optical fiber core wire 50, and an enlarged core portion 60. The optical fiber core 50 has the same configuration as the optical fiber core 10 of the first embodiment except that the hole 13 shown in FIG. 1 is not formed. That is, the constituent material, refractive index distribution, and diameter of the core 51 and the clad 52 are the same as those of the core 11 and the clad 12 of the first embodiment.

  The ferrule 40 </ b> A is a substantially cylindrical member attached to the tip of the optical fiber core wire 50. The ferrule 40A is made of the same material as the ferrule 20 of the first embodiment. The ferrule 40 </ b> A extends along the direction A <b> 2, and the center axis thereof coincides with the center axis of the optical fiber core wire 50. The ferrule 40A has a pair of end surfaces 40a and 40b in the direction A2, and a fiber insertion hole 41 having a circular cross section extending from the end surface 40b to the middle of the ferrule 40A in the direction A2. The tip of the optical fiber core wire 50 is inserted into the fiber insertion hole 41, and the inner surface of the fiber insertion hole 41 and the outer peripheral surface of the optical fiber core wire 50 are fixed to each other. Further, the end face 50a is positioned with respect to the ferrule 40A by the front end face 41a of the fiber insertion hole 41 and the end face 50a of the optical fiber core wire 50 being in contact with each other. Accordingly, also in the present embodiment, the end surface 40a of the ferrule 40A is located on the outer side (front) than the tip 51a of the core 51 in the direction A2.

  Here, a hole 42 is formed in the optical fiber 1B. Unlike the first embodiment, the hole 42 of the present embodiment is formed in the ferrule 40A. The hole 42 has a rotationally symmetric shape around the central axis of the optical fiber core 50, and is formed from the front end surface 41a (that is, the end surface 50a of the optical fiber core 50) to the end surface 40a along the direction A2. .

  The inner diameter L2 of the hole 42 in the cross section perpendicular to the direction A2 is enlarged from the tip 51a of the core 51 toward the end surface 40a. In one example, the hole 42 expands in a tapered shape from the tip 51 a toward the end surface 40 a, and the inner surface thereof has a conical shape centered on the central axis of the optical fiber core wire 50. The opening diameter of the hole 42 in the end surface 40a is, for example, not less than 50 μm and not more than 100 μm. Moreover, the opening diameter of the hole 42 in the front end surface 41a of the fiber insertion hole 41 is, for example, 7 μm or less, and in one example, 3 μm.

  The enlarged core portion 60 is provided inside the hole 42 and is in contact with the tip 51 a of the core 51. The enlarged core portion 60 is made of a transparent resin filled in the holes 42. The resin constituting the enlarged core portion 60 has a refractive index that is equal to or larger than the refractive index of the core 51. Since the shape of this resin follows the shape of the inner surface of the hole 42, the diameter of the enlarged core portion 60 in the cross section perpendicular to the direction A 2 increases from the tip 51 a toward the outside (front) in the direction A 2, as with the hole 42. It will be. Further, the front surface 60a of the enlarged core portion 60 has a substantially spherical shape due to the surface tension of the resin, and incident light is condensed toward the tip 51a by the front surface 60a. In addition, as resin which comprises the expansion core part 60, what was mentioned as a material example of the expansion core part 30 of 1st Embodiment is used suitably.

  FIG. 6A to FIG. 6D are diagrams showing each step of the method of manufacturing the optical fiber 1B of the present embodiment. First, as shown in FIG. 6A, the fiber insertion hole 41 and the hole 42 are formed by processing the ferrule 40A. Next, as shown in FIG. 6B, the end face 50 a is formed by cutting and polishing the optical fiber core wire 50. Subsequently, as shown in FIG. 6C, the optical fiber core wire 50 is inserted into the fiber insertion hole 41, and the inner surface of the fiber insertion hole 41 and the outer surface of the optical fiber core wire 50 are bonded to each other with an adhesive or the like. Fix it. Thereafter, as shown in FIG. 6D, the enlarged core portion 60 is formed by filling the hole 42 with a resin and curing the resin. Through the above steps, the optical fiber 1B of the present embodiment is completed.

  Fig.7 (a)-FIG.7 (d) are figures which show each process of another manufacturing method of the optical fiber 1B. First, as shown in FIG. 7A, the fiber insertion hole 41 and the hole 42 are formed by processing the ferrule 40A. Next, as shown in FIG. 7B, the enlarged core portion 60 is formed by filling the hole 42 with a resin and curing the resin. Subsequently, as shown in FIG. 7C, the end surface 50 a is formed by cutting and polishing the optical fiber core wire 50. Thereafter, as shown in FIG. 7D, the optical fiber core wire 50 is inserted into the fiber insertion hole 41, and the inner surface of the fiber insertion hole 41 and the outer surface of the optical fiber core wire 50 are fixed to each other with an adhesive or the like. To do. Through the above steps, the optical fiber 1B of the present embodiment is completed.

  According to the optical fiber 1B of the present embodiment having the above-described configuration, the same effect as that of the first embodiment can be obtained. In other words, the cost of the optical transmission system can be reduced by adopting the optical fiber 1B instead of the multi-core optical fiber that has been used for optical communication with a wavelength of 1100 nm or less (preferably 850 nm or less) in a relatively short section. .

  Also in the present embodiment, the optical fiber 1 </ b> B includes the enlarged core portion 60. Thereby, even light from a light source having a wide radiation angle can be efficiently incident on the optical fiber 1B having a relatively small diameter of the core 51 (in other words, a small numerical aperture). Further, as in the present embodiment, a hole 42 is formed from the tip 51a of the core 51 to the end face 40a of the ferrule 40A, and the inner diameter L2 of the hole 42 is enlarged from the tip 51a toward the end face 40a. The part 60 may be formed by filling the hole 42 with a resin having a refractive index equal to the refractive index of the core 51. Thereby, the expansion core part 60 can be implement | achieved suitably and since the cutting process of the optical fiber core wire 50 is unnecessary, it can manufacture easily.

(Third embodiment)
FIG. 8 is a cross-sectional view showing the structure near the tip of the optical fiber 1C according to the third embodiment, and shows a cross section along the first direction (that is, the longitudinal direction of the optical fiber 1C) A2 that is the light propagation direction. Show. The difference between the optical fiber 1C of the present embodiment and the optical fiber 1B of the second embodiment is the shape of the hole formed in the ferrule and the configuration of the enlarged core portion.

  In the optical fiber 1C of the present embodiment, a hole 43 is formed instead of the hole 42 of the second embodiment. The hole 43 is formed in the ferrule 40B. The hole 43 has a rotationally symmetric shape around the central axis of the optical fiber core 50, and is formed from the front end surface 41a (that is, the end surface 50a of the optical fiber core 50) to the end surface 40a along the direction A2. . The cross-sectional shape of the hole 43 in the cross section perpendicular to the direction A2 is, for example, a circle. The inner diameter L3 of the hole 43 in the cross section perpendicular to the direction A2 is constant from the tip 51a of the core 51 to the end surface 40a. The inner diameter L3 of the hole 43 is not less than 50 μm and not more than 100 μm, for example.

  The resin portion 80 is provided inside the hole 43 and is in contact with the tip 51 a of the core 51. The resin part 80 is made of resin filled in the holes 43. The resin part 80 includes an enlarged core part 82 and a peripheral part 83. The portion of the resin portion 80 that constitutes the enlarged core portion 82 has a refractive index that is equal to or greater than the refractive index of the core 51. Further, the refractive index of the portion d constituting the peripheral portion 83 in the resin portion 80 is smaller than the refractive index of the core 51, and is substantially equal to the refractive index of the cladding 52 in one example.

  The diameter of the enlarged core portion 82 in the cross section perpendicular to the direction A2 increases from the tip 51a toward the outside (front) in the direction A2. In one example, the enlarged core portion 82 is enlarged in a tapered shape from the tip 51 a toward the end surface 40 a, and the outer surface (interface with the peripheral portion 83) is a conical surface centered on the central axis of the optical fiber core wire 50. It has become a shape. Further, the front surface 82a of the enlarged core portion 82 has a substantially spherical shape due to the surface tension of the resin, and incident light is condensed toward the tip 51a by the front surface 82a.

  FIG. 9A to FIG. 9D are diagrams showing each step of the method of manufacturing the optical fiber 1C of the present embodiment. First, as shown in FIG. 9A, the fiber insertion hole 41 and the hole 43 are formed by processing the ferrule 40B. Next, as shown in FIG. 9B, the end face 50 a is formed by cutting and polishing the optical fiber core wire 50. Subsequently, as shown in FIG. 9C, the optical fiber core wire 50 is inserted into the fiber insertion hole 41, and the inner surface of the fiber insertion hole 41 and the outer surface of the optical fiber core wire 50 are bonded to each other with an adhesive or the like. Fix it. Thereafter, the hole 43 is filled with a photocurable resin that becomes the resin portion 80.

  Subsequently, as illustrated in FIG. 9D, an enlarged core portion 82 and a peripheral portion 83 are formed in the resin portion 80. Specifically, the light that has propagated through the core 51 is irradiated onto the uncured photo-curing resin. At this time, the light emitted from the tip 51a of the core 51 passes through the photocurable resin while spreading. And the area | region where light passed among photocuring resin hardens | cures early, and becomes the expansion core part 82 with a comparatively high refractive index. Further, the region through which light does not pass is hardened slowly and becomes a peripheral portion 83 having a relatively low refractive index.

  According to the optical fiber 1 </ b> C of the present embodiment having the above configuration, the same effect as that of the first embodiment can be obtained. That is, by adopting the optical fiber 1C instead of the multi-core optical fiber that has been used for optical communication with a wavelength of 1100 nm or less (preferably 850 nm or less) in a relatively short section, the cost of the optical transmission system can be reduced. .

  Also in the present embodiment, the optical fiber 1 </ b> C includes the enlarged core portion 82. Thereby, even light from a light source with a wide radiation angle can be efficiently incident on the optical fiber 1C having a relatively small diameter of the core 51 (in other words, a small numerical aperture). Further, as in this embodiment, a hole 43 is formed from the tip 51a of the core 51 to the end surface 40a of the ferrule 40B, and even if the inner diameter L3 of the hole 43 is constant from the tip 51a to the end surface 40a, The resin part 80 may be filled, the enlarged core part 82 may be constituted by a part of the resin part 80, and the refractive index of the other part (peripheral part 83) in the resin part 80 may be smaller than the refractive index of the core 51. Even with such a configuration, the above-described enlarged core portion 82 can be suitably realized, and since the optical fiber core wire 50 is not required to be cut, it can be easily manufactured.

(Fourth embodiment)
FIG. 10 is a diagram conceptually showing the configuration of the optical transmission system 100 as the fourth embodiment of the present invention. The optical transmission system 100 includes the optical fiber 1A of the first embodiment, a light source 91, and a light receiving unit 92. The light source 91 includes a semiconductor laser 91a that outputs light having a wavelength of 1100 nm or less (or light having a wavelength of 850 nm or less) La, and is optically coupled to one end of the optical fiber 1A. The semiconductor laser 91a is, for example, a VCSEL. The light receiving unit 92 includes a photodiode 92a that receives the light La, and is optically coupled to the other end of the optical fiber 1A. The optical transmission system 100 may include the optical fiber 1B of the second embodiment or the optical fiber 1C of the third embodiment instead of the optical fiber 1A.

  The optical transmission system 100 according to the present embodiment is suitably used for optical communication with a wavelength of 1100 nm or less (or wavelength 850 nm or less) in a relatively short section, for example, and is a multicore optical fiber normally used for communication in such a section and wavelength. By providing the optical fiber 1A (1B or 1C) instead of the cost, the cost can be reduced.

(Example)
Here, the measurement result about the bending loss of the optical fiber will be described. In this embodiment, a plurality of optical fibers having various relative refractive index differences Δ (%) and various core diameters d (μm) are prepared, and these optical fibers are circulated in an annular shape having a radius of 7.5 mm. Measurement was performed by winding only.

  FIG. 11 is a chart showing measurement results of bending loss (dB / m) with respect to light having a wavelength of 850 nm. In FIG. 11, a range surrounded by a bold frame is a single mode region, and the other range is a multimode region. When the product of the relative refractive index difference Δ (%) and the core diameter d (μm) is 2 (% · μm) or less, it becomes a single mode region, and the product of Δ and d is 1 (% · μm) or more. If present, bending loss can be kept low. Further, when the product of Δ and d is 1.68 (% · μm) or more, the bending loss is a sufficiently low value (for example, 1 db / m or less).

  FIG. 12 is a chart showing the measurement results of bending loss (dB / m) with respect to light having a wavelength of 1100 nm. In FIG. 12, the range surrounded by a bold frame is a single mode region, and the other range is a multimode region. When the product of the relative refractive index difference Δ (%) and the core diameter d (μm) is 2.9 (% · μm) or less, it becomes a single mode region, and the product of Δ and d is 2.7% or more. When it is, bending loss is suppressed low (for example, 1 db / m or less).

  The optical fiber and the optical transmission system according to the present invention are not limited to the above-described embodiments, and various other modifications are possible. For example, in each of the above-described embodiments, the case where the enlarged core portion is made of resin is exemplified, but the enlarged core portion may be made of another material.

  DESCRIPTION OF SYMBOLS 1A-1C ... Optical fiber, 10, 50 ... Optical fiber core wire, 11, 51 ... Core, 12, 52 ... Cladding, 13, 42, 43 ... Hole, 20 ... Ferrule, 21 ... Fiber insertion hole, 30, 60, 82 ... Expanded core part, 40A, 40B ... Ferrule, 41 ... Fiber insertion hole, 80 ... Resin part, 83 ... Peripheral part, 91 ... Light source, 91a ... Semiconductor laser, 92 ... Light receiving part, 92a ... Photodiode, 100 ... Light Transmission system.

Claims (13)

A core that propagates light having a wavelength of 1100 nm or less in a single mode;
A cladding disposed around the core and having a refractive index lower than that of the core;
With
It has a step type refractive index profile,
An optical fiber having a diameter of the core of 7 μm or less.   2. The optical fiber according to claim 1, wherein a product of a relative refractive index difference between the core and the clad and a diameter of the core is 2.9% · μm or less.   The optical fiber according to claim 1 or 2, wherein a bending loss with respect to light having a wavelength of 1100 nm or less is 1 db / m or less when being wound only once in an annular shape having a radius of 7.5 mm. The core propagates light having a wavelength of 850 nm or less in a single mode,
The optical fiber according to claim 1, wherein the core has a diameter of 6 μm or less.   The optical fiber according to claim 4, wherein a product of a relative refractive index difference between the core and the clad and a diameter of the core is 2% · μm or less.   6. The optical fiber according to claim 4, wherein a bending loss with respect to light having a wavelength of 850 nm or less is 1 db / m or less when being wound only once in an annular shape having a radius of 7.5 mm.   Provided in contact with the tip of the core, having a refractive index equal to or higher than the refractive index of the core, and having a diameter in a cross section perpendicular to the first direction, which is the longitudinal direction of the optical fiber, outward from the tip in the first direction The optical fiber as described in any one of Claims 1-6 further provided with the expansion core part currently expanded toward (1). A ferrule attached to the tip of the optical fiber and having an end face located outside the tip of the core in the first direction;
A hole is formed from the tip of the core to the end face of the ferrule,
An inner diameter of the hole in a cross section perpendicular to the first direction is enlarged from the tip of the core toward the end face of the ferrule;
The optical fiber according to claim 7, wherein the enlarged core portion is formed by filling the hole with a resin having a refractive index equal to a refractive index of the core.   The optical fiber according to claim 8, wherein the hole is formed in the ferrule.   The optical fiber according to claim 8, wherein the hole is formed in the core and the cladding. A ferrule attached to the tip of the optical fiber and having an end face located outside the tip of the core in the first direction;
A hole is formed from the tip of the core to the end face of the ferrule,
An inner diameter of the hole in a cross section perpendicular to the first direction is constant from the tip of the core to the end face of the ferrule;
The resin is filled in the hole, the enlarged core part is constituted by a part of the resin, and a refractive index of the other part excluding the part of the resin is smaller than a refractive index of the core. The optical fiber according to 7. The optical fiber according to any one of claims 1 to 3,
A light source optically coupled to one end of the optical fiber having a semiconductor laser that outputs light having a wavelength of 1100 nm or less;
A light receiving portion having a photodiode and optically coupled to the other end of the optical fiber;
An optical transmission system comprising: The optical fiber according to any one of claims 4 to 6,
A light source having a semiconductor laser that outputs light having a wavelength of 850 nm or less, and optically coupled to one end of the optical fiber;
A light receiving portion having a photodiode and optically coupled to the other end of the optical fiber;
An optical transmission system comprising:

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