JP5346665B2 - Recoating method of double clad optical fiber - Google Patents

Description

  The present invention relates to a recoating method for a double-clad optical fiber that recoats a portion where a second clad layer and a protective coating layer have been removed so that a double-clad optical fiber with a reduced amount of lost light can be obtained.

  A single clad optical fiber has a clad layer surrounding a core, and a typical example is shown in FIG. FIG. 12 is a radial cross-sectional view illustrating a single clad optical fiber. The single clad optical fiber 9 shown here includes a core 91, a clad layer 92 surrounding the core 91, a first protective coating layer 93 surrounding the cladding layer 92, and a second protective coating layer 94 surrounding the first protective coating layer 93. It is roughly composed of In such a single clad optical fiber, the intensity of propagating light is usually extremely small near the surface of a clad layer made of quartz glass. Therefore, even if there is a foreign substance at the interface between the clad layer 92 and the first protective coating layer 93 or in the vicinity thereof, a large loss of propagation light due to scattering does not occur. Therefore, in the single clad optical fiber 9 from which the coating (the first protective coating layer 93 and the second protective coating layer 94) has been removed, the exposed cladding layer 92 is coated with a resin and recoated for recoating the cladding layer 92. Even if there is a foreign substance on the surface of the optical fiber, an optical fiber having good optical characteristics can be obtained. Examples of the recoating method of the single clad optical fiber include a molding method (see Patent Document 1), a die method (see Patent Document 2), and a casting method (see Patent Document 3).

  On the other hand, FIG. 1 is a radial cross-sectional view illustrating a double clad optical fiber. The double clad optical fiber 1 shown here includes a core 11, a first clad layer 12 surrounding the core 11, a second clad layer 13 surrounding the first clad layer 12, and a protective coating layer 14 surrounding the second clad layer 13. It is roughly composed of In general, the core 11 and the first cladding layer 12 are mainly composed of quartz glass, and the second cladding layer 13 and the protective coating layer 14 are mainly composed of a cured product of a curable resin. In the double clad optical fiber 1, the first propagation light propagates in the core 11 and the second propagation light propagates in the first cladding layer 12. However, the intensity of the propagation light is near the surface 12 a of the first cladding layer 12. large. Therefore, if there is a foreign substance at the interface between the first cladding layer 12 and the second cladding layer 13 or in the vicinity thereof, the second propagation light is scattered, and as a result, a large light loss is likely to occur.

FIG. 2 is a longitudinal sectional view of a plane passing through the central axis of the double clad optical fiber 1.
Here, in the double clad optical fiber 1, a part of the second clad layer 13 and the protective coating layer 14 are respectively removed, and the first clad layer 12 is exposed at the removed portion, and the coating removal portion 10 is formed. ing. In the coating removal unit 10, reference numeral 13 a indicates an exposed end portion of the second cladding layer 13, and reference numeral 14 a indicates an exposed end portion of the protective coating layer 14.
Usually, in the coating removal part 10, the foreign material 8 is often present on the surface 12a of the first cladding layer. This is because when the second clad layer 13 and the protective coating layer 14 are removed, the resin waste derived from one or both of these tends to remain without being removed. Further, the exposed end portion 13a of the second cladding layer is easily altered and its surface is easily roughened.

Here, the loss of the second propagation light in the coating removal unit 10 will be described.
FIG. 13 is an enlarged cross-sectional view illustrating the coating removal portion 10 of FIG. 2 in which resin is recoated by a conventional method. FIG. 13A shows the surface 12a of the first cladding layer and the vicinity thereof, and FIG. It is sectional drawing which illustrates the exposed end part 13a and its vicinity of a 2nd clad layer, respectively.

First, as shown in FIG. 13A, the case where the foreign matter 8 is present on the surface 12a of the first cladding layer will be described. The surface of the foreign material 8 usually has complex irregularities, and when the curable resin is laminated in the coating removing unit 10, the curable resin hardly flows into the concave portion 80, and gas such as air tends to remain. When the curable resin is cured in this state, the recoating is completed while the gas is confined in the recess 80, and the void 7 is mixed on the surface 12 a of the first cladding layer in the recoat layer 15. And the 2nd propagation light is scattered by the space | gap part 7, and a big optical loss will arise.
Next, as shown in FIG. 13B, a case where the surface of the exposed end portion 13a of the second cladding layer is rough will be described. The exposed end portion 13a has complicated unevenness, and when the curable resin is laminated at the coating removing portion 10, the curable resin hardly flows into the concave portion 130, and gas tends to remain. When the curable resin is cured in this state, the recoating is completed while the gas is confined in the recess 130 as described above, and the void 7 is mixed in the second cladding layer 13. And when the space | gap part 7 exists in the vicinity of the surface 12a of a 1st cladding layer, 2nd propagation light is scattered by the space | gap part 7, and a big optical loss will arise. Since the exposed end portion 14a of the protective coating layer is located far from the surface 12a of the first cladding layer, there is usually no problem even if the surface is rough.

JP 2003-128440 A JP 09-043446 A Japanese Patent Application Laid-Open No. 60-103053

  As described above, in a double-clad optical fiber, a large optical loss occurs due to the presence of voids on the surface of the first cladding layer or in the vicinity thereof, but this is not seen in a single-clad optical fiber. This is a problem peculiar to clad optical fibers. Conventionally, the same recoating method as that of the single clad optical fiber is applied to the double clad optical fiber, and there is a problem that there is no effective recoating method for solving the above problems.

  The present invention has been made in view of the above circumstances, and provides a method of recoating a double-clad optical fiber so as to reduce the amount of lost light by forming a recoat layer while suppressing the mixing of voids. Is an issue.

To solve the above problem,
The present invention comprises a core, a first cladding layer surrounding the core, and a coating layer covering the first cladding layer, wherein the coating layer has a second cladding layer surrounding the first cladding layer. A method of recoating a clad optical fiber, the step of removing the coating layer to expose the first clad layer, and laminating a curable resin on the exposed first clad layer under reduced pressure, And a step of curing the curable resin under normal pressure to form a recoat layer. A method of recoating a double clad optical fiber is provided.
In the recoating method of the double clad optical fiber of the present invention, it is preferable that the curable resin is laminated on the first clad layer while keeping the temperature at a temperature at which the curable resin is not decomposed and modified.
In the method of recoating a double clad optical fiber according to the present invention, it is preferable to cure the curable resin while measuring the amount of light loss of propagation light in the first clad layer.
In the double-clad optical fiber recoating method of the present invention, the coating layer is preferably removed by laser irradiation.
In the method of recoating a double clad optical fiber of the present invention, it is preferable to remove the coating layer while cooling to a temperature at which the second clad layer is not denatured.
In the double-clad optical fiber recoating method of the present invention, it is preferable that the curable resin is an ultraviolet curable resin and the second cladding layer is formed by curing the ultraviolet curable resin.
Moreover, it is preferable that the recoating method of the double clad optical fiber of this invention forms the said recoat layer by the mold method, the die method, or the casting method.

  According to the present invention, the double clad optical fiber can be recoated so as to reduce the amount of lost light.

It is sectional drawing of the radial direction which illustrates a double clad optical fiber. It is sectional drawing of the longitudinal direction by the plane which passes along the central axis of a double clad optical fiber. It is the schematic for demonstrating the process of removing the coating layer of a double clad optical fiber by laser irradiation. It is the schematic for demonstrating the process of removing the coating layer of a double clad optical fiber by blowing hot air. It is the schematic which illustrates the state which installed this optical fiber in the cooling means via the heat conductor in order to cool a double clad optical fiber. It is an expanded sectional view of the coating removal part by the plane which passes along the central axis of a double clad optical fiber for demonstrating a recoat process, (a) is an expanded sectional view under pressure reduction at the time of curable resin lamination | stacking, (b) Is an enlarged cross-sectional view under normal pressure before the curable resin is cured, and (c) is an enlarged cross-sectional view after the curable resin is cured. It is a schematic block diagram which illustrates the measurement system for measuring the loss light quantity of a double clad optical fiber. It is the schematic for demonstrating the process of applying a dice method and forming a recoat layer. It is the schematic for demonstrating the process of forming a recoat layer by applying a molding method. It is a schematic block diagram which illustrates the clad pump system fiber laser produced by applying the recoat method of the present invention. It is a graph which shows the relationship between the UV light irradiation time with respect to an ultraviolet curable resin in the recoating process of Example 19, and the loss light quantity of the propagation light of a 1st clad layer. It is sectional drawing of the radial direction which illustrates a single clad optical fiber. FIG. 3 is an enlarged cross-sectional view illustrating the coating removal portion of FIG. 2 on which resin is recoated by a conventional method, where (a) shows the surface of the first cladding layer and its vicinity, and (b) shows the exposure of the second cladding layer. It is sectional drawing which illustrates an edge part and its vicinity, respectively.

The double-clad optical fiber recoating method of the present invention comprises a core, a first cladding layer surrounding the core, and a coating layer covering the first cladding layer, and the coating layer includes the first cladding layer. A method of recoating a double clad optical fiber having a surrounding second cladding layer, the step of removing the coating layer to expose the first cladding layer (hereinafter abbreviated as a removal step), and under reduced pressure And a step of laminating a curable resin on the exposed first clad layer and then curing the curable resin under normal pressure to form a recoat layer (hereinafter abbreviated as a recoat step). It is characterized by that.
In the present invention, “recoating” means that the coating layer is removed, a resin is laminated (coated) on the exposed first cladding layer, the resin is cured, and then the coating is performed again. In addition, unless otherwise specified, the “coating layer” refers to not only the protective coating layer but also the entire layer covering the first cladding layer, including the second cladding layer. Further, “denaturation” refers to “losing the original property”.

As a method for coating an optical fiber with a curable resin, for example, a method in which a curable resin is applied to an optical fiber under reduced pressure and cured is known. The purpose of this method is to suppress the remaining of bubbles in the cured resin by removing the gas entrained or dissolved in the resin before curing. This method is applied to the coating of an optical fiber immediately after spinning at the time of manufacturing an optical fiber, and does not particularly take into account the recoating of the resin. Loss cannot be reduced.
On the other hand, the present invention not only removes the gas that is entrained or dissolved in the resin before curing, but also has foreign matter on the surface of the first cladding layer or in the vicinity thereof, Even if the surface of the exposed end portion of the two clad layers is rough, the double-clad optical fiber in which the loss of propagating light is reduced can be obtained by erasing the gap portion of the portion.

  The double clad optical fiber used for recoating may be a known one. That is, it has a core, a first cladding layer, a second cladding layer, and a protective coating layer, for example, the one shown in FIG. 1, wherein the core and the first cladding layer are mainly composed of quartz glass, and the coating layer Includes those having a cured product of a curable resin as a main component. Examples of the curable resin for forming the coating layer include a photocurable resin and a thermosetting resin, and examples thereof include the same curable resins as those for forming a recoat layer described later. Among the coating layers, the second cladding layer is preferably a photocurable resin and more preferably an ultraviolet curable resin because it can be cured in a short time and has good optical characteristics. However, the second cladding layer has a lower refractive index than the first cladding layer.

(Removal process)
In the removing step, the coating layer of the double clad optical fiber is removed to expose the first clad layer.
The covering layer may be removed at least at a part in the central axis direction of the double clad optical fiber. And a part may be removed in the outer peripheral direction of a double clad optical fiber, and all may be removed in an outer peripheral direction.
Hereinafter, the double clad optical fiber shown in FIG. 1 is used to explain the case where the coating layer is completely removed in the outer peripheral direction and the first clad layer is exposed as shown in FIG. 2, but also in other cases, Similarly, the coating layer can be removed.

  What is necessary is just to remove the 2nd clad layer 13 and the protective coating layer 14 which are coating layers by a well-known method. Among them, preferable methods include (a) removal by laser irradiation, (b) removal by blowing hot air, (c) cutting removal using a cutting tool such as a knife or razor (hereinafter referred to as “removal by a cutting tool”) (Abbreviated) and the like. More details are as follows.

(A) Removal by Laser Irradiation FIG. 3 is a schematic diagram for explaining a process of removing the coating layer by laser irradiation.
The laser irradiation apparatus 2 exemplified here includes a laser oscillator 21, a slit 22, a mirror 23, a lens 24, and a stage 25. The stage 25 is movable along the longitudinal direction (direction of arrow A) of the fixed double clad optical fiber 1.
The laser 20 oscillated from the laser oscillator 21 passes through the slit 22 and reaches the mirror 23, is reflected and redirected, and is condensed by the lens 24, and is then applied to a predetermined portion of the double clad optical fiber 1. It comes to be irradiated for hours. The mirror 23 and the lens 24 are installed on the stage 25, and move with the movement of the stage 25. Thereby, the irradiation location of the laser 20 is moved along the longitudinal direction of the fixed double clad optical fiber 1, and the length of the coating removal part 10 can be adjusted easily. The moving distance of the stage 25 is substantially the same as the length of the coating removal unit 10. The double clad optical fiber 1 can be further changed in the irradiation position of the laser 20 by rotating in the outer circumferential direction and changing the direction and fixing.
Note that the laser irradiation apparatus shown here is an example, and any configuration may be used as long as the coating layer can be removed.

  The type of laser and irradiation conditions may be adjusted as appropriate according to the type of coating layer to be removed. For example, an infrared laser, a visible light laser, an ultraviolet laser, an X-ray laser, etc. can be exemplified, and the medium is not particularly limited, and a solid laser such as a ruby laser or a YAG laser; a liquid laser; a carbon dioxide laser, a helium neon laser, an argon gas laser, A gas laser such as an excimer laser; a semiconductor laser or a free electron laser may be used, and these may be irradiated under conditions suitable for removing the coating layer. Of these, excimer lasers such as KrF and ArF, and argon gas lasers are preferable.

(B) Removal by blowing hot air FIG. 4 is a schematic view for explaining a process of removing the coating layer by blowing hot air.
The hot air blowing device 3 illustrated here includes a nozzle 32, a pipe 31 that sends compressed air 30 ′ sent from a compressor (not shown) to the nozzle 32, and a compressed gas that is provided in the nozzle 32 and blown from the nozzle 32. And a heater 33 for heating the air to hot air 30. The nozzle 32 is movable along the longitudinal direction (direction of arrow B) of the fixed double clad optical fiber 1.
The hot air 30 sent from the nozzle 32 is blown to a predetermined portion of the double clad optical fiber 1 for a predetermined time by appropriately setting the direction of the nozzle 32. And by moving the nozzle 32, the location where the hot air 30 is blown can be moved along the longitudinal direction of the fixed double-clad optical fiber 1 so that the length of the coating removal portion 10 can be easily adjusted. It has become. The moving distance of the nozzle 32 is substantially the same as the length of the coating removal unit 10. As in the case of the laser irradiation, the double-clad optical fiber 1 can be rotated in the outer peripheral direction and fixed in a different direction to further change the location where the hot air 30 is blown. Yes.
Note that the hot air blowing device shown here is merely an example, and any configuration may be used as long as the coating layer can be removed.

In order to suppress denaturation of the coating layer, the type of hot air gas is preferably an inert gas, and particularly preferably nitrogen gas.
The amount of hot air is not particularly limited as long as the coating layer can be removed, but is preferably adjusted according to the inner diameter of the nozzle 32. For example, when the inner diameter of the nozzle 32 is 1 to 7 mm, it is preferably 100 to 400 L / min.

  Among them, (a) removal by laser irradiation is preferable because the operation is simple, the removal effect is high, and the high effect of suppressing the generation of resin waste is obtained. In the laser irradiation, the second cladding layer 13 and the protective coating layer 14 can be removed by an ablation effect.

  In the removing step, it is preferable to cool and remove the coating layer to a temperature at which the second cladding layer is not denatured. Thereby, the modification | denaturation of the 2nd cladding layer 13, especially the exposed end part 13a of a 2nd cladding layer is suppressed, and the fluctuation | variation of the refractive index of a coating layer is suppressed. Further, even if foreign matter 8 derived from the second cladding layer is generated and remains on the surface 12a of the first cladding layer or in the vicinity thereof, the foreign matter 8 is not denatured. It is equivalent to the layer 13. Therefore, when the recoat layer is formed using the same curable resin as that used for forming the second cladding layer 13 and the protective coating layer 14, the recoat layer is the second cladding layer 13 and the protective coating layer 14. It has the same optical characteristics as Therefore, the propagation light can be effectively confined, and a higher effect of suppressing the loss of the propagation light can be obtained.

  In order to suppress the modification of the second cladding layer, the temperature of the second cladding layer may be adjusted to a temperature lower than the modification temperature, and cooling is preferable. The temperature to be adjusted at this time varies depending on the type of the second cladding layer and cannot be generally specified. For example, in the case of the second cladding layer formed using a fluorinated acrylate resin, the temperature is 200 ° C. The following is preferable. Further, by cooling to a temperature lower than the glass transition temperature of the resin forming the second cladding layer, a high effect of suppressing deformation of the second cladding layer can be obtained.

  Examples of the method for cooling the coating layer include a method of immersing a target part of a double clad optical fiber in a coolant, a method of installing the target part on a cooling means, and a method of blowing cold air on the target part. Selection may be made according to the method of removing the layer.

When the target portion is immersed in the coolant, for example, the optical fiber may be immersed in the coolant together with the fixing means that fixes the optical fiber.
When the coating layer is removed by laser irradiation, the target portion of the optical fiber in the refrigerant may be directly irradiated. And when irradiating with an excimer laser, a coating layer can be stably removed if it irradiates from a direction substantially perpendicular to the liquid level of the refrigerant. Further, when the target portion of the optical fiber is immersed so that the depth of the coating layer surface from the liquid surface is about 5 mm or less, the coating layer can be removed stably.
A preferable example of the refrigerant is liquid nitrogen.

When the target portion is installed on the cooling means, it may be placed in direct contact with the cooling means, or may be installed via a heat conductor such as metal or alloy.
FIG. 5 is a schematic view illustrating a state in which the double clad optical fiber 1 is installed in the cooling means 28 via the plate-like heat conductor 29. When a thermal conductor is interposed, the double clad optical fiber 1 can be efficiently cooled by using a plate-like one as shown here.
Preferable examples of the cooling means include Peltier elements.

When blowing cool air to the target part, it is preferable to stabilize the blowing part using a fan or a nozzle. Further, the double clad optical fiber needs to be fixed, and for example, it is preferable to apply a tension of about 40 to 200 gf, preferably about 150 gf. By doing so, the shaking of the optical fiber can be prevented and the coating layer can be removed stably.
The kind of gas of the cold air is not particularly limited, and may be appropriately selected from those available such as air and nitrogen. Of these, an inert gas is preferable, and nitrogen, argon, helium, and the like are particularly preferable. By using the inert gas, the chemical reaction between the components of the coating layer and the cold air gas is suppressed, so that the reaction product does not adhere to the surface of the first cladding layer or the vicinity thereof.
The amount of cold air is not particularly limited as long as it has a cooling effect.

By removing the coating layers (second cladding layer 13 and protective coating layer 14) in this way, a part of the surface 12a of the first cladding layer is exposed as shown in FIG.
FIG. 2 shows a double-clad optical fiber from which the coating layer has been removed, in which foreign matter 8 is present on the surface 12a of the first cladding layer and the surface of the exposed end portion 13a of the second cladding layer is rough. However, only one of these conditions may be satisfied, or there may be a foreign material 8 on the surface of the exposed end portion 13a of the second cladding layer.

(Recoat process)
After the removing step, a recoating step is then performed. FIG. 6 is an enlarged cross-sectional view of the coating removal portion by a plane passing through the central axis of the double clad optical fiber 1 for explaining the recoating process, and (a) is an enlarged cross-section under reduced pressure when the curable resin is laminated. FIG. 5B is an enlarged cross-sectional view under normal pressure before the curable resin is cured, and FIG. 5C is an enlarged cross-sectional view after the curable resin is cured.
In the recoating step, first, a curable resin 15 ′ is laminated on the exposed first cladding layer 12 under reduced pressure. At this stage, as shown in FIG. 6A, the curable resin 15 ′ hardly flows into the concave portion 80 on the surface of the foreign material 8 and the concave portion 130 of the exposed end portion 13a of the second cladding layer. 'May be formed. That is, the generation of the void 7 ′ cannot be completely suppressed by simply laminating the curable resin 15 ′ under reduced pressure.

  The curable resin 15 ′ is for forming a recoat layer, and can be arbitrarily selected from known materials such as a photocurable resin and a thermosetting resin according to the purpose. Usually, it is preferable to use a resin capable of forming a layer having the same refractive index as that of the second cladding layer 13, and it is more preferable to use a resin capable of forming the same layer as the second cladding layer 13. By doing in this way, the further high effect which suppresses scattering of propagation light and reduces loss light quantity is acquired. The curable resin 15 'is preferably a photocurable resin and more preferably an ultraviolet curable resin because it can be cured in a short time and has good optical properties.

  The method for laminating the curable resin 15 ′ is not particularly limited as long as it can be performed under reduced pressure, and can be selected from known methods according to the purpose. Specifically, a dipping method, a method using a brush, or the like may be used. Preferred methods include (i) a die method, (ii) a molding method, and (iii) a casting method.

  The pressure under reduced pressure when the curable resin 15 'is laminated is preferably as small as possible, but is preferably 100 Pa or less, more preferably 50 Pa or less, and particularly preferably 20 Pa or less. The lower limit of the pressure is not particularly limited, and may be a vacuum or a realizable pressure. By setting it as such a pressure, the higher effect which suppresses mixing of the cavity part in the hardened resin is acquired so that it may mention later.

  The viscosity of the curable resin 15 ′ at the time of lamination is preferably as low as possible without hindering the lamination, preferably 20 to 3000 mPa · s, and more preferably 1500 to 2500 mPa · s. By setting it as such a viscosity, even if there exists the foreign material 8 in the coating removal part 10 or the surface of the exposed end part 13a of a 2nd cladding layer is rough, these recessed parts 80 and the recessed part 130 are hardened | cured. The curable resin 15 ′ can easily flow in, and a high effect of suppressing the generation of the void portion 7 ′ can be obtained in the layering stage of the curable resin 15 ′. Furthermore, since the curable resin 15 ′ easily flows into the recess 80 and the recess 130, it is possible to shorten the time until the curing is performed after returning to normal pressure, which will be described later.

The viscosity of the curable resin 15 ′ can be adjusted by appropriately selecting the type of resin. Furthermore, you may adjust a viscosity by heating curable resin 15 'at the time of lamination | stacking. Viscosity adjustment by heating is particularly suitable in that the choice of curable resin 15 ′ that can be used is widened.
When heating the curable resin 15 ', it is preferable to set the temperature so that the curable resin 15' is not decomposed and modified. Moreover, when curable resin 15 'is a thermosetting resin, it is preferable to set it as temperature lower than the temperature which hardens | cures further. And it is preferable to laminate | stack curable resin 15 ', heat-retaining at the above temperatures. The temperature at which the curable resin 15 ′ is not decomposed and denatured, and the temperature at which the curable resin 15 ′ is cured vary depending on the type of resin.

  When laminating while keeping the curable resin 15 ′ warm, a part of the laminating apparatus that retains the curable resin 15 ′ and / or a route for transporting it before lamination is kept warm, or the curable resin 15 is laminated before lamination. You can just blow warm air on.

  Next, in the recoating step, the curable resin 15 ′ is cured under normal pressure to form a recoating layer. Thus, in the present invention, before the curable resin 15 ′ is cured, the reduced pressure is released and the pressure is returned to the normal pressure, so that the gap portion 7 ′ becomes the curable resin 15 as shown in FIG. 6B. Filled with 'and disappears. This is because the inside of the gap 7 'is depressurized, whereas the curable resin 15' is placed under normal pressure and a pressure difference is generated. Then, by setting the pressure and viscosity when the curable resin 15 ′ is laminated as described above, the gap 7 ′ can be more easily lost.

  The recoat layer can be formed by curing the curable resin 15 ′ by a method suitable for the type. As a result, as shown in FIG. 6C, the optical fiber 1 in which the curable resin 15 'is the recoat layer 15 is obtained. Thus, according to the present invention, even if the foreign substance 8 is present in the coating removal portion 10 or the surface of the exposed end portion 13a of the second cladding layer is rough, the recoat layer 15 has a mixture of voids. Since it is suppressed, the amount of lost light can be reduced.

  The time from the return to normal pressure to the curing is not particularly limited as long as it is longer than the time required for disappearance of the gap 7 ′, that is, the filling of the curable resin 15 ′ into the gap 7 ′. However, it is preferable to adjust appropriately in consideration of the viscosity of the curable resin 15 ′, the pressure at the time of decompression, the shape of the concave portion 80 and the concave portion 130, and the like. For example, when the viscosity of the curable resin 15 ′ is low, when the pressure during decompression is high, or when the concave portion 80 and the concave portion 130 have a complicated shape (for example, when the concave portion surface has further irregularities), The time until curing may be long, and conversely, when the viscosity of the curable resin 15 ′ is high, when the pressure during decompression is low, when the concave portion 80 and the concave portion 130 have a simple shape (for example, If the surface of the recess is smooth), it may be shorter. Usually, when the viscosity is 1500 to 2300 mPa · s at 25 ° C. and the pressure at the time of reduced pressure is about 50 Pa or less, the time from returning to normal pressure until curing is 2 to 30 minutes. Is preferred.

The refractive index of the recoat layer varies depending on the degree of cure of the curable resin 15 ′. Therefore, it is preferable to adjust the degree of curing so that a desired refractive index is obtained. For that purpose, it is preferable to cure the curable resin 15 ′ while measuring the amount of light loss of the propagation light of the first cladding layer 12. . In general, it is preferable that the first clad layer 12 is cured until the amount of loss of propagating light is zero or minimized.
The loss light amount of the propagation light in the first cladding layer 12 can be measured by a measurement system for measuring the loss light amount of the double clad optical fiber. FIG. 7 is a schematic configuration diagram illustrating such a measurement system.

The measurement system 6 exemplified here is schematically constituted by a light source 61, a double clad optical fiber 1, a photodetector 63 and an optical fiber 62.
The light source 61 is obtained by optically connecting a laser diode controller 61a and a laser diode mount 61b via an optical fiber 61c. Moreover, as the photodetector 63, a power meter etc. can be illustrated. However, the light source and the photodetector are not limited to these, and can be appropriately selected as necessary. Of these, one end of the optical fiber 62 is optically connected to the laser diode mount 61b, and the other end of the optical fiber 62 is fused with one end of the double clad optical fiber 1 to optically. It is connected. Reference numeral 16 denotes a fused portion between the optical fiber 62 and the double clad optical fiber 1. The other end of the optical fiber 1 is disposed so as to face a photometric part (not shown) of the photodetector 63. The light source 61 and the double-clad optical fiber 1 are combined in consideration of the numerical aperture (hereinafter abbreviated as NA) of the optical fiber so that light propagates through the first cladding layer of the optical fiber 1. Further, the optical fiber 1 and the photodetector 63 are combined in consideration of NA so that all the light emitted from the other end of the optical fiber 1 is received by the photodetector 63.
As described above, in the measurement system 6, the light emitted from the light source 61 passes through the recoat portion 17 of the optical fiber 1 and is received by the photodetector 63, and by measuring the amount of received light, the optical fiber is measured. The amount of lost light of 1 can be confirmed.

  The double clad optical fiber 1 on which the recoat layer is formed can have a desired configuration by, for example, covering the recoat layer with a protective coating layer.

  Hereinafter, the recoating process when (i) the die method, (ii) the molding method, and (iii) the casting method are applied will be described more specifically.

(I) Dice Method FIG. 8 is a schematic diagram for explaining a step of forming a recoat layer by applying a die method.
The recoat apparatus 4 illustrated here includes a die 41 and a curing means 42 inside a vacuum chamber 44. The die 41 may be a half-break type or an integrated type so that an optical fiber can be easily installed. The inner diameter D ₄₁ of the resin outlet of the reservoir 41a is in accordance with the outer diameter of the optical fiber to be installed may be slightly larger than it. The curing means 42 may be selected according to the type of the curable resin 15 ′, and examples thereof include a light source or a heat source. A vacuum pump 47 is connected to the vacuum chamber 44, a leak valve 441 is provided, and a pipe 45 is communicated from the outside to the inside. Inside the pipe 45 is a die storage section. It is arranged on 41a. A valve 46 is inserted in the pipe 45 so as to be located outside the vacuum chamber 44.

On the die 41, the double clad optical fiber 1 from which the coating has been removed is installed. Then, when the leak valve 441 is closed, the vacuum pump 47 is operated to decompress the inside of the vacuum chamber 44. In this state, the valve 46 is opened, and the curable resin 15 ′ is injected into the die storage portion 41 a through the pipe 45. After injecting the curable resin 15 ′, the valve 46 is closed, and the double clad optical fiber 1 is pulled down at a constant speed by the driving means (not shown) (in the direction of arrow C). A curable resin 15 ′ is laminated on the surface 12 a of one cladding layer.
Next, the double clad optical fiber 1 is stopped, the leak valve 441 is opened to release the reduced pressure, and the inside of the vacuum chamber 44 is returned to normal pressure. Then, after a predetermined time has elapsed, the curing means 42 is operated to cure the laminated curable resin 15 ′, thereby forming a recoat layer.

(Ii) Molding Method FIG. 9 is a schematic diagram for explaining a process of forming a recoat layer by applying a molding method.
A mold 51 illustrated in FIG. 9 is arranged in place of the die 41 in the vacuum chamber 44 of the recoat apparatus 4 in FIG. The mold 51 may be one that is normally used, and is composed of an upper mold 51a and a lower mold 51b. A split groove 510a is formed on the bonding surface (split surface) of the upper mold 51a, and the same is applied to the bonding surface of the lower mold 51b. In addition, a split groove 510b is formed, and these split grooves 510a and 510b are aligned so as to form one through hole 510 when the upper mold 51a and the lower mold 51b are joined. The through-hole 510 is for inserting an optical fiber and injecting a curable resin, and the length in the central axis direction depends on the length of the coating removing portion 10 of the double clad optical fiber 1. It should be a little longer than that. Further, the inner diameter D ₅₁₀ of the through hole 510 may be the same as the inner diameter D ₄₁ . When the upper mold 51a and the lower mold 51b use a photocurable resin as the curable resin, the upper mold 51a and the lower mold 51b are made of a light-transmitting material such as quartz glass, and use a thermosetting resin as the curable resin. For this, it is preferable to use a material having high thermal conductivity such as a metal or an alloy. The curing means 42 may be arranged in the same manner as in the case of the recoating apparatus 4, or may be arranged so as to be easily cured in accordance with the form of the mold 51, so as to sandwich the mold 51. An example of opposing arrangement is given.

In the molding method, as in the case of the above-described die method, the inside of the vacuum chamber is depressurized, and as shown in FIG. 9A, the coating removal portion 10 is aligned with the split grooves 510a and 510b, and double clad light is obtained. The fiber 1 is installed. Then, as shown in FIG. 9B, the upper mold 51a and the lower mold 51b are joined, and the curable resin 15 ′ is injected into the through-hole 510 through which the optical fiber 1 is inserted, and the coating removal unit 10, a curable resin 15 ′ is laminated on the surface 12 a of the first cladding layer.
Next, the inside of the vacuum chamber 44 is returned to normal pressure, and after a predetermined time has elapsed, the curing means 42 is operated to cure the laminated curable resin 15 ′ as shown in FIG. A recoat layer is formed. After forming the recoat layer, as shown in FIG. 9D, the upper mold 51a and the lower mold 51b may be divided and the double clad optical fiber 1 may be taken out.

(Iii) Casting method In the casting method, a recoat layer may be formed using a normal mold (not shown). The mold preferably has a groove having an inner diameter larger than the outer diameter of the double clad optical fiber and a length slightly longer in the central axis direction than the length of the coating removal portion of the double clad optical fiber. The material of the mold may be the same as that of the mold.
Then, a recoating apparatus in which such a mold is arranged instead of the die 41 is used inside the vacuum chamber 44 of the recoating apparatus 4 in FIG. The double clad optical fiber 1 is installed in Next, as in the case of the molding method, the inside of the vacuum chamber is decompressed, the curable resin is injected into the groove where the optical fiber is installed, and the curable resin is applied to the surface of the first cladding layer at the coating removal portion. Laminate. Next, as in the case of the molding method, the inside of the vacuum chamber is returned to normal pressure, and after a predetermined time has elapsed, the laminated curable resin is cured to form a recoat layer.

  The amount of light loss of the recoated double clad optical fiber can be measured using, for example, the measurement system 6.

The recoating method of the present invention can be applied to, for example, recoating a fiber Bragg grating (hereinafter abbreviated as FBG) for constituting a fiber laser resonator. For example, FBG can be produced by removing the coating of the double clad optical fiber and then irradiating the coating removal part with ultraviolet light. However, in order to maintain the strength and optical characteristics, the coating removal part needs to be recoated. Yes, it is suitable as an application target of the recoating method of the present invention.
The recoating method of the present invention can also be applied to the connection between double clad optical fibers or between a double clad optical fiber and another optical fiber. Such connection of optical fibers is usually performed by fusing the end portions of the optical fibers, but at the time of fusing, it is necessary to remove the coating layer, and the fused portion has no coating layer. Therefore, recoating is necessary to maintain the strength and optical characteristics of the fused part, which is suitable as an application target of the recoating method of the present invention.
The FBG or fused portion recoated by the method of the present invention reduces the amount of light lost at the recoated portion, and can efficiently use the excitation light of the fiber laser propagating through the first cladding layer.

FIG. 10 is a schematic configuration diagram illustrating a clad pump type fiber laser manufactured by applying the recoating method of the present invention to the recoating of the FBG and the connection as described above.
The fiber laser 60 shown here is roughly composed of an optical fiber 1 including an FBG 18, a pump laser diode 601, and an amplification optical fiber 602. Both ends of the amplification optical fiber 602 are optically connected to one end of the optical fiber 1, and the other optical fiber 1 is not fused to the amplification optical fiber 602. The end is fused and optically connected to the pump laser diode 601. The other end of the other optical fiber 1 that is not fused to the amplification optical fiber 602 is the laser emission light 1a. Reference numeral 603 denotes a fused portion between the optical fiber 1 and the amplification optical fiber 602 or between the optical fiber 1 and the pump laser diode 601. The fused portion 603 and the FBG 18 are recoated by the method of the present invention.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.
[Examples 1 to 9]
1. The core and the first cladding layer have quartz glass as a main component, the second cladding layer is made of a fluorinated acrylate resin having a refractive index lower than that of the first cladding layer, and the protective coating layer has the cross section shown in FIG. Using a double clad optical fiber made of urethane acrylate resin, recoating was performed according to the following procedure. The outer diameter of the first cladding layer of the optical fiber is 125 μm, and the outer diameter of the protective coating layer is 250 μm.
Nine of the above-mentioned double-clad optical fibers were prepared by cutting so that the length in the longitudinal direction was 5 m.
Next, (a) removal by laser irradiation, (b) removal by blowing hot air, (c) cutting removal by a cutting tool, and removing the second cladding layer and the protective coating layer (removal step), Three were made. Specifically, it is as follows.

<Recoating of double clad optical fiber>
(Removal process)
(A) Removal by laser irradiation A double clad optical fiber was irradiated with a KrF excimer laser having a wavelength of 248 nm using the laser irradiation apparatus shown in FIG. The intensity of the irradiated laser beam was 0.78 mJ / cm ² . The double-clad optical fiber was fixed, and the laser was irradiated while moving the stage along the longitudinal direction of the fiber at a speed of 0.05 mm / second for 600 seconds. Furthermore, the fiber is rotated 90 ° in the outer circumferential direction, the laser is irradiated in the same manner, the same operation is repeated twice more, and the laser is irradiated from a total of four directions every 90 °, and the second cladding in the irradiation section All layers and coating layers were removed. As a result, the surface of the first clad layer and the surface of the exposed end of the second clad layer in the coating removal portion have resin scraps having minute irregularities, and further the exposed end of the second clad layer. The surface was also rough.

(B) Removal by blowing hot air Using a hot air blowing device shown in FIG. 4, hot air at 850 ° C. was blown onto the double clad optical fiber to remove the coating layer. The amount of hot air to be blown was 250 L / min. The double clad optical fiber was fixed, and hot air was blown while moving the nozzle along the longitudinal direction of the fiber for 3 seconds at a speed of 10 mm / second. The blowing direction was only one direction, and the second cladding layer and the coating layer in the blowing section were all removed. As a result, resin scraps having minute irregularities were present on the surface of the first cladding layer in the coating removal portion, and the exposed end surface of the second cladding layer was also rough.

(C) Cutting removal with a cutting tool The second clad layer and the coating layer were all removed in a predetermined section in the longitudinal direction of the double clad optical fiber by a manual operation using a single-blade razor. As a result, as in the case of (a) above, the surface of the first cladding layer and the surface of the exposed end portion of the second cladding layer have resin scraps having minute irregularities, and the second cladding layer The exposed end surface was also rough.

  Recoating by applying (i) the die method, (ii) the molding method, and (iii) the casting method to the coating removal portion of the double clad optical fiber processed by the methods (a) to (c) above. A layer was formed (recoat process). Specifically, it is as follows.

(Recoat process)
(I) Dice method The recoat layer was formed using the recoat apparatus shown in FIG. The die was made of stainless steel, and D ₄₁ was 260 μm. Further, a fluorinated acrylate resin, which is an ultraviolet curable resin, was used as the curable resin, which was laminated at room temperature (24 ° C.), and a mercury lamp, which was a UV light source, was used as the curing means. The inside of the vacuum chamber was decompressed to 10 Pa with a rotary pump. Then, after the decompression was released and the pressure was returned to the normal pressure, the mercury lamp was turned on after 5 minutes, and the curing of the resin was started. The completion of the curing of the resin was judged visually, and as a result, the UV light irradiation time was 150 seconds.

(Ii) Molding Method A recoat layer was formed using the recoat apparatus shown in FIG. The mold was made of quartz glass, and the one having D ₅₁₀ of 260 μm was used. Further, a fluorinated acrylate resin, which is an ultraviolet curable resin, was used as the curable resin, which was laminated at room temperature (24 ° C.), and a mercury lamp, which was a UV light source, was used as the curing means. The inside of the vacuum chamber was decompressed to 10 Pa with a rotary pump. Then, after the decompression was released and the pressure was returned to the normal pressure, the mercury lamp was turned on after 5 minutes, and the curing of the resin was started. The completion of the curing of the resin was judged visually, and as a result, the UV light irradiation time was 160 seconds.

(Iii) Casting method A recoat layer was formed using a cylindrical mold having an inner diameter of 1 mm and made of quartz glass. A fluorinated acrylate resin, which is an ultraviolet curable resin, was used as the curable resin, which was laminated at room temperature (24 ° C.), and a mercury lamp, which was a UV light source, was used as the curing means. The inside of the vacuum chamber was decompressed to 10 Pa with a rotary pump. Then, after the decompression was released and the pressure was returned to the normal pressure, the mercury lamp was turned on after 5 minutes, and the curing of the resin was started. The completion of the curing of the resin was judged visually, and as a result, the UV light irradiation time was 600 seconds.

<Measurement of transmitted light power with recoated double clad optical fiber>
Using the measurement system shown in FIG. 7, the amount of light loss in the recoated double clad optical fiber was confirmed. A light source including a laser diode having a wavelength of 980 nm was used, and a power meter was used as the photodetector.
First, the transmitted light power of the nine types of recoating been double-clad optical fiber is measured, the measured value was P _0. Then cut double-clad optical fiber in the vicinity of the midpoint between the fused portion 16 and the recoated section 17 measures the transmitted light power, and the measured value was P _1. Then, the amount of light loss at the cut portion calculated by the expression “P _LOSS = P ₀ −P ₁ ” was obtained for each of the nine types of optical fibers. “P _LOSS ” includes the amount of light loss due to the transmission loss of the double-clad optical fiber itself in addition to the amount of light loss at the coating removal portion and the recoat portion, but the double-clad optical fiber used for measurement has a length of 5 m. With such a short length, the amount of light lost due to the transmission loss is sufficiently smaller than the amount of light lost at the coating removal portion and the recoat portion, and can be ignored. Accordingly, “P _LOSS ” can be regarded as being equal to “the amount of light lost at the coating removal portion and the recoat portion”.
Table 1 shows the calculation result of P _LOSS by this method.

[Comparative Examples 1 to 9]
In the recoating step, the curable resin is laminated on the surface of the first cladding layer under normal pressure without reducing pressure, and recoating is carried out in the same manner as in Examples 1 to 9 except that curing is started immediately after lamination. _LOSS was calculated. The calculation results are shown in Table 1.

  As is clear from the results in Table 1, in Examples 1 to 9 in which the curable resin was laminated under reduced pressure regardless of the method for removing the coating layer and the method for forming the recoat layer, the amount of light loss was 0.08 dB or less. In contrast, in Comparative Examples 1 to 9 in which the curable resin was laminated under normal pressure without reducing the pressure, the amount of lost light was 0.52 dB or more, and the amount of lost light could not be reduced.

[Examples 10 to 18]
Except that a heating wire heater is provided in a die, a mold, or a mold, and these are heated by the heater, so that the curable resin is laminated on the surface of the first cladding layer while keeping the temperature at 60 ° C. Recoating was performed in the same manner as in 9, and P _LOSS was calculated. The calculation results are shown in Table 1. It was confirmed in advance that the fluorinated acrylate resin used as the curable resin did not undergo modification or decomposition such as curing at 60 ° C.

  As is clear from the results in Table 1, in Examples 10 to 18 in which the curable resin was laminated while being kept at 60 ° C., the amount of lost light could be further reduced as compared with Examples 1 to 9 in which the curable resin was laminated at room temperature. . Even if there is a foreign substance in the coating removal part or the surface of the exposed end part of the second cladding layer is rough, even if there are recesses in these, the curable resin that is kept warm easily flows into the recesses. It is thought that the occurrence was suppressed.

[Example 19]
1. The core and the first cladding layer have quartz glass as a main component, the second cladding layer is made of a fluorinated acrylate resin having a refractive index lower than that of the first cladding layer, and the protective coating layer has the cross section shown in FIG. Using a double clad optical fiber made of urethane acrylate resin, recoating was performed according to the following procedure. The outer diameter of the first cladding layer of the optical fiber is 125 μm, and the outer diameter of the protective coating layer is 250 μm.
As shown in FIG. 5, the double clad optical fiber was installed in a Peltier element as a cooling means via a metal plate as a heat conductor. Then, the temperature of the metal plate is maintained at 10 ° C. by the Peltier element to cool the double clad optical fiber, and a KrF excimer laser having a wavelength of 248 nm is doubled by using the laser irradiation apparatus shown in FIG. The fiber was irradiated. The intensity of the irradiated laser beam was 0.78 mJ / cm ² . The double-clad optical fiber was fixed, and the laser was irradiated while moving the stage along the longitudinal direction of the fiber at a speed of 0.05 mm / second for 600 seconds. Furthermore, the fiber is rotated 90 ° in the outer circumferential direction, the laser is irradiated in the same manner, the same operation is repeated twice more, and the laser is irradiated from a total of four directions every 90 °, and the second cladding in the irradiation section All layers and coating layers were removed (removal step).
Next, using the recoat apparatus shown in FIG. 8, a recoat layer was formed by the (i) dice method on the coating removal portion of the double clad optical fiber in the same manner as in Example 1 (recoat process). In other words, the die was made of stainless steel and D ₄₁ was 260 μm. Further, a fluorinated acrylate resin, which is an ultraviolet curable resin, was used as the curable resin, which was laminated at room temperature (24 ° C.), and a mercury lamp, which was a UV light source, was used as the curing means. The inside of the vacuum chamber was decompressed to 10 Pa with a rotary pump. Further, when the resin was cured, a measurement system as shown in FIG. 7 was used so that the amount of loss of propagation light in the first cladding layer could be measured. And after canceling | reducing pressure reduction and returning to a normal pressure, the mercury lamp was turned on 5 minutes afterward, hardening of resin was started, and it hardened | cured, measuring the loss light quantity. A similar optical fiber was used in advance to investigate the relationship between the UV light irradiation time and the amount of loss of light propagated through the first cladding layer, and the results are shown in FIG. As is clear from FIG. 11, it was confirmed that the amount of lost light decreased as the UV light irradiation time increased, and thereafter the amount of lost light started to increase. Therefore, in this example, curing was performed while measuring the amount of lost light, and the mercury lamp was quickly turned off when the amount of lost light started to increase, and the curing was stopped to minimize the amount of lost light.
Next, as a result of calculating P _LOSS , it was 0.03 dB, and the loss light amount could be reduced to an extremely small value.

[Reference Example 1]
In the removal step, the double clad optical fiber is not cooled, and in the recoating step, the ultraviolet curable resin (fluorinated acrylate resin) is laminated to the surface of the first clad layer under normal pressure without reducing the pressure. Recoating was performed in the same manner as in Example 19, and P _LOSS was calculated. As a result, P _LOSS was 0.45 dB.

[Comparative Example 10]
In the removal process, the double-clad optical fiber is not cooled, and in the recoating process, the UV curable resin (fluorinated acrylate resin) is laminated to the surface of the first cladding layer under normal pressure without reducing the pressure, and the resin is cured. Except that it was judged visually, recoating was performed in the same manner as in Example 19, and P _LOSS was calculated. As a result of repeating this 10 times, the average value of P _LOSS was 0.60 dB.

  The present invention is applicable to various optical components using a double clad optical fiber that requires recoating.

  DESCRIPTION OF SYMBOLS 1 ... Double clad optical fiber, 11 ... Core, 12 ... 1st clad layer, 13 ... 2nd clad layer, 14 ... Protective coating layer, 15 ... Recoat layer, 15 ' ... Curable resins

Claims (7)

A double clad optical fiber comprising a core, a first cladding layer surrounding the core, and a coating layer covering the first cladding layer, wherein the coating layer has a second cladding layer surrounding the first cladding layer Recoating method,
Removing the coating layer to expose the first cladding layer;
Laminating a curable resin on the exposed first cladding layer under reduced pressure, and then curing the curable resin under normal pressure to form a recoat layer; and
A method of recoating a double clad optical fiber, comprising:   The method for recoating a double-clad optical fiber according to claim 1, wherein the curable resin is laminated on the first cladding layer while keeping the temperature at a temperature at which the curable resin is not decomposed or modified.   The method for recoating a double-clad optical fiber according to claim 1 or 2, wherein the curable resin is cured while measuring the amount of light loss of propagation light in the first clad layer.   The said coating layer is removed by laser irradiation, The recoat method of the double clad optical fiber as described in any one of Claims 1-3 characterized by the above-mentioned.   5. The method of recoating a double-clad optical fiber according to claim 1, wherein the coating layer is removed while being cooled to a temperature at which the second clad layer is not denatured.   6. The double clad according to claim 1, wherein the curable resin is an ultraviolet curable resin, and the second clad layer is formed by curing an ultraviolet curable resin. Optical fiber recoating method.   The method for recoating a double clad optical fiber according to any one of claims 1 to 6, wherein the recoat layer is formed by a molding method, a die method, or a casting method.

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