JP2002277669A - Double clad fiber and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a core 11 extending in a central axis direction of a fiber 1 and transmitting a signal light, and a first clad 12 transmitting an excitation light.
A double clad fiber 1 having a first clad 13 and a second clad 13, the double clad fiber 1 having an improved numerical aperture for pumping light, and a method for manufacturing the same. A core rod (21) is provided in a support pipe (24).
a, a first cladding rod 22a, and a second cladding capillary 23a, respectively.
Is formed into a porous structure having a large number of holes 13a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double clad fiber used for a fiber laser or a fiber amplifier and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, a double clad fiber used for a fiber laser or a fiber amplifier has been known. The double clad fiber is composed of a core (single mode core) doped with an excitation light active material and a single mode core. It has a first clad that covers the periphery and a second clad that covers the periphery of the first clad. In this double-clad fiber, while the signal light propagates in the core, the pump light for exciting the signal light propagates in the first clad, so that the pump light crosses the core each time the pump light crosses the core. The photoactive substance is activated, so that the signal light is amplified.

[0003]

By the way, in the double-clad fiber having the first and second claddings, the second cladding fiber is required to have a
It is necessary to make the refractive index of the cladding lower than that of the first cladding. For this reason, in the conventional double clad fiber, the core and the first clad are made of quartz (SiO ₂ ).
However, the second clad is formed of, for example, an ultraviolet curable resin.

However, even if the second clad is formed of an ultraviolet curable resin, the numerical aperture for excitation light is about 0.5, and further improvement of the numerical aperture cannot be expected. In addition, forming the second clad with a resin may cause a problem of thermal stability of the double clad fiber.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a double clad fiber having an improved numerical aperture for pumping light and a method of manufacturing the same.

[0006]

According to a first aspect of the present invention, there is provided a core which extends in a central axis direction of a fiber and through which a signal light propagates, and which covers the periphery of the core and excites the signal light. For a double-clad fiber having a first clad through which pumping light to be propagated and a second clad covering the periphery of the first clad, the second clad is formed by a large number of thin fibers extending in the central axis direction of the fiber. It is a specific matter to configure a porous structure having holes.

According to a second aspect of the present invention, there is provided a core extending in the central axis direction of the fiber, through which the signal light propagates, a first cladding which covers the periphery of the core, and through which the pump light for exciting the signal light propagates, A double clad fiber having a second clad covering the periphery of the first clad;
And the second clad is configured to have a porous structure having a large number of pores extending in the central axis direction of the fiber.

Here, it is preferable that the core is doped with an excitation light active substance (for example, a rare earth element or the like) and is doped with, for example, Ge or the like in order to increase the refractive index more than the first cladding.

According to the first or second aspect of the present invention, since the second clad has a porous structure having a large number of pores extending in the direction of the central axis of the fiber, the refractive index of the second clad ( The effective refractive index is determined by the refractive index of the air in each hole and the refractive index of the portion other than the holes, and the effective refractive index of the second clad changes according to the porosity. That is, when the porosity is increased, the effective refractive index of the second cladding decreases. Accordingly, by making the second clad a porous structure, the relative refractive index difference between the second clad and the first clad can be made relatively large, and the numerical aperture of pumping light in the double clad fiber is greatly increased. It becomes possible to do.

In addition, if not only the second cladding but also the first cladding has a porous structure having a large number of pores extending in the central axis direction of the fiber, the porosity can be adjusted. The refractive index (effective refractive index) of the first clad changes only by the above. Therefore, if the relative refractive index difference between the core and the first clad is made relatively small,
Even if the core diameter is increased, a single mode is achieved. That is, by making the first clad a porous structure,
An enlarged core structure of a double clad fiber is obtained.

Further, since the relative refractive index difference between the second clad and the first clad can be increased only by making the second clad a porous structure, it is necessary to form the second clad with a resin or the like. Disappears. Therefore, the problem of thermal stability of the double clad fiber can be solved by forming the second clad from SiO ₂ .

Here, in the second aspect of the present invention, it is preferable that the porosity of the first cladding is set lower than the porosity of the second cladding. Thereby, the effective refractive index of the second clad becomes smaller than the effective refractive index of the first clad, and the pumping light can propagate in the first clad.

In the second aspect of the present invention, it is preferable that the porosity of the first cladding is set to 2.5% or less, for example.

As a result, the relative refractive index difference between the core and the first clad becomes relatively small. As described above, even if the core diameter is enlarged, a single mode is formed.

Further, according to the second aspect of the present invention, for example, the pores of the first clad may be periodically arranged in the cross section of the fiber.

As a result, the effective refractive index of the first clad has wavelength dependence, and there is a condition that a single mode is obtained at all wavelengths.

The cross section of the first clad may be formed in a polygonal shape such as a triangle, a quadrangle or a hexagon, a circle, an ellipse, or the like.

In addition, the core may be doped with a rare earth element. Here, the rare earth element may be at least one of erbium (Er), neodymium (Nd) and ytterbium (Yb).

The inventions according to claims 1 to 6 relate to a double-clad fiber, but the inventions according to claims 7 to 10 relate to a method for manufacturing the same.
According to a seventh aspect of the present invention, there is provided a core which extends in the central axis direction of the fiber and through which the signal light propagates, a first clad which covers the periphery of the core and through which pumping light for exciting the signal light propagates, and a periphery of the first clad. A method for manufacturing a double-clad fiber having a second clad covering the core, the core comprising the core of the fiber, and the first clad covering the periphery of the core and serving as the first clad of the fiber. A first preform producing step of producing a first preform having the first preform, and the first preform is disposed in a tubular support tube such that the core portion is located substantially at the center of the support tube; By disposing a plurality of tubular capillaries between the first preform and the support tube, a second clad portion serving as a second clad of the fiber is formed to form a second clad.
A second preform producing step of producing a preform;
And a drawing step of drawing the second preform by heating and drawing the fiber into a fiber shape.

According to the present invention, in the first preform manufacturing step, a first preform having a core portion and a first clad portion surrounding the core portion is manufactured. This first preform is, for example, a VAD method,
A preform having a core portion and a clad portion (first clad portion) may be manufactured by a known method such as an OVD method or a rod-in-tube method, and the preform may be manufactured by grinding.

Then, in the second preform preparation step, a second preform is prepared. That is, the first preform is disposed in a cylindrical support tube so that the core portion is located substantially at the center of the support tube, and a plurality of first preforms are provided between the first preform and the support tube. By disposing a cylindrical capillary, a second clad portion is formed to produce a second preform.

The thus prepared second preform is
In the drawing step, the fiber is heated and drawn to draw a fiber. Thereby, while the portion of the first preform becomes the core and the first clad of the double clad fiber, the holes of the capillaries in the second clad portion are oriented in the fiber central axis direction in the second clad of the double clad fiber. By forming an extending hole, the second clad is configured to have a porous structure. Thus, the second clad having a porous structure can be easily manufactured only by disposing a plurality of capillaries in the support tube.

By the way, the above-mentioned first preform manufacturing process is a process which has been conventionally performed when manufacturing a double clad fiber. However, since the preform is ground, it is extremely complicated. However, there is an inconvenience that it is difficult to form a complicated cross-sectional shape.

The invention according to claim 8 is a method for producing a double-clad fiber more easily from such a viewpoint. More specifically, a core extending in the central axis direction of the fiber and through which signal light propagates. A double clad fiber manufacturing method comprising: a first clad that covers the periphery of the core and through which pump light for exciting the signal light propagates; and a second clad that covers the periphery of the first clad. It is assumed that the method includes a preform manufacturing step of manufacturing a reform and a drawing step of heating and stretching the preform to draw a fiber.

In the preform manufacturing step, at least one rod-shaped core rod is disposed substantially at the center of the cylindrical support tube, thereby forming a core portion serving as a core of the fiber. Forming a first clad portion for forming a first clad portion to be a first clad of the fiber by forming a plurality of rod-shaped first clad rods around the core portion in the support tube; A second clad forming a second clad part to be a second clad of the fiber by disposing a plurality of tubular second clad capillaries between the first clad part and the support tube; And a part forming step.

According to the invention described in claim 8, in the core portion forming step in the preform manufacturing step, at least one rod-shaped core rod is disposed substantially at the center of the cylindrical support pipe. A core part to be a fiber core is formed. Here, the number of core rods to be provided may be set according to the diameter of the rod and the core diameter of the fiber. The core rod may be one doped with, for example, an excitation light active substance or Ge.

In the first cladding portion forming step, a plurality of rod-shaped first cladding rods are disposed around the core portion in the support tube, so that the first cladding of the fiber becomes the first cladding. A clad portion is formed.

In the second cladding portion forming step, a plurality of cylindrical second cladding capillaries are arranged between the first cladding portion and the support tube, so that the second cladding of the fiber is formed. A second clad portion is formed.

As described above, since the first clad portion to be the first clad is formed by a plurality of rods for the first clad, the first clad portion (the first clad portion) can be formed only by adjusting the arrangement of the rods. ) Can be easily formed into a desired cross-sectional shape, for example, a polygonal shape, a circular shape, or an elliptical shape. Therefore, a grinding process is not required in the production of the double clad fiber, and the production cost is significantly reduced.

When the first clad portion of the preform is formed by a plurality of rods, the first clad portion and the second clad portion in the cross section of the preform may be formed in the preform manufacturing step. And a mixed layer in which first cladding rods and second cladding capillaries are alternately arranged along the boundary.

As a result, the interface between the first clad and the second clad in the cross section of the drawn double-clad fiber becomes uneven, and the excitation light is randomly reflected at this interface. For this reason, the ratio of the pumping light crossing the core is further improved, and the pumping efficiency of the double clad fiber is improved.

On the other hand, the invention according to claim 10 is suitable for manufacturing the double-clad fiber according to claim 2, specifically, a core extending in the central axis direction of the fiber and propagating signal light; A preform is manufactured for a method of manufacturing a double-clad fiber including a first clad that covers the periphery of the first cladding and in which the excitation light for exciting the signal light propagates, and a second clad that covers the periphery of the first clad. The method includes a preform manufacturing step and a drawing step of heating and stretching the preform to draw a fiber.

In the preform manufacturing step, at least one rod-shaped core rod is disposed substantially at the center of the cylindrical support tube, thereby forming a core portion serving as a core of the fiber. Forming a first cladding portion that forms a first cladding portion of the fiber by disposing a plurality of cylindrical first cladding capillaries around the core portion in the support tube; By forming and forming a plurality of cylindrical second cladding capillaries between the first cladding portion and the support tube,
And forming a second clad portion for forming a second clad portion serving as a second clad of the fiber.

According to the tenth aspect of the present invention, since the first clad portion of the preform is formed by a plurality of the first clad capillaries, the first clad capillary is formed in the same manner as in the seventh aspect. By simply adjusting the arrangement of the first clad portion (first clad portion) having a desired cross-sectional shape,
Cladding) can be easily formed. Therefore, the manufacturing cost of the double clad fiber is greatly reduced.

By adjusting the arrangement of the first cladding capillary, the first cladding hole is formed as follows.
It is also easy to form double-clad fibers periodically arranged in the fiber cross section.

[0036]

As described above, according to the double clad fiber of the present invention, at least the second clad has a porous structure having a large number of pores extending in the central axis direction of the fiber. The relative refractive index difference between the clad and the first clad can be made relatively large,
It is possible to increase the numerical aperture for the excitation light in the double clad fiber. Further, the second clad is made of Si.
Even if it is formed of O ₂ , a desired effective refractive index can be obtained by forming a porous structure, so that the thermal stability of the double clad fiber can be improved as compared with the case where the second clad is formed of resin. it can.

Further, according to the method for producing a double-clad fiber of the present invention, a large number of holes extending in the central axis direction of the fiber can be provided only by disposing a plurality of cylindrical capillaries in a cylindrical support tube. The second clad having a porous structure can be easily manufactured.

Further, the first clad portion of the preform is
The first clad portion (the first clad of the double clad fiber) can be easily formed to have a desired cross-sectional shape by being formed by a plurality of first clad rods or capillaries.

[0039]

Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment> FIG. 1 shows a double clad fiber 1 according to a first embodiment of the present invention, which comprises a core 11 through which signal light propagates, and a signal light which covers the core 11 to cover the core. A first clad 12 through which excitation light for exciting light propagates, a second clad 13 covering the periphery of the first clad 12, and a support portion 14 covering the periphery of the second clad 13 are provided.

The core 11 is a single mode core made of SiO ₂ , which is disposed substantially at the center of the double clad fiber 1 and extends in the direction of the central axis of the double clad fiber 1. Is doped with, for example, Ge so that the refractive index is higher than that of the first cladding 12, and the signal light in the core 11 is amplified by pumping light propagating through the first cladding 12. For example, rare earth elements (specifically, Er, N
d, Yb, etc.).

The first cladding 12 covers the periphery of the core 11, extends in the direction of the central axis of the double clad fiber 1, and is made of SiO ₂ .
Although the cross-sectional shape of the first clad 12 is formed in a substantially regular hexagonal shape in the illustrated example, it is not limited to the regular hexagonal shape, but may be formed in a triangular shape, a polygonal shape such as a rectangular shape, a circular shape, or an elliptical shape. You may.

The second clad 13 is disposed so as to extend in the central axis direction of the double-clad fiber 1 while covering the periphery of the first clad 12, and the second clad 13 is also made of SiO ₂ . . The second clad 13 has a porous structure having a large number of holes 13a, 13a,... Extending in the central axis direction of the double clad fiber 1, and the holes 13a, 13a,. The fibers 1 are arranged so as to be almost closest in the cross section. The holes 13a, 13
a may be arranged so as to be substantially uniform in the circumferential direction in the cross section of the fiber.

The support portion 14 is provided so as to cover the second clad 13 so as to protect the second clad 13 having a porous structure and improve the mechanical strength of the double clad fiber 1. Has become.

Although not shown, a coating material (eg, a coating material such as an ultraviolet curable resin) provided on a normal communication optical fiber or the like is provided around the support portion 14. Good.

As described above, since the second clad 13 of the double clad fiber 1 has a porous structure having a large number of pores extending in the central axis direction of the fiber, the refractive index of the second clad 13 is large. (Effective refractive index) is
Are determined by the refractive index of the air in each of the holes 13a, 13a,... And the refractive index of the SiO ₂ portion other than the holes (the portion between the holes). The hole 13 with respect to the total volume (cross-sectional area) in the clad 13
The effective refractive index of the second clad 13 changes according to the volume (the ratio of the cross-sectional area) of the second clad 13. That is,
The smaller the porosity is, the larger the effective refractive index of the second cladding 13 is, and the larger the porosity is, the smaller the effective refractive index of the second cladding 13 is.

.. Are arranged so that the distance (pitch) 最 between the centers of a pair of adjacent holes 13a, 13a is constant. The porosity is defined by the ratio (d / Λ) of the pitch Λ to the diameter d of each hole 13a. Therefore, d
/ Λ is smaller, the porosity is smaller and the second clad 1
3, the larger the effective refractive index of d.
The porosity increases and the effective refractive index of the second cladding 13 decreases.

As described above, by making the second clad 13 have a porous structure, the effective refractive index can be changed to about 1 to 1.46 in accordance with the porosity. Along with this,
The numerical aperture of the double clad fiber 1 for the excitation light can be changed to about 0.01 to 0.99. For this reason, compared with a conventional double clad fiber in which the second clad is formed of an ultraviolet curable resin (about 0.5 numerical aperture), it is possible to greatly increase the numerical aperture for excitation light. As a result, it becomes possible to make high power pumping light incident on the double clad fiber 1, and the amplification efficiency of signal light is improved. Further, since the second clad 13 is made of SiO ₂ , the thermal stability of the double clad fiber 1 can be improved.

Next, a method of manufacturing the double clad fiber 1 will be described with reference to FIG. FIG.
Shows a preform 2 of the double clad fiber 1, and the production of the double clad fiber 1 is performed by a preform manufacturing step of manufacturing the preform 2, and the preform 2 is heated and drawn into a fiber shape. And a drawing step of drawing. Since the preform 2 is symmetric, the right half of the preform 2 is not shown in FIG.

First, the preform manufacturing process will be described. A cylindrical support tube 24 having a substantially circular cross section is prepared.
At the approximate center of the support tube 24, seven round rod-shaped rods 21a made of SiO ₂ are arranged to form the core portion 21 which becomes the core 11 of the double clad fiber 1 (core portion forming step). . The core 21 is formed in a substantially circular shape by disposing six core rods 21a around one core rod 21a disposed substantially at the center of the support tube 24 so as to be in contact with each other. What is necessary is just to form so that it may become. The core rod 21a is preferably doped with Ge or the like for increasing the refractive index in advance and doped with an excitation light active substance in advance. The number of core rods 21a forming the core portion 21 may be appropriately adjusted according to the diameter of the core rod 21a and the diameter of the core 11 of the double clad fiber 1.

By arranging a plurality of round rod-shaped first cladding rods 22a made of SiO ₂ around the core portion 21 in the support tube 24, the first cladding 12 of the double clad fiber 1 is provided. Is formed (first clad portion forming step). At this time, the first clad rod 22a may be arranged in the closest density so that the first clad portion 22 has a substantially regular hexagonal cross section. However, the first clad rod 22a in the double clad fiber 1 after the drawing step is formed. If the solid 12 has a solid shape, the first cladding rod 22a need not be arranged in the closest density. The first cladding rod 22a may be doped with, for example, Ge.

By disposing a plurality of cylindrical SiO ₂ second cladding capillaries 23 a between the first cladding portion 22 and the support tube 24, the second cladding fiber 2 of the double clad fiber 1 is provided. The second clad portion 23 to be 13 is formed (second clad portion forming step). The second cladding capillary 23a may also be arranged at the closest density. However, even if the second cladding capillary 23a is not arranged at the closest density, the second cladding capillary 23a may have the holes 13a, .. May be arranged so as to be distributed substantially uniformly in the circumferential direction of the double clad fiber 1. However, the second cladding capillary 23a
Is preferably disposed such that the pitch Λ is 5 μm or less in the second clad 13 of the double clad fiber 1.

Thus, the preform 2 in which the core rod 21a, the first cladding rod 22a, and the second cladding capillary 23a are disposed in the support tube 24 is produced. ).
Note that there is no limitation on the order in which the core part forming step, the first clad part forming step, and the second clad part forming step are performed.

The preform 2 is heated and drawn in a drawing furnace (not shown) to form a fiber (drawing step). In this drawing step, a coating material may be provided around the fiber 1. By performing this drawing step, the holes of the second cladding capillary 23a form the holes of the second cladding 13, and the second cladding 13 has a porous structure, and the first cladding rods 22a are fused with each other. 1 to form the first clad 12, and the core rods 21a are fused together to form the core 11, whereby the double clad fiber 1 as shown in FIG. 1 is manufactured. The support tube 24
Forms the support portion 14 of the double clad fiber 1.

As described above, only by disposing a plurality of second cladding capillaries 23a, 23a,...
The clad 13 can be configured to have a porous structure. Also, when producing the preform, the inner diameter of the second cladding capillary 23a and the second cladding capillary
3a (a pair of capillaries 23a, 23a)
The diameter d and the pitch 孔 of the hole 13a of the second clad 13 can be changed only by changing the distance between “a”, and the effective refractive index of the second clad 13 can be changed.

Since the first clad portion 22 of the preform 2 is also formed by a plurality of first clad rods 22a, the first clad portion 22 (first clad 12) can be formed without performing a grinding step. It can be formed in a desired cross-sectional shape. That is, in the illustrated example, the cross-sectional shape of the first clad 12 (the first clad portion 22) is a substantially regular hexagonal shape, but the first clad 12 is merely adjusted by adjusting the arrangement of the first clad rod 22a. 12 can be formed in a polygonal shape such as a triangular shape or a rectangular shape, a circular shape, an elliptical shape, or the like. Therefore, the double clad fiber 1
Can significantly reduce the manufacturing cost.

The boundary between the first cladding part 22 and the second cladding part 23 in the preform 2 is, for example, as shown in FIG.
As shown in the figure, only the first cladding rod 22a is arranged closest to one side of the boundary (boundary surface) S,
The first cladding rod 22a and the second cladding capillary 23a are arranged so that only the second cladding capillary 23a is disposed closest to the other side of the boundary surface S.
and a may be individually arranged to form the boundary surface S.

In contrast to this, for example, as shown in FIG.
A mixed layer 25 in which the first cladding rods 22a and the second cladding capillaries 23a are alternately arranged along the boundary (boundary surface) S at the boundary between the second cladding portion 23 and the second cladding portion 23. May be provided. By providing the mixed layer 25 in this manner, the interface between the first clad 12 and the second clad 13 in the double clad fiber 1 becomes uneven, and the excitation light is randomly reflected at this interface. Thereby, the ratio of the excitation light crossing the core 11 is further increased, and the excitation efficiency can be improved.

The first cladding rod 22a is not limited to a round bar, but may be a bar-shaped member 22b having a regular hexagonal cross section as shown in FIG. The second cladding capillary 23a is also a cylindrical member 2 having a regular hexagonal cross section.
3b may be used. In the illustrated example, the shape of the hole of the second cladding capillary 23b is formed in a circular shape, but the shape of the hole is not limited to a circular shape. When the first cladding rod 22b or the second cladding capillary 23b having such a regular hexagonal cross-sectional shape is disposed in the support tube 24, the first cladding rod 22b or the second cladding capillary 23b is used. What is necessary is just to arrange side by side so that sides may mutually adhere.
In this case, the first clad rod 22b and the like are disposed without forming a gap between the adjacent first clad rods 22b and the like. Further, in the second cladding portion 23 in which the second cladding capillary 23b is provided, the holes of the capillary 23b are arranged in the closest density.

The first clad rod 22b and the second clad rod 22b
Even when the cross-sectional shape of the cladding capillary 23b is a regular hexagon, as shown in FIG. 6, the first cladding rod is provided at the boundary between the first cladding portion 22 and the second cladding portion 23 in the preform 2. A mixed layer 25 in which the caps 22b and the second cladding capillaries 23b are alternately arranged along the boundary surface S may be provided.

(Modification) Next, a method of manufacturing the double clad fiber 1 different from the above will be described with reference to FIG. This figure also shows the double clad fiber 1
2, but the illustration of the right half of the preform 3 is omitted as in FIG.

The method for manufacturing the double-clad fiber 1 according to this modification includes a first preform manufacturing step, a second preform manufacturing step, and heating and heating the second preform.
Drawing and drawing in a fiber shape.

In the first preform manufacturing step, the core portion 31 serving as the core 11 of the double clad fiber 1 is formed.
And a step of producing a first preform 35 having a first clad part 32 covering the periphery of the core part 32 and serving as the first clad 12 of the double clad fiber 1.
The first preform 35 is, for example, a VAD method, an OV
A preform having a core portion 31 and a clad portion (first clad portion 32) is manufactured by a known method such as the D method or the rod-in-tube method, and the preform is ground to have a substantially hexagonal cross section. What is necessary is just to produce by processing.

Next, a tubular support tube 34 is prepared, and the core portion 31 of the first preform 35 is attached to the support tube 3.
4 so that the first preform 3
5 is disposed in a cylindrical support tube 34, and a plurality of second cylindrical cladding capillaries 33 a are disposed between the first preform 35 and the support tube 34, thereby providing a second clad. The clad part 33 is formed. The second cladding capillary 33a may be the same as the second cladding capillary 23a in the first embodiment. Further, the second cladding rod 33a is arranged closest in the illustrated example, but is not limited to this.

Thus, the second preform 3 in which the first preform 35 and the plurality of second cladding capillaries 33a are disposed in the support tube 34 is manufactured (second preform manufacturing step). . The order in which the first preform 35 and the second cladding capillary 33a are arranged in the support tube 34 is not limited.

The process of drawing the second preform 36 is the same as in the first embodiment.

According to this manufacturing method, as shown in FIG. 1, the double clad fiber 1 having the porous structure in which the second clad 13 has a large number of holes 13a extending in the central axis direction of the fiber is manufactured. can do.

<Second Embodiment> FIG. 8 shows a double clad fiber 5 according to a second embodiment of the present invention. The present embodiment is different from the first embodiment in that it has a porous structure having a large number of holes 52a, 53a extending in the direction of the central axis. In the illustrated example, the first cladding 52 is formed in a substantially rectangular cross section, but may be formed in another polygonal shape, a circular shape, or an elliptical shape in the same manner as in the first embodiment. is there.

Here, a preferred embodiment of the first clad 52 having a porous structure will be described. For example, considering the case where the holes 52a and 53a of the first and second claddings are arranged closest, the diameter of each hole 52a in the first cladding 52 is d1, the pitch is Λ1, and the second
When the diameter of each hole 53a in the cladding 53 is d2 and the pitch is Λ2, it is preferable that the following formula is satisfied: d2 / Λ2 ≦ d1 / Λ1 (1). That is, the first
It is preferable that the porosity of the cladding 52 be lower than the porosity of the second cladding 53. Thereby, the second
The effective refractive index of the clad 53 becomes smaller than the effective refractive index of the first clad 52, and the pumping light can be propagated in the first clad 52.

Further, it is preferable that the first cladding 52 be configured so as to satisfy d1 / ｄ1 ≦ 0.15 (2). Thereby, the porosity of the first clad 52 is set to 2.5% or less, the relative refractive index difference between the core 51 and the first clad 52 becomes relatively small, and the diameter of the core 51 of the double clad fiber 5 is reduced. Even if it is enlarged, it becomes single mode. That is, an enlarged structure of the core 51 of the double clad fiber 5 is obtained.

Further, the holes 52a of the first clad 52 are formed.
Are preferably arranged periodically in the fiber cross section. This periodic arrangement includes the arrangement at the closest density. As a result, the effective refractive index of the first cladding 52 has wavelength dependency, and there is a condition that a single mode is obtained at all wavelengths.

Next, a method of manufacturing the double-clad fiber 5 according to the second embodiment will be described with reference to FIG. 9. Compared with the method of manufacturing the preform 2 of the double-clad fiber 1 according to the first embodiment, Then (see FIG. 2), the preform 4 of the double-clad fiber according to the second embodiment is formed by disposing the first clad portion 42a having a circular cylindrical shape for the first clad 42a. The only difference is that the formation of the core portion 41 by disposing the core rod 41a in the support tube 44 and the formation of the second cladding portion 43 by disposing the second cladding capillary 43a are the same as those described above. This is the same as the production of the preform 2 according to one embodiment.

The above formulas (1) and (2) can be satisfied by appropriately selecting the first cladding capillary 42a and the second cladding capillary 43a used for producing the preform 4. For example, as an example, as the first cladding capillary 42a,
When the outer diameter is the same as the outer diameter of the second cladding capillary 43a, and the inner diameter is smaller than the inner diameter of the second cladding capillary 43a, the above expression (1) is satisfied. Can be.

As described above, the double clad fiber 5 according to the second embodiment can easily change the relative refractive index difference between the core 51 and the first clad 52 by configuring the first clad 52 to have a porous structure. Thus, the double clad fiber 5 having the enlarged diameter of the core 51 while satisfying the condition of the single mode is configured.

A double-clad fiber 5 in which both the first and second claddings 52.53 have a porous structure is also provided.
It can be formed simply by disposing the first and second cladding capillaries 42a and 43a in the support tube 44.
Further, the first clad is connected to the first clad capillary 42.
Since it is formed by a, it can be easily formed in the cross-sectional shape of the first clad 52.

[0076]

Next, an embodiment in which a double clad fiber is manufactured by the manufacturing method according to the present invention will be described.

<First Example> In the first example, a double-clad fiber was manufactured by the manufacturing method according to the first embodiment, and the following was used as a support tube and the like.

Support tube: φ40mm × 5t × 300m
mL core rod (relative index difference Δ1.0% (Ge-doped),
Yb dope (3000 ppm): φ500 μm × 30
0 mmL First cladding rod (Δ0.5% (Ge-doped)):
φ500μm × 300mmL Capillary for second cladding: outer diameter φ500, inner diameter φ20
As shown in FIG. 2, seven core rods 21 a are disposed substantially at the center of the support tube 24 to form the core part 21, and the core part 21 is formed around the core part 21. The first clad portion 22 was formed by arranging the rods 22a for one clad in the closest density so as to have a substantially regular hexagonal cross section. Further, the second cladding capillary 23a was arranged between the first cladding portion 22 and the support tube 24 in the closest density to form the second cladding portion 23. The second cladding capillary 23a is:
A chlorine treatment is performed, and a sealing process for sealing both ends is performed.

The preform 2 thus produced was set on a lathe and subjected to a chlorine treatment. Further, the inside of the support tube 24 was depressurized, and then both ends of the support tube 24 were sealed.

Then, the preform 2 was drawn to produce a double clad fiber having an outer diameter of 125 μm. At the time of this drawing step, a coating process of providing a coating material around the fiber was performed.

The double clad fiber 1 (see FIG. 1) manufactured in this manner has a core 11 diameter of about φ5 μm, a size of the first clad 12 (a length of a diagonal line of a regular hexagonal cross section) of about 60 μm, and a second clad fiber. 13 is 94 μm, and the diameter d of each hole 13 a of the second clad 13 is 1.
1 μm and the pitch Λ was 1.6 μm. The numerical aperture for the excitation light was 0.85 to 0.86.

<Second Example> In a second example, a double-clad fiber was manufactured by the manufacturing method according to the second embodiment, and the following was used as a support tube and the like.

Support tube: φ40mm × 5t × 300m
mL Core rod (Δ1.0% (Ge-doped), Yb-doped (3000 ppm)): φ500 μm × 300 mmL First cladding capillary: outer diameter φ500, inner diameter φ10
0 μm × 300 mmL Capillary for second cladding: outer diameter φ500, inner diameter φ35
0 μm × 300 mmL As shown in FIG. 9, the core rod 41 a is disposed substantially at the center of the support tube 44 to form the core portion 41, and the first cladding is formed around the core portion 41. The first cladding part 42 was formed by disposing the capillary for use 42a so as to have a substantially rectangular cross section. Further, the second cladding capillary 43a was disposed between the first cladding portion 42 and the support tube 44 to form the second cladding portion 43.

The first and second cladding capillaries 42a and 43a are subjected to a chlorine treatment and a sealing treatment for sealing both ends thereof.

The preform 4 thus manufactured is set on a lathe and subjected to a chlorine treatment. Further, after the inside of the support tube 44 is depressurized, both ends of the support tube 44 are sealed. By drawing
The production of a double clad fiber having an outer diameter of 125 μm is the same as in the first embodiment.

The double clad fiber 5 (see FIG. 8) manufactured as described above has a core 51 having a diameter of approximately φ2.
4 μm, and the diameter d1 of each hole 52a of the first cladding 52 is set to 0.
9 μm, the pitch # 1 is 1.6 μm, the diameter d2 of each hole 53a of the second cladding 53 is 1.4 μm, and the pitch # 2 is 1.6.
μm. The numerical aperture for the excitation light is 0.1.
85-0.86.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a double clad fiber according to a first embodiment.

FIG. 2 is a sectional view showing a preform of the double clad fiber according to the first embodiment.

FIG. 3 is an enlarged cross-sectional view showing an interface between a first and a second cladding.

FIG. 4 is an enlarged cross-sectional view showing an example in which a mixed layer is provided at an interface between a first cladding part and a second cladding part.

FIG. 5 is a cross-sectional view showing, in an enlarged scale, an interface between a first and a second clad portion when a capillary and a rod having a regular hexagonal cross section are used.

FIG. 6 is an enlarged cross-sectional view showing an example in which a mixed layer is provided at the interface between the first and second clad portions when a capillary and a rod having a regular hexagonal cross section are used.

FIG. 7 is a cross-sectional view showing a preform of a double clad fiber according to a modification of the first embodiment.

FIG. 8 is a diagram corresponding to FIG. 1, illustrating a double clad fiber according to a second embodiment.

FIG. 9 is a cross-sectional view showing a preform of a double clad fiber according to a second embodiment.

[Explanation of symbols]

 1,5 double clad fiber 2,4 preform 3 second preform 11,51 core 12,52 first clad 13,53 second clad 14,54 support part 21-41 core part 22-42 first clad part 23 To 43 second clad part 24 to 44 support tube 25 mixed layer 35 first preform 13a, 53a hole (second clad) 21a, 41a core rod 22a, 22b first clad rod 23a to 43a, 23b second clad Capillary 42a Capillary for first cladding 52a Hole (first cladding) S Interface (boundary)

Continued on the front page (72) Inventor Norio Naka, 4-chome Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Minoru Yoshi ▼ 4-chome 3, Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries In Itami Works, Ltd. (72) Inventor, Moriyuki Fujita 4-3-1, Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. In Itami Works, Ltd. (72) Inventor, Masataka Nakazawa 2-3-1, Otemachi, Chiyoda-ku, Tokyo Japan Within Telegraph and Telephone Co., Ltd. (72) Inventor Hirokazu Kubota 2-3-1, Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Co., Ltd. F-term in Nippon Telegraph and Telephone Corporation (reference) 2H050 AB04Y AB05X AB18X AC01 AC09 AC36 AC71 AD00 4G021 BA02 HA00

Claims (10)

[Claims] 1. A core extending in a central axis direction of a fiber, through which a signal light propagates, a first cladding that covers the periphery of the core, and through which an excitation light that excites the signal light propagates, and a first cladding that covers the periphery of the first cladding. A double-clad fiber comprising two clads, wherein the second clad has a porous structure having a large number of pores extending in a central axis direction of the fiber. 2. A core extending in the central axis direction of the fiber, through which a signal light propagates, a first cladding that covers the periphery of the core, and through which an excitation light for exciting the signal light propagates, and a first cladding that covers the periphery of the first cladding. A double-clad fiber comprising: two claddings, wherein the first and second claddings have a porous structure having a large number of pores extending in a central axis direction of the fiber. fiber. 3. The double clad fiber according to claim 2, wherein the porosity of the first cladding is set lower than the porosity of the second cladding. 4. The double clad fiber according to claim 2, wherein the porosity of the first clad is set to 2.5% or less. 5. The double clad fiber according to claim 2, wherein the pores of the first clad are periodically arranged in a cross section of the fiber. 6. The double-clad fiber according to claim 1, wherein the core is doped with a rare earth element. 7. A core extending in the central axis direction of the fiber, through which the signal light propagates, a first cladding that covers the periphery of the core, and through which the pumping light for exciting the signal light propagates, and a first cladding that covers the periphery of the first cladding. A method of manufacturing a double-clad fiber comprising two clads, comprising: a first core portion serving as a core of the fiber; and a first clad portion covering a periphery of the core portion and serving as a first clad of the fiber. A first preform producing step of producing a preform; and disposing the first preform in a cylindrical support tube such that the core portion is located substantially at the center of the support tube. By arranging a plurality of cylindrical capillaries between the preform and the support tube, a second clad portion serving as a second clad of the fiber is formed to form a second preform. And renovation manufacturing process manufacturing method of a double clad fiber, characterized in that it comprises a drawing step of drawing in the second by heating and drawing the preform fibrous. 8. A core that extends in the central axis direction of the fiber and through which signal light propagates, a first cladding that covers the periphery of the core, and through which excitation light that excites the signal light propagates, and a first cladding that surrounds the first clad. A method for producing a double-clad fiber, comprising: a preform producing step of producing a preform; and a drawing step of heating and stretching the preform to draw the fiber into a fiber shape. The manufacturing step includes a core part forming step of forming a core part that becomes a core of the fiber by arranging at least one rod for a rod-shaped core substantially at the center of the cylindrical support pipe; A first cladding portion forming a first cladding portion serving as a first cladding of the fiber by disposing a plurality of rod-shaped first cladding rods around the core portion; Forming a second clad part to be a second clad of the fiber by disposing a plurality of cylindrical second clad capillaries between the first clad part and the support tube. A method for producing a double-clad fiber, comprising a step of forming a clad portion. 9. The preform manufacturing process according to claim 8, wherein the first clad rod and the second clad capillary are formed at the boundary between the first clad and the second clad in the cross section of the preform. A method of manufacturing a double-clad fiber, comprising providing mixed layers alternately arranged along a portion. 10. A core extending in the central axis direction of the fiber, through which signal light propagates, a first cladding that covers the periphery of the core, and through which pumping light for exciting the signal light propagates, and a first cladding that covers the periphery of the first cladding. A method for producing a double-clad fiber, comprising: a preform producing step of producing a preform; and a drawing step of heating and stretching the preform to draw the fiber into a fiber shape. The manufacturing step includes: a core part forming step of forming a core part serving as a core of the fiber by arranging at least one rod-shaped core rod substantially at the center of the cylindrical support pipe; By disposing a plurality of cylindrical first cladding capillaries around the core portion, a first cladding portion serving as a first cladding portion of the fiber is formed.
Forming a second clad portion to be a second clad of the fiber by disposing a plurality of cylindrical second clad capillaries between the first clad portion and the support tube; A method for manufacturing a double-clad fiber, comprising: a second clad portion forming step.