JP2003329869A - Double-clad fiber and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To arrange a first clad layer and a second clad layer concentrically with high accuracy in a double-clad fiber having the first clad layer of a noncircular shape including a polygonal shape. <P>SOLUTION: The first clad layer 2 made of a material whose refractive index is smaller than that of a core layer 1, covering the outer circumference of the core layer and having a noncircular shape at its cross section is provided. A cylindrical second clad layer 3 made of a material whose refractive index is smaller than that of the first clad layer 2 is provided on the outer circumference of the first clad layer 2. The first clad layer 2 is brought into line contact with the inner surface of the second clad layer 3 to be held. The second clad layer 3 is provided with a cylindrical inner tube 3a made of the same material as that of the first clad layer 2 and brought into line contact with the first clad layer 2 and a cylindrical resin layer 3b arranged concentrically on the outer circumference of the cylindrical inner tube 3a and brought into area contact with the cylindrical inner tube 3a. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a core layer having two layers.
The present invention relates to a double clad fiber provided with first and second clad layers and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, there is an optical fiber called a double clad fiber. Double clad fiber
Mainly used for amplification optical fiber. As shown in FIG. 4, the double clad fiber includes a core layer 100 and first and second clad layers 101 and 102. The core layer 100 is usually composed of a single mode core. Then, a first cladding layer 101 (having a smaller refractive index than the core layer 100) and a second cladding layer 102 (having a smaller refractive index than the first cladding layer 101) are sequentially arranged concentrically around the core layer 100. There is.

The double clad fiber has a core layer 100.
While the signal light is propagated to the first cladding layer 101, the excitation light is propagated to the first cladding layer 101. Then, the first cladding layer 101
The excitation light propagating through the laser beam traverses the core layer 100 during the propagation, thereby amplifying the signal light in the core layer 100.

According to the structure of the double-clad fiber,
The first cladding layer 101 having a larger diameter than the core layer 100
High-power pumping can be performed by injecting the light into the optical fiber, and as a result, the gain as the optical amplifier or the output as the light source is improved as compared with the configuration in which the pumping light is input to the single mode core.

The double clad fiber is conventionally manufactured by sequentially performing the following first to third steps. First Step A first clad layer glass preform composed of a glass rod having a core layer glass preform arranged on its axis is formed. Second Step The core layer 100 and the first cladding layer 101 are formed by drawing the first cladding layer glass preform and the core layer glass preform by heating drawing. Third Step The second clad layer 102 is formed by coating the second clad layer material made of a polymer resin around the first clad layer 101.

A resin protective layer is usually coated around the second cladding layer 102.

In the double clad fiber thus manufactured, the core layer 100 and the first clad layer 10
The first and second cladding layers 102 each have a circular cross section and are arranged concentrically with each other.

Conventionally, in the double clad fiber thus constructed, as shown in FIG. 5, the first clad layer 101 is formed into a polygonal cross section (square in the figure). This is a structural improvement carried out for the following purposes. That is, in the double clad fiber, the pumping light is transmitted to the first clad layer 10
It is held in the first cladding layer 101 by reflecting at the interface between the first cladding layer 102 and the first cladding layer 102. At that time, by making the first cladding layer 101 into a polygonal shape, the variation of the reflection angle at each reflection is increased, thereby increasing the frequency of the excitation light crossing the core layer 100 (that is, reducing the skew light). It improves the amplification efficiency.

The shaping (polygonal shape treatment) of the first cladding layer 101 is performed by polishing and shaping the glass base material of the first cladding layer.

[0010]

However, in the double clad fiber, if the first clad layer 101 has a polygonal shape, the following problems occur. That is, when the double clad fiber is manufactured, the first clad layer 101 may be manufactured due to the structural characteristics of the first clad layer 101.
A second cladding layer 102 is formed around the core layer 100 (including the core layer 100).
It becomes difficult to form the uniform. This is because the first clad layer 101 having a polygonal shape cannot be uniformly coated around the second clad layer material (polymer resin). The reason why the second clad layer material cannot be uniformly coated around the first clad layer 101 is that the second clad layer 10 having a polygonal shape is used.
The position of the outer surface of No. 1 varies along the circumferential direction with respect to its axis.

Since the second clad layer 102 cannot be uniformly formed around the first clad layer 101 (including the core layer 100), the conventional double clad fiber has the same structure as the first clad layer 101 (including the core layer 100). The second clad layer 102 could not be accurately aligned with the axis, and eccentricity was generated between the two.

In this way, both cladding layers 101, 10
If the double clad fiber is formed in a state where the two are eccentric to each other, the following problems occur. That is, depending on the degree of eccentricity, the corners of the first cladding layer 101 (the corners are present because of the polygonal shape) may be exposed from the outer surface of the second cladding layer 102. If the first clad layer 101, which is a glass member, is exposed and exposed to the outside, there arises a problem that the strength of the optical fiber in that portion is weakened and the optical fiber is easily broken. If the optical fiber is broken during use and the excitation light is emitted into the air, it is extremely dangerous.

Further, in the state where the eccentricity occurs, it becomes difficult to align the axes of the double clad fibers with each other with high accuracy when the double clad fiber is fused and connected to another optical fiber. If the connection is made in a state where accurate axis alignment is not possible, the light transmission efficiency will be reduced. Further, in a state where the alignment accuracy is low, the double-clad fiber may not be able to be melt-spliced to another optical fiber.

Therefore, a main object of the present invention is to accurately and concentrically arrange the first cladding layer and the second cladding layer in a double-clad fiber having a non-circular first cladding layer including a polygonal shape. .

[0015]

In order to achieve the above-mentioned object, the double-clad fiber of the present invention comprises a core layer and a material having a refractive index smaller than that of the core layer, and the outer periphery of the core layer is A first clad layer having a non-circular cross section provided so as to cover, and a cylindrical second clad layer formed of a material having a refractive index smaller than that of the first clad layer and provided on the outer periphery of the first clad layer. And is characterized in that the first cladding layer is held in line contact with the inner surface of the second cladding layer.

Further, the present invention is characterized by the following steps in the method for producing a double-clad fiber.

That is, the manufacturing method of the present invention comprises a core layer glass preform and a first cladding layer glass preform having a refractive index smaller than that of the core layer glass preform, and the first glass layer around the core layer glass member. Forming a first glass preform by concentrically arranging the first cladding layer glass members; shaping the peripheral surface of the first glass preform into a polygonal cross-section;
A cylindrical inner tube glass preform having an inner diameter equal to the circumscribed circle of the glass preform and having a refractive index equal to or smaller than that of the first cladding layer glass preform is prepared, and the first glass preform is A step of forming a second glass preform by inserting and mounting it into this cylindrical inner tube glass preform, and a core layer sequentially arranged concentrically from the inside by stretching the second glass preform by heating drawing. A step of forming a first clad layer and a cylindrical inner tube, and a cylindrical transparent resin layer having a refractive index smaller than that of the first clad layer in a cylindrical shape around the cylindrical inner tube. And a step of forming a second clad layer including the cylindrical inner tube.

According to the double-clad fiber and the method for manufacturing the same of the present invention, the first cladding layer is held by being in line contact with the second cladding layer. With such a structure, the following configuration can be easily realized. That is, it is possible to make the first clad layer have a polygonal cross section and hold the corner portion of the first clad layer in line contact with the inner surface of the second clad layer. Further, the second cladding layer is made of the same material as that of the first cladding layer and is in line contact with the first cladding layer. It can be configured by including a cylindrical resin layer that contacts.

According to this structure, the second cladding layer can be accurately and concentrically arranged with respect to the first cladding layer.

This is because: -A cylindrical inner tube having an inner diameter equivalent to the circumscribed circle of the first cladding layer can be easily concentrically arranged with respect to the first cladding layer-A cylindrical inner tube having a cylindrical shape However, there are two reasons that the cylindrical transparent resin layer can be easily arranged concentrically.

The inner diameter of the cylindrical inner tube is preferably the same as the circumscribed circle of the first cladding layer. According to this structure, the cylindrical inner tube can be more easily arranged concentrically with the first cladding layer. Furthermore, in this state,
By providing the cylindrical resin layer around the cylindrical inner tube, the second clad layer including the cylindrical resin layer and the cylindrical inner tube can be concentrically arranged with higher accuracy with respect to the first clad layer. .

When the second clad layer is concentrically arranged with respect to the first clad layer, it is preferable that the first clad layer has a polygonal shape as described above. that way,
This makes it easier to grasp the axial center position of the first cladding layer, facilitates axial alignment of both cladding layers, and improves the alignment accuracy.

Further, it is preferable that the cylindrical inner tube is made of a material having a refractive index equal to or smaller than that of the first cladding layer, which increases the light confinement efficiency in the first cladding layer. be able to. In order to increase the light confinement efficiency in the first cladding layer,
It is more preferable that the cylindrical inner tube is made of a material having a refractive index smaller than that of the first cladding layer.

A void is formed between the polygonal first cladding layer and the cylindrical second cladding layer. In that case, an air cladding having a lower refractive index than the second cladding layer is formed in the void. It is possible to provide layers. Then, the light (excitation light) to be confined in the first cladding layer is less likely to diverge to the surroundings, and the confinement efficiency is improved.
Furthermore, if an air clad layer is provided, N
A (numerical aperture) becomes large, and the amount of light propagating through the first cladding layer can be correspondingly increased. By thus improving (increasing) the confinement efficiency and the amount of light (excitation light) in the first cladding layer, it is possible to improve the amplification efficiency when the double-clad fiber is used in an optical amplifier. Becomes

[0025]

1 is a sectional view showing the structure of a double-clad fiber according to an embodiment of the present invention. This double clad fiber includes a core layer 1 and first and second clad layers 2 and 3. The core layer 1 is usually composed of a single mode core. Around the core layer 1, a first cladding layer 2 (having a smaller refractive index than the core layer 1) and a second cladding layer 3 (having a smaller refractive index than the first cladding layer 2) are sequentially arranged concentrically. There is.

The core layer 1 and the first cladding layer 2 are both made of a glass material (eg, quartz glass material). In this double clad fiber used as an amplification optical fiber, the core layer 1 is doped with a rare earth element (for example, erbium). First clad layer 2
Is made of a glass material having a lower refractive index than the core layer 1. The first cladding layer 2 has a rectangular cross section.
The core layer 1 is at the axial center position of the first cladding layer 2 (the center position, which is the intersection of the diagonal lines of the rectangular first cladding layer 2).
It is located in.

The second clad layer 3 has a cylindrical inner tube 3a and a cylindrical transparent resin layer 3b. The cylindrical inner tube 3a has a first
It is composed of a cylindrical glass cylindrical body having a refractive index equal to or smaller than that of the cladding layer 2. The inner diameter r1 of the cylindrical inner tube 3a is the diameter r of the circumscribed circle of the first cladding layer 2.
It is set to the same diameter as 2.

The first clad layer 2 having a rectangular shape is housed and arranged inside the cylindrical inner tube 3a thus constructed. Since the first cladding layer 2 and the inner cylindrical pipe 3a satisfy the above-mentioned shape condition, the following is obtained. That is,
The corner 2a of the first cladding layer 2 is held in line contact with the inner surface of the cylindrical inner tube 3a. Further, the first cladding layer 2 is arranged concentrically with the cylindrical inner tube 3a.

Since the rectangular first cladding layer 2 is housed inside the cylindrical inner tube 3a, gaps are formed between the four circumferential surfaces of the first cladding layer 2 and the cylindrical inner tube 3a. . Air has entered this gap. As a result, the void forms the air clad layer 4. Since the air clad layer 4 is composed of the air layer, it has a smaller refractive index than the first clad layer 2.

The cylindrical transparent resin layer 3b is formed concentrically on the peripheral surface of the cylindrical inner tube 3a. Specifically, the cylindrical transparent resin layer 3b is attached to the peripheral surface of the cylindrical inner tube 3a so as to have a uniform thickness. As a result, the cylindrical transparent resin layer 3b is concentrically arranged in surface contact with the peripheral surface of the cylindrical inner tube 3a.

The second clad layer 3 is functionally constituted by a substantially cylindrical transparent resin layer 3b. Therefore, the cylindrical inner tube 3a may functionally hold the first cladding layer 2. Therefore, the thickness of the cylindrical inner tube 3a is preferably as thin as possible in order to reduce the diameter of the double clad fiber. However, the cylindrical inner tube 3a needs to have a thickness capable of holding the first cladding layer 2 without being damaged during manufacturing. Based on such a viewpoint, the cylindrical inner tube 3
The thickness of a is set.

The thickness of the cylindrical transparent resin layer 3b is set to such an extent that it can exhibit the function of confining the excitation light in the first cladding layer 2 and the function of mechanically retaining the first cladding layer 2 and the core layer 1. .

The outer peripheral side of the cylindrical transparent resin layer 3b is
Although not shown, it may be covered with a protective sheath.

In this double-clad fiber, the second clad layer 3 can be accurately and concentrically arranged with respect to the first clad layer 2. It depends on the following two reasons. The first reason is that the cylindrical inner tube 3a has an inner diameter r1 equivalent to the circumscribed circle r2 of the first cladding layer 2,
That is, the cylindrical inner tube 3a can be easily arranged concentrically with respect to the first cladding layer 2. The second reason is that the cylindrical transparent resin layer 3b is used for the cylindrical inner tube 3a.
That is, they can be easily arranged concentrically.

Furthermore, the fact that the first clad layer 2 has a polygonal shape is another reason why the axial centers of the clad layers 2 and 3 can be easily aligned.

The cylindrical inner tube 3a is connected to the first cladding layer 2 by
Since it is made of a material having a refractive index equal to or smaller than, it has the following advantages. That is,
When light (for example, pumping light) is introduced into the first cladding layer 2 in the double-clad fiber thus configured, the efficiency of confining the light in the first cladding layer 2 is increased.

Since the inner cylindrical tube 3a is made of a material having a refractive index smaller than that of the first cladding layer 2, the efficiency of confining light in the first cladding layer 2 is further enhanced.

Since the air clad layer 4 is provided between the first clad layer 2 and the second clad layer 3, the light (excitation light) confined in the first clad layer 5 is less likely to diverge into the surroundings. As a result, the light confinement efficiency is further improved. Furthermore, by providing the air clad layer 4, the NA (numerical aperture) with respect to the incident light is increased, and the amount of light propagating through the first clad layer 2 can be increased by that amount.

Next, a method of manufacturing the double clad fiber of the present invention will be described with reference to FIG.

First, as shown in FIG. 2A, the first glass base material 22 is manufactured. The first glass base material 22 includes a core layer glass base material 20 and a first clad layer glass member 21. The first cladding layer glass base material 21 is made of a glass material having a smaller refractive index than the core layer glass base material 20. The first glass base material 22 in which the core layer glass base material 20 and the first cladding layer glass member 21 are concentrically arranged has a well-known VAD method (Vapor-phase axial deposition).
Method). The core layer glass preform 20 and the first cladding layer glass preform 21 produced by the VAD method are formed concentrically.

Next, as shown in FIG. 2B, the peripheral surface of the first glass base material 22 (specifically, the peripheral surface of the first clad layer glass base material 21) is polished to obtain the first Glass base material 22
Shape (specifically, the shape of the first cladding layer glass base material 21) is shaped into a polygonal shape (square shape in the drawing). In the figure,
The reference numeral 23 is given to the first glass base material after being shaped into a polygon.

Next, the glass preform 24 for the cylindrical inner tube is prepared. The cylindrical inner tube glass preform 24 has an inner diameter equal to the circumscribed circle of the first glass preform 23 (specifically, the circumscribed circle of the first cladding layer glass preform 21 after being shaped into a polygon). To prepare. Further, the cylindrical inner tube glass base material 2
For No. 4, a glass material having a refractive index equal to or smaller than that of the first cladding layer glass base material 21 is prepared.

Then, as shown in FIG. 2C, the first glass preform 23 is inserted and mounted in the cylindrical inner tube glass preform 24. After inserting and mounting, the cylindrical inner tube glass preform 24 is heat-treated to be contracted in the radial direction so that both preforms 23, 24
Firmly integrates with no gap. As a result, the second glass base material 25 is formed. In the second glass base material 25,
Between the first glass base material 23 and the cylindrical inner tube glass base material 24, voids 26 having a crescent-shaped cross section are formed in all directions.
Both ends of the void 26 are open to the outside, and the void 26 is filled with air.

Next, as shown in FIG. 2D, while the second glass preform 25 is being heated in the heating furnace 27, the double-clad fiber material wire 28 is drawn (drawing) from its tip. To make. Although not shown, the double clad fiber material wire 28 has a configuration including a core layer 1, a first clad layer 2 and a cylindrical inner tube 3a.

Next, the double clad fiber material wire 28
Is coated with a transparent polymer resin by a resin coating device 29. As the transparent polymer resin to be coated, a polymer resin material having a smaller refractive index than the first clad layer 2 is selected. At the time of resin coating, the peripheral surface of the double-clad fiber material wire 28 to be coated (specifically, the peripheral surface of the cylindrical inner tube 3a) is a circumferential surface.
Therefore, the polymer resin to be coated, that is, the cylindrical transparent resin layer 3b is formed concentrically with the peripheral surface of the double-clad fiber material wire 28 (specifically, the peripheral surface of the cylindrical inner tube 3a). After that, a resin protective layer (not shown) is formed around the second cladding layer 3. This completes the double-clad fiber.

In the double clad fiber thus manufactured, the core layer 1, the first clad layer 2 and the second clad layer 3 (the cylindrical inner tube 3a and the cylindrical transparent resin layer 3) are used.
b) means that they are concentrically arranged with high precision.

In the heating drawing process, in order to maintain a high drawing accuracy, it is preferable that the melting point of the first cladding layer glass preform 21 and the melting point of the cylindrical inner tube glass preform 24 be set to substantially the same value. . Both glass base materials 21, 24
The melting point of can be arbitrarily controlled by adjusting the type and amount of the trace element to be added.

In the above-described embodiment, the first cladding layer 2 has a rectangular cross section. The present invention is not limited to the one having such a first cladding layer 2. For example, as shown in FIG. 3A, the first cladding layer 30 having a triangular cross section can be provided. Further, as shown in FIG. 3B, a first clad layer 31 having a pentagonal cross section can be provided. Further, as shown in FIG. 3C, a first clad layer 32 having an elliptical cross section can be provided. Further, as shown in FIG. 3D, the first clad layer 33 having a non-circular shape (in the figure, a polygonal shape such as a square) is formed on the inner surface of the cylindrical inner tube 3 a via the line contact holding member 34. It may be configured to be held in line contact with.
The point is that the non-circular first cladding layer including the polygonal shape may be held in the cylindrical inner tube by line contact.

[0049]

As described above, according to the present invention,
In a double clad fiber having a non-circular first clad layer including a polygonal shape, a first clad layer and a second clad layer are provided.
It is possible to accurately and concentrically arrange the clad layer.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a double clad fiber according to an embodiment of the present invention.

FIG. 2 is a diagram showing each step of the method for manufacturing the double-clad fiber according to the embodiment.

FIG. 3 is a sectional view showing a modified example of the present invention.

FIG. 4 is a sectional view showing a first conventional example.

FIG. 5 is a sectional view showing a second conventional example.

[Explanation of symbols]

1 core layer 2 1st clad layer 2a corner part 3 2nd clad layer 3a cylindrical inner tube 3b cylindrical transparent resin layer 4 air clad layer r1 inner diameter r of cylindrical inner tube 3a
2 circumscribed circle diameter 20 core layer glass base material 21 first clad layer glass base material 22 first glass base material (circle) 23 first glass base material (square) 24 cylindrical inner tube glass base material 26 second glass base material 26 Air gap 27 Heating furnace 28 Double-clad fiber material wire 29 Resin coating device

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor ▲ Yoshi ▼ Minoru Ta             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works F-term (reference) 2H050 AA01 AB03Z AC13 AC36                       AC62 AD00                 5F072 AB09 AK06 YY17

Claims (7)

[Claims] 1. A first non-circular cross-section formed of a core layer and a material having a refractive index smaller than that of the core layer and provided to cover the outer periphery of the core layer.
A clad layer, and a second tubular member formed of a material having a refractive index smaller than that of the first clad layer and provided on the outer periphery of the first clad layer.
A double-clad fiber having a clad layer, wherein the first clad layer is held in line contact with the inner surface of the second clad layer. 2. The double-clad fiber according to claim 1, wherein the first cladding layer has a polygonal cross-section, and the corners of the first cladding layer are in line contact with the inner surface of the second cladding layer. A double-clad fiber characterized by being held. 3. The double clad fiber according to claim 2, wherein the second clad layer is made of a material similar to that of the first clad layer, and has an inner cylindrical tube that is in line contact with the first clad layer. A cylindrical resin layer that is concentrically arranged on the outer circumference of the tube and is in surface contact with the cylindrical inner tube. 4. The double-clad fiber according to claim 3, wherein the cylindrical inner tube has an inner diameter equivalent to the circumscribed circle of the first cladding layer. 5. The double-clad fiber according to claim 3, wherein the cylindrical inner tube is made of a material having a refractive index equal to or smaller than that of the first cladding layer. Clad fiber. 6. The double clad fiber according to claim 1, wherein an air clad layer is provided in a gap provided between the first clad layer and the second clad layer. Double clad fiber. 7. A core layer glass base material and a first clad layer glass base material having a refractive index smaller than that of the core layer glass base material, and the first clad layer glass member is concentrically provided around the core layer glass member. A step of forming the arranged first glass preform, a step of shaping the peripheral surface of the first glass preform into a polygonal cross-section, and an inner diameter equivalent to the circumscribed circle of the first glass preform. A cylindrical inner tube glass preform having a refractive index equal to or smaller than that of the first clad layer glass preform is also prepared, and the first glass preform is inserted and mounted in the cylindrical inner tube glass preform. A step of forming a second glass preform, and a step of forming a core layer, a first cladding layer, and a cylindrical inner tube sequentially arranged from the inner side by stretching the second glass preform by heating drawing. , Around the cylindrical inner tube A transparent resin having a refractive index smaller than that of the first clad layer is mounted in a cylindrical shape to form a second clad layer including the cylindrical transparent resin layer and the cylindrical inner tube. A method for producing a double-clad fiber.