JPS6110036A - Preform for optical fiber - Google Patents

Abstract

PURPOSE:An intermediate layer with a larger deformation resistance is allowed to lie between the core and the clad layer to improve the roundness of the resultant optical fiber and increase resolving power in transmitted images. CONSTITUTION:The objective preform for optical fiber is obtained by allowing an intermediate layer 3 of quartz which has an intermediate refractive index between the core 2 and the clad layer 3 and larger deformation resistance in fiber drawing to lie between the core 2 of doped quartz glass of high refractive index and the clad layer 4 of doped quartz glass of low refractive index. Thus, the intermediate layer function, when fibers are formed, as a suppressor against free deformation of the softened core part 2 to give an optical fiber with the core of almost complete round cross section. When these fibers are used as image guides, more clearly transmitted images are obtained.

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention provides an optical fiber with excellent core roundness by interposing an intermediate layer (N) with higher deformation resistance between the core portion and the cladding layer. This invention relates to a silica glass-based optical fiber preform that can be used.

BACKGROUND OF THE INVENTION Multiple optical fibers made by bundling together a large number of optical fibers are known. This is expected to be applied in various technical fields such as the field of medical optical instruments.

Recently, the development of multiple optical fibers made of silica glass-based optical fibers, in which adjacent optical fibers are fused to each other, has been progressing. This is due to the MM points of conventional multi-component glass optical fibers that are simply gathered together and bundled.□ There is a lot of optical loss and the optical transmission distance is short, and the optical fibers are not flexible enough. This overcomes the disadvantages of □, such as the fact that wires break easily and therefore have a short service life.

Up to this IL, the multi-glue optical fiber made of silica glass-based optical fiber is one in which a core made of doped silica glass or pure silica glass with a high refractive index is coated with a cladding layer made of doped silica glass with a lower refractive index. I have a suggestion. These are manufactured by bundling a large number of preforms each consisting of a corresponding core part and claride layer, and heating them to a state where adjacent fibers of each group are fused together.

Problems to be Solved by the Invention However, when a preform consisting of a core made of doped quartz glass and a cladding layer is bundled and drawn, it is difficult to obtain the desired result for each optical fiber in the multiple optical fiber. The problem with Kametsu 9 is that the core is highly deformed. That is, the cross-sectional shape of the core of each optical fiber is generally irregular, such as a circular shape, a triangular shape, or other polygonal or complicated shapes.

When a multi-glue optical fiber consisting of a bundle of optical fibers whose cores have irregular cross-sectional shapes is used, for example, as an image guide, problems such as poor field clarity or resolution of the obtained transmitted image occur.

On the other hand, preform #' having a core made of pure silica glass
There was a problem in that the difference in refractive index between the core and the crano layer in the resulting optical fiber was small, and as a result, the numerical aperture was small.

Means for Solving the Problems The present invention provides a core portion made of doped quartz glass with a high refractive index, based on the refractive index of the intermediate layer made of silica-based glass, and an intermediate layer covering the core portion. , a cladding layer made of doped silica glass having a low refractive index covering the intermediate layer, and the intermediate layer has a higher deformation resistance at a drawing temperature than the core portion and the cladding layer. Ruru provides renovation services.

As shown in FIG. 1, the preform IFi of the present invention, the core part 2
．． It consists of an intermediate layer 8 and a cladding layer 4.

Further, the core portion 2 is made of doped quartz glass having a higher refractive index with respect to the refractive index of the intermediate layer 8, and the cladding layer 4
is made of doped silica glass with a lower refractive index.

That is, the core portion 2 is made of silica glass containing a dopant that increases the refractive index v, such as germanium-based or aluminum-based dopant.

The cladding layer is made of silica glass containing a dopant that lowers the refractive index, such as boron-based or fluorine-based dopant. On the other hand, the intermediate layer 8ii interposed between the core part and the cod layer is made of silica glass, and has a higher deformation resistance at the preform 10 drawing temperature than the core part and the cod layer. It is formed in I.

The deformation resistance at drawing temperature refers to the resistance force when a preform is heated to soften it and then drawn into a wire to make it thinner.

Therefore, the intermediate layer in the preform of the present invention is
When drawing the wire, it is formed so that it has better shape retention than the core and cladding layers and is less likely to form. Therefore, in the present invention, the material of the fine silica glass forming the intermediate layer is relatively determined based on the deformation resistance at the drawing temperature of the doped quartz glass forming the core portion or cladding layer. In general, the higher the purity of quartz glass, the greater the deformation resistance, so examples of silica-based glass forming the intermediate layer include natural quartz glass, synthetic quartz glass, and quartz glass with a lower dopant content than the core or cladding layer. is used. When the doped quartz glass is completely bent, it is desirable to set the dopant content to 172 F, which is the smaller of the dopant contents in the doped quartz glass forming the core part or the quad layer, from the viewpoint of the difference in the degree of deformation resistance. The quartz glass that can be preferably used to form the intermediate layer has a purity of 99.9.
9% 1'J, the upper quartz glass. However, when a germanium-based dopant is contained, for example, the deformation resistance of the doped silica glass increases. In such a case, it is appropriate to suppress an increase in the deformation resistance by forming the core part using a phosphorus-based dopant, for example.

The preform of the present invention can be manufactured by, for example, the VAD method, C
The VD method, the τ17 do-in-tube method, or a suitable combination of these methods can be used in any way. The preform in the present invention may be manufactured by the method described above, and may be in the state of seven pieces, or may be made thinner by further drawing the preform.

Further, a support N5 made of quartz glass may be provided on the outside of the preform glass layer of the present invention (
(FIG. 2) As the quartz-based glass, the same glass as in the case of the above-mentioned intermediate layer is used. However, it is not necessary that all intermediate layers and support layers in the preform be made of the same material. It is preferable that the support layer has a deformation resistance of the same level as the intermediate layer or a little higher than the intermediate layer, but it may be even smaller than the intermediate layer by 1 and is not limited thereto. When the support Mt is provided, there is an advantage that when the preform gold wire is drawn to form an optical fiber, the core is easily located at the center of the optical fiber.

In addition, by treating the preform having a support layer with a hydrofluoric acid solution or the like to remove the support layer,
It is also possible to provide a preform consisting only of a core portion, an intermediate layer, and a cladding layer.

In the preform of the present invention, the ratio of the outer layer of the core: the thickness of the intermediate layer: the thickness of the cladding layer is 0:0.05 to 2:
0.5-6%, preferably 10:0.1-1:1-4, more preferably 10:0.2-0.4:2-8.

If the thickness ratio of the intermediate layer is less than 0.05, it will be difficult to control the thickness of the intermediate layer in the resulting optical fiber, and the purpose of providing the intermediate layer will be lost.
If it exceeds n, the core space factor of the resulting optical fiber will become too small, which is not preferable. On the other hand, if the cladding layer thickness ratio is less than 0.6, it will not substantially function as a cladding layer in the resulting optical fiber, and on the other hand, if it exceeds 6, the core space factor in the resulting optical fiber will decrease. is undesirable because it is too small.

In addition, the ratio of the outer diameter of the core part to the thickness of the support layer provided as necessary is 10:0.05 to 2, preferably 10:
:0.1-1. More preferably 10:0.2-0.8
It is.

The preform of the present invention is preferably used for forming multiple optical fibers as a bundle of a large number of optical fibers. In particular, as shown in FIG.
It is preferably used to form multiple optical fibers in which the optical fibers are fused together.

For example, the production of multi-1 to optical fibers is 100 μm in diameter.
-Multiple preforms of 1 san, e.g. 100'0-5
For example, one end of a bundle obtained by bundling 0,000
This can be done by heating it to a temperature of 800 to 2200°C to soften it, and drawing it to a predetermined thickness, for example, 0.1 to 8 cm. Desired multiple optical fibers can be easily obtained by appropriately changing the preform used, the number of bundled fibers, the drawing conditions, etc. In the multiple optical fibers formed using the preform of the present invention, the degree of irregularity of the core 2' in each optical fiber 1' is small, and its cross-sectional shape is close to a perfect circle (Fig. 4). When used as a transmitter, the sharpness or resolution of the resulting transmitted image is immediately improved. In particular, when the preform is formed using a preform having a support percentage (Fig. 5), the degree of irregularity of each optical fiber 1' in the multiple optical fibers is small - [, regular polygon in cross section (normal case: regular hexagonal An ideal configuration in which optical fibers (nearly square) are arranged regularly and in a close-packed state, and the core 2' and intermediate layer 8' are concentrically located at the center of the optical fiber with a perfectly circular cross section. It has the advantage that it is easier to obtain something similar. Multiple optical fibers that are close to the ideal form will quickly improve the sharpness and resolution of the transmitted image. In addition, 4'
is the cladding layer of the optical fiber 1', and 5' is its support layer.

The above-mentioned multiple optical fiber can also be manufactured by drawing only a bundle of preforms, but it is preferable that the synthetic fiber is a pipe made of natural quartz (hereinafter referred to as an F1 quartz skin pipe) and a preform TA array. This is a manufacturing method in which it is filled and then drawn. In this case, the obtained multiple optical fibers have a structure in which a quartz skin layer that is attached to the quartz skin vibe is fused and exists on the outer periphery of a group of mutually fused optical fibers. Multiple - If the surface of an optical fiber has irregularities or scratches, it will easily break when bent, reducing its flexibility. Draw a line only at the bundle of preforms! Although unevenness tends to occur on the outer peripheral surface of multidel optical fibers, when it has a quartz skin layer, unevenness and scratches are less likely to occur. The thickness of the quartz skin layer is 10 to 800111 nm, preferably Vi 80 to 200 μm, particularly 50 to 11007 μm.

Quartz skin layer t - The quartz glass constituting the layer has an inner diameter of 28 mm,
When a pipe with an outer diameter of 26w1 is melted by heating and drawn at a speed of 0.5rnZ, the drawing force is 50Of or more when drawing it out as a pipe with an inner diameter of 2.8 m and an outer diameter of 2.6 m.
It is preferable that the minimum temperature at which this occurs is at least 1800°C, particularly at least 1900°C.

In addition, in the bundle of preforms to be drawn into a multidel optical fiber, the obtained multidel optical fiber is
Since the optical fiber u1j in the optical fiber does not contain holes or the like, it is desirable that the preform be interposed between the preforms in the entire material bundle that becomes liquid at the drawing temperature. The holes may locally bend the optical fiber in the multidel optical fiber or cause the diameter of the optical fiber to vary locally, which causes scattering loss and, as a result, produces dark spots in the multidel optical fiber. That will happen.

Functions and Effects In the preform Hc of the present invention, the intermediate layer B is the core part 2
Based on the fact that the deformation resistance at the drawing temperature is higher than that of Kurasod wI4, it functions as a restraining wall against the free deformation of the core portion softened during wire drawing, and completely prevents or suppresses deformation of the core portion, and The cross-sectional shape of the core in the obtained optical fiber is made to be close to a perfect circle.

As a result, constraints such as the softening temperature difference between the core part and the cladding layer due to wire drawing can be relaxed, making it possible to contain even more dopants. This makes it possible to increase the optical fiber's numerical aperture, reducing photometry from the core to the cladding layer, resulting in an optical fiber whose numerical aperture is close to I.

When the preform of the present invention is used to fuse adjacent optical fibers to each other to form a multidel optical fiber in the following state, a multidel optical fiber consisting of very small optical fibers can be obtained. , by the way, 6000
Diameter t of multiple optical fibers made of wood optical fibers
``It is also possible to set it to 0.5 m, and the resolution of the fL7j mano V triple optical fiber, which can be obtained by 0%, is also TT-C.

Examples Example 1 GeC14 (100cq4) 1, POCI B (15
cc/min) G(l) formed by fully bent internal CVD method
Pure silica glass Vt'' having a quartz glass layer containing a P-based dopant on the inner wall, the same <BFgt- Dovan of B-based and F yarns formed by internal CVD method using %-
Quartz glass P containing gold? The core part (n
An intermediate layer made of pure silica glass (n D : 1.455, thickness 0.8 m) is placed on the outside of the inner layer (L 47 , outer diameter 12.6 m).

On the outside thereof, a crondo layer (nD: 1.44, thickness 'J 8.0 m) made of B-based and F-based doped silica glass,
Furthermore, a support M (n p.

1.455. ! ! "g 0.6m) outer diameter 20
．． A primary preform of 8 m was obtained.

Next, a secondary preform (length 6°α) of 800 μm outer diameter scrap was formed by drawing the above primary preform.
000 pieces were bundled so that the cross section was in the shape of a close-packed optical fiber, one end of which was inserted into a quartz glass tube with an inner diameter of 85 m+, and then heated and fused. It was washed in distilled water while being subjected to ultrasonic waves, and then dried.

Next, one end of the bundle of secondary preforms on the quartz glass tube side was heated to 1900°C and drawn.
Multi-optical fibers having an outer diameter of 0.8+W were obtained by fusion-bonding 12,000 adjacent silica glass optical fibers to each other.

When the end face of the obtained multidel optical fiber was observed under a microscope, it was found that the core of the optical fiber was located at the center of the optical fiber, and its cross-sectional shape was uniformly close to a perfect circle. In addition, the flexibility of the multi-optical fibers was extremely good. Furthermore, an objective lens (40 degree viewing angle, manufactured by Yataka Seiki Co., Ltd.) consisting of a pair of lenses 6 as shown in Fig. 6 is attached to one end of the multiple optical fiber with a length of 50 mm, and an eyepiece lens ( When an image guide with a focal length of 15 m) was attached and had a magnification of 10 times, and an image 80 cm ahead of the end of the objective lens was observed, the point resolution Fio was 4wn5.

Example 2 Example 1 except that 6000 secondary preforms with the support layer removed using a 25% by volume hydrofluoric acid aqueous solution were used.
In the same manner as above, a multi-optical fiber having an outer diameter of 0.68+m was obtained. The core of this optical fiber rarely had a uniform cross-sectional shape close to a perfect circle. Further, its flexibility is extremely good, and the point resolution of the image guide produced in the same manner as in Example 1 is 0.5. I failed. In addition,
The objective lens has an outer diameter of 0.8 mm and an outer diameter of o, ssm.
Q was prepared by soaking.

The core part without intermediate NI is 999 using the comparison and exception CVD method.
Multiple optical fibers were obtained in the same manner as in Example 2, except that a primary preform made of 9% silica glass was formed. The core of this type of optical fiber has a large degree of irregularity, including triangular cross-sectional shapes.
Multi-1 - Optical fibers were not of practical use as image guides.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views showing structural examples of the preform of the present invention, and FIG. 8 is a cross-sectional view of multiple optical fibers.
4 and 6 are enlarged sectional views of the optical fibers in the multi-optical fiber 1, and FIG. 6 is a sectional view of the objective lens.

Claims (1)

[Claims] 1. Based on the refractive index of the intermediate layer made of silica-based glass, a core made of doped silica glass with a high refractive index, an intermediate layer covering the core, and the intermediate layer 1. A preform for an optical fiber, comprising a cladding layer made of doped silica glass having a low refractive index, the intermediate layer having a higher deformation resistance at a drawing temperature than the core portion and the cladding layer. 2. The preform according to claim 1, which has a support layer made of quartz glass on the outside of the cladding layer. 8. The preform according to claim 1, which is used as a multiple optical fiber as a bundle of a large number of optical fibers. 4. The preform according to claim 8, which is a multiple optical fiber in which adjacent optical fibers are fused to each other.