CA1245488A - Optical multiple fiber - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An optical multiple fiber comprising a multiplicity of optical fibers which are fused together with each other, each optical fiber comprising a core made of pure silica glass and a cladding layer disposed on the core and made of a dopant-containing silica glass, characterized in that the thickness of the cladding layer satisfies the following equation (I):
2 D1 ? T1 ? 1.0 µm. (I) wherein T1 is the thickness of the cladding layer in µm.
and D1 is the outer diameter of the core in µm., in order to improve the image-transmitting capacity of the multiple fiber, including the sharpness and brightness of transmitted image.

Description

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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view of a mul-tiple fiber in accordance with the present invention and il-lus-trates the disposition of cons-tituent op-tical fibers therein.
Fig. 2 is a cross-sec-tional view of an optical fiber included in a mul-tiple fiber in accordance with -the present invention and having a two-layer construc-tion.
Fig. 3 is a cross-sectional view of a multiple fiber, illus-trating the state that there is a lacking portion that is not occupied by an optical fiber.
F'ig. 4 is a cross-sec-tional view of an op-tical fiber included in a multiple fiber in accordance wi-th the presen-t invention and having a three-layer cons-truction.
BACKGROUND OF THE INVENTION
The present invention relates to an optical multiple fiber (hereinafter referred to as "multiple fiber") having a construction that a multiplicity of silica glass optical fibers are fused together with each other and more particularly to a multiple fiber, each constituent optical fiber of which consists of a pure silica glass core and a cladding layer thereon made of a silica glass containing a dopant and having a lower refractive indeY~ than pure silica glass.
The above-mentioned type of multiple fiber which consists of optical fibers each having a core of pure silica glass retains desirable properties such as excellent heat resistance and radiation resistance in-herent in pure silica glass, and is useful as an image-guide for monitoring the inside of a high-temperature furnace, a nuclear reactor, and the like.
Generally, a silica glass multiple fiber is produced by bundling a multiplici-ty of optical fiber preforms or -those obtained by drawing the preform -to reduce it in diameter (the above-mentioned optical fi-ber preform and those obtained by drawing the preform to reduce i-t in diameter are included in -the scope of ' ~

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"preform" intended in the present invention) and drawing -the resulting bundle at a high temperature, whereby each preform is reduced in diameter into an optical fiber and simultaneously adjacent optica:l fibers are fused -together with each other.
As compared with pure silica glass which con-stitutes the core, the dopant-containing silica glass of the claddina layer has a very low softening point and a very low resistance to deformation in the sof-tened state.
For that reason, in the case of a multiple fiber produced by the above-mentioned drawing technique, as illus-trated in Fig. 1, the core 2 of each optical fiber 1 included in the multiple fiber retains a circular cross-section similar -to that of the core of the preform, while the cladding layer 3 on -the core 2 acquires a hexagonal outer shape in cross-section and the cladding layer 3 having such outer shape is fused together with the cladding lay-ers of the adjacent optical fibers, most of the optical fibers -thus being arranged in such a manner that hexa-gons are most closely packed.
However, the production of the multiple fiberof silica glasses is very difficult as compared with mul-tiple fibers of multi-component glasses becasue -the soft-ening point and melt viscosity of silica glass are much higher than -those of multi-component glass. It is only recently that it became promising to produce such a sil-ica glass multiple fiber. Under these circumstances, there are few studies dealing with such problems as how to improve various properties required for multiple fib-er, such as image-transmitting capacity.
The present inventors have found that the sec-tional structure of each optical fiber included in a sil-ica glass multiple fiber, especially -the interrelation-ship between the core diameter and the cladding layer thickness, has a great influence on the various proper-ties of the multiple fiber, including the image-transmit-ting capaci-ty. This finding and an intensive study based thereon have led to the present invention.
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SUMMARY OF THE INVENTION
The present invention provides a multiple fi-ber comprising a multiplicity of optical fibers fused to-gether with each other, each optical fiber comprising a 5 pure silica glass core and a cladding layer thereon maAe of a dopant-containing silica glass and having a thick-ness which sa-tisfies the following equation (I~:

2 Dl ~ T ~ 1.0 ~m- (I) wherein Tl is the thickness of the claddi.ng layer in ~m.
and Dl is the outer diameter of the core in ~m In the above, the cladding layer thickness Tl means -the shortest dis-tance between a side of the hexa-gon defined by the conto~lr of the deformed cladding lay-er and the core surface, as illus-trated in Fig. 2.

DETAILED DESCRIPTION
In a mul-tiple fiber having a construction that a mul-tiplicity of optical fibers are fused together and each optical fiber consists of a core made of a pure sil-ica glass and a cladding layer made of a dopant-containing silica glass, most of the optical fibers have a hexagonal outer shape (namely, the cladding layer has the hexagonal outer shape) and they are arranged in the mos-t closely packed state, as described above.
In the present invention, it has been found -that when the thickness Tl of the cladding layer of each optical fiber included in a multiple fiber and having the above-mentioned shape satisfies the equation (I) mentioned above, the sharpness of transmitted images is improved and the flexibility of the multiple fiber is ensured.
In case that the optical fibers included in a multiple fiber are fused together with each other, the chance of leak of light from individual optical fibers is not nil and the light leakage affects adversely the sharpness of -transmit-ted image, and consequently the thickness of the cladding layer must be more than a B

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certain value. On the other hand, the thermal expansion coefficient of the silica glass of the cladding layer increases with an increasing content of dopant, as compared with pure silica glass which constitutes the core.
From the standpoint of preventing the light leakage, it is desirable to enlarge the difference between the refractive index of the core and that of the cladding layer by increasing the dopant content. However, an increased dopant content results in a very great difference in thermal expansion coefficient between the core and the cladding layer, which causes cracking of the cladding layer and thereby makes the production of preforms themselves difficult. Accordingly, the difference in refractive index between the core and the cladding layer cannot be very great. For that reason, the cladding layer thickness must be increased in order to prevent the light leakage.
As a result of research by the present inventors, it has been found that when the thickness Tl of the cladding layer of each optical fiber included in a multiple fiber is at least 1.0 ~m., preferably at least 1.3 ~m., more preferably at least 1.5 ~m., and most preferably at least 1.8 ~m., the above-mentioned light leakage is markedly reduced, hence the sharpness of transmitted images is improved. Moreover, when the cladding layer thickness Tl is not less than the above limit, leak of light can be prevented efficiently even when the difference in refractive index between the core and the cladding layer is small, for example, 0.012 i 0.006. As a result, the production of preforms becomes facilitated. From the standpoints of further reducing light leakage and narrowing the differences in thermal expansion coefficient, softening point, thermal deformation resistance and other properties between the core and the cladding layer, the above-mentioned refractive index difference is preferably 0.012 ~ 0.004.
On the other hand, when the thickness of the cladding layer of each optical fiber included in a ~z~s~

multiple fiber is too large, the core volume ratio (~) of the optical fiber as defined by the formllla:

Sectional area of core Sectional area of optical fiber becomes small, hence the brightness of transmitted images is desreased and moreover the sharpness of transmitted images is decreased due to stray light resulting from an increased quantity of light entering the cladding layer from the multiple fiber end face. Furthermore, an excessive cladding layer thickness results in an increased outer diameter of optical fiber, which in trun leads to an increased outer diameter multiple fiber especially when the number of constituent optical fibers is great. The result is a decreased flexibility o~ the multiple fiber.
For improving the multiple fiber flexibility, it is prererable to employ an outer diameter of multiple fiber of not more than 5 mm.
In view of the above, it is preferable that the cladding layer thickness Tl is not more than 2 times the outer diameter Dl of the core in accordnace with the present invention. The cladding layer thickness Tl is more preferably not more than 1 time the core outer diameter Dl, and most preferably not more than 0.5 time the core outer diameter.
The core outer diameter Dl is usually about 5 to about 15 ~m.
When the core outer diameter Dl is in the above range with the multiple fiber outer diameter being from 0.5 to 3 mm. and the number of constituent optical fibers contained in the multiple fiber being from 1,000 to 30,000, it is preferable that the cladding layer thickness Tl satisfies the following equation (II):

3.0 ~m. 2 Tl > 1.0 llm- (II) in particular, the following equation (III):
2.3 llm. > Tl > 1.5 ~m. (III) In the presen-t invention, it is also preEerable that the core volume ratio is in the range of 20 to 60 %.
A smaller core volume ratio results in a decreased quantity of light transmitted. In case that a multiple fiber is used as an image-guide, the multiple fiber is required to have a core volume ratio of at least 20 % in order that transmitted images have a practically required minimum level of brightness. On the other hand, a greater core volume ratio is desirable from the standpoint of brightness of transmitted images. However, for obtaining a multiple fiber having a high core volume ratio, preforms having small cladding layer thickness must be preparedO
In order that the thic~ness Tl of the cladding layer of each constituent optical fiber included in a multiple fiber obtained from such preforms can securely be not less than the above-mentioned minimum value, namely 1.0 ~m., the bundle of preforms must be drawn in such a manner that each consitutent optical fiber have a large outer diameter as a necessity, but in that case a problem arises wlth respect to the flexibility of the multiple fiber. In view of the foregoing, the core volume ratio is preferably not more than 60 %, more preferably from 25 to 50 ~, and most preferably from 35 to 45 %.
As already mentioned above, when a multiple fiber having a construction that a multiplicity of optical fibers are fused together and each optical fiber consists of a pure silica glass core and a cladding layer of a dopant-containing silica glass is produced by the drawing technique, each constituent optical fiber in the multiple fiber obtaind bears a hexagonal contour in cross section, with the constituent optical fibers being arranged in such a manner that such hexagons are most closely packed, as shown in Fig. 1.
However, if preforms are bundled irregularly, there may arise in some cases a portion 5 where an optical fiber is lacking in the multiple fiber obtained, as illustrated in Fig. 3. Although this lacking portion 5 is filled up with part of the cladding layer-constituting ~;~45 ~

silica glass coming from the surrounding constituent optical fibers, there remains a problem that this filling up reduces the thickness of the cladding layer of the surrounding optical fibers, whexeby the function of the cladding layer becomes poor or is lost. Therefore, if such a lacking portion 5 once is formed, the multiple fiber composed of optical fibers eac'n having a pure silica glass core inevitably con-tains optical fibers deficient with respect to the cladding layer thickness mentioned above. Such multiple fiber, when used as an image-guide, is not always satisfactory with respect to image-transmitting capacity or, in other words, sharpness of transmitted images.
In view of the above, the present invention further proposes a multiple fiber composed of optical fibers each having a sufficiently thick cladding layer even if the cladding layer silica glass would flow into the lacking portion 5 as shown in Fig. 3. This object is accomplished when the thickness Tl of the cladding layer of the constituent optical fiber satisfies the following equation (IV):
2 Dl 2 Tl >0.05 Dl + 1.0 ~m. (IV) preferably the following equation (V):
2 Dl 2 T12 0.06 Dl + 1.0 ~m. (V) In cases where optical fibers are arranged in a multiple fiber in the manner of most closely packed hexagons without any lacking portion 5, as shown in Fig.
1, the extent of light leakage is markedly reduced and the sharpness of transmitted images is improved, as mentioned previously, when the thickness Tl of the cladding layer of each constituent optical fiber is at least 1.0 ~m., preferably at least 1.3 ~m., more preferably at least 1.5 ~m. If the thickness Tl of the cladding layer of each constituent optical fiber satisfies the above equation (IV) or (V), sharpness of transmitted images can be secured even when there is a lacking portion 5 as shown in Fig. 3. This is because even those constituent optical fibers which exist adjacent to the lacking portion 5 and whose cladding layer has partly flowed into the lacking portion 5 mostly still have a cladding layer thickness of at least 1.0 ~m.
In accordance with the present invention, each optical fiber constituting the multiple fiber may have either a two-layer construction composed of a core and a cladding layer or three-layer construction having further on the cladding layer a third thin layer, for example, a support layer made of natural or synthetic silica glass.
When two-layered preforms consis-ting of a core and cladding layer are bundled and drawn, the structure of the cladding layer of each optical fiber included in the resulting multiple fiber ma~ in some cases deviate from the hexagonal structure as shown in Fig. 1 due to excessive fluidization of the cladding layer material.
When three-layered preforms having the above third thin layer are used, such excessive fluidization of the cladding layer is prevented and the cladding layer advantageously acquires a section more close to a regular hexagon. In the case of a multiple fiber composed of three-layered optical fibers, the optical fibers are fused together by fusion of the third thin layer on the cladding layer. In that case, each optical fiber has such a structure that a third layer 4 having a hexagonal contour in cross section exists on a cladding layer 3 having a hexagonal contour in cross section, as shown in Fig. 4.
Constituent optical fibers having such a structure are arranged, as in the case of two-layered optical fibers, in the manner of most closely packed hexagons as shown in Fig. 1.
It is preferable that the thickness T2 of the third layer 4 satisfies the following equation (VI):
1.0 ~m. > T2 2 0.01 ~m. (VI~
The thickness T2 of the third layer 4 means, as illustrated in Fig. 4, the distance between a side of the hexagon of the cladding layer 3 and the side parallel thereto of the hexagon of the third layer 4.
~ hen the thickness of the third layer is too small, the cladding layer expands with heat during drawing, whereby the third layer is broken and the provision of the third layer becomes meaningless. For avoiding such breakage, it is prererable that the third layer thickness T2 is at least 0.01 ~m., more preferably at least 0.05 ~m~, and most preferably at least 0Ol ~m.
On the other hand, an excessively thick third layer reduces the core volume ratio to lower the brightness of transmitted images and further allows an increased quantity of light to enter the third layer and behave as stray light, making transmitted images indistinct or blurred. Therefore, it is desirable that the thickness T2 of the third layer is not more than 1.0 ~m., more preferably noi more than 0.7 ~m., and most preferably not more than 0.5 ~m.
Even when each optical fiber contained in a multiple fiber has the above-mentioned three-layer construction, the cladding layer thickness Tl is required to satisfy the above-mentioned equation (I), (II) or ~III). It is also necessary that the core volume ratio is frorn 20 to 60 ~, preferably from 25 to 50 %, more preferably from 35 to 45 %.
Even in a multiple fiber composed of three-layered optical fibers, there may exist a lacking portion 5 as shown in Fig. 3. In that case, the lacking portion 5 is filled up with the materiales which flow thereinto from the cladding layer and the third layer thereon. If the cladding layer thickness Tl satisfies the above-mentioned equation (IV) or (V), the cladding layer o~ those optical fibers that are located around the lacking portion 5 can have a thickness of at least 1.0 ~m.
The multiple fiber of the present invention can be produced by bundling a required number of two-layered preforms composed of a core and a cladding la~er alone or 3i three-layered preforms composed of a core, a cladding layer and a third thin layer and drawing the bundle of the preforms a-t a temperature of 1,900 to 2,200 C. Although the outer diameter of the resulting multiple fiber and the ~59~

size of each constituent optical fiber contained in the multiple fiber may vary depending on the dimensional proportions of the layers of the preforms used and the extent of drawing, a multiple fiber having a desired structure can be easily produced by performing simple trials using several preforms different in structure and varying the extent of drawing.
Although the multiple fiber of the present invention can be produced by drawing a bundle of preforms alone, it is advantageous to produce the multiple fiber by filling a pipe made of a synthetic or natural silica glass (hereinafter referred to as "silica glass skin pipe" ) with preforms arranged orderly and drawing the preforms and the pipe as a whole. The multiple fiber obtained in that way has a structure such that a silica glass skin layer resulting from the silica glass skin pipe is present surrounding the group of constituent optical fibers fused together. Presence of unevenness or flaws on the surface of the multiple fiber may lead to breakage upon bending, or reduce the flexibiity to be mentioned hereinafter. The multiple fiber obtained by drawing a bundle of preforms alone tends to have an uneven outer surface. When the multiple fiber has a silica glass skin layer, uneveness or flaws are sparingly formed thereon and constant flexibility can be obtained easily. These are advantageous features.
In accordance with the present invention, the silica glass skin layer has a thickness of 10 to 300 ~m., preferably 30 to 200 ~m., and more preferably 50 to 100 ~m. The silica glass constituting the silica glass skin layer has preferably a drawable temperature of at least 1,800C., more preferably at least 1,900C. The drawable temperature is defined as follows: A pipe made of the same silica glass material as that to be used for forming the skin layer and having an inner diameter of 23 mm. and an outer diameter of 26 mm. is drawn to reduce the pipe in diameter, giving a pipe having an inner diameter of 2.3 mm. and an outer diameter of 2.6 mm. The drawable L~

temperature means the lowest temperature that permits to take up the pipe of the reduced diameter at a rate of 0.5 m./min. with a drawing tension of not more than 500 g.
The method of the present invention is explained by means of -the following Examples. These Examples are intended to illustrate the invention and not be construed to limit the scope of the invention. It is understood that various changes and modifications may be rnade in the invention without departing from the spirit and scope thereof.
In the following Examples and Comparative Examples, the materials use~ ~or forming the cladding layer~s were a pure silica glass doped with B2O3 and fluorine and the refractive index of each material was adjusted by varying the amounts of the dopants. In the followings, the difference in refractive index between the pure silica glass of the core and the dopant-containing silica glass of the cladding layer is represented by ~n.
The image-resolving power and flexibility of each multiple fiber were determined and evaluated by the following methods:
(1) Image-resolving power A black paint was applied to the whole surface of a multiple fiber having a length of 50 cm. for excluding external light. Several millimeter portions on both ends were then cut off, and both the end faces were optically polished. A rod-shaped converging type image-forming lens having an outer diameter of 2 mm. and a visual field angle of 35 ("Selfoc Lens" made by Nippon Sheet Glass Company, Limited, SLS: 200 mm., pitch: 0.25) as an objective lens was brought into close contact with one polished end face of the multiple fiber. An external visual field ~as imaged on the end face of the multiple fiber and the image was allowed to be transmitted to the other end face of the multiple fiber, where the image was enlarged by means of a convex lens having a focal length of 15 mm. as an eye lens for observation.
As the object of observation, there was prepared ~2~5~8 a test chart carrying thereon the letter "A" in blue (line width: 2 mm.~ on a red ground. The test chart was placed at a distance of 100 mm. from the object lens and the boundary between the blue letter "A" and the red ground S was observed through the eye lens. The observation results were graded according to the following criteria:
Grade E: The boundary exhibited an excellent contrast.
Grade G: The boundary exhibited a good contrast.
Grade F: The boundary was somewhat blurred but the letter "A" could be identified distinctly.
Grade P: A color between red and blue was observed in the boundary due to a great degree of light leakage.
Grade ~7P: No boundary could be observed.
(2) Flexibility A multiple fiber was bent to a loop~ e form and the loop di~meter was reduced gradually. The flexibility is defined as the loop diameter (mm.) when the multiple fiber was broken.

Example 1 A silica glass skin pipe of 20 cm. in lengt'n was filled with about 12,000 preforms arranged orderly and each having a core outer diameter/cladding layer thickness/silica support layer thickness ratio of lG ~

4.0 : 0.4, ~n of 0.012 and an outer diameter of 260 ~m.
and the whole was drawn at about 2,000C. to give a multiple fiber composed of constituent optical fibers fused together with each other and each having a core outer diameter of 6.1 ~m., a cladding layer thickness Tl of 2.2 ~m., a support layer thickness T2 of .~ ~m. and a core volume ratio of 28.3 ~ and further carrying a silica glass skin layer having a thickness of 120 ~m. and fusedly surrounding the optical fiber aggregate. The outer diameter of the multiple fiber was 1.5 mm. The image-resolving power and flexibility of the multiple fiber were grade E and 290 mm., respectively.

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Exam~les 2 to 13 and Comprative Examples 1 to 4 The procedures oE Example ]. were repeated using an adequate number of preforms of each species and varing the extent of drawing to give various multiple fibers having the structures and characteristics as shown in the following table. As an additional finding, there is mentioned the fact that the transmitted image was very dark when the multiple fibers obtained in Comprative Example 2 and Example 9 were tested.

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Claims (24)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An optical multiple fiber comprising a multiplicity of optical fibers which are fused toge-ther with each other, each optical fiber comprising a core made of pure silica glass and a cladding layer disposed on the core and made of a dopant-containing silica glass, and the cladding layer having a thick-ness which satisfies the following equation (I):

2 D1 ? T1 ? 1.0 µm (I) wherein T1 is the thickness of the cladding layer in µm and D1 is the outer diameter of the core in µm.

2. The multiple fiber of claim 1, wherein the cladding layer thickness T1 satisfies the follow-ing equation (II):

3.0 µm ? T1 ? 1.0 µm (II)

3. The multiple fiber of claim 1, wherein the cladding layer thickness T1 satisfies the above equation (II) and the core volume ratio is from 20 to 60%.

4. The multiple fiber of claim 1, wherein the cladding layer thickness T1 satisfies the follow-ing equation (IV):

2 D1 ? T1 ? 0.05 D1 + 1.0 µm (IV)

5. The multiple fiber of claim 1, wherein each optical fiber further has a silica glass support layer disposed on the cladding layer.

6. The multiple fiber of claim 2, wherein each optical fiber further has a silica glass support layer disposed on the cladding layer.

7. The multiple fiber of claim 3, wherein each optical fiber further has a silica glass support layer disposed on the cladding layer.

8. The multiple fiber of claim 4, wherein each optical fiber further has a silica glass support layer disposed on the cladding layer.

9. The multiple fiber of claim 1, 2 or 3, which has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer of the multiple fiber.

10. The multiple fiber of claim 4, which has a silica glass skin layer having a thickness of 10 to 300 µm as an outermost layer of the multiple fiber.

11. The multiple fiber of claim 5, which has a silica glass skin layer having a thickness of 10 to 300 µm as an outermost layer of the multiple fiber.

12. The multiple fiber of claim 6, which has a silica glass skin layer having a thickness of 10 to 300 µm as an outermost layer of the multiple fiber.

13. The multiple fiber of claim 7, which has a silica glass skin layer having a thickness of 10 to 300 µm as an outermost layer of the multiple fiber.

14. The multiple fiber of claim 8, which has a silica glass skin layer having a thickness of 10 to 300 µm as an outermost layer of the multiple fiber.

15. The multiple fiber of claim 1, 2 or 3, wherein the difference in refractive index between the core and the cladding layer is 0.012 ? 0.004;
the multiple fiber has a silica glass skin layer hav-ing a thickness of 10 to 300 µm as an outermost layer thereof; and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

16. The multiple fiber of claim 4, wherein the difference in refractive index between the core and the cladding layer is 0.012 + 0.004; the multiple fiber has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer thereof;
and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

17. The multiple fiber of claim 5, wherein the difference in refractive index between the core and the cladding layer is 0.012 + 0.004; the multiple fiber has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer thereof;
and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

18. The multiple fiber of claim 6, wherein the difference in refractive index between the core and the cladding layer is 0.012 + 0.004; the multiple fiber has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer thereof;
and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

19. The multiple fiber of claim 7, wherein the difference in refractive index between the core and the cladding layer is 0.012 ? 0.004; the multiple fiber has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer thereof;
and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

20. The multiple fiber of claim 8, wherein the difference in refractive index between the core and the cladding layer is 0.012 ? 0.004; the multiple fiber has a silica glass skin layer having a thick-ness of 10 to 300 µm as an outermost layer thereof;
and the outer diameter of the multiple fiber is from 0.5 to 3 mm.

21. The multiple fiber of claim 11, wherein the multiple fiber is composed of 1,000 to 30,000 op-tical fibers each having a silica glass support layer thickness T2 of 0.01 to 1.0 µm; the silica glass skin layer is made of a silica glass having a drawable tem-perature of not less than 1,800°C and having a thick-ness of 10 to 300 µm; and the outer diameter of the multiple diameter is from 0.5 to 3 mm.

22. The multiple fiber of claim 12, wherein the multiple fiber is composed of 1,000 to 30,000 op-tical fibers each having a silica glass support layer thickness T2 of 0.01 to 1.0 µm; the silica glass skin layer is made of a silica glass having a drawable tem-perature of not less than 1,800°C and having a thick-ness of 10 to 300 µm; and the outer diameter of the multiple diameter is from 0.5 to 3 mm.

23. The multiple fiber of claim 13, wherein the multiple fiber is composed of 1,000 to 30,000 op-tical fibers each having a silica glass support layer thickness T2 of 0.01 to 1.0 µm; the silica glass skin layer is made of a silica glass having a drawable tem-perature of not less than 1,800°C and having a thick-ness of 10 to 300 µm; and the outer diameter of the multiple diameter is from 0.5 to 3 mm.

24. The multiple fiber of claim 14, wherein the multiple fiber is composed of 1,000 to 30,000 op-tical fibers each having a silica glass support layer thickness T2 of 0.01 to 1.0 µm; the silica glass skin layer is made of a silica glass having a drawable tem-perature of not less than 1,800°C and having a thick-ness of 10 to 300 µm; and the outer diameter of the multiple diameter is from 0.5 to 3 mm.