JP4976265B2 - Fusion splicing method of optical fiber - Google Patents

Description

  The present invention relates to an optical fiber fusion splicing method for splicing optical fibers having holes formed along a longitudinal direction.

  An optical fiber having holes formed along the longitudinal direction is attracting attention because it has characteristics that cannot be realized by an optical fiber having a core and a clad having a normal structure. In particular, the broadband characteristics and low bending loss characteristics of optical fibers having holes show values that overturn the common sense of optical fibers having a normal structure, and are attracting attention as future optical fibers. As a fiber having a hole, for example, a holey is provided around the center axis of an optical fiber having a substantially uniform refractive index, and a holey fiber that guides light using a region near the center axis without a hole as a core. And a so-called hole assist fiber with a hole around the core of an optical fiber of a normal structure having a core and a cladding, and a hole serving as a core on the central axis. There is a so-called photonic bandgap fiber that guides light by generating a photonic bandgap by periodically arranging holes. In addition, the optical fiber which has these void | holes is also called the microstructure fiber (refer patent document 1).

  On the other hand, fusion bonding is a technique for permanently connecting optical fibers with high reliability and low loss. The fusion splicing is a technique for physically and optically connecting optical fibers by aligning two optical fibers and bringing these ends into contact with each other after being exposed to a high temperature. At the time of fusion splicing, the optical fiber is exposed to a high temperature that is higher than the softening point of the glass, which is the material, so that the connection portion can be connected smoothly, and as a result, a connection with low loss can be realized.

  As described above, since the temperature of the optical fiber reaches or exceeds the glass softening point temperature at the time of fusion splicing, the optical fiber is locally deformed. In the case of an optical fiber with a normal structure, there is no problem even if local deformation occurs, and since the optical fiber with a normal structure is cylindrical, its shape is maintained by surface tension even when the glass is softened. Therefore, it will not be a problem.

  However, when an optical fiber having holes is fusion-spliced, the hole shape is deformed due to the softening of the glass. Furthermore, the effect of surface tension, which has been working to maintain the shape of the optical fiber with the normal structure, causes a problem that the inner diameter of the hole is reduced, and the light confinement effect by the hole is insufficient. However, there is a problem that the confinement loss increases and a low-loss fusion splice cannot be performed.

  In order to solve this problem, when connecting the holey fiber to another optical fiber, the fusion strength connection is performed by adjusting the discharge intensity and discharge time so that the hole of the holey fiber is not crushed. A method for realizing the connection is disclosed (see Patent Document 2).

JP 2006-264997 A JP 2004-177804 A

  However, the conventional method of fusion-connecting holey fibers so that the holes are not crushed has a problem that complicated adjustment of discharge conditions such as discharge intensity and discharge time is required.

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber fusion splicing method that can more easily fuse and connect optical fibers having holes with low loss.

  In order to solve the above-described problems and achieve the object, a fusion splicing method of optical fibers according to the present invention includes a hole optical fiber having holes formed along a longitudinal direction, and the hole optical fiber. The connection optical fiber to be connected to the fusion optical fiber is fused and connected, and the inner diameter of the hole is expanded from the initial inner diameter at the connection side end of the hole optical fiber to form a diameter-expanded portion. A diameter-enlarged portion forming step, the connection-side end of the holey optical fiber in which the diameter-expanded portion is formed, and the connection-side end of the connection optical fiber are brought close to each other, and each connection-side end is heated. And a fusion splicing step for fusion splicing.

  Further, in the fusion splicing method of an optical fiber according to the present invention, in the above invention, the expanded diameter portion forming step includes immersing the connection side end portion of the hole optical fiber in an etching solution. It is characterized by expanding the diameter of the hole of the optical fiber.

  Further, in the fusion splicing method of optical fibers according to the present invention, in the above invention, the inner diameter of the expanded portion is estimated by measuring the outer diameter of the portion of the hole optical fiber immersed in the etching solution. Including an estimation step.

  Further, the fusion splicing method of the optical fiber according to the present invention is the hole optical fiber according to the above invention, wherein the enlarged diameter portion forming step is performed in a state where the air pressure in the hole of the hole optical fiber is increased. A diameter-enlarged region forming step of softening a part of the hole to form an expanded region where the inner diameter of the hole is expanded, and a hole optical fiber formed with the expanded region are perpendicular to the longitudinal direction. And a cutting step of cutting along a plane crossing the enlarged diameter region.

  According to the present invention, the inner diameter of the hole is expanded from the initial inner diameter at the connection side end of the hole optical fiber to form the expanded diameter portion, and the connection side end is heated to be fusion-bonded. Therefore, there is an effect that the two optical fibers having holes can be fused and connected with low loss more easily.

  DESCRIPTION OF EMBODIMENTS Embodiments of an optical fiber fusion splicing method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view of an optical fiber that is fusion spliced by the fusion splicing method according to Embodiment 1 of the present invention, cut along a plane perpendicular to the longitudinal direction. FIG. 2 is a schematic cross-sectional view of the optical fiber shown in FIG. 1 cut along the longitudinal direction. As shown in FIGS. 1 and 2, the holey optical fiber 1 includes a core portion 1 a located in the center, and a plurality of holes formed along the longitudinal direction around the core portion 1 a and located on the outer periphery of the core portion 1 a. A so-called holey fiber structure including a cladding portion 1b having a plurality of holes 1c. Reference numeral 1d denotes a connection side end of the optical fiber 1, and the hole optical fiber 1 is connected to another optical fiber at the connection side end 1d.

The core portion 1a and the cladding portion 1b of the holey optical fiber 1 are made of pure silica glass not containing a dopant that changes the refractive index, but are not particularly limited as long as they are made of the same glass material. For example, the core portion 1a has a diameter of about 10 μm, and the cladding portion 1b has an outer diameter of about 125 μm. The holes 1c are formed in a triangular lattice pattern around the core portion 1a. In this hole optical fiber 1, Λ is 10 μm and d / Λ is 0.50, where d is the inner diameter of the hole 1c, and the distance between centers of the holes 1c, that is, the lattice constant of the triangular lattice is Λ. For example, while having the possibility of broadband transmission with a cutoff wavelength of 1000 nm or less, the effective core area of the 1.55 μm band can be made 110 μm ² or more. It has characteristics that can be suitably used as an optical fiber.

  Next, an optical fiber fusion splicing method according to the first embodiment will be described. 3-6 is explanatory drawing explaining the fusion splicing method of the optical fiber which concerns on this Embodiment 1. FIG. First, the inner diameter of the hole 1c is increased with respect to the initial inner diameter at the connection side end 1d of the hole optical fiber 1 to form the diameter-increased part 1ca. The enlarged diameter portion 1ca is formed by immersing the connection side end portion 1d of the hole optical fiber 1 in an etching solution such as hydrofluoric acid and etching the inner wall of the hole 1c.

  Next, as shown in FIG. 4, a hole optical fiber 2 having the same structure as the hole optical fiber 1 is prepared as a connection optical fiber to be connected to the hole optical fiber 1. That is, this hole optical fiber 2 is a clad having a core part 2a located at the center and a plurality of holes 2c located on the outer periphery of the core part 2a and formed along the longitudinal direction around the core part 2a. And a diameter-enlarged portion 2ca formed on the connection-side end portion 2d. Then, by using an optical fiber fusion splicer (Furukawa Electric Fitel S175), the hole optical fibers 1 and 2 are arranged so that the connection side end portions 1d and 2d face each other and come close to each other.

  Next, as shown in FIG. 5, the arc discharge A1 is emitted from the arc electrode rod E to the connection point C while the connection side ends 1d and 2d of the hole optical fibers 1 and 2 are abutted, and the connection side end The parts 1d and 2d are heated, and the hole optical fibers 1 and 2 are fusion-spliced. The discharge intensity and discharge time at this time are such that the optical hole fibers 1 and 2 are connected with sufficient mechanical strength, and the expanded diameter portions 1ca and 2ca are substantially the same as the initial inner diameter, that is, the hole 1c or 2c. Set to reduce to the inner diameter. For example, since the hole optical fibers 1 and 2 have the same outer diameter as that of a standard single mode optical fiber, it is possible to fuse with sufficient mechanical strength by setting a discharge condition for fusion splicing of the standard single mode optical fiber. Incoming connection is possible.

  As a result, as shown in FIG. 6, the hole optical fibers 1 and 2 are fusion-bonded at the connection point C. On the other hand, since the diameter-expanded portions 1ca and 2ca are reduced in diameter to substantially the same inner diameter as the initial inner diameter, there is no increase in confinement loss due to deformation of the holes or reduction in diameter in the vicinity of the connection point C. Fusion splicing can be performed.

  Note that the inner diameter of the enlarged diameter portions 1ca and 2ca and the length of the enlarged portions 1ca and 2ca formed from the end face are determined as follows, for example. First, a preliminary experiment is performed in which a hole optical fiber having the same structure as that of the hole optical fibers 1 and 2 is fusion-bonded under a discharge condition that can sufficiently secure the mechanical strength of the connection point. Thereafter, it is measured how much the hole has been reduced in diameter at the connection point, and the enlarged diameter portions 1ca and 2ca having an inner diameter sufficient to compensate for the reduced diameter are formed over the reduced length. do it.

  When the formation of the enlarged diameter portions 1ca and 2ca is performed by etching, the etching rate of the etching liquid with respect to the glass material of the hole optical fiber 1 is checked in advance, and based on this etching rate, the hole optical fiber 1 The immersion time in the etching solution is adjusted as appropriate. Thereby, a desired diameter expansion amount can be easily realized. When the hole optical fiber 1 is immersed in the etching solution, not only the hole 1c is expanded, but the hole optical fiber 1 itself is etched at the same etching rate. The diameter is reduced by the same amount as the diameter of the hole 1c. Therefore, when it is desired to confirm how much the hole 1c has been expanded, it is better to measure the outer diameter of the hole optical fiber 1 and calculate the diameter expansion amount of the hole 1c from the reduced diameter. It is easier than directly measuring the inner diameter of 1c.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Two hole optical fibers having a structure as shown in FIG. 1 and having an inner diameter d of 5.0 μm and an inter-hole distance Λ of 10 μm were prepared. Next, a commercially available fusion splicer was prepared, and these holey optical fibers were fusion spliced at a discharge intensity of 60 mA and a discharge time of 1.5 seconds, which are the same discharge conditions as a standard single mode optical fiber. The connection loss was 1.0 dB. When this connection part was cut and the inner diameter of the hole was confirmed with a microscope, the inner diameter, which was 5.0 μm before the fusion, was reduced to about 4.0% and about 20%. Further, the diameter reduction occurred over a length of 1 mm (0.5 mm per optical fiber).

  Next, 60 hole optical fibers were prepared based on the above experiment. The average of the inner diameters of the 60 hole optical fibers was 4.99 μm, and the standard deviation was 0.11 μm. Thirty hole optical fibers arbitrarily selected from these were used as hole optical fibers according to Example 1, and the diameter of the holes at these ends was increased. Specifically, the end portion of the hole optical fiber is immersed in a 50% hydrofluoric acid solution having a silica glass etching rate of 0.2 mm / h for 20 seconds to a depth of 0.5 mm from the end surface, and 0.5 mm from the end surface. Etching was performed so that the inner diameter of the holes until the end was about 6.25 μm, which was about 20% larger than the initial inner diameter, and then washed with pure water. When the inner diameter of the holes at the end of the etched hole optical fiber was measured, the average value was 6.25 μm and the standard deviation was 0.15 μm. Next, these hole optical fibers were fused and connected. Then, the connection loss at the 15 connection points was an average of 0.2 dB, and the standard deviation was 0.1 dB, which was a stable and sufficiently low loss value. Regarding these hole optical fibers, when the connecting portion was cut and the inner diameter of the holes was confirmed with a microscope, the average inner diameter was 4.8 μm and the standard deviation was 0.4 μm, which is almost the same as the initial inner diameter. It was value.

  On the other hand, when the remaining 30 hole optical fibers were used as hole optical fibers according to Comparative Example 1 and fusion splicing was performed under the same discharge conditions as described above, the average connection loss at 15 connection points was 1. The standard deviation was 6 dB and the standard deviation was 0.6 dB, and the connection loss had a large value and a large variation. Regarding these hole optical fibers, when the connecting portion was cut and the inner diameter of the holes was confirmed with a microscope, the average inner diameter was 3.9 μm and the standard deviation was 0.3 μm, which was smaller than the initial inner diameter. It was diameter. Note that the hole diameter was reduced to 3.4 μm at the connection portion of the hole optical fiber having the largest connection loss.

  When the outer diameters of the hole optical fiber according to Example 1 before and after the etching process were confirmed using a video analyzer, the average was 125.0 μm before the process and 123. It was 8 μm. The standard deviation of the outer diameter was 0.2 μm before and after the treatment. That is, the outer diameter of the hole optical fiber was reduced by about 1.2 μm by the etching process, which was almost the same value as the diameter of the hole. Accordingly, it was confirmed that in the step of expanding the hole, when it is desired to check the amount of hole expansion, the outer diameter of the hole optical fiber is measured and the amount of diameter reduction is calculated.

  Next, as Examples 2 to 5, each of the 30 hole optical fibers in each example was used in the same manner as in Example 1 except that the concentration of the hydrofluoric acid solution used for the etching treatment and the immersion time were changed. Fusion spliced. The concentration of hydrofluoric acid used was 5%, 10%, 15%, and 20% in each of Examples 2 to 5, and the immersion time at each concentration was 200 seconds, 100 seconds, 67 seconds, and 50%, respectively. Seconds. As a result, in each of the hole optical fibers according to Examples 2 to 5, the average inner diameter of the expanded holes was 6.26 μm, but the standard deviation was Example 2 having a concentration of 5 to 15%. -4 was 0.12 μm, and Example 5 having a concentration of 20% was 0.14 μm. Furthermore, the average connection loss in the case of fusion splicing was 0.1 dB in Examples 2 to 4, and 0.2 dB in Example 5. Therefore, it was confirmed that when the hydrofluoric acid concentration was 15% or less, the variation in the diameter expansion of the holes was small and the connection loss was small. The reason for this is thought to be that when the concentration of hydrofluoric acid is high, the controllability of the etching amount is lowered, for example, the etching proceeds in the time from the etching process to the subsequent cleaning with pure water.

(Embodiment 2)
In the first embodiment, the enlarged diameter portion 1ca is formed by immersing the connection side end 1d of the hole optical fiber 1 in an etching solution such as hydrofluoric acid and etching the inner wall of the hole 1c. The present invention is not limited to this. Hereinafter, an optical fiber fusion splicing method according to Embodiment 2 of the present invention in which the method for forming the enlarged diameter portion is different will be described.

  7 and 8 are explanatory diagrams for explaining the fusion splicing method of optical fibers according to the second embodiment. As shown in FIG. 7, in the second embodiment, a hole optical fiber having a core 3a, a cladding 3b, and a hole 3c similar to those of the optical fiber 1 shown in FIG. 3 is prepared, the end portion on the side connected to the connection optical fiber is heated to seal the hole 3c, and the sealing portion 3e is formed. Next, the arc discharge A2 is discharged from the arc electrode rod E to the position near the sealing portion 3e and heated using a commercially available optical fiber fusion splicer. Then, since the sealing portion 3e exists in the vicinity of the heated portion, the air in the air holes 3c expands, the internal atmospheric pressure becomes high, and the viscosity of the heated portion of the glass decreases and softens. The hole 3c is expanded in diameter to form an expanded region 3cb. Thereafter, the hole optical fiber 3 is cut at a plane L perpendicular to the longitudinal direction and crossing the enlarged diameter region 3cb, thereby forming an enlarged diameter portion 3ca at the connection side end 3d as shown in FIG. The hole optical fiber 3 is formed. The inner diameter and the length from the end face of the enlarged diameter portion 3ca can be determined by conducting a preliminary experiment in the same manner as in the first embodiment.

  Thereafter, a hole optical fiber having a diameter-expanded portion formed by the same method as that of the hole optical fiber 3 or the method of the first embodiment is prepared, and the hole optical fiber and the hole optical fiber 3 are formed as the first embodiment. Fusion splicing is performed by the method described above. As a result, as in the first embodiment, since the confinement loss does not increase due to the deformation of the holes at the connection portion of the hole optical fiber, the fusion connection can be performed with a low loss.

  Next, as Example 6 of the present invention, 50 hole optical fibers similar to Example 1 were prepared, and the end of each hole optical fiber was heated and sealed with a commercially available fusion splicer, Furthermore, the part 5 mm away from the sealing part was heated, the heating time was adjusted, and the diameter was expanded so that the inner diameter of the hole in that part was about 6.25 μm. Next, the expanded diameter region of the hole was cut along the plane perpendicular to the longitudinal direction at a position where the length of the expanded diameter portion was about 0.5 mm. When the inner diameter of the holes at the end of the cut hole optical fiber was measured, the average value was 6.25 μm, which was the target, but the standard deviation was slightly large, 0.5 μm.

  Next, among the 50 hole optical fibers whose holes are expanded, the average value of the inner diameters of the holes is 6.25 μm and the standard deviation is 0.15 μm as in the case of Example 1. One hole optical fiber was selected. The 18 hole optical fibers thus selected were connected to each other under the same discharge conditions as the standard single mode optical fiber as in Example 1. As a result, the average connection loss was 0.2 dB and the standard deviation was 0.1 dB. As in Example 1, the value was stable and sufficiently low loss. In addition, when the hole optical fibers that were not selected were connected to each other under the same fusion conditions, the connection loss was 0.5 dB as a whole, including the selected one, and the standard deviation was 0.3 dB. The value was stable and sufficiently low loss.

  In the above embodiments and examples, the end of the hole optical fiber is sealed in order to increase the pressure inside the hole, but gas is injected from the end face of the hole optical fiber into the hole and pressurized. May be. Further, although the fusion splicer is used for heating the optical fiber, a heater or a torch may be used.

  Further, in the above-described embodiments and examples, holey optical fibers are fused and connected as holey fibers. However, the present invention is applicable to the case where the same type or different types of optical fibers having other holes are fused and connected. Is also applicable. The present invention can also be applied to a case where a hole optical fiber and an optical fiber having a normal structure as a connection optical fiber are fusion-connected.

FIG. 3 is a schematic cross-sectional view of an optical fiber that is fusion spliced by the fusion splicing method according to the first embodiment, cut along a plane perpendicular to the longitudinal direction. It is typical sectional drawing which cut | disconnected the optical fiber shown in FIG. 1 along the longitudinal direction. It is explanatory drawing explaining the fusion splicing method of the optical fiber which concerns on Embodiment 1. FIG. It is explanatory drawing explaining the fusion splicing method of the optical fiber which concerns on Embodiment 1. FIG. It is explanatory drawing explaining the fusion splicing method of the optical fiber which concerns on Embodiment 1. FIG. It is explanatory drawing explaining the fusion splicing method of the optical fiber which concerns on Embodiment 1. FIG. 5 is an explanatory diagram for explaining a fusion splicing method for optical fibers according to Embodiment 2. FIG. 5 is an explanatory diagram for explaining a fusion splicing method for optical fibers according to Embodiment 2. FIG.

Explanation of symbols

1 to 3 Hole optical fiber 1a to 3a Core portion 1b to 3b Cladding portion 1c to 3c Hole 1ca to 3ca Expanded portion 1d to 3d Connection side end portion 3cb Expanded region 3e Sealing portion A1, A2 Arc discharge C connection Point E Arc electrode rod L surface

Claims (4)

A method of fusion splicing a hole optical fiber having holes formed along a longitudinal direction and a connection optical fiber to be connected to the hole optical fiber,
A diameter-enlarged portion forming step of expanding the inner diameter of the hole with respect to the initial inner diameter at the connection side end of the hole optical fiber, and forming an expanded diameter portion;
A fusion splicing in which the connection-side end of the hole optical fiber in which the diameter-expanded portion is formed and the connection-side end of the connection optical fiber are brought close to each other and the connection-side ends are heated and fusion-bonded. Process,
An optical fiber fusion splicing method comprising:   2. The diameter expansion portion forming step of expanding the hole diameter of the hole optical fiber by immersing a connection side end portion of the hole optical fiber in an etching solution. An optical fiber fusion splicing method.   3. The method of claim 2, further comprising estimating an inner diameter of the expanded portion by measuring an outer diameter of a portion of the hole optical fiber immersed in the etching solution. Incoming connection method. The expanded diameter portion forming step includes
A diameter-enlarged region forming step of forming a diameter-enlarged region in which the inner diameter of the hole is increased by heating and softening a part of the hole-shaped optical fiber in a state where the pressure in the hole of the hole-shaped optical fiber is increased When,
A cutting step of cutting the holey optical fiber in which the diameter-enlarged region is formed, at a plane perpendicular to the longitudinal direction and crossing the diameter-enlarged region;
The method for fusion splicing optical fibers according to claim 1, comprising:

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