CA1296528C - Method for producing glass preform for optical fiber - Google Patents
Method for producing glass preform for optical fiberInfo
- Publication number
- CA1296528C CA1296528C CA000509253A CA509253A CA1296528C CA 1296528 C CA1296528 C CA 1296528C CA 000509253 A CA000509253 A CA 000509253A CA 509253 A CA509253 A CA 509253A CA 1296528 C CA1296528 C CA 1296528C
- Authority
- CA
- Canada
- Prior art keywords
- fluorine
- preform
- atmosphere
- soot
- containing compound
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 239000011521 glass Substances 0.000 title claims abstract description 39
- 239000013307 optical fiber Substances 0.000 title claims abstract description 17
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 16
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 claims abstract description 70
- 229910052731 fluorine Inorganic materials 0.000 claims abstract description 70
- 239000011737 fluorine Substances 0.000 claims abstract description 70
- 239000004071 soot Substances 0.000 claims abstract description 54
- 239000012298 atmosphere Substances 0.000 claims abstract description 40
- 150000001875 compounds Chemical class 0.000 claims abstract description 34
- 229940060037 fluorine Drugs 0.000 claims description 66
- 235000019000 fluorine Nutrition 0.000 claims description 66
- 238000000034 method Methods 0.000 claims description 24
- ABTOQLMXBSRXSM-UHFFFAOYSA-N silicon tetrafluoride Chemical compound F[Si](F)(F)F ABTOQLMXBSRXSM-UHFFFAOYSA-N 0.000 claims description 13
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 claims description 12
- 239000000460 chlorine Substances 0.000 claims description 12
- 229910052801 chlorine Inorganic materials 0.000 claims description 12
- 238000010438 heat treatment Methods 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 11
- 230000018044 dehydration Effects 0.000 claims description 8
- 238000006297 dehydration reaction Methods 0.000 claims description 8
- 238000007792 addition Methods 0.000 claims 3
- 208000005156 Dehydration Diseases 0.000 claims 1
- 239000001307 helium Substances 0.000 description 15
- 229910052734 helium Inorganic materials 0.000 description 15
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 15
- 229910004014 SiF4 Inorganic materials 0.000 description 11
- 238000004017 vitrification Methods 0.000 description 11
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 9
- 230000005540 biological transmission Effects 0.000 description 8
- SFZCNBIFKDRMGX-UHFFFAOYSA-N sulfur hexafluoride Chemical compound FS(F)(F)(F)(F)F SFZCNBIFKDRMGX-UHFFFAOYSA-N 0.000 description 8
- 238000009826 distribution Methods 0.000 description 6
- 125000002887 hydroxy group Chemical group [H]O* 0.000 description 6
- 239000011261 inert gas Substances 0.000 description 6
- 235000012239 silicon dioxide Nutrition 0.000 description 6
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 4
- 239000010453 quartz Substances 0.000 description 4
- 229910007260 Si2F6 Inorganic materials 0.000 description 3
- 230000002093 peripheral effect Effects 0.000 description 3
- SDNBGJALFMSQER-UHFFFAOYSA-N trifluoro(trifluorosilyl)silane Chemical compound F[Si](F)(F)[Si](F)(F)F SDNBGJALFMSQER-UHFFFAOYSA-N 0.000 description 3
- 229910018503 SF6 Inorganic materials 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 229910052681 coesite Inorganic materials 0.000 description 2
- 239000011162 core material Substances 0.000 description 2
- 229910052906 cristobalite Inorganic materials 0.000 description 2
- 239000012024 dehydrating agents Substances 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 229910052742 iron Inorganic materials 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 239000000377 silicon dioxide Substances 0.000 description 2
- 229910052682 stishovite Inorganic materials 0.000 description 2
- VZGDMQKNWNREIO-UHFFFAOYSA-N tetrachloromethane Chemical compound ClC(Cl)(Cl)Cl VZGDMQKNWNREIO-UHFFFAOYSA-N 0.000 description 2
- TXEYQDLBPFQVAA-UHFFFAOYSA-N tetrafluoromethane Chemical compound FC(F)(F)F TXEYQDLBPFQVAA-UHFFFAOYSA-N 0.000 description 2
- 229910052905 tridymite Inorganic materials 0.000 description 2
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- LDDQLRUQCUTJBB-UHFFFAOYSA-N ammonium fluoride Chemical compound [NH4+].[F-] LDDQLRUQCUTJBB-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 150000001768 cations Chemical class 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 238000005253 cladding Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000000151 deposition Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 239000002019 doping agent Substances 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 125000004435 hydrogen atom Chemical group [H]* 0.000 description 1
- GVGCUCJTUSOZKP-UHFFFAOYSA-N nitrogen trifluoride Chemical compound FN(F)F GVGCUCJTUSOZKP-UHFFFAOYSA-N 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- OBCUTHMOOONNBS-UHFFFAOYSA-N phosphorus pentafluoride Chemical compound FP(F)(F)(F)F OBCUTHMOOONNBS-UHFFFAOYSA-N 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 238000005245 sintering Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Landscapes
- Manufacture, Treatment Of Glass Fibers (AREA)
Abstract
Abstract:
A method for producing a glass preform for use in the fabrication of an optical fiber comprises adding fluorine to a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot pre-form is in the porous state. The soot preform is then heated in an atmosphere containing a fluorine-containing compound at a higher temperature to vitrify it to form a glass preform. An optical fiber homogeneously containing fluorine can be fabricated from this glass preform.
A method for producing a glass preform for use in the fabrication of an optical fiber comprises adding fluorine to a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot pre-form is in the porous state. The soot preform is then heated in an atmosphere containing a fluorine-containing compound at a higher temperature to vitrify it to form a glass preform. An optical fiber homogeneously containing fluorine can be fabricated from this glass preform.
Description
~2965~8 Method for producing glass preform for optical fiber The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber, more particularly, an optical fiber that homogeneously contains fluorine and an extremely small amount of impurities.
For the production of a glass preform with added fluorine, methods have been proposed comprising depositing glass soot particles to form a soot preform and then dehydrating and vitrifying the soot preform by inserting it into a furnace containing an atmosphere comprising fluorine.
To enable the prior art to be described with the aid of diagrams, the figures of the drawings will first be listed.
Figs. 1 and 3 schematically show furnaces in which a soot preform is heated, Figs. 2 and 4 are graphs showing distributions of the refractive index of conventional glass preforms, Figs. 5 and 6 are graphs showing distributions of the refractive index of glass preforms produced according to the present invention.
A first of such methods is described and claimed in Japanese Patent Kokai Publication (unexamined) No. 67533/
1980 and comprises heating a soot preform 2 (Fig. 1) in an atmosphere containing a fluorine-containing compound at a P~ ~
12965Z~
temperature not higher than l,000C, and then heating and vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400C in a furnace 31. Example 3 of Japanese Patent Kokai Publication (unex-amined) No. 60938/1985 describes a similar method compris-ing heating a soot preform in an atmosphere containing SF6 at l,000C and vitrifying it in an atmosphere not containing SF6 at 1,600C. In addition, Japanese Patent Kokai Publi-cation (unexamined) No. 86045/1985 describes a method com-prising heating a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature not lower than 1000C and lower than 1,400C and then vitrifying the soot preform in an atmosphere of an inert gas at a temper-ature not lower than 1,400C.
A second method comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound and an inert gas at a temperature not lower than 1,400C
to produce a glass preform containing fluorine, in which the soot preform 2 is inserted in a zone furnace 41 equipped with a heater 40 as shown in Fig. 3. A similar method is described in Japanese Patent Kokai Publication (unexamined) No. 5035/1986 and comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,650C. This publication states that "since the dopant is absorbed in the soot preform during vitrification of the glass, the treatment becomes easier at a higher temperature and vitrification proceeds at a higher rate than at a lower temperature so that the production cost is reduced". Examples 1, 2 and 4 of Japanese Patent Kokai Publication (unexamined) 30 No. 60938/1985 comprise heating a soot preform in an atmos-phere comprising a fluorine-containing compound at 1,600C.
Further, Japanese Patent K~kai Publication No. 86049/1985 describes a method for producing dense glass by diffusing fluorine at a temperature in a sintering temperature range.
As the result of a study of these two types of methods, the following has been discovered.
.~ . . .
~ 12965Z8 When, according to the first method, the soot preform is heated in an atmosphere comprising a fluorine-containing compound in an apparatus as shown in Fig. 1 at a temperature not higher than l,000C, the porosity of the soot preform is reserved. Thereafter, the soot preform is vitrified in an atmosphere of the inert gas. An analysis of the distribution of the refractive index of the produced glass preform has revealed that the refractive index at a peripheral portion of the preform is greater than at the central portion, as shown in Fig. 2 in which R stands for the outer diameter of the glass preform. This means that the added amount of fluo-rine in the peripheral portion is less than that in the central portion. The reason for this may be that, since at the end of the addition of fluorine at l,000C, the preform 15 is still porous, the added fluorine is dissipated during subsequent heating in the inert gas atmosphere at a higher temperature.
When, according to the second method, the soot preform is heated in an atmosphere comprising a fluorine-containing 20 compound and an inert gas at a temperature not lower than 1,400C and then vitrified, the produced glass preform has a distribution of refractive index as shown in Fig. 4, which shows that the added amount of fluorine is smaller in the central portion. The reason for this may be that, since 25 the addition of fluorine and vitrification proceed simul-taneously, the period of time to add fluorine is not long, so that an insufficient amount of the fluorine-containing compound reaches the central portion. When such a soot pre-form was inserted in the furnace at a rate less than half of 30 the normally employed rate, fluorine was homogeneously added to the central portion of the preform. However, the total treating time was extremely extended.
One object of the present invention is to provide a method for producing a glass preform for use in the fabri-35 cation of an optical fiber, in which fluorine is homogene-ously added.
129652~
Another object of the present invention is to provide a method for producing a glass preform for use in the fabric-ation of an optical fiber which contains a smaller amount of impurities.
Accordingly, the present invention provides a method for producing a glass preform for use in the fabrication of an optical fiber, which method comprises adding fluorine to a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot pre-form is in the porous state, and then heating the soot pre-form in an atmosphere containing a fluorine-containing com-pound at a higher temperature to vitrify it to form a glass preform.
A soot preform to be treated by a method of the present invention may be produced by any of the conventional methods, such as the VAD method, the OVD method, the OVPD method, or the like.
According to the present invention, fluorine is added to the soot preform at a temperature at which the soot is still in the porous state. Thereafter, the soot preform with added fluorine is kept in or inserted into a furnace having an atmosphere comprising a fluorine-containing com-pound at a higher temperature at which the soot preform is vitrified to produce a transparent glass preform. Prefer-ably, the fluorine-containing compounds used in the fluorine-adding step and the vitrifying step are the same. Specific examples of the fluorine-containing compound are SiF4, Si2F6, SF6, NH4F, NF3, PF5 and CF4, and chlorofluorocarbons such as CC12F2. Among them, silicon fluorides such as SiF4 and Si2F6 are preferred, since they do not form bubbles in the qlass preform even when the soot preform is vitrified at a higher temperature and a higher rate.
The addition of fluorine is preferably carried out at a temperature not lower than l,100C but lower than 1,400C. If this temperature is too low, no reaction pro-ceeds between the fluorine-containing compound and the soot --` 1296S2~3 preform. On the other hand, if it is too high, the soot preform shrinks too quickly. Since the fluorine is added at a temperature at which the soot preform is in the por-ous state, the fluorine is sufficiently diffused into the central portion of the preform.
At a higher temperature, the fluorine-containing compound is decomposed, so as to dehydrate the soot preform.
Therefore, a dehydrating agent such as a chlorine-containing compound is not necessarily used in the method of the present invention. For the purpose of dehydration, a fluorine-containing compound having no hydrogen atom, such as SiF4, Si2F6, SF6 and CF4, is preferred.
If no dehydrating agent is used, impurities such as iron tends to remain in the vitrified preform, and an optical fiber fabricated from such a preform shows an increased attenuation of light transmission at a wavelength of Q.8 to 1.3 ~m. Therefore, it is preferred to use a chlorine-containing compound for dehydrating water and/or removing impurities. This dehydration with a chlorine-containing compound can be carried out prior to or simultaneous with the addition of fluorine.
The vitrification of the preform is preferably carried out in the absence of the chlorine-containing compound in view of improvement of hydrogen resistance or radiation resistance. As the chlorine-containing compound, chlorine, as such, or carbon tetrachloride are preferred.
The addition of fluorine and the vitrification of the soot preform can be carried out in the same furnace or in separate furnaces.
Since the vitrification of the soot preform is carried out in the presence of a fluorine-containing compound, fluorine added to the peripheral portion of the preform in the fluorine adding step is not dissipated in the vitrify-ing step. In addition, since the soot preform to which fluorine has been added is vitrified, the vitrification rate is not influenced by the period of time used for adding fluorine, so that the preform can be vitrified at a higher rate.
-12965~3 When a chlorine-containing compound is used, dehydration and removal of impurities are effectively performed, so that it is possible to produce a glass preform containing a very small amount of impurities such as iron.
Practically and presently preferred embodiments of the present invention will be illustrated by the following examples.
Example 1 A soot preform consisting of SiO2 having a diameter of 90 mm and a length of 500 mm was produced by the VAD method and dehydrated, had fluorine added and was vitrified in a uniform heating furnace.
The preform was first heated to 1,100C in a pure helium atmosphere. After reaching 1,100C and until the completion of vitrification (at 1,550C?, the soot preform was heated in an atmosphere consisting of 97 ~ helium and 3 ~ SF6.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 % as shown in Fig. 5.
Example 2 The same soot preform as used in Example 1 was heated by traversing it at a rate of 6 mm/min. in a zone furnace containing an atmosphere consisting of 92 % helium and 8 ~
SiF4 while keeping the furnace surface temperature at 1,350C.
The soot preform with added fluorine was then heated by traversing it at a rate of 6 mm/min. in the same atmosphere while keeping the heater surface temperature at 1,650C to vitrify it. `No chlorine-containing compound was used.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.4 ~ as shown in Fig. 6. The content of hydroxyl groups in the 2965~
glass preform was less than the limit of detection by a IR
absorption method.
Comparative Example 1 The same soot preform as used in Example 1 was heated by traversing it at a rate of 3 mm/min. in the zone furnace containing an atmosphere consisting of 92 % helium and 8 %
SiF4 while keeping the furnace surface temperature at 1,650C
to add fluorine and simultaneously vitrify the soot preform.
Although the total treating time was substantially the same as in Example 2, the produced glass preform had a distri-bution of refractive index as shown in Fig. 4.
Example 3 The same soot preform as used in Example 1 was heated in a furnace containing an atmosphere consisting of 97 %
helium and 3 % SF6 kept at 1,200C. Then, the fluorine-added preform was inserted at a rate of 6 mm/min. in another zone furnace with a surface temperature of 1,650C and con-taining an atmosphere consisting of 97 % helium and 3 % SF6 to vitrify it.
The produced glass preform homogeneously contained fluorine and the content of the hydroxyl groups was less than the limit of detection by the IR absorption method.
The distribution of the refractive index was as shown in Fig. 5.
Example 4 A soot preform consisting of SiO2 having a diameter of 120 mm and a length of 550 mm was produced by the VAD
method and dehydrated, had fluorine added and was vitrified in a uniform heating furnace.
The preform was first heated to 1,100C in a pure helium atmosphere and from l,100C to 1,400C in an atmos-phere consisting of 99.6 % helium and 0.4 % SF6 to perform dehydration and the addition of fluorine. The fluorine-added preform was then vitrified at 1,550C in the latter atmosphere.
-"` 1296~28 Fluorine was homogeneously added to the preform in an amount corresponding to a specific refractive index difference of -0.18 %.
By using the thus produced glass preform as a core material and a dehydrated quartz glass containing fluorine in an amount corresponding to a specific refractive index difference of -0.5 %, an optical fiber was fabricated. The optical fiber had a transmission loss of 0.8 dB/km due to the hydroxyl groups at a wavelength of 1.38 ~m.
Example 5 The same soot preform as used in Example 4 was treated in the zone furnace under the following conditions:
First step [Dehydration and removal of impurities]
Atmosphere: 98 % helium, 2 % C12 Heater surface temperature: 1,150C
Traversing rate: 6 mm/min.
Second step [addition of fluorine]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Third step [Vitrification]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 %. The transparent glass preform contained no bubbles.
The glass prefrom was bored by means of a ultrasonic borer. A highly pure quartz rod was inserted into the bore and drawn to fabricate an optical fiber which had an attenu-ation of light transmission of 0.35 dB/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group 35 of 0.5 dB/km at a wavelength of 1.38 ~m.
" '~
' . .
~29~;5~
g - Example 6 The same soot preform as used in Example 4 was treated in the zone furnace under the following conditions:
First step [Dehydration, removal of impurities and addition of fluorine]
Atmosphere: 89 % helium, 3 % SiF4, 7 % oxygen, 1 % CC14 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Second step [Vitrification]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 %. The transparent glass preform contained no bubbles.
The glass preform was bored by means of a ultrasonic borer. A highly pure quartz rod was inserted into the bore and drawn to fabricate an optical fiber which had an attenu-ation of light transmission of 0.35 ds/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group of 0.5 dB/km at a wavelength of 1.38 ~m.
Example 7 Around a transparent glass rod consisting of a pure quartz made core having an outer diameter of 2 mm and a cladding made of quartz with added fluorine in an amount corresponding to a specific refractive index difference of -0.3 % and having an outer diameter of 10 mm, glass soot particles were deposited to an outer diameter of 120 mm.
The produced composite was treated in the zone furnace under the following conditions:
---` 129652~3 First step [Dehydration removal of impurities and addition of fluorine]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Second step [Vitrification]
Atmosphere: 97 % helium, 3% SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The vitrified preform contained no bubbles. The transparent glass preform was drawn to fabricate an optical fiber which had an attenuation of light transmission of 0.34 dB/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group of 0.6 dB/km at a wavelength of 1.38 ~m.
The presence of impurities that were not removed by the fluorine-containing compound did not materially influ-ence the quality of the optical fiber.
For the production of a glass preform with added fluorine, methods have been proposed comprising depositing glass soot particles to form a soot preform and then dehydrating and vitrifying the soot preform by inserting it into a furnace containing an atmosphere comprising fluorine.
To enable the prior art to be described with the aid of diagrams, the figures of the drawings will first be listed.
Figs. 1 and 3 schematically show furnaces in which a soot preform is heated, Figs. 2 and 4 are graphs showing distributions of the refractive index of conventional glass preforms, Figs. 5 and 6 are graphs showing distributions of the refractive index of glass preforms produced according to the present invention.
A first of such methods is described and claimed in Japanese Patent Kokai Publication (unexamined) No. 67533/
1980 and comprises heating a soot preform 2 (Fig. 1) in an atmosphere containing a fluorine-containing compound at a P~ ~
12965Z~
temperature not higher than l,000C, and then heating and vitrifying the soot preform in an atmosphere of an inert gas at a temperature not lower than 1,400C in a furnace 31. Example 3 of Japanese Patent Kokai Publication (unex-amined) No. 60938/1985 describes a similar method compris-ing heating a soot preform in an atmosphere containing SF6 at l,000C and vitrifying it in an atmosphere not containing SF6 at 1,600C. In addition, Japanese Patent Kokai Publi-cation (unexamined) No. 86045/1985 describes a method com-prising heating a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature not lower than 1000C and lower than 1,400C and then vitrifying the soot preform in an atmosphere of an inert gas at a temper-ature not lower than 1,400C.
A second method comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound and an inert gas at a temperature not lower than 1,400C
to produce a glass preform containing fluorine, in which the soot preform 2 is inserted in a zone furnace 41 equipped with a heater 40 as shown in Fig. 3. A similar method is described in Japanese Patent Kokai Publication (unexamined) No. 5035/1986 and comprises heating a soot preform in an atmosphere comprising a fluorine-containing compound at 1,650C. This publication states that "since the dopant is absorbed in the soot preform during vitrification of the glass, the treatment becomes easier at a higher temperature and vitrification proceeds at a higher rate than at a lower temperature so that the production cost is reduced". Examples 1, 2 and 4 of Japanese Patent Kokai Publication (unexamined) 30 No. 60938/1985 comprise heating a soot preform in an atmos-phere comprising a fluorine-containing compound at 1,600C.
Further, Japanese Patent K~kai Publication No. 86049/1985 describes a method for producing dense glass by diffusing fluorine at a temperature in a sintering temperature range.
As the result of a study of these two types of methods, the following has been discovered.
.~ . . .
~ 12965Z8 When, according to the first method, the soot preform is heated in an atmosphere comprising a fluorine-containing compound in an apparatus as shown in Fig. 1 at a temperature not higher than l,000C, the porosity of the soot preform is reserved. Thereafter, the soot preform is vitrified in an atmosphere of the inert gas. An analysis of the distribution of the refractive index of the produced glass preform has revealed that the refractive index at a peripheral portion of the preform is greater than at the central portion, as shown in Fig. 2 in which R stands for the outer diameter of the glass preform. This means that the added amount of fluo-rine in the peripheral portion is less than that in the central portion. The reason for this may be that, since at the end of the addition of fluorine at l,000C, the preform 15 is still porous, the added fluorine is dissipated during subsequent heating in the inert gas atmosphere at a higher temperature.
When, according to the second method, the soot preform is heated in an atmosphere comprising a fluorine-containing 20 compound and an inert gas at a temperature not lower than 1,400C and then vitrified, the produced glass preform has a distribution of refractive index as shown in Fig. 4, which shows that the added amount of fluorine is smaller in the central portion. The reason for this may be that, since 25 the addition of fluorine and vitrification proceed simul-taneously, the period of time to add fluorine is not long, so that an insufficient amount of the fluorine-containing compound reaches the central portion. When such a soot pre-form was inserted in the furnace at a rate less than half of 30 the normally employed rate, fluorine was homogeneously added to the central portion of the preform. However, the total treating time was extremely extended.
One object of the present invention is to provide a method for producing a glass preform for use in the fabri-35 cation of an optical fiber, in which fluorine is homogene-ously added.
129652~
Another object of the present invention is to provide a method for producing a glass preform for use in the fabric-ation of an optical fiber which contains a smaller amount of impurities.
Accordingly, the present invention provides a method for producing a glass preform for use in the fabrication of an optical fiber, which method comprises adding fluorine to a soot preform in an atmosphere comprising a fluorine-containing compound at a temperature at which the soot pre-form is in the porous state, and then heating the soot pre-form in an atmosphere containing a fluorine-containing com-pound at a higher temperature to vitrify it to form a glass preform.
A soot preform to be treated by a method of the present invention may be produced by any of the conventional methods, such as the VAD method, the OVD method, the OVPD method, or the like.
According to the present invention, fluorine is added to the soot preform at a temperature at which the soot is still in the porous state. Thereafter, the soot preform with added fluorine is kept in or inserted into a furnace having an atmosphere comprising a fluorine-containing com-pound at a higher temperature at which the soot preform is vitrified to produce a transparent glass preform. Prefer-ably, the fluorine-containing compounds used in the fluorine-adding step and the vitrifying step are the same. Specific examples of the fluorine-containing compound are SiF4, Si2F6, SF6, NH4F, NF3, PF5 and CF4, and chlorofluorocarbons such as CC12F2. Among them, silicon fluorides such as SiF4 and Si2F6 are preferred, since they do not form bubbles in the qlass preform even when the soot preform is vitrified at a higher temperature and a higher rate.
The addition of fluorine is preferably carried out at a temperature not lower than l,100C but lower than 1,400C. If this temperature is too low, no reaction pro-ceeds between the fluorine-containing compound and the soot --` 1296S2~3 preform. On the other hand, if it is too high, the soot preform shrinks too quickly. Since the fluorine is added at a temperature at which the soot preform is in the por-ous state, the fluorine is sufficiently diffused into the central portion of the preform.
At a higher temperature, the fluorine-containing compound is decomposed, so as to dehydrate the soot preform.
Therefore, a dehydrating agent such as a chlorine-containing compound is not necessarily used in the method of the present invention. For the purpose of dehydration, a fluorine-containing compound having no hydrogen atom, such as SiF4, Si2F6, SF6 and CF4, is preferred.
If no dehydrating agent is used, impurities such as iron tends to remain in the vitrified preform, and an optical fiber fabricated from such a preform shows an increased attenuation of light transmission at a wavelength of Q.8 to 1.3 ~m. Therefore, it is preferred to use a chlorine-containing compound for dehydrating water and/or removing impurities. This dehydration with a chlorine-containing compound can be carried out prior to or simultaneous with the addition of fluorine.
The vitrification of the preform is preferably carried out in the absence of the chlorine-containing compound in view of improvement of hydrogen resistance or radiation resistance. As the chlorine-containing compound, chlorine, as such, or carbon tetrachloride are preferred.
The addition of fluorine and the vitrification of the soot preform can be carried out in the same furnace or in separate furnaces.
Since the vitrification of the soot preform is carried out in the presence of a fluorine-containing compound, fluorine added to the peripheral portion of the preform in the fluorine adding step is not dissipated in the vitrify-ing step. In addition, since the soot preform to which fluorine has been added is vitrified, the vitrification rate is not influenced by the period of time used for adding fluorine, so that the preform can be vitrified at a higher rate.
-12965~3 When a chlorine-containing compound is used, dehydration and removal of impurities are effectively performed, so that it is possible to produce a glass preform containing a very small amount of impurities such as iron.
Practically and presently preferred embodiments of the present invention will be illustrated by the following examples.
Example 1 A soot preform consisting of SiO2 having a diameter of 90 mm and a length of 500 mm was produced by the VAD method and dehydrated, had fluorine added and was vitrified in a uniform heating furnace.
The preform was first heated to 1,100C in a pure helium atmosphere. After reaching 1,100C and until the completion of vitrification (at 1,550C?, the soot preform was heated in an atmosphere consisting of 97 ~ helium and 3 ~ SF6.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 % as shown in Fig. 5.
Example 2 The same soot preform as used in Example 1 was heated by traversing it at a rate of 6 mm/min. in a zone furnace containing an atmosphere consisting of 92 % helium and 8 ~
SiF4 while keeping the furnace surface temperature at 1,350C.
The soot preform with added fluorine was then heated by traversing it at a rate of 6 mm/min. in the same atmosphere while keeping the heater surface temperature at 1,650C to vitrify it. `No chlorine-containing compound was used.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.4 ~ as shown in Fig. 6. The content of hydroxyl groups in the 2965~
glass preform was less than the limit of detection by a IR
absorption method.
Comparative Example 1 The same soot preform as used in Example 1 was heated by traversing it at a rate of 3 mm/min. in the zone furnace containing an atmosphere consisting of 92 % helium and 8 %
SiF4 while keeping the furnace surface temperature at 1,650C
to add fluorine and simultaneously vitrify the soot preform.
Although the total treating time was substantially the same as in Example 2, the produced glass preform had a distri-bution of refractive index as shown in Fig. 4.
Example 3 The same soot preform as used in Example 1 was heated in a furnace containing an atmosphere consisting of 97 %
helium and 3 % SF6 kept at 1,200C. Then, the fluorine-added preform was inserted at a rate of 6 mm/min. in another zone furnace with a surface temperature of 1,650C and con-taining an atmosphere consisting of 97 % helium and 3 % SF6 to vitrify it.
The produced glass preform homogeneously contained fluorine and the content of the hydroxyl groups was less than the limit of detection by the IR absorption method.
The distribution of the refractive index was as shown in Fig. 5.
Example 4 A soot preform consisting of SiO2 having a diameter of 120 mm and a length of 550 mm was produced by the VAD
method and dehydrated, had fluorine added and was vitrified in a uniform heating furnace.
The preform was first heated to 1,100C in a pure helium atmosphere and from l,100C to 1,400C in an atmos-phere consisting of 99.6 % helium and 0.4 % SF6 to perform dehydration and the addition of fluorine. The fluorine-added preform was then vitrified at 1,550C in the latter atmosphere.
-"` 1296~28 Fluorine was homogeneously added to the preform in an amount corresponding to a specific refractive index difference of -0.18 %.
By using the thus produced glass preform as a core material and a dehydrated quartz glass containing fluorine in an amount corresponding to a specific refractive index difference of -0.5 %, an optical fiber was fabricated. The optical fiber had a transmission loss of 0.8 dB/km due to the hydroxyl groups at a wavelength of 1.38 ~m.
Example 5 The same soot preform as used in Example 4 was treated in the zone furnace under the following conditions:
First step [Dehydration and removal of impurities]
Atmosphere: 98 % helium, 2 % C12 Heater surface temperature: 1,150C
Traversing rate: 6 mm/min.
Second step [addition of fluorine]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Third step [Vitrification]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 %. The transparent glass preform contained no bubbles.
The glass prefrom was bored by means of a ultrasonic borer. A highly pure quartz rod was inserted into the bore and drawn to fabricate an optical fiber which had an attenu-ation of light transmission of 0.35 dB/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group 35 of 0.5 dB/km at a wavelength of 1.38 ~m.
" '~
' . .
~29~;5~
g - Example 6 The same soot preform as used in Example 4 was treated in the zone furnace under the following conditions:
First step [Dehydration, removal of impurities and addition of fluorine]
Atmosphere: 89 % helium, 3 % SiF4, 7 % oxygen, 1 % CC14 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Second step [Vitrification]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The refractive index of the produced glass preform was measured by means of a preform analyzer. Fluorine was homo-geneously added to the preform in an amount corresponding to a specific refractive index difference of -0.3 %. The transparent glass preform contained no bubbles.
The glass preform was bored by means of a ultrasonic borer. A highly pure quartz rod was inserted into the bore and drawn to fabricate an optical fiber which had an attenu-ation of light transmission of 0.35 ds/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group of 0.5 dB/km at a wavelength of 1.38 ~m.
Example 7 Around a transparent glass rod consisting of a pure quartz made core having an outer diameter of 2 mm and a cladding made of quartz with added fluorine in an amount corresponding to a specific refractive index difference of -0.3 % and having an outer diameter of 10 mm, glass soot particles were deposited to an outer diameter of 120 mm.
The produced composite was treated in the zone furnace under the following conditions:
---` 129652~3 First step [Dehydration removal of impurities and addition of fluorine]
Atmosphere: 97 % helium, 3 % SiF4 Heater surface temperature: 1,350C
Traversing rate: 6 mm/min.
Second step [Vitrification]
Atmosphere: 97 % helium, 3% SiF4 Heater surface temperature: 1,650C
Traversing rate: 6 mm/min.
The vitrified preform contained no bubbles. The transparent glass preform was drawn to fabricate an optical fiber which had an attenuation of light transmission of 0.34 dB/km at a wavelength of 1.3 ~m and a transmission loss due to the hydroxyl group of 0.6 dB/km at a wavelength of 1.38 ~m.
The presence of impurities that were not removed by the fluorine-containing compound did not materially influ-ence the quality of the optical fiber.
Claims (5)
1. A method for producing a glass preform for use in the fabrication of an optical fiber, which method com-prises adding fluorine to a soot preform in an atmosphere comprising a flubrine-containing compound at a temperature at which the soot preform is in the porous state, and then heating the soot preform in an atmosphere containing a fluorine-containing compound at a higher temperature to vitrify it to form a glass preform.
2. A method according to claim 1, wherein the fluo-rine-containing compound is a silicon fluoride.
3. A method according to claim 1, wherein the addi-tion of fluorine is carried out at a temperature not lower than 1,100°C and lower than 1,400°C.
4. A method according to claim 1, wherein the soot preform is dehydrated with a chlorine-containing compound prior to the addition of fluorine, and it has the fluorine added and is vitrified in an atmosphere not containing any chlorine-containing compound.
5. A method according to claim 1, wherein dehydra-tion of the soot preform and removal of impurities with a chlorine-containing compound is carried out simultaneously with the addition of fluorine, the preform being vitrified in an atmosphere not containing any chlorine-containing compound.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP103997/1985 | 1985-05-17 | ||
| JP7399785U JPS6249483U (en) | 1985-05-17 | 1985-05-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1296528C true CA1296528C (en) | 1992-03-03 |
Family
ID=13534276
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000509253A Expired - Lifetime CA1296528C (en) | 1985-05-17 | 1986-05-15 | Method for producing glass preform for optical fiber |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS6249483U (en) |
| CA (1) | CA1296528C (en) |
-
1985
- 1985-05-17 JP JP7399785U patent/JPS6249483U/ja active Pending
-
1986
- 1986-05-15 CA CA000509253A patent/CA1296528C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6249483U (en) | 1987-03-27 |
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