JP7640382B2 - Optical fiber connection structure - Google Patents

Description

The present invention relates to an optical fiber connection structure.

A holey-core optical fiber is an optical fiber in which the core that propagates light is made up of holes that do not contain glass or other materials. Hole-core optical fiber has attracted a lot of attention as a fiber with new properties that are difficult to achieve with conventional solid-type silica-based optical fiber, 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 up of a solid medium such as glass.

One of the challenges in putting hole-core optical fibers to practical use was the control of higher-order modes, but various studies are underway, as disclosed in Patent Document 1 and Non-Patent Document 1, and work toward practical use is accelerating. Meanwhile, another major challenge in putting hole-core optical fibers to practical use is the terminal processing and the connection mechanism with other optical fibers. Various studies are also underway regarding terminal processing and connection mechanisms, as disclosed in Patent Document 2 and Non-Patent Document 2, but compared to conventional solid optical fibers, there are challenges in terms of connection loss, handling, etc.

International Publication No. 2013/152243 Patent No. 3870713

J. M. Fini, “Aircore microstructured fibers with suppressed higher-order modes” Opt. Express 14, pp11354-11361, (2006). 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).

The present invention has been made in view of the above, and its purpose is to provide an optical fiber connection structure that reduces the connection loss of a hole-core optical fiber.

In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical fiber connection structure comprising: a hole-core type optical fiber having a hole core that is approximately polygonal in a cross section perpendicular to the longitudinal direction and an effective core area greater than 80 ^μm2 at a predetermined wavelength; and a solid type optical fiber connected to the hole-core type optical fiber and having an effective core area greater than 80 ^μm2 at the predetermined wavelength, wherein the solid core of the solid type optical fiber in a portion connected to the hole-core type optical fiber has an approximately polygonal shape corresponding to the shape of the hole core in a cross section perpendicular to the longitudinal direction.

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

The hole-core optical fiber and the solid optical fiber may be physically connected.

The hole-core optical fiber and the solid optical fiber may be fusion spliced.

The solid optical fiber may be a single-mode optical fiber at the specified wavelength.

The hole-core optical fiber may be an effectively single-mode optical fiber at the specified wavelength.

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

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

The air-hole core optical fiber may be a 19-cell type photonic bandgap fiber.

The solid-type optical fiber may be connected to the side opposite to the side connected to the hole-core type optical fiber, and may include a normal single-mode optical fiber having a smaller effective core area than the solid-type optical fiber.

The solid optical fiber and the normal single mode optical fiber may be fusion spliced.

The fusion connection may be a thermal diffusion type fusion connection.

The present invention has the effect of realizing an optical fiber connection structure that reduces the connection loss of a hole-core optical fiber.

FIG. 1 is a schematic cross-sectional view of an optical fiber connection structure according to an embodiment, taken along a plane along the longitudinal direction. FIG. 2 is a schematic cross-sectional view of the photonic bandgap fiber shown in FIG. 1 taken along a plane perpendicular to the longitudinal direction. FIG. 3 is a schematic cross-sectional view of the Aeff enlarged optical fiber shown in FIG. 1 taken along a plane perpendicular to the longitudinal direction. FIG. 4 is a schematic cross-sectional view of the normal single mode optical fiber shown in FIG. 1 taken along a plane perpendicular to the longitudinal direction. FIG. 5 is a diagram showing an example of a field distribution in a photonic bandgap fiber. FIG. 6 is a diagram showing the splice loss between a holey-core optical fiber and a solid-type optical fiber in the embodiment. 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.

The following describes in detail an embodiment of the present invention 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.

(Embodiment)
1 is a schematic cross-sectional view of an optical fiber connection structure according to an embodiment taken along a longitudinal direction thereof. 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 hole-core type optical fiber, and the expanded Aeff optical fiber 20 is an example of a solid type optical fiber.

Figure 2 is a schematic cross-sectional view of a photonic bandgap fiber 10 in a plane perpendicular to the longitudinal direction. Figure 3 is a schematic cross-sectional view of an Aeff-enhanced optical fiber 20 in a plane perpendicular to the longitudinal direction. Figure 4 is a schematic cross-sectional view of a normal single-mode optical fiber 30 in a plane perpendicular to the longitudinal direction.

The photonic bandgap fiber 10 comprises 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 formed on the outer periphery of the main core 11a and the side cores 11b and 11c and having regularly arranged air holes 12a. The cladding 12 is made of, for example, a silica-based glass, and is preferably made of pure silica glass that does not contain a dopant for adjusting the refractive index.

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

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 specified wavelength, and one hole 12a in the center of the triangular lattice and the surrounding approximately hexagonal region in which 18 holes 12a should be arranged are replaced with holes that become the main core 11a as a crystal defect. The specified wavelength is, for example, 1550 nm. With the above basic structure, the photonic bandgap fiber 10 strongly confines light in the main core 11a and transmits it. Note that a structure in which such a region of 19 holes is replaced with a hole core is sometimes called a 19-cell type core.

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 higher-order propagation mode of a predetermined wavelength in the main core 11a. As a result, light propagating in the higher-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 a predetermined wavelength with low loss, so the photonic bandgap fiber 10 is essentially a single-mode optical fiber. As a result, the field distribution of light of a predetermined wavelength propagating through 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.

The effective core area (Aeff) at a predetermined wavelength in the main core 11a of the photonic bandgap fiber 10 is preferably larger than 80 ^μm2 , and more preferably larger than 100 ^μm2 . ^{80 μm2} is a typical effective core area of a normal single mode optical fiber conforming to the regulations of, for example, ITU-T G.652. In the photonic bandgap fiber 10, a larger effective core area is preferable because it is possible to reduce the light intensity at the core interface and it is easy to reduce transmission loss.

Figure 5 shows an example of the field distribution of a photonic bandgap fiber. However, Figure 5 shows an example of a 7-cell type air hole core. As shown in Figure 5, the field distribution of this type of photonic bandgap fiber is approximately hexagonal in accordance with the shape of the air hole core. In addition, since the photonic bandgap fiber in Figure 5 is a single mode optical fiber, the radial distribution of the field is Gaussian.

The Aeff-enhanced optical fiber 20 is connected to the photonic bandgap fiber 10 at the end face 23. The Aeff-enhanced optical fiber 20 and the photonic bandgap fiber 10 are physically connected without a thermal melting process and are in physical contact. An anti-reflection coating may be formed on the end face 23 of the Aeff-enhanced optical fiber 20. It is preferable that the anti-reflection coating is provided so that the reflection loss is -30 dB or less.

The Aeff-enhanced optical fiber 20 and the photonic bandgap fiber 10 are fixed to each other by a connecting member 40. The connecting 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 span the joint between the Aeff-enhanced optical fiber 20 and the photonic bandgap fiber 10. The connecting member 40 may be a heat-shrink tube.

The Aeff-expanded optical fiber 20 is made of silica glass and includes a core 21 and a cladding 22 that surrounds the outer circumference of the core 21. The core 21 has a constant diameter portion 21a with a substantially constant core diameter in the longitudinal direction, and an expanding diameter portion 21b that is connected to the constant diameter portion 21a and gradually expands in diameter from the constant diameter portion 21a toward the end face 24. The cross section of the core 21 shown in Figure 3 is the cross section of the constant diameter portion 21a. As shown in Figure 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.

The effective core area at a predetermined wavelength in the constant diameter portion 21a of the Aeff expanded optical fiber 20 is preferably larger than 80 ^μm2 , and more preferably larger than 100 ^μm2 . A larger effective core area of the Aeff expanded optical fiber 20 is preferable in terms of reducing the connection loss with the photonic bandgap fiber 10.

The normal single mode optical fiber 30 is connected to the end face 24 of the Aeff-expanded 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 with 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 Aeff-expanded 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.

The normal single mode optical fiber 30 is, for example, a solid type optical fiber conforming to the regulations of ITU-T G. 652. The effective core area of the normal single mode optical fiber 30 is smaller than that of the expanded Aeff optical fiber 20, for example, 80 ^μm2 . The normal 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, etc.

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

For example, in the technology of Non-Patent Document 2, when a solid optical fiber having a circular core cross section and an effective core area of 165 ^μm2 is connected to a photonic bandgap fiber having a substantially hexagonal core cross section and an effective core area of 200 ^μm2 with an FC/PC connector, the connection loss is about 0.3 dB, and when physically connected, it is 0.25 dB. In contrast, with 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.

In order to achieve a desirable low connection loss, it is preferable that 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 Aeff expanded optical fiber 20 be within about 20%.

The Aeff-expanded optical fiber 20 and the normal single mode optical fiber 30 are fusion spliced. Specifically, the Aeff-expanded optical fiber 20 and the normal single mode optical fiber 30 are thermal diffusion fusion spliced. The expanded diameter portion 21b of the Aeff-expanded 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 thermal diffusion fusion splicing of the Aeff-expanded optical fiber 20 and the normal single mode optical fiber 30, 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 Aeff-expanded optical fiber 20 becomes closer to a circle, so that the splice loss is reduced, and as a specific example, it is about 0.05 dB or less, as described below.

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

(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 ² at a wavelength of 1550 nm and had single-mode propagation characteristics.

The Aeff enlarged optical fiber was fabricated as follows. First, a core preform containing germania was fabricated using the VAD (Vapor-phase Axial Deposition) method, and the core preform was processed into a hexagonal column shape. Next, silica glass to become a 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 Aeff enlarged optical fiber. The fabricated Aeff enlarged optical fiber had a step-type refractive index profile, the relative refractive index difference of the core to the cladding was 0.16%, the diagonal length of the approximately hexagonal core in the cross section perpendicular to the longitudinal direction was 15.5 μm, the effective core cross-sectional area was 198 ^μm2 , and the cutoff wavelength was 1478 nm. That is, the Aeff enlarged optical fiber was a single mode optical fiber at a wavelength of 1550 nm.

Next, the Aeff-enhanced 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 Aeff-enhanced optical fiber and the photonic bandgap fiber, and then fixed by UV-curing. As a result, the reflection loss at the connection was reduced to -30 dB or less.

After carrying out this kind of physical connection and fixing, we performed an experiment to measure the connection loss 10 times, and obtained the results shown in Figure 6. As shown in Figure 6, the average connection loss measured over the 10 times was an extremely low value of 0.048 dB. Also, in many cases, a connection loss of 0.05 dB was obtained.

Next, the fabricated Aeff-enhanced 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 shown in Figure 7 were obtained. As shown in Figure 7, the average splice loss measured 10 times was an extremely low value of 0.041 dB. Also, in many cases, a splice loss of 0.05 dB or less was obtained.

From the above results, it was confirmed that the optical fiber connection structure 100 according to the embodiment can achieve a total connection loss of 0.1 dB or less.

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

In addition, in the above embodiment, the expanded Aeff optical fiber 20 and the photonic bandgap fiber 10 are physically connected, but they may be fusion spliced. However, in the case of fusion splicing, it is desirable to optimize the fusion splicing conditions so that the hole structure of the photonic bandgap fiber 10 is not disturbed. This is because disturbance of the hole structure may increase the connection loss.

In addition, in the above embodiment, the end face where the Aeff-enhanced optical fiber 20 and the photonic bandgap fiber 10 are connected may be an oblique end face to reduce reflection loss.

In addition, 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 approximately hexagonal in shape, but they may be other polygonal shapes such as square or octagonal.

In addition, in the above embodiment, a ferrule or connector may be provided at the connection between the expanded Aeff optical fiber 20 and the photonic bandgap fiber 10.

Furthermore, the present invention is not limited to the above-mentioned embodiment. The present invention also includes configurations in which the above-mentioned components are appropriately combined. Furthermore, 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.

10: Photonic band gap fiber 11a: Main core 11b, 11c: Side core 12, 22, 32: Cladding 12a: Hole 20: Aeff enlarged optical fiber 21, 31: Core 21a, 31a: Fixed diameter portion 21b, 31b: Enlarged diameter portion 23, 24: End surface 30: Normal single mode optical fiber 40: Connection member 100: Optical fiber connection structure

Claims (7)

a hole-core optical fiber having a substantially polygonal hole core in a cross section perpendicular to the longitudinal direction and an effective core area at a predetermined wavelength greater than 80 μm2 ^;
a solid-type optical fiber connected to the holey-core optical fiber and having an effective core area of more than 80 ^μm2 at the predetermined wavelength;
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,
an optical fiber connection structure, wherein a solid core of the solid type optical fiber in a portion connected to the hole-core type optical fiber has a substantially polygonal shape corresponding to a shape of the hole core in a cross section perpendicular to the longitudinal direction. The optical fiber connection structure according to claim 1 , wherein the air core and the solid core are substantially hexagonal in shape. 3. The optical fiber connection structure according to claim 1, wherein the solid optical fiber is a single mode optical fiber at the predetermined wavelength. 4. The optical fiber connection structure according to claim 1 , wherein the hole-core type optical fiber is a substantially single mode optical fiber that propagates only a fundamental mode at the predetermined wavelength with low loss. 5. The optical fiber connection structure according to claim 4 , wherein 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 light propagating in the higher-order propagation mode leaks out while propagating . 6. The optical fiber connection structure according to claim 1 , wherein the hole-core optical fiber is a photonic bandgap fiber. 7. The optical fiber connection structure according to claim 6 , wherein the hole-core optical fiber is a 19-cell type photonic bandgap fiber.

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