Optical fiber connection structure

Abstract

To provide an optical fiber connection structure with a reduced connection loss of a pore core type optical fiber.SOLUTION: An optical fiber connection structure 100 includes: a pore core type optical fiber 10 having a pore core 11a of a substantially polygonal shape in a cross section vertical to a longer direction with an effective core area larger than 80 μm2 at a prescribed wavelength; and a solid type optical fiber 20 connected to the pore core type optical fiber with an effective core area larger than 80 μm2 at a prescribed wavelength. A solid core 21 of the solid type optical fiber at a portion connected to the pore core type optical fiber has a substantially polygonal shape in accordance with a shape of the pore core in a cross section vertical to a longer direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical fiber connection structure. [Background technology]

[0002] A holey-core optical fiber is an optical fiber in which the core that propagates light is made of holes that do not contain glass, etc. Hole-core optical fibers have attracted much attention as fibers with new properties that are difficult to achieve with conventional solid-type silica-based optical fibers, such as ultra-low nonlinearity and ultra-low latency. Here, a solid-type optical fiber is an optical fiber in which the core that propagates light is made of a solid medium, such as glass.

[0003] One of the challenges for practical application of holey-core optical fibers was the control of higher-order modes, but various studies have been conducted as disclosed in Patent Document 1 and Non-Patent Document 1, and the study for practical application is accelerating. Meanwhile, another major challenge for practical application of holey-core optical fibers is the terminal processing and the connection mechanism with other optical fibers. Various studies have been conducted on the terminal processing and connection mechanism as disclosed in Patent Document 2 and Non-Patent Document 2, but compared with conventional solid-type optical fibers, there are challenges in terms of connection loss, handling, etc. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 152243 [Patent Document 2] Patent No. 3870713 [Non-patent literature]

[0005] [Non-Patent Document 1] JM Fini, “Aircore microstructured fibers with suppressed higher-order modes” Opt. Express 14, pp11354-11361, (2006). [Non-Patent Document 2] W. Nicholson et al., “Low-loss low return-loss coupling between SMF and single-mode, hollow-core fibers using connectors”, CLEO Applications and Technology, paper JTu4A, (2014). Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber connection structure in which the connection loss of a hole-core type optical fiber is reduced. [Means for solving the problem]

[0007] In order to solve the above problems and achieve the object, one aspect of the present invention is a fiber having an air hole core with a substantially polygonal shape in a cross section perpendicular to the longitudinal direction, and an effective core area at a predetermined wavelength of 80 μm 2 and a holey-core optical fiber having an effective core area of ​​80 μm at the predetermined wavelength connected to the holey-core optical fiber. 2 and a solid-type optical fiber larger than 1 mm in diameter, wherein a solid core of the solid-type optical fiber at a portion connected to the hole-core-type optical fiber has a substantially polygonal shape corresponding to the shape of the hole core in a cross section perpendicular to the longitudinal direction.

[0008] The hollow core and the solid core may be substantially hexagonal in shape.

[0009] The holey-core optical fiber and the solid-type optical fiber may be physically connected.

[0010] The hole-core type optical fiber and the solid type optical fiber may be fusion-spliced.

[0011] The solid optical fiber may be a single mode optical fiber at the predetermined wavelength.

[0012] The holey core optical fiber may be an effectively single mode optical fiber at the predetermined wavelength.

[0013] The hole-core optical fiber may have a resonant coupling mechanism that optically couples with a higher-order propagation mode of the hole core.

[0014] The hole-core optical fiber may be a photonic bandgap fiber.

[0015] The hole-core optical fiber may be a 19-cell type photonic bandgap fiber.

[0016] The optical fiber may further comprise a normal single mode optical fiber having an effective core area smaller than that of the solid type optical fiber, connected to the side of the solid type optical fiber opposite to the side connected to the holey core type optical fiber.

[0017] The solid type optical fiber and the normal single mode optical fiber may be fusion spliced.

[0018] The fusion splice may be a thermal diffusion type fusion splice. Effect of the Invention

[0019] The present invention has an effect of realizing an optical fiber connection structure in which the connection loss of a hole-core type optical fiber is reduced. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic cross-sectional view along a longitudinal direction of an optical fiber connection structure according to an embodiment. [Diagram 2] FIG. 2 is a schematic cross-sectional view of the photonic bandgap fiber shown in FIG. 1 in a plane perpendicular to the longitudinal direction. [Diagram 3] FIG. 3 is a schematic cross-sectional view of the Aeff enlarged optical fiber shown in FIG. 1 in a plane perpendicular to the longitudinal direction. [Figure 4] FIG. 4 is a schematic cross-sectional view of the ordinary single-mode optical fiber shown in FIG. 1 in a plane perpendicular to the longitudinal direction. [Diagram 5] FIG. 5 is a diagram showing an example of a field distribution of a photonic bandgap fiber. [Figure 6] FIG. 6 is a diagram showing the splice loss between a holey-core optical fiber and a solid-type optical fiber in the embodiment. [Figure 7] FIG. 7 is a diagram showing the splice loss between the solid type optical fiber in the embodiment and a normal single mode optical fiber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment described below. In addition, in each drawing, the same or corresponding components are appropriately assigned the same reference numerals. In addition, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). In addition, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0022] (Embodiment) 1 is a schematic cross-sectional view of an optical fiber connection structure according to an embodiment taken along a longitudinal direction. The optical fiber connection structure 100 includes a photonic bandgap fiber 10, an expanded Aeff optical fiber 20, a normal single mode optical fiber 30, and a connection member 40. The photonic bandgap fiber 10 is an example of a holey core type optical fiber, and the expanded Aeff optical fiber 20 is an example of a solid type optical fiber.

[0023] Fig. 2 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the photonic bandgap fiber 10. Fig. 3 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the expanded Aeff optical fiber 20. Fig. 4 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the normal single mode optical fiber 30.

[0024] The photonic bandgap fiber 10 includes a main core 11a which is an air hole core, side cores 11b and 11c which are air hole cores arranged at positions on either side of the main core 11a, and a cladding 12 which is formed on the outer periphery of the main core 11a and the side cores 11b and 11c and has regularly arranged air holes 12a. The cladding 12 is made of, for example, a silica-based glass, and in particular, it is preferably made of pure silica glass which does not contain a dopant for adjusting the refractive index.

[0025] The main core 11a and the side cores 11b and 11c are hollow cores having a generally polygonal shape, specifically a generally hexagonal shape, in a cross section perpendicular to the longitudinal direction. Here, the generally polygonal shape includes polygonal shapes with rounded sides and corners.

[0026] The photonic bandgap fiber 10 has a basic structure in which holes 12a are arranged in a triangular lattice to form a photonic crystal for forming a photonic bandgap at a predetermined wavelength, and one hole 12a at the center of the triangular lattice and a substantially hexagonal region around it where 18 holes 12a should be arranged are replaced with holes that become main cores 11a as crystal defects. The predetermined wavelength is, for example, 1550 nm. The photonic bandgap fiber 10 transmits light by tightly confining it in the main core 11a due to the above basic structure. Note that a structure in which such a region of 19 holes is replaced with an air hole core is sometimes called a 19-cell type core.

[0027] Furthermore, in the photonic bandgap fiber 10, two regions of a substantially hexagonal shape in which seven of the holes 12a should be arranged are replaced with holes that become side cores 11b and 11c as crystal defects. The side cores 11b and 11c are 7-cell type cores. The side cores 11b and 11c are configured to be optically coupled with a high-order propagation mode of a predetermined wavelength in the main core 11a. As a result, light propagating in the high-order propagation mode in the main core 11a transfers to the side cores 11b and 11c and leaks while propagating. As a result, the main core 11a propagates only the fundamental mode at the predetermined wavelength with low loss, so that the photonic bandgap fiber 10 is substantially a single-mode optical fiber. As a result, the field distribution of the light of the predetermined wavelength propagating in the main core 11a is Gaussian in the radial direction. The side cores 11b and 11c are also an example of a resonant coupling mechanism.

[0028] The effective core area (Aeff) of the main core 11a of the photonic bandgap fiber 10 at a given wavelength is 80 μm 2 Preferably larger than 100 μm 2 Larger is more preferable. 80 μm 2is a typical size of the effective core area of ​​a normal single-mode optical fiber conforming to the standard of ITU-T G.652. In the photonic bandgap fiber 10, a larger effective core area is preferable because it is possible to reduce the optical intensity at the core interface and it is easy to reduce the transmission loss.

[0029] Fig. 5 is a diagram showing an example of the field distribution of a photonic bandgap fiber. However, Fig. 5 shows an example of a 7-cell type air hole core. As shown in Fig. 5, the field distribution of this type of photonic bandgap fiber is approximately hexagonal according to the shape of the air hole core. In addition, since the photonic bandgap fiber in Fig. 5 is a single mode optical fiber, the radial distribution of the field is Gaussian.

[0030] The large Aeff optical fiber 20 is connected to the photonic bandgap fiber 10 at an end face 23. The large Aeff optical fiber 20 and the photonic bandgap fiber 10 are physically connected without a thermal melting process and are in physical contact with each other. An antireflection coating may be formed on the end face 23 of the large Aeff optical fiber 20. The antireflection coating is preferably provided so that the reflection loss is −30 dB or less.

[0031] The large Aeff optical fiber 20 and the photonic bandgap fiber 10 are fixed to each other by a connection member 40. The connection member 40 is made of, for example, an ultraviolet curable resin or glass formed by a sol-gel method, and is provided so as to straddle the joint between the large Aeff optical fiber 20 and the photonic bandgap fiber 10. The connection member 40 may be a heat shrink tube.

[0032] The expanded Aeff optical fiber 20 is made of silica glass and includes a core 21 and a cladding 22 surrounding the outer periphery of the core 21. The core 21 has a constant diameter portion 21a having a substantially constant core diameter in the longitudinal direction, and an expanding diameter portion 21b connected to the constant diameter portion 21a and gradually expanding in diameter from the constant diameter portion 21a toward the end face 24. The cross section of the core 21 shown in FIG. 3 is a cross section of the constant diameter portion 21a. As shown in FIG. 3, the cross section of the constant diameter portion 21a has a substantially hexagonal shape. The core 21 is an example of a solid core.

[0033] The effective core area of ​​the constant diameter portion 21a of the Aeff enlarged optical fiber 20 at a predetermined wavelength is 80 μm 2 Preferably larger than 100 μm 2 A larger effective core area of ​​the expanded Aeff optical fiber 20 is preferable in terms of reducing the splice loss with the photonic bandgap fiber 10.

[0034] The normal single mode optical fiber 30 is connected to the end face 24 of the expanded Aeff optical fiber 20. The normal single mode optical fiber 30 is made of silica glass and includes a core 31 and a cladding 32 surrounding the outer periphery of the core 31. The core 31 has a constant diameter portion 31a having a substantially constant core diameter in the longitudinal direction, and an expanding diameter portion 31b connected to the constant diameter portion 31a and gradually expanding in diameter from the constant diameter portion 31a toward the end face 24 of the expanded Aeff optical fiber 20. The cross section of the core 31 shown in FIG. 4 is the cross section of the constant diameter portion 31a. As shown in FIG. 4, the cross section of the constant diameter portion 31a has a substantially circular shape.

[0035] The normal single mode optical fiber 30 is a solid type optical fiber conforming to the standard of ITU-T G.652. The effective core area of ​​the normal single mode optical fiber 30 is smaller than that of the Aeff enlarged optical fiber 20, and is, for example, 80 μm 2 Generally, the single mode optical fiber 30 is widely used as an optical fiber for communication, and therefore has good handling properties and good connectability with optical fibers used in other devices, transmission cables, and the like.

[0036] Here, the core 21 (constant diameter portion 21a) of the expanded Aeff optical fiber 20 at the portion connected to the photonic bandgap fiber 10 has a substantially polygonal shape, specifically a substantially hexagonal shape, in a cross section perpendicular to the longitudinal direction, corresponding to the shape of the main core 11a of the photonic bandgap fiber 10. As a result, the optical field of the photonic bandgap fiber 10 and the optical field of the expanded Aeff optical fiber 20 become highly consistent, thereby reducing the connection loss.

[0037] For example, in the technology of Non-Patent Document 2, the cross section of the core is circular and the effective core area is 165 μm 2 and a solid optical fiber having a core with a substantially hexagonal cross section and an effective core area of ​​200 μm 2 When the photonic bandgap fiber is connected to the optical fiber with an FC / PC connector, the connection loss is about 0.3 dB, and when the optical fiber is physically connected, the connection loss is 0.25 dB. In contrast, in the optical fiber connection structure 100 according to the present embodiment, the connection loss can be reduced to about 0.05 dB or less as described below, and a significantly reduced connection loss can be realized.

[0038] In order to achieve a desirable low connection loss, the difference between the effective core area of ​​the photonic bandgap fiber 10 and the effective core area of ​​the constant diameter portion 21a of the large Aeff optical fiber 20 is preferably within about 20%.

[0039] The expanded-Aeff optical fiber 20 and the normal single-mode optical fiber 30 are fusion-spliced. Specifically, the expanded-Aeff optical fiber 20 and the normal single-mode optical fiber 30 are thermal diffusion fusion-spliced. The expanded diameter portion 21b of the expanded-Aeff optical fiber 20 and the expanded diameter portion 31b of the normal single-mode optical fiber 30 are portions formed by expanding the diameter of the fixed diameter portion 21a or 31a during the thermal diffusion fusion splicing process. As a result of the expanded-Aeff optical fiber 20 and the normal single-mode optical fiber 30 being thermal diffusion fusion-spliced ​​in this way, the effective core areas of the two become closer to each other, and the cross-sectional shape of the expanded diameter portion 21b at the end face 24 of the expanded-Aeff optical fiber 20 becomes closer to a circle, so that the splice loss is reduced, and as a specific example, it becomes about 0.05 dB or less, as described later.

[0040] As described above, the optical fiber connection structure 100 according to this embodiment has reduced connection loss and is easy to handle.

[0041] (Experimental Example) A photonic bandgap fiber having the same structure as that of the embodiment was fabricated using a conventional method. The fabricated photonic bandgap fiber had an effective core area of ​​about 200 μm at a wavelength of 1550 nm. 2 and had single-mode propagation characteristics.

[0042] The large Aeff optical fiber was fabricated as follows. First, a germania-containing core preform was fabricated using the VAD (Vapor-phase Axial Deposition) method, and this core preform was processed into a hexagonal column shape. Next, silica glass to become the cladding was formed around the outer periphery of the processed core preform using the VAD method to form an optical fiber preform. Next, the optical fiber preform was drawn to fabricate an large Aeff optical fiber. The fabricated large Aeff optical fiber had a step-type refractive index profile, with a relative refractive index difference of the core to the cladding of 0.16%, a diagonal length of the approximately hexagonal core in a cross section perpendicular to the longitudinal direction of 15.5 μm, and an effective core area of ​​198 μm.2 The cutoff wavelength was 1478 nm. In other words, the large Aeff optical fiber was a single-mode optical fiber with a wavelength of 1550 nm.

[0043] Next, the large Aeff optical fiber with an anti-reflection coating on its end face was physically connected to the photonic bandgap fiber using a jig, and then a UV-curable resin was applied across the large Aeff optical fiber and the photonic bandgap fiber, and then the resin was cured with UV light to fix the fiber in place. As a result, the reflection loss at the connection was reduced to -30 dB or less.

[0044] When this kind of physical connection and fixing was performed, the connection loss was measured 10 times, and the results shown in Figure 6 were obtained. As shown in Figure 6, the average connection loss measured 10 times was an extremely low value of 0.048 dB. Also, in many cases, a connection loss of 0.05 dB was obtained.

[0045] Next, the fabricated large-Aeff optical fiber was spliced ​​with a normal single-mode optical fiber conforming to the ITU-T G.652 standard by thermal diffusion fusion splicing, and the splice loss was measured 10 times. The results are shown in Figure 7. As shown in Figure 7, the average splice loss measured 10 times was an extremely low value of 0.041 dB. In many cases, a splice loss of 0.05 dB or less was obtained.

[0046] From the above results, it was confirmed that in the optical fiber connection structure 100 according to the embodiment, a total connection loss of 0.1 dB or less can be achieved.

[0047] In the above embodiment, the hole-core optical fiber is the photonic bandgap fiber 10, but it may be a Bragg-type or antiresonant-type hole-core optical fiber.

[0048] In the above embodiment, the large Aeff optical fiber 20 and the photonic bandgap fiber 10 are physically connected, but they may be fusion spliced. In the case of fusion splicing, however, it is desirable to optimize the fusion splicing conditions so as not to cause structural disturbance in the hole structure of the photonic bandgap fiber 10. This is because structural disturbance in the hole structure may increase the connection loss.

[0049] In the above embodiment, the end face where the large Aeff optical fiber 20 and the photonic bandgap fiber 10 are connected may be an oblique end face to reduce reflection loss.

[0050] In the above embodiment, the core 21 of the expanded Aeff optical fiber 20 and the main core 11a of the photonic bandgap fiber 10 are both substantially hexagonal in shape, but may be other polygonal shapes such as a square or octagon.

[0051] In the above embodiment, a ferrule or a connector may be provided at the connection portion between the large Aeff optical fiber 20 and the photonic bandgap fiber 10.

[0052] Furthermore, the present invention is not limited to the above-mentioned embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-mentioned embodiment, and various modifications are possible. [Explanation of symbols]

[0053] 10: Photonic bandgap fiber 11a: Main core 11b, 11c: Side core 12, 22, 32: Clad 12a:Vacancy 20: Aeff enlarged optical fiber 21, 31: Core 21a, 31a: Fixed diameter part 21b, 31b: Expanded diameter part 23, 24: End face 30: Normal single mode optical fiber 40: Connection parts 100: Optical fiber connection structure

Claims

1. The cross section perpendicular to the longitudinal direction has a substantially polygonal air hole core, and the effective core area at a given wavelength is 80 μm ２ and a hole-core optical fiber having a diameter greater than 100 nm. The effective core area at the predetermined wavelength is 80 μm ２ and a solid optical fiber having a diameter greater than 100 nm. a normal single mode optical fiber connected to the solid type optical fiber on the opposite side to the side connected to the holey core type optical fiber, the effective core area of ​​which is smaller than that of the solid type optical fiber and which complies with the provisions of ITU-T G.652 of the International Telecommunications Union (ITU); Equipped with the hole-core optical fiber and the solid optical fiber are physically connected; the solid optical fiber and the normal single mode optical fiber are thermal diffusion fusion spliced ​​together, The solid core of the solid optical fiber at the portion to be connected to the hole-core optical fiber has a substantially polygonal shape corresponding to the shape of the hole core in a cross section perpendicular to the longitudinal direction. Optical fiber connection structure.

2. The hollow core and the solid core are substantially hexagonal in shape.

2. The optical fiber connection structure according to claim 1.

3. The solid optical fiber is a single mode optical fiber at the predetermined wavelength.

3. The optical fiber connection structure according to claim 1 or 2.

4. The hole-core optical fiber is a substantially single-mode optical fiber that propagates only the fundamental mode at the predetermined wavelength with low loss. The optical fiber connection structure according to any one of claims 1 to 3.

5. The hole-core optical fiber has a resonant coupling mechanism that is optically coupled to a higher-order propagation mode of the hole core, and the light propagating in the higher-order propagation mode leaks out while propagating.

5. The optical fiber connection structure according to claim 4.

6. The hole-core optical fiber is a photonic bandgap fiber. The optical fiber connection structure according to any one of claims 1 to 5.

7. The air-core optical fiber is a 19-cell type photonic bandgap fiber. The optical fiber connection structure according to claim 6.

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