Ultraviolet resistant silica glass fiber

Abstract: A silica glass optical fiber (1) superior in resistance to ultraviolet rays, which fiber (1) comprises a doped silica glass cladding (3) layer formed on a silica glass core (2), said silica glass core having an OH content of 10-1000 ppm, a fluorine content of 50-5000 ppm, and being substantially free of chlorine.

Description: FIELD OF THE INVENTION The present invention relates to a silica glass fiber, such as a single optical fiber and a fiber bundle consisting of a number of optical fibers, which is superior in resistance to ultraviolet rays at the ultraviolet region, and specifically to a fiber bundle suitable for spectral analysis. Also, the present invention relates to a method of transmitting energy or signals by ultraviolet rays, which comprises transmitting ultraviolet rays through said ultraviolet resistant silica glass fiber. BACKGROUND OF THE INVENTION An optical fiber, particularly the one composed of a silica has been used at the ultraviolet region, not to mention at the visible light region. As the core material, used are silica glasses containing OH group but substantially free of fluorine, which may or may not contain chlorine. Said core material is mainly characterized by OH group intentionally contained therein, and is advantageous in that its output is great and initial characteristic is superior. On the other hand, its ultraviolet deterioration resistance characteristic, namely, resistance to the deterioration of core material caused by ultraviolet rays, is not, in general, entirely superior, and specifically said characteristic is insufficient at the wavelength of 215 nm. Such a fiber is known from the paper "Improvement in radiation resistivity of pure-silica core fibers", T. Suzuki et al, Proceedings of Fiber Optics Reliability: Benign and Adverse Environments II, Boston, September 1988, SPIE, pages 36-42. In the meantime, silica glass core materials for optical fibers which have been used at the visible light region have received increasing demand from the field involving exposure to radiation, such as for nuclear power, and resistance to radiation has recently been strongly demanded of the optical fiber for use in such fields, see for instance US-A-4 690 504. A silica glass optical fiber for use in the ultraviolet region, which is composed of a doped silica glass cladding layer formed on a silica glass core, said silica glass core having an OH group content of 10-1000 ppm and containing fluorine, and a method of transmitting ultra-violet light using such a fiber, are known from US-A-4 799 754. In view of the background as described, the present inventors developed a new optical fiber to be used at the visible light region, specifically an optical fiber composed of a silica glass core made of a core material intentionally containing fluorine, but substantially free of chlorine and OH group. This core material exhibits extremely superior resistance to radiation at the visible light region, whereas when used at the ultraviolet region, shows small initial characteristic and small output: substantially it is not practical for use at 250 nm or below. SUMMARY OF THE INVENTION An object of the present invention is to provide a silica glass fiber having excellent resistance to ultraviolet rays, which is for use at the ultraviolet region. Another object of the present invention is to provide a method of transmitting energy or signals using a silica glass fiber having resistance to the deterioration caused by ultraviolet rays. Further objects of the present invention will become clear from the description to follow hereunder. The present inventors have continuously studied in an effort to solve the conventional problems which this kind of optical fiber composed of silica core materials have at the ultraviolet region, and to find out the composition of the silica glass which can reduce degree of deterioration by ultraviolet rays when used at the ultraviolet region and permits efficient transmission of energy or signals. The studies have led the present Inventors to a new idea that fluorine, chlorine and OH group contents in the core material greatly influence ultraviolet resistance characteristic, and that an excellent ultraviolet resistance may be obtained by appropriately adjusting their contents. The present Invention rests in the achievement of the aforementioned objects, motivated by this new idea. The present inventors have found that efficient transmission of energy or signals using ultraviolet rays can be achieved by a single optical fiber composed of a doped silica glass cladding layer formed on a silica glass core or a fiber bundle consisting of a number of said fibers, said silica glass core having (1) an OH group content of 10-1000 ppm and (2) a fluorine content of 50-5000 ppm, and (3) being substantially free of chlorine, i.e. containing at most 1 ppm of chlorine, with greatly reduced deterioration of the silica glass fiber by ultraviolet rays. That said silica glass fiber has excellent resistance to ultraviolet rays is unknown to those skilled in the art from the prior art, and one of ordinary skill in the art would not envision use of said silica glass fiber for the transmission of energy or signals by ultraviolet rays. The present invention relates to a silica glass optical fiber or a fiber bundle for use in the ultraviolet region, wherein the optical fiber is composed of a doped silica glass cladding layer formed on a silica glass core, said silica glass core having (1) an OH group content of 10-1000 ppm and (2) a fluorine content of 50-5000 ppm, and (3) containing at most 1 ppm of chlorine. Also, the present invention relates to a method of transmitting energy or signals using ultraviolet rays, which comprises transmission of energy or signals using ultraviolet rays through an ultraviolet resistant silica glass fiber composed of a doped silica glass cladding layer formed on a silica glass core, said silica glass core having (i) an OH group content of 10-1000 ppm and (2) a fluorine content of 50-5000 ppm, and (3) containing at most 1 ppm of chlorine, or a fiber bundle consisting of a number of said fibers. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a single optical fiber of the present invention as described in Examples, Fig. 2 is also a cross sectional view of a fiber bundle, and Fig. 3 depicts the method for testing the resistance to ultraviolet rays possessed by the optical fiber in the atmosphere. DETAILED DESCRIPTION OF THE INVENTION It is highly characteristic of the core material for a silica glass fiber of the present Invention that it contains OH group and fluorine, but does not substantially contain chlorine as described above. As mentioned earlier, while the conventional core materials for ultraviolet rays contain OH group, they do not substantially contain fluorine, thus resulting in superior initial characteristic but poor ultraviolet deterioration resistance characteristic. On the other hand, the core material for visible light could be improved in radiation resistance by adding fluorine. The core material for a silica glass fiber of the present invention, in contrast, contains OH group as do the conventional core materials for ultraviolet silica glass fibers, and additionally contains fluorine such that the resistance to deterioration caused by ultraviolet rays can be improved. When the fluorine and OH group are contained in a specific proportion of 50-5000 ppm, and 10-1000 ppm, respectively, superior ultraviolet resistance characteristic, particularly remarkable improvements in initial transmission loss at the ultraviolet region and ultraviolet deterioration resistance characteristic can be revealed. The optical fiber of the present invention can be roughly divided into a single fiber and a fiber bundle consisting of a number of said fibers. Fig. 1 is a cross sectional view of a single fiber, wherein 1 is a single optical fiber, 2 is a core layer, 3 is a cladding layer, and 4 is a supporting layer. The supporting layer is a protecting layer which is formed only when necessary and is not always formed. The single fiber 1 in Fig. 1 can be produced, for example, In such a manner that a doped silica glass which becomes the cladding layer 3 is externally applied on a silica glass rod which becomes the core 2, or a cladding layer is internally applied, and the supporting layer 4 of a three-layer structure preforms obtained by the Rod-In-Tube method is removed by fire polishing to give a preform of a two-layer structure of the core 2 and the cladding layer 3. A fiber bundle 10 which is another embodiment of the present invention is produced by bundling a number of fibers, as shown in Flg. 2. The fiber bundle 10 at an output end 11 has normally one or two rows of fibers adhered by an appropriate means such as adhesives, and at a light input end 12, has fibers adhered to each other nearly in a round shape by adhesives. etc., as shown in Fig. 2. The fibers I in an Intermediate part 13 between the output end 11 and the input end 12 are not adhered to each other but are apart. The fiber bundle 10 having this structure, it is flexible as a whole, and is vastly advantageous for use. While the number of the fibers 1 in the fiber bundle is appropriately determined according to the object of use and places to be located, a representative fiber bundle contains 12-24 fibers of 1-3 m in length. The core of the optical fiber of the present invention is composed of a pure silica glass having an OH group content of 10-1000 ppm, preferably 20-600 ppm, more preferably 30-400 ppm, and a fluorine content of 50-5000 ppm, preferably 200-3000 ppm, more preferably 300-2000 ppm, and being substantially free of chlorine, meaning that chlorine may be contained only in a slight proportion of, for example, 1 ppm at most. Such pure silica glass for core can be produced by a production method for synthetic silica glass, comprising, for example, burning at least one member of silicon compounds of the formula R 1 m Si(OR 2 ) 4-m [hereinafter referred to as Compound (I)] and at least one member of fluorine compounds of the formula C a H b F c [hereinafter referred to as Compound (II)] with oxyhydrogen flame, and accumulating the resulting synthetic silica fine particles on a rotating heat-resistant base substance to give a porous sintered silica which is then heat-fused for vitrification. In the above-mentioned formulas, R 1 and R 2 are each a lower alkyl group having 1 to 4 carbon atoms such as methyl, ethyl, n-propyl, n-butyl, iso-butyl, and so on, and 0≦m≦4 (preferably 2≦m≦4), 1≦a≦3 (preferably 1≦ a≦2), 0≦b≦7 (preferably 0≦b≦3), and 1≦c≦8 (preferably 2≦c≦4). The molar ratio of Compound (I) to Compound (II) is normally 100: 0.05-10, preferably 100:0.1-8, more preferably 100:0.1-5. As the heat-resistant base substance, those conventionally known may be used. The cladding layer 3, as a dopant, is made of, for example, a silica glass containing B and/or F. Such doped silica glass can be produced by the well-known Chemical Vapor Deposition method (CVD method) from a gas mixture of BF 3 , SiCl 4 , and oxygen, BCl 3 , BF 3 , SiCl 4 , and oxygen, BCl 3 , SiF 4 , and oxygen, or BF 3 or BCl 3 , SiF 4 , and oxygen as a starting material. Of the aforementioned material gas mixtures, particularly preferable is a gas mixture of BF 3 , SiCl 4 , and oxygen. When the material constituting the support layer 4 to be formed as necessary is a silica glass containing a high degree of impurities, it may exert undesirable effect on the ultraviolet resistance to the extent that it can be expressed numerically. Therefore, the material constituting the support layer 4 is preferably a synthetic silica glass having a drawing temperature of at least 1800°C which is exemplified by natural silica glasses and synthetic silica glasses, with preference given to high purity synthetic silica glass having a purity of not less than 99% by weight, particularly not less than 99.9% by weight. The single optical fiber or the fiber bundle consisting of a number of said fibers are superior in the characteristic at the ultraviolet region as described earlier, and are extremely useful as an optical fiber for use under ultraviolet irradiation, or as a fiber bundle for medical use. The present invention is hereinbelow described in detail by illustrating examples. Example 1 Methyltrimethoxysilane (CH 3 Si(OCH 3 ) 3 ) as a silicon compound and tetrafluorocarbon (CF 4 ) as a fluorine compound were burnt for reaction while supplying 700 N ℓ/h hydrogen gas, 800 N ℓ/h oxygen gas, 500 g/h CH 3 Si(OCH 3 ) 3 , and 0.44 g/h CF 4 . The synthesized silica fine particles were accumulated on a synthetic silica base substance to give a porous silica sintered product having an outer diameter of 60 mm and length of 230 mm. This sintered product was heated in a helium gas at 1550°C under atmospheric pressure into molten glass to give a silica glass rod having an outer diameter of about 35 mm, and length of 200 mm. This glass rod had a chlorine content of 0.1 ppm or below, an OH content of 100 ppm, a fluorine content of 1100 ppm, other impurity content of 5 ppm, and a refractive index at 20°C of 1.4575. The chlorine content of silica glass was measured by activation analysis, the fluorine content was measured by ion analysis, and the OH content was estimated from the absorption loss at a wavelength of 2.73 µm by Fourier transform infrared spectrometer. Explaining the determination of the OH content, it can be calculated by the following formula (1) wherein transmissivity (%) when the OH content at a wavelength of 2.73 µm is 0, is T 0 , actual transmissivity of the measurement subject is T 1 , and the thickness (mm) of the measurement subject is L. Inserting a core rod (outer diameter: 15 mm) obtained by drawing the above-mentioned pure silica glass rod into a glass tube having on its inner surface a B-F-doped silica glass layer (n 20 :1.4465) which was prepared from SiCl 4 , BF 3 , O 2 , and a synthetic silica glass tube (outer diameter: 26 mm, thickness: 1 .5 mm, n 20 :1.459), there was obtained a three-layer structure preform (outer diameter: 16.5 mm) by the Rod-In-Tube method, which was then drawn while heating at 2000°C to give an optical fiber having an outer diameter of 200 µm. Thirteen aforementioned optical fibers (1.5 m in length) were bundled to give a fiber bundle. Examples 2-6, Comparative Examples 1-5 Various kinds of core materials were prepared in the same manner as in Example 1 except that the OH content and F content In the core material were adjusted by changing the CF 4 content in the material gas, and that tetrachlorosilane (SiCl 4 ) was used in place of methyltrimethoxysilane (material gas) in Comparative Examples 4, 5, 9, and 10 in order to adjust the chlorine content, and fiber bundles were produced from respective fibers In the same manner as above. Then, initial characteristic and resistance to ultraviolet-induced deterioration of each fiber bundle obtained were determined, the results of which are summarized in Table 1 and Table 2. Test Method The measurement was conducted by the method shown in Fig. 3. In Fig. 3, 21 is a lamp light source, 22 is a D 2 lamp, 23 is an optical fiber to be tested, and 24 is an instantaneous measurement multi-system, in which used were the following. Lamp light source : POWER SUPPLY C-1316 D 2 lamp : DEUTERIUMLAMP MC 962A Multi-system : MCPD-1100, PC-9801 (personal computer) Example 1 2 3 Composition of core material (ppm) Cl <0.1 <0.1 <0.1 OH 100 100 60 F 1100 1400 1000 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 100 250 nm 233 218 160 230 nm 310 278 232 215 nm 405 309 259 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 95 88 93 1h 230 nm 97 95 95 3h 230 nm 96 94 93 6h 230 nm 94 92 91 12h 230 nm 93 90 90 1h 215 nm 98 96 99 3h 215 nm 97 94 97 6h 215 nm 94 89 90 12h 215 nm 89 80 83 Example 4 5 6 Composition of core material (ppm) Cl <0.1 <0.1 <0.1 OH 30 200 450 F 3300 900 200 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 100 250 nm 131 258 336 230 nm 188 343 431 215 nm 206 397 469 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 94 90 91 1h 230 nm 97 94 91 3h 230 nm 95 93 90 6h 230 nm 94 91 89 12h 230 nm 92 90 87 1h 215 nm 98 96 96 3h 215 nm 95 91 90 6h 215 nm 91 85 85 12h 215 nm 85 78 77 Comparative Example 1 2 3 Composition of core material (ppm) Cl 650 <0.1 <0.1 OH 140 <10 600 F <10 3000 <10 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 100 250 nm 241 4 341 230 nm 341 35 352 215 nm 432 40 306 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 79 output power unmeasurably small 95 1h 230 nm 89 92 3h 230 nm 86 74 6h 230 nm 82 69 12h 230 nm 72 60 1h 215 nm 93 output power unmeasurably small 87 3h 215 nm 86 59 6h 215 nm 75 50 12h 215 nn 59 39 Comparative Example 4 5 Composition of core material (ppm) Cl 200 650 OH 550 140 F <10 <10 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 250 nm 302 327 230 nm 319 368 215 nm 275 317 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 95 98 1h 230 nm 93 97 3h 230 nm 80 95 6h 230 nm 74 84 12h 230 nm 67 75 1h 215 nm 88 96 3h 215 nm 68 92 6h 215 nm 57 78 12h 215 nm 46 60 Examples 7-12, Comparative Examples 6-10 The optical fibers bundled in the fiber bundles of Table 1 and Table 2 were measured for their characteristics in the same manner, the results of which are summarized in Table 3 and Table 4, respectively. Example 7 8 9 Composition of core material (ppm) Cl <0.1 <0.1 <0.1 OH 100 100 60 F 1100 1400 1000 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 100 250 nm 226 214 155 230 nm 290 271 229 215 nm 324 302 252 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 83 86 92 1h 230 nm 96 94 94 3h 230 nm 94 91 93 6h 230 nm 90 87 88 12h 230 nm 82 80 80 1h 215 nm 99 95 99 3h 215 nm 95 91 98 6h 215 nm 88 85 88 12h 215 nm 81 78 82 Example 10 11 12 Composition of core material (ppm) Cl <0.1 <0.1 <0.1 OH 30 200 450 F 3300 900 200 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 100 250 nm 120 260 330 230 nm 177 340 420 215 nm 198 380 465 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 92 90 89 1h 230 nm 96 93 90 3h 230 nm 90 87 84 6h 230 nm 88 84 80 12h 230 nm 84 81 78 1h 215 nm 99 97 95 3h 215 nm 96 89 87 6h 215 nm 89 85 82 12h 215 nm 83 79 74 Comparative Example 6 7 8 Composition of core material (ppm) Cl <0.1 <0.1 <0.1 OH <10 <10 600 F 3000 6000 <10 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm . 100 100 100 250 nm 12 12 307 230 nm 5 9 363 215 nn 12 47 422 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm absolute value of output power unreliably small, unmeasurable absolute value of output power unreliably small, unmeasurable 89 1h 230 nm 82 3h 230 nm 78 6h 230 nm 65 12h 230 nm 47 1h 215 nm absolute value of output power unreliably small, unmeasurable absolute value of output power unreliably small, unmeasurable 82 3h 215 nm 61 6h 215 nm 32 12h 215 nm 21 Comparative Example 9 10 Composition of core material (ppm) Cl 200 650 OH 550 140 F <10 <10 Initial characteristic (ratio to the output at 400 nm) (%) 400 nm 100 100 250 nm 325 154 230 nm 382 201 215 nm 445 231 Resistance to deterioration by ultraviolet rays (ratio to the initial characteristic at the same wavelength) (%) 3h 250 nm 93 83 1h 230 nm 86 87 3h 230 nm 73 75 6h 230 nm 60 63 12h 230 nm 43 47 1h 215 nm 82 82 3h 215 nm 58 69 6h 215 nm 38 27 12h 215 nm 20 20

Claims: A silica glass optical fiber for use in the ultraviolet region, which is composed of a doped silica glass cladding layer formed on a silica glass core, said silica glass core having an OH group content of 10-1000 ppm and containing fluorine, and characterised in that the fluorine content of said silica glass core is 50-5000 ppm, and in that said silica glass core contains at mmost 1ppm of chlorine. A silica glass fiber according to claim 1, wherein said fluorine content is of 200-3000 ppm. The silica glass fiber for use in the ultraviolet region as claimed in claim 1 or 2, wherein the silica glass for the core is a synthetic silica glass produced by burning at least one member selected from the group consisting of silicon compounds of the formula R L / mSi(OR 2 ) 4-m , and at least one member selected from the group consisting of fluorine compounds of the formula C a H b F c , wherein R 1 and R 2 are each a lower alkyl group having 1 to 4 carbon atoms, and 0 ≦ m ≦ 4, 1 ≦ a ≦ 3, 0 ≦ b ≦ 7, and 1 ≦ c ≦8, with oxyhydrogen flame, and vitrifying the synthesized silica fine particles. The silica glass fiber for use in the ultraviolet region as claimed in claim 3, wherein the molar ratio of the silicon compound to the fluorine compound is 100:0.05-10. The silica glass fiber for use in the ultraviolet region as claimed in any one of claim 1 to 4, wherein the cladding layer is made of a silica glass containing B and/or F as a dopant. A fiber bundle for use in the ultraviolet region, wherein a number of optical fibers accarding to one of claims 1 to 5 are bundled. A method of transmitting energy or signals using ultraviolet rays, which comprises transmission of energy or signals by ultraviolet rays through an ultraviolet resistant silica glass optical fiber composed of a doped silica glass cladding layer formed on a silica glass core or a fiber bundle consisting of a number of said fibers, said silica glass core having containing an OH group content of 10-1000 ppm and characterised in that the fluorine content of said silica glass core is 50-5000 ppm, and in that said silica glass core contains at most 1ppm of chlorine. A method of transmitting energy or signals according to claim 7, wherein said fluorine content is of 200-3000 ppm. The method of transmitting energy or signals as claimed in claim 7 or 8, which comprises transmission of ultraviolet rays through a silica glass fiber for use in the ultraviolet region, wherein the silica glass for the core is obtained by burning at least one member selected from the group consisting of silicon compounds of the formula R L / mSi(OR 2 ) 4-m , and at least one member selected from the group consisting of fluorine compounds of the formula C a H b F c wherein R 1 and R 2 are each a lower alkyl group having 1 to 4 carbon atoms, and 0 ≦ m ≦ 4, 1 ≦ a ≦ 3, 0 ≦ b ≦ 7, and 1 ≦ c ≦ 8, with oxyhydrogen flame, and vitrifying the synthesized silica fine particles. The method of transmitting energy or signals according to claim 9, wherein the molar ratio of the silicon compound to the fluorine compound is 100:0.05-10. The method of transmitting energy or signals according to any one of claims 8 to 10, wherein the cladding layer contains B end/or F as a dopant. A method of transmitting energy or signals wherein a fiber bundle for use in the ultraviolet region, which consists of a number of optical fibers according to any one of claims 1 to 5 bundled therein, is used for transmitting ultraviolet rays.

Inventors: HAYAMI HIROYUKI, SUZUKI KOICHI, TAKITA MASATOSHI, HAYAMI, HIROYUKI, SUZUKI, KOICHI, TAKITA, MASATOSHI, Hayami, Hiroyuki, c/o Itami Plant, Suzuki, Koichi, c/o Itami Plant
Applicants: MITSUBISHI CABLE IND LTD, SHINETSU CHEMICAL CO, MITSUBISHI CABLE INDUSTRIES, LTD., SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6