JP6496695B2 - Polymer clad fiber and method for producing polymer clad fiber - Google Patents

Description

  The present invention relates to a polymer clad fiber and a method for producing a polymer clad fiber.

  An optical fiber used for a fiber laser needs to transmit high-density light. In recent years, the output of fiber lasers has been increased, and fiber lasers having a kW-class optical output that are used for metal cutting and welding have been commercialized.

  An amplification optical fiber used for a fiber laser includes a core and a glass cladding. In the core, the same light (signal light) as the output light propagates. The glass cladding is formed around the core, and light (excitation light) for amplifying signal light propagates in the glass cladding. When the excitation light propagating in the glass cladding is linked with the core, the excitation light is absorbed as energy in the core, and the signal light is amplified. It is desirable that all the excitation light is absorbed in the core. However, in practice, part of the excitation light remains in the glass clad and becomes skew light that does not contribute to the excitation of the signal light. This skew light becomes clad mode light that continues to propagate in the glass clad. A part of the signal light propagating in the core also leaks into the glass clad due to disturbance applied to the optical fiber, and becomes clad mode light.

  The clad mode light causes the beam quality of the fiber laser to deteriorate. Therefore, Patent Document 1 below discloses a leaky light processing structure for removing clad mode light. This leakage light processing structure emits cladding mode light to the outside of the optical fiber through a leakage light guide member provided around the glass cladding.

JP 2008-268747 A

  By the way, in the structure described in Patent Document 1, clad mode light is concentrated from a specific portion and emitted to the outside of the optical fiber. For this reason, there is a possibility that the portion where the clad mode light is emitted locally generates heat and the optical fiber is burned.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a polymer clad optical fiber capable of removing clad mode light while suppressing local heat generation.

  In order to solve the above problems, a polymer-clad fiber according to one aspect of the present invention includes a bare optical fiber having a core and a glass clad surrounding the core, and a resin having a refractive index lower than the refractive index of the glass clad. And a polymer cladding layer that covers the bare optical fiber, and is configured to have an excitation loss of 0.1 dB / m or more at a wavelength of 915 nm.

  According to the polymer clad fiber according to the above aspect, since the excitation loss is 0.1 dB / m or more, the clad mode light can be gradually removed in the longitudinal direction. Thereby, compared with the case where clad mode light is concentrated and emitted from a specific portion, local heat generation can be suppressed.

  The polymer clad fiber according to the above aspect may include a protective coating layer that covers the polymer cladding layer, and the protective coating layer may be formed of a thermosetting resin.

  In this case, since the protective coating layer is formed of a thermosetting resin, when the polymer coated fiber is cured in a state where the protective coating layer is thermally expanded at a high temperature and then cooled, the protective coating layer is Heat shrinks. Due to the thermal contraction of the cured protective coating layer, a lateral pressure is applied to the polymer cladding layer, and the excitation loss can be easily increased.

  Moreover, the film thickness of the protective coating layer may be greater than or equal to the film thickness of the polymer clad layer.

  In this case, since the film thickness of the protective coating layer is large, it is possible to suppress the occurrence of cracking or the like when the protective coating layer is thermally contracted, and to reliably apply a lateral pressure to the polymer cladding layer.

  Further, the type D hardness of the thermosetting resin forming the protective coating layer may be 50 or more.

  In this case, since the hardness of the protective coating layer is large, a lateral pressure can be reliably applied to the polymer cladding layer when the protective coating layer is thermally contracted.

  The method for producing a polymer-clad fiber according to one aspect of the present invention includes a resin having a refractive index lower than a refractive index of the glass clad in a bare optical fiber having a core and a glass clad surrounding the core. A polymer clad fiber having a pumping loss of 0.1 dB / m or more at a wavelength of 915 nm is manufactured.

  According to the method for producing a polymer clad fiber according to the above aspect, a polymer clad fiber having an excitation loss of 0.1 dB / m or more is produced. This polymer clad fiber can gradually remove clad mode light in the longitudinal direction. For this reason, compared with the case where clad mode light is concentrated and emitted from a specific portion, local heat generation can be suppressed.

  The method for producing a polymer clad fiber includes a step of applying a thermosetting resin around the polymer clad layer, and then heating and curing the thermosetting resin to form a protective coating layer. May be.

  In this case, when the polymer clad fiber is manufactured, the thermosetting resin serving as the protective coating layer is cured in a thermally expanded state and then thermally contracts when cooled. Due to the thermal contraction of the cured protective coating layer, a lateral pressure is applied to the polymer cladding layer, and the excitation loss can be easily increased.

  In the step of forming the protective coating layer, the thermosetting resin to be the protective coating layer may be cured at a temperature of 300 ° C. or higher.

  In this case, since the thermosetting resin to be the protective coating layer is cured in a state where it is thermally expanded at a high temperature of 300 ° C. or higher, the amount of thermal shrinkage of the protective coating layer is increased during cooling, and a lateral pressure is reliably applied to the polymer cladding layer. can do.

  According to the above aspect of the present invention, it is possible to provide a polymer clad optical fiber capable of removing clad mode light while suppressing local heat generation.

It is a schematic diagram which shows the manufacturing process of the polymer clad fiber 10 which concerns on this embodiment. It is a schematic diagram of the measuring apparatus of excitation loss. It is the graph which showed the relationship between film thickness ratio and excitation loss.

Hereinafter, the polymer clad fiber according to the present embodiment will be described with reference to FIGS. In each drawing used in the following description, the scale is appropriately changed to make each member a recognizable size.
FIG. 1A to FIG. 1F are schematic views showing manufacturing steps of the polymer clad fiber 10 according to the present embodiment. Among these, FIG.1 (f) is sectional drawing of the manufactured polymer clad fiber 10. FIG.
As shown in FIG. 1 (f), the polymer clad fiber 10 includes an optical fiber bare wire 1, a polymer clad layer 2, and a protective coating layer 3. The polymer cladding layer 2 covers the bare optical fiber 1. The protective coating layer 3 covers the polymer cladding layer 2.

The bare optical fiber 1 is made of, for example, quartz glass. The bare optical fiber 1 includes a core 1a and a glass clad 1b. The glass clad 1b covers the core 1a.
The material of the polymer cladding layer 2 may be a UV curable resin or a thermosetting resin. The polymer cladding layer 2 is formed of a material having a refractive index lower than that of the glass cladding 1b.

  The protective coating layer 3 is formed of a thermosetting resin. As a material of the protective coating layer 3, for example, an epoxy-modified silicone resin can be used. The epoxy-modified silicone resin is suitable as a material for the protective coating layer 3 from the viewpoints of hardness, viscosity, and curing speed. The linear expansion coefficient of the material forming the protective coating layer 3 is larger than the linear expansion coefficient of the material forming the bare optical fiber 1 and the polymer cladding layer 2.

  As will be described later, the protective coating layer 3 is cured in a state of being thermally expanded at the time of manufacture, and then thermally contracts to apply a lateral pressure to the polymer cladding layer 2. For this reason, it is desirable that the protective covering layer 3 is made of a material that is hard to some extent. Specifically, the material for forming the protective coating layer 3 is a hardness of a type D durometer (hereinafter referred to as “type D hardness” or the like in this specification) measured based on JIS-K-6253. It is desirable that it is 50 or more. Further, when the protective coating layer 3 is formed using a material having a type D hardness of 70 or more, a lateral pressure can be applied to the polymer cladding layer 2 more reliably.

  Next, the manufacturing process of the polymer clad fiber 10 will be described.

When manufacturing the polymer clad fiber 10, first, the optical fiber bare wire 1 is formed (FIG. 1A). The bare optical fiber 1 has a desired outer diameter (for example, about several hundred μm) by pulling out a glass base material heated to about 2000 ° C., for example.
Next, a resin to be the polymer cladding layer 2 is applied around the bare optical fiber 1 (FIG. 1B). Thereafter, the applied resin is cured by irradiation with UV light or heating to form the polymer cladding layer 2.
Next, a thermosetting resin to be the protective coating layer 3 is applied around the polymer clad layer 2 (FIG. 1C).
Next, the polymer clad fiber 10 is heated by a crosslinking apparatus such as an electric furnace (FIG. 1D). Thereby, the thermosetting resin is cured and the protective coating layer 3 is formed. The temperature at which the thermosetting resin is cured is, for example, 300 ° C. or higher. At this time, the protective coating layer 3 is cured in a thermally expanded state (FIG. 1 (e)).
Next, the polymer clad fiber 10 is cooled to room temperature. Thereby, the protective coating layer 3 shrinks thermally (FIG. 1 (f)).

Here, since the protective coating layer 3 is thermally contracted in a cured state, a lateral pressure in the compression direction acts on the polymer cladding layer 2. This lateral pressure, it is possible to increase the excitation loss. The pumping loss is a transmission loss of pumping light propagating through the core 1a and the glass cladding 1b. The excitation loss can be measured by the cutback method using, for example, the measuring apparatus 100 shown in FIG.

FIG. 2 is a schematic diagram of an excitation loss measuring apparatus 100. The measuring apparatus 100 includes a light source 101 and a calorimeter 102.
The light source 101 includes a laser diode 103, an NA conversion unit 104, and a dummy fiber 105. The laser diode 103 emits measurement laser light. The measurement laser light is incident on the NA conversion unit 104 via the lead fiber 103 a of the laser diode 103. The NA conversion unit 104 converts the NA of the lead fiber 103a so as to be equal to the NA of the polymer clad fiber 10 to be measured. The dummy fiber 105 connects the NA conversion unit 104 and the polymer clad fiber 10 to be measured. The dummy fiber 105 and the polymer clad fiber 10 to be measured are fused and connected to each other. The calorimeter 102 measures the output power of the light emitted from the polymer clad fiber 10 to be measured.

When measuring the excitation loss, the polymer clad fiber 10 to be measured is set in the measuring apparatus 100 and light having a wavelength of 915 nm is emitted from the laser diode 103. At this time, the output power P _{2 of the} light emitted from the polymer clad fiber 10 is measured by the calorimeter 102. Then, the polymer clad fiber 10 to be measured and a predetermined quantity cut (cut back), measure the subsequent output power P _1. Cutting back length (P ₂ difference obtained by subtracting the fiber length of the P ₁ when measured from the fiber length of the measurement) is L, the following equation (1), excitation loss of the polymer clad fiber 10 is calculated .
Excitation loss = | 10 log (P ₁ / P ₂ ) | / L (1)

  Next, a description will be given of the results of a test in which a plurality of polymer clad fibers 10 having different film thicknesses of the protective coating layer 3 are prepared and the excitation loss is measured.

(Test Example 1)
The outer diameter of the bare optical fiber 1 was set to 125 μm. As the material of the polymer cladding layer 2, a thermosetting silicone resin having a refractive index of 1.41 was used. As the material of the protective coating layer 3, a thermosetting silicone resin having a type D hardness of 75 was used.
Under the above conditions, a plurality of polymer clad fibers 10 having different film thicknesses of the polymer clad layer 2 and the protective coating layer 3 were prepared. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 1. The film thickness ratio in Tables 1 and 2 is the ratio of the film thickness of the protective coating layer 3 to the film thickness of the polymer clad layer 2. For example, when the film thickness of the polymer cladding layer 2 is 25 μm and the film thickness of the protective coating layer 3 is 37.5 μm, the film thickness ratio is 37.5 μm ÷ 25 μm = 1.5.
In Test Example 1, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.2 to 1.5. The excitation loss of each polymer clad fiber 10 was in the range of 0.003 to 0.13 dB / m.

(Test Example 2)
In Test Example 2, a thermosetting silicone resin having a type D hardness of 50 was used as the material of the protective coating layer 3. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 1. Other conditions are the same as in Test Example 1.
In Test Example 2, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.2 to 1.5. Moreover, the excitation loss of each polymer clad fiber 10 was in the range of 0.0027 to 0.131 dB / m.

(Test Example 3)
In Test Example 3, the outer diameter of the bare optical fiber 1 was 80 μm. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 1. Other conditions are the same as in Test Example 1.
In Test Example 3, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.2 to 1.5. The excitation loss of each polymer clad fiber 10 was in the range of 0.0015 to 0.13 dB / m.

(Test Example 4)
In Test Example 4, the outer diameter of the bare optical fiber 1 was 400 μm. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 2. Other conditions are the same as in Test Example 1.
In Test Example 4, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.2 to 2.0. The excitation loss of each polymer clad fiber 10 was in the range of 0.0028 to 0.145 dB / m.

(Test Example 5)
In Test Example 5, a thermosetting silicone resin having a type D hardness of 20 was used as the material for the protective coating layer 3. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 2. Other conditions are the same as in Test Example 1.
In Test Example 5, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.4 to 2.0. Moreover, the excitation loss of each polymer clad fiber 10 was in the range of 0.01 to 0.09 dB / m.

(Test Example 6)
In Test Example 6, a thermosetting silicone resin having a type A hardness of 80 was used as the material for the protective coating layer 3. The film thicknesses of the polymer clad layer 2 and the protective coating layer 3 of the produced polymer clad fiber 10 are as shown in Table 2. Other conditions are the same as in Test Example 1.
In Test Example 6, the film thickness ratio of each polymer clad fiber 10 was in the range of 0.5 to 2.6. Moreover, the excitation loss of each polymer clad fiber 10 was in the range of 0.0029 to 0.075 dB / m.

FIG. 3 is a graph showing the relationship between the film thickness ratio and the excitation loss for the polymer clad fibers 10 of Test Examples 1 to 6. The horizontal axis is the film thickness ratio, and the vertical axis is the excitation loss (dB / m).
As shown in FIG. 3, the excitation loss increases as the film thickness ratio increases. This is because the thicker the protective coating layer 3 is, the more reliably the lateral pressure is applied to the polymer cladding layer 2 when the protective coating layer 3 is cured and thermally shrinks.

  Next, the results of an experiment for confirming whether or not the excitation light (clad mode light) has been sufficiently removed from the polymer clad fiber 10 prepared in Test Examples 1 to 6 will be described.

  In this experiment, the polymer clad fiber 10 prepared in Test Examples 1 to 6 was connected 10 m as an output fiber of a fiber laser, and a clad mode removing portion was formed downstream thereof. An optical device described in Japanese Patent No. 5850992 was used as the cladding mode removing unit. The temperature in the coating removal section of this optical device was measured to determine whether or not there was a temperature rise. The determination result is shown in the column of “temperature rise at the cladding mode removing portion” shown in Tables 1 and 2. Here, ◯ indicates that there was no temperature increase, Δ indicates that there was a slight temperature increase, and × indicates that there was a significant temperature increase. When there is no temperature increase, it can be estimated that the removal of the excitation light by the polymer clad fiber 10 is sufficient.

From Table 1 and Table 2, it can be seen that the higher the excitation loss, the more the temperature rise at the cladding mode removal portion is suppressed. Specifically, there was a significant temperature rise in the range where the excitation loss was 0.065 dB / m or less. There was a slight increase in temperature when the excitation loss was in the range of 0.07 to 0.09 dB / m. In the range where the excitation loss was 0.1 dB / m or more, there was no temperature rise.
From the above results, the cladding mode light can be sufficiently removed by setting the excitation loss of the polymer clad fiber 10 to 0.1 dB / m.

  Next, the conditions for setting the excitation loss of the polymer clad fiber 10 to 0.1 dB / m or more will be considered.

As shown in Tables 1 and 2, for Test Example 5 and Test Example 6 in which the hardness of the material forming the protective coating layer 3 is relatively small, a polymer-clad fiber 10 having an excitation loss of 0.1 dB / m or more is obtained. I couldn't. On the other hand, for Test Examples 1 to 4 having a Type D hardness of 50 or more, a polymer clad fiber 10 having an excitation loss of 0.1 dB / m or more was obtained.
From the above, the use of a material having a type D hardness of 50 or more as the protective coating layer 3 is a condition for setting the excitation loss to 0.1 dB / m or more.
The reason why the excitation loss increases as the hardness of the protective coating layer 3 is increased is that the side pressure applied to the polymer cladding layer 2 increases as the protective coating layer 3 is hardened when thermally contracted. .

  Next, the film thickness ratio for setting the excitation loss to 0.1 dB / m or more will be considered. Among Test Examples 1 to 4, the excitation loss is 0.1 dB / m or more when the film thickness ratio is 1.0 or more (see Tables 1 and 2). Therefore, the condition for setting the film thickness ratio to 1.0 or more, that is, to set the film thickness of the protective coating layer 3 to be equal to or greater than the film thickness of the polymer cladding layer 2 is to make the excitation loss 0.1 dB / m or more. It is.

  As described above, according to the polymer clad fiber 10 of the present embodiment, the cladding mode light can be gradually removed in the longitudinal direction by setting the excitation loss to 0.1 dB / m or more. Thereby, compared with the case where clad mode light is concentrated and emitted from a specific portion, local heat generation can be suppressed.

  In addition, since the protective coating layer 3 is formed of a thermosetting resin, when the polymer cladding fiber 10 is manufactured, the protective coating layer 3 is cured in a state where the protective coating layer 3 is thermally expanded at a high temperature and is then cooled. Layer 3 heat shrinks. Due to the thermal shrinkage of the cured protective coating layer 3, a lateral pressure is applied to the polymer cladding layer 2, and the excitation loss of the polymer cladding layer 2 can be easily increased.

  Further, when the thickness of the protective coating layer 3 is set to be equal to or larger than the thickness of the polymer cladding layer 2, it is possible to suppress the occurrence of cracking or the like when the protective coating layer 3 is thermally contracted. A lateral pressure can be reliably applied.

  Further, when the type D hardness of the thermosetting resin forming the protective coating layer 3 is 50 or more, when the protective coating layer 3 is thermally contracted, it is possible to reliably apply a lateral pressure to the polymer cladding layer 2. it can.

  Further, in the step of forming the protective coating layer 3, when the thermosetting resin to be the protective coating layer 3 is cured in a state of thermal expansion at a high temperature of 300 ° C. or higher, the thermal contraction of the protective coating layer 3 during cooling is performed. The amount is increased, and a lateral pressure can be reliably applied to the polymer cladding layer 2.

  The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

  For example, in the above-described embodiment, the method of setting the excitation loss of the polymer clad fiber 10 to 0.1 dB / m or more by forming the protective coating layer 3 with a thermosetting resin has been described. However, the present invention is not limited to this, and the excitation loss may be 0.1 dB / m or more by other methods. For example, the excitation loss of the polymer clad fiber 10 may be set to 0.1 dB / m or more by providing a scattering source such as a bubble in the glass clad 1b.

  Moreover, in the said embodiment, the method which makes the excitation loss of the polymer clad fiber 10 0.1 dB / m or more by forming the protective coating layer 3 with thermosetting resin whose type D hardness is 50 or more was demonstrated. However, the present invention is not limited to this. For example, by increasing the film thickness ratio, the polymer clad fiber 10 is formed so that the excitation loss becomes 0.1 dB / m or more while forming the protective coating layer 3 with a thermosetting resin having a Type D hardness of less than 50. It may be configured.

  In the above-described embodiment, the method of setting the excitation loss of the polymer clad fiber 10 to 0.1 dB / m or more by setting the film thickness ratio to 1.0 or more has been described. However, the present invention is not limited to this. For example, by forming the protective coating layer 3 using a thermosetting resin having a large type D hardness, the polymer cladding is formed so that the excitation loss is 0.1 dB / m or more while the initial film ratio is less than 1.0. The fiber 10 may be configured.

  Moreover, in the said embodiment, it demonstrated that the thermosetting resin used as the protective coating layer 3 was hardened at the temperature of 300 degreeC or more. However, the present invention is not limited to this, and the thermosetting resin that becomes the protective coating layer 3 may be cured at a temperature of less than 300 ° C.

  In addition, the constituent elements in the above-described embodiment can be appropriately replaced with known constituent elements without departing from the gist of the present invention, and the above-described embodiments and modifications may be appropriately combined.

  DESCRIPTION OF SYMBOLS 1 ... Bare optical fiber 1a ... Core 1b ... Glass clad 2 ... Polymer clad layer 3 ... Protective coating layer 10 ... Polymer clad fiber

Claims (8)

A bare optical fiber having a core and a glass cladding surrounding the core;
A polymer cladding layer covering the optical fiber bare wire, formed of a resin having a refractive index lower than that of the glass cladding;
A protective coating layer covering the polymer cladding layer ,
The protective coating layer is that the action of lateral pressure to the polymer cladding layer, at the wavelength 915 nm, the total value of the pump loss of the core and the glass cladding that Do the above 0.1 dB / m,
Polymer clad fiber. Before Symbol protective coating layer is formed by thermosetting resin, polymer clad fiber according to claim 1.   The polymer clad fiber according to claim 2, wherein a film thickness of the protective coating layer is equal to or greater than a film thickness of the polymer clad layer.   The polymer clad fiber according to claim 2 or 3, wherein the thermosetting resin forming the protective coating layer has a type D hardness of 50 or more. The polymer clad fiber according to any one of claims 1 to 4, wherein a scattering source is included in the glass clad. Applying a resin having a refractive index lower than the refractive index of the glass cladding to a bare optical fiber having a core and a glass cladding surrounding the core, and forming a polymer cladding layer ;
Forming a protective coating layer around the polymer cladding layer and applying a lateral pressure to the polymer cladding layer ,
Wherein by the action of lateral pressure, at a wavelength of 915 nm, to produce the core and the polymer cladding fiber total value that Do the above 0.1 dB / m of the pump loss of the glass cladding,
Production method of polymer clad fiber. After applying the thermosetting resin on the periphery of the polymer cladding layer, a step of forming the protective coating layer was cured by heating the thermosetting resin,
The manufacturing method of the polymer clad fiber of Claim 6 . In the step of forming the protective coating layer, the thermosetting resin to be the protective coating layer is cured at a temperature of 300 ° C. or higher.
The manufacturing method of the polymer clad fiber of Claim 7 .

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