JP2018045168A - Pigtail fiber module and manufacturing method thereof, and light transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the variation in intensity distribution of output light from a large mode area fiber at a connection between the large mode area fiber and a connection component.SOLUTION: A pigtail fiber module 1 includes: a large mode area fiber 2 which includes a fiber core wire 3 comprising a core 3a transmitting single-mode light and a clad 3b surrounding the circumference of the core 3a, and a protective covering material 4 covering the circumference of the fiber core wire 3; an insertion hole 6 with which an end of the large mode area fiber 2 is connected, and into which is insertable an exposed part 3c of the fiber core wire 3 that is exposed by removing the protective covering material 4 located near the end of the large mode area fiber 2; and a connection component 5 which has the exposed part 3c of the fiber core wire 3 inserted into the insertion hole 6 and adhesively fixes the exposed part. The circumferential surface of the clad 3b of the exposed part 3c of the fiber core wire 3 is covered with a metal coating layer 9.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a pigtail fiber module using a large mode area fiber used for, for example, a fiber laser, and more particularly, to a pigtail fiber module having improved connection structure between a large mode area fiber and a connection component, a method for manufacturing the same, and an optical device. The present invention relates to a transmission apparatus.

The pigtail fiber module is a type of optical transmission medium used in optical communications, laser processing machines, and the like. In recent years, the power of transmitted light has increased in laser processing machines and the like, and various optical fibers have been used. A large mode area fiber (Large Mode Area Fiber, hereinafter abbreviated as LMA fiber if necessary) is currently the mainstream as an optical fiber capable of high output.
A pigtail fiber module using this LMA fiber is described in Patent Document 1, for example. The LMA fiber has a large mode size and a structure that prevents high-order mode guiding, and the refractive index difference between the cladding and the core is smaller than that of a normal optical fiber. When stress is applied to the optical fiber, the refractive index is modulated by the photoelastic effect, but in the pigtail fiber module using the LMA fiber, when external stress is generated around the cladding due to curing shrinkage of the adhesive used for module assembly, etc. There is a concern that the refractive index distribution of the optical fiber is modulated and the desired light intensity distribution cannot be maintained.
Patent Document 1 includes a pigtail fiber (corresponding to an LMA fiber), a ferrule, an end cap, and a thermosetting adhesive, and an end cap is attached to the end face in the through hole of the ferrule. The ferrule and the pigtail fiber are fixed with an adhesive, and the through hole is fixed with the adhesive and the clad with the end portion of the end cap inserted and the end surface of the end cap and the first end surface of the ferrule coincided with each other. A pigtail fiber module having an inner diameter in which the average gap ((inner diameter of the through hole−cladding diameter) / 2) is less than 1 μm is disclosed.
That is, in Patent Document 1, when the LMA fiber is fixed to a ferrule having a through hole with a thermosetting adhesive, the gap between the LMA fiber clad and the ferrule through hole is made as small as possible to achieve a thermosetting type. There is disclosed a pigtail fiber module in which stress load on an optical fiber due to curing shrinkage of an adhesive is suppressed and variation in output light intensity distribution of an LMA fiber does not occur.

JP2012-255999A (form for carrying out the invention, FIG. 4)

However, it has been confirmed that the method of making the average gap between the ferrule and clad bonding sites in Patent Document 1 less than 1 μm is very difficult in view of the processing accuracy of the ferrule and LMA fiber.
Since there are variations in the actual LMA fiber cladding diameter and ferrule through-hole diameter, a large number of LMA fibers and ferrules are prepared in order to keep the gap between the LMA fiber cladding and the ferrule through-hole within a certain range. It is necessary to search for a combination of compatible materials, and such an assembly method is expensive. Moreover, even if the average gap between the adhesion points of the clad and the ferrule exceeds 1 μm, after curing the adhesive, laser light is incident on the pigtail fiber module and the output light intensity distribution is not measured. It is impossible to find a problem that the average gap exceeds 1 μm.

  A technical problem to be solved by the present invention is to suppress variation in intensity distribution of output light from an LMA fiber at a connection portion between an LMA fiber and a connection component of a pigtail fiber module.

  A first technical feature of the present invention is a large mode area fiber having a core for propagating a single mode light, a fiber core wire comprising a clad surrounding the outer periphery of the core, and a protective covering material covering the outer periphery of the fiber core wire. And an insertion hole into which an exposed portion of the fiber core wire, which is exposed by removing the protective covering located near the end of the large mode area fiber, is connected to the end of the large mode area fiber. And a connecting component for inserting and fixing the exposed portion of the fiber core wire into the insertion hole, and covering the cladding peripheral surface of the exposed portion of the fiber core wire with a metal coating layer. Pigtail fiber module.

A second technical feature of the present invention is a pigtail fiber module having the first technical feature, wherein the cladding diameter of the fiber core wire is d (μm), and the thickness of the metal coating layer is t (μm). Then, it is a pigtail fiber module characterized by satisfying t ≧ d / 10.
According to a third technical feature of the present invention, in the pigtail fiber module having the first technical feature, if the cladding diameter of the fiber core wire is 125 (μm), the thickness of the metal coating layer is 13 μm. This is a pigtail fiber module characterized by the above.
A fourth technical feature of the present invention is a pigtail fiber module characterized in that in the pigtail fiber module having the first technical feature, the metal coating layer is formed by metal plating.
According to a fifth technical feature of the present invention, in the pigtail fiber module having the first technical feature, an end cap is fixed to an end portion of the exposed portion of the fiber core wire, and the end cap is an insertion hole of a connection component. The pigtail fiber module is characterized in that when disposed inside, the metal coating layer also covers the peripheral surface of the end cap.

A sixth technical feature of the present invention is a large mode area fiber having a core that propagates a single mode light, a fiber core wire comprising a clad surrounding the outer periphery of the core, and a protective covering material that covers the outer periphery of the fiber core wire. And an insertion hole into which an exposed portion of the fiber core wire, which is exposed by removing the protective covering located near the end of the large mode area fiber, is connected to the end of the large mode area fiber. A connecting part for inserting and fixing the exposed portion of the fiber core wire into the insertion hole, and before connecting the large mode area fiber and the connecting part. The above-mentioned face exposed by removing the protective covering material located near the end of the large mode area fiber. A plating step of covering the cladding peripheral surface of the exposed portion of the core wire with a metal coating layer made of metal plating, and inserting the exposed portion of the fiber core wire of the large mode area fiber after the plating step into the insertion hole of the connection component And a connecting step of bonding and fixing the pigtail fiber module.
A seventh technical feature of the present invention is characterized by comprising a pigtail fiber module having any one of the first to fifth technical features and an optical component assembled to the pigtail fiber module. It is an optical transmission device.

According to the first technical feature of the present invention, it is possible to suppress variations in the intensity distribution of the output light from the large mode area fiber at the connection portion with the large mode area fiber.
According to the second technical feature of the present invention, the minimum necessary thickness for the metal coating layer can be easily selected from the relationship with the cladding diameter.
According to the third technical feature of the present invention, a minimum necessary thickness can be selected as a metal coating layer for a fiber having a cladding diameter of 125 μm.
According to the fourth technical feature of the present invention, a metal coating layer having a substantially uniform thickness in the circumferential direction can be easily formed on the circumferential surface of the clad.
According to the fifth technical feature of the present invention, the fiber core wire with the end cap can be connected to the insertion hole of the connection component in a state in which the stress load due to the adhesive is suppressed.
According to the sixth technical feature of the present invention, a pigtail fiber module capable of suppressing variations in intensity distribution of output light from the large mode area fiber at the connection portion with the large mode area fiber is easily manufactured. can do.
According to the seventh technical feature of the present invention, an optical transmission including a pigtail fiber module capable of suppressing variations in intensity distribution of output light from the large mode area fiber at the connection portion with the large mode area fiber. An apparatus can be provided.

(A) is explanatory drawing which shows the outline | summary of embodiment of the pigtail fiber module to which this invention was applied, (b) is components decomposition | disassembly explanatory drawing of the pigtail fiber module shown to (a), (c) is in (a) CC sectional view explanatory drawing of this, (d) is CC sectional view explanatory drawing in (a) in the comparison form. (A) is explanatory drawing which shows the structural example of the pigtail fiber module which concerns on Embodiment 1, (b) is explanatory drawing which shows the structural example of a large mode area fiber, (c) is CC line in (a) FIG. (A) is explanatory drawing which shows the dimension and action stress of the exposed part of the fiber core wire coat | covered with the metal coating layer among the pigtail fiber modules which concern on Embodiment 1, (b) is the fiber core wire coat | covered with the metal coating layer. The schematic diagram which shows a distortion state when radial direction load F acted on the exposed part of (c), (c) is a distortion state when radial direction load F acts on the exposed part of the cylindrical fiber core wire except a metal coating layer (D) is a schematic diagram which shows a distortion state when the radial load F acts on the cylindrical metal coating layer formed in the exposed part of the fiber core wire. In the pigtail fiber module which concerns on Example 1, it is explanatory drawing which shows the example of arrangement | positioning of the exposed part of the fiber core wire with a metal coating layer with respect to the through-hole of a metal ferrule with a horizontal hole, and the thickness distribution example of the periphery adhesive agent. (A) shows the photographic image before hardening of a thermosetting adhesive among the photographic images which projected the light radiate | emitted from the end cap end surface on the beam profiler in the pigtail fiber module which concerns on Example 1, (b) These are explanatory drawings which show the photographic image after hardening of a thermosetting adhesive among the photographic images. (A) shows the photographic image before hardening of a thermosetting adhesive among the photographic images which projected the light radiate | emitted from the end cap end surface on the beam profiler in the pigtail fiber module which concerns on the comparative example 1, (b) These are explanatory drawings which show the photographic image after hardening of a thermosetting adhesive among the photographic images.

Outline of Embodiment FIG. 1A shows an outline of an embodiment of a pigtail fiber module to which the present invention is applied, and FIG. 1B shows an exploded view of parts of the pigtail fiber module.
In FIG. 1, a pigtail fiber module 1 is a large core having a core 3 a that propagates single mode light and a fiber core wire 3 that includes a clad 3 b that surrounds the outer periphery of the core 3 a, and a protective covering material 4 that covers the outer periphery of the fiber core wire 3. An exposed portion 3c of the fiber core wire 3 exposed by removing the protective coating 4 located near the end of the LMA fiber 2 is connected to the mode area fiber (LMA fiber) 2 and the end of the LMA fiber 2. A connecting part 5 having an insertion hole 6 that can be inserted, and inserting and fixing the exposed portion 3c of the fiber core wire 3 into the insertion hole 6, and the peripheral surface of the cladding 3b of the exposed portion 3c of the fiber core wire 3 Is coated with the metal coating layer 9. In FIGS. 1A and 1B, reference numeral 7 denotes a thermosetting adhesive made of, for example, a thermosetting epoxy resin.

In such technical means, the LMA fiber 2 has a fiber core wire 3 (core 3a + cladding 3b) and a protective covering material 4 surrounding the periphery. For example, by removing the protective covering material 4, the LMA fiber 2 near the end of the fiber is removed. The fiber core wire 3 is exposed and connected to the connection component 5, whereby the pigtail fiber module 1 is manufactured.
Further, the connecting part 5 corresponds to a part called a ferrule, for example, and FIGS. 1A and 1B show an aspect using a part having one insertion hole 6 for simplifying the description. However, as shown in the first embodiment to be described later, a typical embodiment is composed of a plurality of parts having insertion holes 6 having different inner diameters, such as an embodiment having a zirconia ferrule at the end of a metal ferrule with a horizontal hole. Is often used.
Furthermore, the connection portion between the connection component 5 and the LMA fiber 2 needs to have a structure in which at least the exposed portion 3 c of the fiber core wire 3 is inserted into the insertion hole 6. This is because it is essential to remove the protective covering material 4 at the end portion of the optical fiber. For example, when the end cap 8 (see FIG. 1A) is fused to the end of the fiber core wire 3, it is necessary to remove the protective coating material 4 to expose the fiber core wire 3, and to For example, the end face of an optical fiber may be processed into a mirror surface so that the end face of the fiber is free from distortion or scattering of light. Polishing, mechanical cleaving, and laser cleaving have been put to practical use for this type of method. However, in any method, it is necessary to remove the protective coating material 4 and expose the fiber core wire 3. 1A and 1B show a mode in which only the exposed portion 3c of the fiber core wire 3 from which the protective covering material 4 has been removed is inserted into the insertion hole 6 of the connection component 5, but this is not limitative. Of course, a mode in which a part of the region of the fiber core wire 3 covered with the protective coating material 4 is included in the insertion hole 6 of the connection component 5 may be used.

Further, the metal coating layer 9 is formed of, for example, a metal plating layer, and is a film having a rotationally symmetrical and substantially uniform thickness on the peripheral surface of the cladding 3b. There are few concerns about the resulting stress loading. In this example, since the metal coating layer 9 is formed on the peripheral surface of the cladding 3b of the exposed portion 3c of the fiber core wire 3, the adhesive 7 is applied to the cladding 3b when the LMA fiber 2 is bonded and fixed to the connection component 5. Since the contact with the peripheral surface of the metal coating layer 9 is not performed directly, the stress load due to the curing shrinkage of the adhesive 7 is effectively buffered by the metal coating layer 9, and as shown in FIG. It is distributed over the circumferential direction of the core wire 3 and acts as a substantially equivalent stress p _{0 in} the radial direction of the fiber core wire 3, thereby suppressing the stress load on the fiber core wire 3. For this reason, the output light intensity distribution of the LMA fiber 2 is unlikely to vary. In the region where the fiber core wire 3 that is still covered with the protective coating material 4 is inserted into the insertion hole 6 of the connection component 5, the protective coating material 4 is usually made of an elastic material, so that the adhesive 7 is cured. Even if contracted, the deformation is absorbed by the protective covering material 4 and the stress load on the fiber core wire 3 is suppressed.
In this respect, in the comparative form shown in FIG. 1D (the aspect not including the metal coating layer 9 shown in FIG. 1C), the thermosetting adhesive 7 extends in the circumferential direction of the fiber core wire 3. When the filling amount varies, the degree of cure shrinkage of the adhesive 7 is different, so that different stresses p _{1 to} p ₃ (for example, p ₁ > p ₂ > p ₃ ) act on the peripheral surface of the fiber core wire 3. As a result, a biased stress load acts on the peripheral surface of the fiber core wire 3, and the output light intensity distribution of the LMA fiber 2 is likely to vary. That is, in thermosetting adhesives such as thermosetting epoxy resins, curing shrinkage of about several percent is unavoidable, and the adhesive hardness is large, so that a large residual stress is generated on the exposed portion 3c of the fiber core wire 3. To do. The fiber core wire 3 is usually sensitive to the residual stress as described above because the difference in refractive index between the core 3a and the clad 3b is small, and the output light intensity distribution from the LMA fiber 2 is frequently varied. It was.
Furthermore, in this example, the metal coating layer 9 is formed on the peripheral surface of the fiber core wire 3, but there is little concern that the metal coating layer 9 affects the refractive index of the clad 3b, and light transmission by the fiber core wire 3 is performed. The performance is kept good.
Here, taking an example in which the metal coating layer 9 is a metal plating layer, for example, it is known that, as a general theory, plating generates stress of expansion and contraction, which causes peeling and swelling. . When considering the plating of the optical fiber peripheral surface, it is thought that the prescription that suppresses the generation of stress is to select a material that has high adhesiveness and does not easily peel off due to heating or the like, and to set appropriate conditions. . In addition, even if some expansion and contraction stress occurs, the stress distribution is rotationally symmetric, and it is assumed that the refractive index of the cladding 3b and the refractive index of the core 3a are increased or decreased equally. However, the beam is not distorted and the size change is expected to be slight. In this respect, in the aspect in which there is no coating with the metal coating layer 9, as described above, the stress of the thermosetting adhesive acts directly on the peripheral surface of the fiber core wire 3, but the stress of the thermosetting adhesive is It is considered that the problem is uneven stress with poor symmetry. That is, it is presumed that the metal coating layer 9 has less influence on the light transmission performance than the thermosetting adhesive having a non-uniform stress distribution.

Next, typical aspects or preferred aspects used in this embodiment will be described.
First, as a preferable aspect of the thickness of the metal coating layer 9, when the diameter of the clad 3b of the fiber core wire 3 is d (μm) and the thickness of the metal coating layer 9 is t (μm), t ≧ d / 10 is satisfied. An embodiment is mentioned. In this example, as a condition for the deformation of the cladding 3b to be relaxed by the metal coating layer 9, a condition for reducing the amount of elastic deformation of the cladding 3b alone supported by the metal coating layer 9 under a certain stress is obtained. It was found that t ≧ d / 10 was satisfied.
For example, if the cladding 3b diameter d of the fiber core wire 3 is 125 (μm), the thickness t of the metal coating layer 9 may be 13 μm or more.

Moreover, the aspect currently formed by metal plating as a typical aspect of the metal coating layer 9 is mentioned. Here, the metal plating may be appropriately selected including nickel, and when the metal coating layer 9 is formed by metal plating, the metal coating layer 9 having a substantially uniform thickness can be easily formed on the peripheral surface of the clad 3b. Is possible.
Furthermore, as a typical aspect of the pigtail fiber module 1, when the end cap 8 is fixed to the end of the exposed portion 3c of the fiber core wire 3 and the end cap 8 is disposed in the insertion hole 6 of the connecting component 5, The metal coating layer 9 includes an embodiment in which the peripheral surface of the end cap 8 is also covered. In this example, as a typical aspect of the pigtail fiber module 1, an end cap 8 is fixed to the end of the exposed portion 3 c of the fiber core wire 3. At this time, when the end cap 8 is disposed in the insertion hole 6 of the connection component 5, the metal cap layer 9 is also formed on the peripheral surface of the end cap 8, whereby the end cap 8 is cured and contracted by the adhesive 7. It is preferable at the point which can also suppress the stress load to.

Further, as a manufacturing method of the pigtail fiber module 1, when the pigtail fiber module 1 is manufactured, the protective covering material 4 located near the end of the LMA fiber 2 is removed before the connection between the LMA fiber 2 and the connection component 5. The plating step of covering the cladding 3b peripheral surface of the exposed portion 3c of the exposed fiber core wire 3 with a metal coating layer 9 made of metal plating, and the exposed portion 3c of the fiber core wire 3 of the LMA fiber 2 after the plating step are connected. And a connecting step of inserting and fixing the component 5 into the insertion hole 6.
In this example, the plating process is performed before the connection between the LMA fiber 2 and the connection component 5, and then the connection process for connecting the two is performed. Here, the plating step may be performed after removing the protective coating material 4 by removing the protective coating material 4 so that metal plating is formed on the portion where the fiber core wire 3 is exposed.
Furthermore, the pigtail fiber module 1 of the present embodiment is configured as various optical transmission devices by assembling optical components (for example, a light source such as an LD or a polarization-independent optical isolator core).

Hereinafter, the present invention will be described in more detail based on embodiments shown in the accompanying drawings.
Embodiment 1
FIG. 2A shows a configuration example of the pigtail fiber module according to the first embodiment.
In FIG. 1, a pigtail fiber module 10 includes an LMA fiber 20 and a ferrule 30 as a connection component connected to the LMA fiber 20.
In this example, as shown in FIGS. 2A and 2B, the LMA fiber 20 includes a core 21 a that propagates single-mode light and a fiber core 21 that includes a clad 21 b that surrounds the outer periphery of the core 21 a, and a fiber core 21. And a loose tube 22 as a protective covering material covering the outer periphery.
In this example, the loose tube 22 is removed from the vicinity of one end of the LMA fiber 20 to form an exposed portion 21 c of the fiber core wire 21. An end cap 23 made of, for example, a quartz glass rod is fused to the tip of the exposed portion 21c of the fiber core wire 21. This end cap 23 is for, for example, reducing the energy density at the output end of high-power light propagating through the fiber core wire 21 and preventing damage to the end surface of the fiber core wire 21. In adopting such a structure, a method of fusing the end cap 23 to the end portion of the LMA fiber 20 is often employed, and in order to fuse the end cap 23, the loose tube 22 of the LMA fiber 20 is removed. Thus, it is necessary to expose the fiber core wire 21, and the exposed portion 21 c of the fiber core wire 21 as described above becomes indispensable.

In this example, the ferrule 30 includes a metal ferrule 31 with a horizontal hole to which the vicinity of the end of the LMA fiber 20 is connected, and a portion surrounded by the loose tube 22 of the LMA fiber 20 can be inserted into the metal ferrule 31. A through hole 32 having an inner diameter h ₁ is formed, a stepped hole portion 33 is formed on the tip side of the through hole 32, and a thermosetting adhesive 38 is provided at a location other than the stepped hole portion 33 of the through hole 32. Is formed, and a cylindrical small ferrule 35 made of, for example, zirconia is fitted into and fixed to the stepped hole 33 of the metal ferrule 31 so that the inner diameter h ₂ ( In this example, the majority of the exposed portion 21c of the fiber core wire 21 and the end cap 23 of the LMA fiber 20 are inserted and disposed in the through hole 36 of h ₂ <h _1. It has become.
The ferrule 30 and the LMA fiber 20 are surrounded by the through hole 32 of the metal ferrule 31 near one end of the LMA fiber 20 (in this example, the exposed portion 21c of the fiber core wire 21 and the loose tube 22 adjacent thereto). In the region from the through-hole 32 of the metal ferrule 31 to the through-hole 36 of the small-diameter ferrule 35 by inserting the adhesive 38 from the lateral hole 34 of the metal ferrule 31. The gap between the through holes 32 and 36 and the peripheral surface near the end of the LMA fiber 20 is filled with an adhesive 38, and the vicinity of the end of the LMA fiber 20 is connected and fixed to the ferrule 30. Manufactured as a fiber module 10.

  Furthermore, in this embodiment, in order to avoid the light reflected by the tip surface of the end cap 23 from returning to the LMA fiber 20, the tip surface of the end cap 23 is predetermined as shown in FIG. Polishing is performed at an angle θ (several degrees). In this polishing process, the LMA fiber 20 to which the end cap 23 has been fused is inserted into the through holes 32 and 36 of the ferrule 30 and cured with a thermosetting adhesive 38 to produce the pigtail fiber module 10. In the ferrule 30, the tip surface of the end cap 23 is obliquely machined together with the tip surface of the small-diameter ferrule 35.

In particular, in the present embodiment, as shown in FIGS. 2A to 2C, a plating layer as a metal coating layer is formed on the exposed portion 21 c of the fiber core wire 21 and the peripheral surface of the end cap 23 of the LMA fiber 20. 40 is formed.
The reason why such a plating layer 40 is formed will be described below.
Now, the exposed portion 21c of the fiber core wire 21 with the end cap 23 of the LMA fiber 20 is inserted into the pigtail fiber module 10 according to the comparative embodiment 1 in which the plating layer 40 is not formed, that is, the through holes 32 and 36 of the ferrule 30. In the embodiment in which the thermosetting adhesive 38 is filled in the through holes 32 and 36 of the ferrule 30 in which the exposed portion 21c of the fiber core wire 21 is inserted, the intensity distribution of the output light is examined. It was found that the intensity distribution of the output light from the module 10 varies. The present inventors paid attention to the thermosetting adhesive 38 used for bonding and fixing the LMA fiber 20 and the ferrule 30 with respect to these factors. This type of thermosetting adhesive 38 inevitably undergoes cure shrinkage of several percent, and has a high adhesive hardness. Therefore, a large residual stress is generated on the exposed portion 21 c of the fiber core wire 21, and the fiber core wire 21. In the exposed portion 21c, the difference in refractive index between the core 21a and the clad 21b is small, so that it is sensitive to the residual stress as described above, and it is assumed that the intensity distribution of the output light from the LMA fiber 20 varies. .

Thus, when the stress generated in the LMA fiber 20 is subject to variations in the intensity distribution of the output light, the thermosetting adhesive is applied to the peripheral surface of the clad 21b of the exposed portion 21c of the fiber core wire 21. 38, the distribution of filling of the adhesive 38 on the circumferential surface of the clad 21b usually varies in the circumferential direction of the clad 21b, so that the shrinkage due to curing shrinkage of the adhesive 38 on the circumferential surface of the clad 21b. It is presumed that the stress acts as a local load, and a local tensile stress acts on the peripheral surface of the clad 21b, causing a variation in the intensity distribution of the output light.
Here, if the adhesive 38 is filled with a completely rotationally symmetric distribution with respect to the peripheral surface of the clad 21b, the stress distribution applied to the clad 21b is such that the stress applied to the core 21a is also rotationally symmetric. Thus, the roundness of the intensity distribution of the output light of the LMA fiber 20 is not impaired. However, the center of the fiber core 21 and the center of the through-holes 32 and 36 of the ferrule 30 do not usually coincide with each other, and the thickness of the adhesive 38 is often uneven in a crescent shape. As a result, the thick portion of the adhesive 38 undergoes significant curing shrinkage, resulting in a stress distribution in which the clad 21b is deformed into an ellipse, and a local stress load acts on the peripheral surface of the clad 21b.

  As a technique for relieving such stress load, a method using an adhesive having a small elastic modulus can be considered. However, if an adhesive having a small elastic modulus is used, the tensile strength of the LMA fiber 20 can be maintained at a sufficient value. There wasn't. Tensile strength was measured using various adhesives, but an adhesive that can withstand a tensile stress of 5 N or more could not be found with a soft adhesive having a small elastic modulus.

Therefore, in the present embodiment, as a technique for relieving the stress load described above, a technique of protecting the cladding 21b peripheral surface with a highly rigid buffering buffer layer is considered, and as shown in FIG. When a plating layer 40 made of, for example, Ni plating is applied to the surface as a metal coating layer, as shown in FIG. 3A, even if the adhesive 38 adheres and cures on the plating layer 40, there is no comparison with the plating layer 40. It was confirmed that the variation in the intensity distribution of the output light from the LMA fiber 20 hardly occurred as compared with the first embodiment. As described above, the thickness of the adhesive 38 on the peripheral surface of the clad 21b is not uniform, and the stress load F (F ₁ ≠ F ₂ ≠ F ₃ ≠ F ₄ ) varies in the circumferential direction of the clad 21b. even now things, biased stress load F is reduced by the plating layer 40, the stress p ₀ substantially equal distributed on the peripheral surface of the cladding 21b is assumed to act.
In this example, in the through holes 32 and 36 of the ferrule 30, the adhesive 38 is also attached to the portion of the LMA fiber 20 surrounded by the loose tube 22 adjacent to the exposed portion 21 c of the fiber core wire 21. Although it is cured, it has been confirmed that it does not affect the variation in the intensity distribution of the output light from the LMA fiber 20. A material with low hardness is selected for the loose tube 22, and it is considered that the residual stress of the adhesive 38 is sufficiently relaxed and does not directly act on the peripheral surface of the clad 21 b of the fiber core wire 21.

Further, as shown in FIG. 3A, even when the thickness t of the plating layer 40 is increased, the roundness of the intensity distribution of the output light of the fiber core wire 21 is impaired by the stress of the plating layer 40 itself. There was no. This is considered because the plating layer 40 forms a film having a uniform thickness t with rotational symmetry on the peripheral surface of the clad 21b. Since the plating layer 40 itself does not generate stress, it is considered that the buffer layer effect (buffer effect) increases as the plating layer 40 becomes thicker.
However, since the processing time is long to form the thick plating layer 40 and it is not preferable in terms of increasing the cost, in the present embodiment, what is the minimum thickness t necessary for the plating layer 40 should be set? We decided to examine the selection criteria for good.
In order for the deformation of the clad 21b to be alleviated by the plating layer 40, a condition is required in which the amount of elastic deformation of the clad 21b alone is supported by the plating layer 40 and significantly reduced under a certain stress.

Now, as shown in FIG. 3 (b), in the embodiment in which the plating layer 40 is applied around the fiber core wire 21, the fiber core wire 21 is a cylindrical member having a diameter d, and the plating layer 40 has an outer diameter D and an inner diameter d. Assuming a cylindrical member, the amount of strain δ when a vertical load F is applied from the radially outer side is obtained.
Here, as shown in FIG. 3C, when the strain amount of the cylindrical fiber core wire 21 is δ ₁ , the cross-sectional secondary moment is I, the Young's modulus is E _g , and a is a constant, It can be expressed like
δ ₁ = −ad ³ / E _g I (Formula 1)
And the cross-sectional secondary moment I of a cylinder becomes like the following formula 2.
I = πd ^4/64 ··· (Equation 2)
Further, as shown in FIG. 3D, when the strain amount of the cylindrical plating layer 40 is δ ₂ , the cross-sectional secondary moment is I, the Young's modulus is E _m , and a is a constant, It can be expressed as
δ ₂ = −aD ³ / E _m I (Formula 3)
Then, the cross-sectional secondary moment I of the cylinder is as shown in Equation 4 below.
I = π (D ⁴ −d ⁴ ) / 64 (Formula 4)
At this time, the strain amount δ ₁ of the cylindrical fiber core wire 21 is equal to the strain amount δ _{2 of the} cylindrical plating layer 40, and the strain amount δ ₁ of the fiber core wire 21 is reduced by half by the plating layer 40. When the condition is obtained, it can be expressed as the following Expression 5.
δ ₁ = δ ₂ (Formula 5)
Here, according to Equations 1 to 4,
δ ₁ = − (ad ³ / E _g ) · (64 / πd ⁴ ) = − (64a / π) / E _g d
δ ₂ = − (aD ³ / E _m ) · 64 / π (D ⁴ −d ⁴ )
=-(64a / π) · D ³ / E _m (D ⁴ -d ⁴ )
Therefore, when these are applied to the expression 5, the following expression 5 ′ is obtained.
1 / E _g d = D ³ / E _m (D ⁴ −d ⁴ ) (Formula 5 ′)
This can be summarized as shown in Equation 6 below.
E _m D ⁴ −E _g d D ³ −E _m d ⁴ = 0 (Formula 6)
In other words, the thickness t ((D -D) / 2) may be selected.

In the present embodiment, Young's modulus E _g of the fiber core wire 21 (for example, Young's modulus of glass: 7.1 × 10 ³ N / mm ² ), Young's modulus E _m of the plating layer 40 (for example, Young's modulus of Ni: 20. 4 × 10 ³ N / mm ² ), the results shown in Table 1 below were obtained.
In Table 1, it was calculated whether or not Expression 6 was satisfied while changing the thickness t of the plating layer 40 for each cladding diameter d, and the minimum necessary value was obtained as the thickness t of the plating layer 40. .
According to Table 1, it was confirmed that the cladding diameter d and the thickness (plating thickness) t of the plating layer 40 are in a proportional relationship, and at least the plating thickness should be 1/10 or more of the cladding diameter d.

In addition, in order to confirm the results in Table 1, the following tests were performed using LMA fibers having each cladding diameter d.
An LMA fiber having a clad diameter d of, for example, 100 μm was used, and the plating thickness t was changed to 2 μm, 5 μm, 10 μm, 15 μm, and 20 μm, and was bonded and fixed to a metal ferrule with an inner diameter of 250 μm with a two-component thermosetting epoxy resin. After the adhesive was heat-cured, the LMA fiber end face was obliquely polished.
When the roundness was measured with respect to the intensity distribution of the output light of the test product thus produced, a satisfactory value was obtained when the plating thickness t was 10 μm or more. In addition, when the same test was done using the LMA fiber of other clad diameter d (125 micrometers-500 micrometers), the result shown in Table 1 was supported.
Therefore, it is considered that the light output of the LMA fiber can be kept good if the stress caused by the curing shrinkage of the adhesive 38 is reduced by about half by the plating layer 40.

Example 1
This example embodies the pigtail fiber module according to the first embodiment.
First, the loose tube 22 as a protective coating material was removed from the LMA fiber 20 (core diameter φ10 μm, clad diameter φ125 μm), and an end cap 23 was fused to the tip of the exposed portion 21c of the fiber core wire 21 as the coating removal portion. Next, a plating layer 40 by nickel (Ni / Au in this example) plating was applied to the coating removal portion of the LMA fiber 20 and the peripheral surface of the end cap 23 so that the plating thickness was 13 μm.
The plated LMA fiber 20 is inserted into a metal ferrule 31 with a horizontal hole having a maximum inner diameter of 0.25 mm into which a zirconia ferrule as a small-diameter ferrule 35 having an inner diameter of a through-hole 36 of 0.2 mm is inserted, for resin filling An adhesive 38 made of a thermosetting epoxy resin was poured from the lateral hole 34 as an adhesive fixing resin, and the adhesive 38 was heated and cured, and then the tip of the zirconia ferrule 35 was polished together with the end cap 23 to produce the pigtail fiber module 10. .

FIG. 4 shows a cross section of a through hole portion having a maximum inner diameter of 0.25 mm of the metal ferrule 31 with a horizontal hole of the pigtail fiber module 10 actually manufactured in this example.
According to the figure, the LMA fiber diameter including the plating layer 40 having a thickness of 13 μm is 151 μm, but the maximum inner diameter of the through hole 32 of the metal ferrule 31 with a horizontal hole is as large as 0.25 mm. It is understood that the center of is not coaxial and the thickness of the adhesive 38 is biased. At this time, since the thickness of the adhesive 38 deviates from perfect rotational symmetry, it is considered that the curing shrinkage due to the adhesive 38 generates a stress that causes the plating layer 40 of the LMA fiber 20 to deform flatly. Since the pigtail fiber module 10 according to the example suppresses the stress load on the LMA fiber 20 by the metal coating layer made of the plating layer 40, the intensity distribution of the output light from the LMA fiber 20 does not vary.

Here, the results of measuring the output light from the LMA fiber 20 before and after curing of the adhesive with a beam profiler (apparatus for measuring the intensity distribution and shape of the output light) are shown in FIGS. Since the output light from the LMA fiber 20 is originally single mode light, the ideal shape is a perfect circle, and the intensity distribution is a Gaussian beam represented by a Gaussian distribution.
Before and after curing the adhesive, the roundness (1 is ideal) in the intensity distribution of the output light from the LMA fiber 20 is 0.98, and the Gaussian fitting value (how close to the ideal Gaussian distribution of the X and Y components) 1 is an ideal index indicating whether or not) is 0.95 and no change is seen, and by using the structure of this example, the intensity distribution of the output light from the LMA fiber 20 is almost unchanged. It was.
In addition, in the pigtail fiber module 10 according to the present embodiment, when the LMA fiber 20 is generally pulled from the module 10, a tensile strength of about 10 N (that is, when the LMA fiber 20 is pulled from the module 10 to be pulled out, What degree of tensile resistance is required). When the tensile strength of the prototype of this example was measured, a result of 20 N or more was obtained.

◎ Comparative Example 1
In Comparative Example 1, a pigtail fiber module was produced in the same manner as in Example 1 except that the coating removal portion of the LMA fiber was not plated.
Similar to Example 1, the results of measuring the output light from the LMA fiber before and after curing of the adhesive with a beam profiler are shown in FIGS.
In the state before the adhesive is cured, as shown in FIG. 6A, the roundness in the intensity distribution of the output light from the LMA fiber is 0.99 and the Gaussian fitting value is 0.95, which are high values. In contrast, as shown in FIG. 6B, it is understood that the roundness is 0.94 and the Gaussian fitting value is 0.89 after the adhesive is cured. As described above, when the intensity distribution of the output light from the LMA fiber varies, there is a concern that a large loss may occur when the output light from the LMA fiber is condensed and reentered into the optical fiber.

  According to the pigtail fiber module according to the present embodiment, since the intensity distribution of the output light from the LMA fiber does not vary, it is applied as, for example, a part of a laser head in a processing laser device or a part of an optical connector. It is possible to construct an optical transmission device widely by combining with other optical components.

1 Pigtail fiber module 2 Large mode area fiber (LMA fiber)
DESCRIPTION OF SYMBOLS 3 Fiber core wire 3a Core 3b Clad 3c Exposed part of fiber core wire 4 Protective coating material 5 Connection component 6 Insertion hole 7 Adhesive 8 End cap 9 Metal coating layer 10 Pigtail fiber module 20 Large mode area fiber (LMA fiber)
21 Fiber core wire 21a Core 21b Clad 21c Exposed portion of fiber core wire 22 Loose tube 23 End cap 30 Ferrule 31 Metal ferrule 32 Through hole 33 Stepped hole 34 Horizontal hole 35 Small diameter ferrule 36 Through hole 38 Adhesive 40 Plating layer (metal coating) layer)

Claims (7)

A large mode area fiber having a core that propagates a single mode light and a fiber core wire made of a clad surrounding the outer periphery of the core, and a protective coating material that covers the outer periphery of the fiber core wire;
An end of the large mode area fiber is connected, and there is an insertion hole into which the exposed portion of the fiber core wire exposed by removing the protective coating located near the end of the large mode area fiber can be inserted A connection part for inserting and fixing the exposed portion of the fiber core wire into the insertion hole, and
A pigtail fiber module, wherein a cladding peripheral surface of an exposed portion of the fiber core wire is coated with a metal coating layer. The pigtail fiber module according to claim 1, wherein
A pigtail fiber module satisfying t ≧ d / 10, where d (μm) is the cladding diameter of the fiber core wire and t (μm) is the thickness of the metal coating layer. The pigtail fiber module according to claim 1, wherein
If the clad diameter of the fiber core wire is 125 (μm), the pigtail fiber module is characterized in that the thickness of the metal coating layer is 13 μm or more. The pigtail fiber module according to claim 1, wherein
The pigtail fiber module, wherein the metal coating layer is formed by metal plating. The pigtail fiber module according to claim 1, wherein
An end cap is fixed to the end of the exposed portion of the fiber core wire,
The pigtail fiber module, wherein when the end cap is disposed in the insertion hole of the connection component, the metal coating layer also covers the peripheral surface of the end cap. A large mode area fiber having a core that propagates a single mode light and a fiber core wire made of a clad surrounding the outer periphery of the core, and a protective coating material that covers the outer periphery of the fiber core wire;
An end of the large mode area fiber is connected, and there is an insertion hole into which the exposed portion of the fiber core wire exposed by removing the protective coating located near the end of the large mode area fiber can be inserted , When manufacturing a pigtail fiber module comprising a connecting part for inserting and fixing the exposed portion of the fiber core wire into the insertion hole,
Before connecting the large mode area fiber and the connecting component, the cladding peripheral surface of the exposed portion of the fiber core wire exposed by removing the protective coating located near the end of the large mode area fiber is made of metal. A plating step of coating with a metal coating layer made of plating;
A method for manufacturing a pigtail fiber module, comprising: a connecting step of inserting and fixing an exposed portion of the fiber core wire of the large mode area fiber that has undergone the plating step into an insertion hole of the connecting component. Pigtail fiber module according to any of claims 1 to 5,
And an optical component assembled to the pigtail fiber module.