CA1261127A - Optical waveguide manufacture - Google Patents
Optical waveguide manufactureInfo
- Publication number
- CA1261127A CA1261127A CA000522402A CA522402A CA1261127A CA 1261127 A CA1261127 A CA 1261127A CA 000522402 A CA000522402 A CA 000522402A CA 522402 A CA522402 A CA 522402A CA 1261127 A CA1261127 A CA 1261127A
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- Prior art keywords
- silica
- support tube
- fluorine
- layer
- deposited
- Prior art date
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- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
- C03B37/01446—Thermal after-treatment of preforms, e.g. dehydrating, consolidating, sintering
- C03B37/01453—Thermal after-treatment of preforms, e.g. dehydrating, consolidating, sintering for doping the preform with flourine
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
- C03B37/01225—Means for changing or stabilising the shape, e.g. diameter, of tubes or rods in general, e.g. collapsing
- C03B37/01228—Removal of preform material
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
- C03B37/01225—Means for changing or stabilising the shape, e.g. diameter, of tubes or rods in general, e.g. collapsing
- C03B37/01248—Means for changing or stabilising the shape, e.g. diameter, of tubes or rods in general, e.g. collapsing by collapsing without drawing
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
- C03B37/01446—Thermal after-treatment of preforms, e.g. dehydrating, consolidating, sintering
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
- C03B37/01466—Means for changing or stabilising the diameter or form of tubes or rods
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
- C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
- C03B37/01861—Means for changing or stabilising the diameter or form of tubes or rods
- C03B37/01869—Collapsing
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B2201/00—Type of glass produced
- C03B2201/06—Doped silica-based glasses
- C03B2201/08—Doped silica-based glasses doped with boron or fluorine or other refractive index decreasing dopant
- C03B2201/12—Doped silica-based glasses doped with boron or fluorine or other refractive index decreasing dopant doped with fluorine
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B2203/00—Fibre product details, e.g. structure, shape
- C03B2203/10—Internal structure or shape details
- C03B2203/22—Radial profile of refractive index, composition or softening point
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B2207/00—Glass deposition burners
- C03B2207/36—Fuel or oxidant details, e.g. flow rate, flow rate ratio, fuel additives
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Geochemistry & Mineralogy (AREA)
- Manufacturing & Machinery (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Glass Compositions (AREA)
Abstract
OPTICAL WAVEGUIDE MANUFACTURE
Abstract of the Disclosure Optical waveguide having a fused silica or doped silica core and a fluorine doped fused silica cladding is made by depositing particulate cladding silica onto the outside of a support tube and then drying, fluorine doping, and sintering the silica. The support tube is then etched out and further particulate silica is deposited and sintered inside the resulting tube to form a core. The resulting tubular preform is heated to collapse it into a rod from which waveguide is drawn, the waveguide having a fluorine doped silica cladding.
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Abstract of the Disclosure Optical waveguide having a fused silica or doped silica core and a fluorine doped fused silica cladding is made by depositing particulate cladding silica onto the outside of a support tube and then drying, fluorine doping, and sintering the silica. The support tube is then etched out and further particulate silica is deposited and sintered inside the resulting tube to form a core. The resulting tubular preform is heated to collapse it into a rod from which waveguide is drawn, the waveguide having a fluorine doped silica cladding.
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Description
OPTICAL WAVEGUIDE ~lANUFACTURE
The invention relates to a method for manuFacturing optical waveguideO It has particular application -to the manufacture of optical waveguide having a fluorine doped silica cladding and a doped or undoped silica core.
Optical waveguide having an undoped silica core and a fluorine doped silica cladding is described in Uni-ted States patent 4,082,420 (Shiraishi et al). The optical waveguide is made by a flame hydrolysis method in which silicon tetrachloride and silicon tetrafluoride are fed to an oxygen-hydrogen burner where -the silicon compounds dissociate and a fluorine doped silica is formed and deposited as a soot onto the surface of a pure fused silica rod. The Fluorine dopant lowers the refractive index of silica. The rod and deposited soot are then heated to consolidate the soot into a composite glass preform and fiber is drawn from -the preform. The -Fiber has an undoped fused silica core and a fluorine doped cladding of lower refractive index than the core which is a necessary characteristic for a fiber to func-tion as an optical waveguide.
Using this method, it is not possible to incorporate sufficien-t fluorine to achieve a refractive index depression greater than 0.003. This is the level required for a fiber with an undoped silica core to function as a single mode optical waveguide in the 1.3 micron wavelength range. To achieve the required refractive index difference between core and cladding, an index raising dopant such as germania must be added to the core. ~lowever a dopan-t within the core increases Rayleigh scattering and has a negative impact on lifetime.
Any process which can be used to make silica based optical waveguide ~1~
with lit-tle or no core dopant is intrinsically superior.
Fluorine in glass lowers the refractive index while most other dopants such as germanium, phosphorus and aluminum raise the index. To make a single mode waveguide with a difference in index between core and cladding of 0.003 or greater, some combination oF
core and cladding doping is generally used. In fact, a waveguide can be made having a pure silica core and a fluorine doped cladding, a pure silica cladding and a doped core or any proportion in between these two. The only modification that is necessary in the process is that one mus-t be able to control the relative flow of fluorine doping ma-terial during cladding sintering as well as that of the core dopant during core deposition. While a pure silica core structure is preferred for low loss and environmental stability, it has an outer cladding (i.e. non-waveguide) of pure silica which can strip the light from the lower index cladding. This design therefore requires a larger proportion of waveguide cladding, than a pure silica cladding (matched) cladding design. This extra cladding is expensive to make (as much as twice the price) and therefore structures such as pure silica cladding have cost advantages over pure silica core designs which made them attractive for applications in which loss and environmental hardiness are not crucial.
Our copending Canadian Patent Application Serial number ~76,84~, filed on 19 March 1985 in the name of Pe-ter P. Pilon and enti-tled OPTICAL WAVEGUIDE MANUFACTURE, describes an alternative method of fabricating a fluorine doped silica clad fiber. A silica cylinder is made by depositing particulate core silica onto the outside of a fused silica support tube, drying the core silica in chlorine, heating the silica to densify it and removing the support tube by etching. Further particulate silica is deposited and is then heated in a fluorine-containing gas to dry, fluorine diFfuse, and sinter the porous outer part of the silica. The resulting tubular silica preForm is heated to collapse the tubular preform into a rod from which optical waveguide is drawn, the waveguide having a fluorine doped silica cladding. The par-ticulate silica can be subjected to a chlorine drying step before fluorine drying, diffusion and sintering.
This technique is relatively time consuming and it has been found difficult to obtain a clean interface between the glass core and cladding soot. Also the process is not easily applicable to the manuFacture of single mode fiber due to the large cladding to core ratio needed.
A fabrication method is now proposed which overcomes these disadvantages of known processes for making fluorine doped clad silica core fiber. The method is readily modified to make doped core, undoped silica clad fiber or fiber having both core and cladding dopants. The process is amenable to scaling to large pre-Form sizes which can have a significant cost lowering effect on the production oF
such waveguide.
According to the invention, there is provided a method of manufacturing optical waveguide comprising depositing a layer of particulate silica on an outer surFace of a fused silica cylindrical support tube, drying the deposited silica and difFusing fluorine into it, sintering the deposited silica to cause -Fusion thereof, etching away the support tube, depositing a layer of particulate silica on the inside sur-Face of -the resulting tube, collapsing the composite tubular preform obtained into a rod preform, heating the rod preform to a drawing temperature and drawing op-tical waveguide from the rod pre-form, such waveguide having a core and a cladding derived from the deposited silica, the cladding being derived from the fluorine diffused region thereof.
The deposited silica is preFerably dried by subjecting the soot to a mixture of chlorine and helium. Preferably the support tube is etched away by passing a gaseous mix-ture containing sulphur hexafluoride through the center of the suppor-t tube. Advantageously the support tube is itself, supported by a graphite rod so that the tube is not distor-ted when the fluorine doped silica is sin-tered. Any residual graphite within the tube can be remo.ed prior to the step of etching away the support tube by subjecting the center of the -tube to a wet etch, for example within a mixture of nitric and hydrochloric acids.
One embodiment of the invention will now be described with re-Ference to the accompanying drawings in which:-Figure 1 shows the end part of an optical waveguidemade by a method according to the invention, the Figure also illus-trating a refractive index profile across the fiber, and Figures 2 to 9 are schematic views of stages in the manufacture of opt-ical waveguide by one method according to the invention.
Referring -to Figure 1~ an optical fiber has a core 10 of high purity Fused silica, a cladding 12 of fluorine doped silica, and a silica jacket 14. The single mode optical fiber has an outer diameter of 125 microns with a core diameter of about 9 microns. The fluorine is present in an amount sufficient that the refract;ve index of the cladding region is 1.444 or less compared -to 1.447 for the core region at a wavelength of 1300 mm.
To make a fiber having the structure and composi-tion shown in Figure 1, a tube is made from particulate silica. The tube is dried and fluorine is diFfused into it. The tube is then sintered into a fused silica tube and a layer cf pure fused silica is deposited onto the interior surface of the tube. The resulting tube is collapsed and then drawn to produce fiber having a relatively low refractive index cladding corresponding to the fluorine doped region.
Referring particularly to Figures 2 to 8, Figure 2 shows a tubular fused silica support tube 16, 120 centimeters in length with an internal diameter of 5 millimeters and an external diameter of 7 millimeters. The ends of the silica support tube are fixed in the chucks 18 of a glass working lathe. A silica soot producing burner 20 is mounted to direct a flame at the support tube 16. Silicon tetrachloride entrained within a stream of oxygen by bubbling the oxygen through silicon tetrachloride is fed to a central tubular chamber within the burner 200 Argon, which separates the silicon tetrachloride vapour from the burner gases within the burner itself is fed to a second surrounding annular chamber, hydrogen is fed to a third annular chamber, and a mixture of argon and oxygen is fed to an outer burner chamber. The flow rates are 2 to 4 1 itres per minute of oxygen to the first chamber, 2 litres per minute of argon to the second chamber, 10 to 20 litres per minute of hydrogen to the third chamber and 15 to 30 litres per minute of argon with 3 litres per minute of oxygen to the outer chamber. The burner is moved along the length of the support tube at 8 cen-timeters per minute and the support tube 16 is ro-tated at 30 revolutions per minute.
Particulate cladding silica 24 is deposited onto one end of the support tube 16 to a diameter of 4.0 centimeters, length of 40 centimeters and a density of 0.30 grams cm~3. The rate of growth depends on the diameter of the support tube 16 as supplemented by previously deposited particulate silica. The deposited silica 24 has a very high moisture content which is untenable if the silica is to become part of an optical waveguide since the moisture results in a large absorption peak near 1400 nanometers. This reduces the transmission at 1300 and 1550 nanometers which are the output wavelengths of long wavelength light emitting devices of interest in fiber optic communications systems.
To remove this OH moisture absorption peak, the particula-te silica 24 is dried in a chlorine-containing atmosphere at high temperature. As shown in Figure 3, the support -tube 15 toge-ther with the deposited silica 24 is mounted within a 100 millimeter internal diameter silica tube 26 which sits in a vertical furnace 28 having a heating element 42 that is shorter than the 40 cm long soot 20 boule 24. The furnace temperature is held at 1300C while the soot boule is slowly lowered through the heat zone 42 -to cause drying of the soot. A mixture of chlorine (150 cc/min), and oxygen (3000 cc/min) is fed -through the tube 26. Hydrogen contained within the porous silica as -the hydroxyl species reacts with the chlorine -to produce volatile hydrogen chloride which is entrained within -the gas stream and removed. Removal of hydroxyl species renders subsequently formed fused silica very highly -transmissive.
r7 In a subsequent sin-tering or consolidation s-tep (Figure 4), the Furnace 2~ is heated to a tempera-ture of 1580C and the soot is driven through the hea-t zone 42 at a rate o-F 0.2 cm/min -to sin-ter the soot 24 into clear glass 43. After a three hour furnace traversal period in an atmosphere of 2 (3000 cc/min); Cl2 (150 cc/min); He (1500 cc/min) and SF6 (150 cc/min) the soot is consolidated to a fused silica -tube 32 about 30 centimeters in length having an external diameter of 1.~ centimeters.
The refractory rod 22 is removed and the support tube 16 is then etched away. Initially, a wet etchant, 1:2 nitric acid:hydrochloric acid, is used to remove carbon and outgassing contaminants from the inside of the tube 16 (Figure 5).
The composition of the support tube 16 can be silica doped with the OH group, for example, to improve etchability. However care must be taken to ensure that such an impurity is not absorbed into the cladding silica 24 while it is porous. Moreover any impurity used should not cause a marked change in the coefficient of thermal expansion of -the silica since otherwise there will be untenable stress introduced at the support tube/cladding silica interface during temperature fluctuations associated with processing.
This is followed by a gas phase etching step (Figure 6). Tubular silica handles 34 are attached to both ends of the cladding and support tube structure such that a continuous center hole runs down the axis of the systern. This is placed in a glass working lathe and a 1:1:1 mixture of helium, oxygen and sulphur hexafluoride is fed through the center hole. At the same time, the preform is subjected to a number of passes of the localized zone furnace 44 a-t a rate of 1.0 cm/min to es-tablish a temperature at the radial center of the preform of about 1600C. At this temperature the silica of the initial support tube 16 is etched away. The amount of material removed from the tube is determined by the temperature and relative -Flow rate of the SF6.
Referring to Figure 7, further fused silica 36 is deposited to a depth of 0.01 cm using a modified chemical vapour deposition (~lCVD) process described in MacChesney et al. A furnace 44 heats the tube 32 to a temperature of 1600 to 1700C and downwardly traverses the tube at a ra-te of 5 cm/min as a mixture of SiC14 (60 cc/min) He (1000 cc/min) and 2 (1000 cc/min) is passed down the tube and the tube is rotated at 30 revolutions/min. As the gaseous mixture passes through the hot zone~ the SiC14 dissociates and reacts with the oxygen to form particulate silica. The silica soot 46 settles on the inside surface of the tube 32 and is subsequently consolidated as the hot zone reaches it.
In an alternative method, the gas mixture and the hot zone are directed in opposite directions and the silica ls deposited as silica soot to a depth of 2 to 3 mm with a density of approximately 0.1 g/cm3. A drying gas mixture of chlorine and oxygen ls then passed through the tube and the tube is heated to approximately 1200C
for 1 hour to reduce the OH moisture content. The silica tube is then placed in a furnace at a temperature 1600 to 1700C to sinter the centrally deposited silica.
The composite tubular preform obtained is then collapsed (Figure 8) by heating to a temperature in the range 1850 to 2200C and traversing the burner or furnace at 1 cm/min towards the inlet encl while maintaining the Cl2/He atmosphere in central bore 38.
In another alternative approach, fluorine alone can be used both to dry and dope. By using the chlorine and fluorine drying techniques at various stages in the fabrication process, a mois-ture level oF less than 0.1 parts per million in the fused silica is achieved.
The diameter of the tubular rod is then increased from about 1.6 cm to about 2.5 cm by adding a silica jacket 14. To do this a silica tube 40 is heated to collapse it down onto -the outside of the fluorine doped silica 32 (Figure 8).
Referring to Figure 9, the composite rod preform finally obtained is subsequently placed in a vertical orientation drawing tower with a furnace zone at which -the preform temperature is raised to about 2000C which is higher than the silica softening point. Fiber 42 is pulled from the lower end oF the preform by a drum onto which the fiber is wound after being cooled and coated with a protective acrylate or silicone layer. The fiber has a high purity silica core and a relatively lower refractive index fluorine doped silica cladding.
The waveguide produced using the dimensions indicated is single mode fiber having a core diameter of about 9 microns, a cladding diameter of about 80 microns, and an overall diameter o-f 125 microns. As previously indicated the refractive index difference is 25 approximately 0.0045.
A refractive index difference oF the cladding relative to that of pure silica is controlled by the ratio of helium to SF6 in the mixture used during the fluorine doping stage of Figure 5 assuming that the chlorine and oxygen levels are held constant. The amount of Fluorine necessary to lower the refractive index of silica to 1.439 at a wavelength of 1300 mm is about the limiting level at which fluorine can be incorporated into silica using this method. To obtain a refractive index difference larger than OnO08 for a silica based fiber, the refractive index oF the core can be increased above the value of that of the pure silica. Most dopants increase the refractive index of silica, so by incorporation of the dopant material into the silica internally deposited within the tube, the refractive index of the resulting waveguide core is increased above that of pure silica.
Germanium can be included within the MCVD deposited silica by entraining germanium tetrachloride with the silicon tetrachloride injected into the tube.
The invention relates to a method for manuFacturing optical waveguideO It has particular application -to the manufacture of optical waveguide having a fluorine doped silica cladding and a doped or undoped silica core.
Optical waveguide having an undoped silica core and a fluorine doped silica cladding is described in Uni-ted States patent 4,082,420 (Shiraishi et al). The optical waveguide is made by a flame hydrolysis method in which silicon tetrachloride and silicon tetrafluoride are fed to an oxygen-hydrogen burner where -the silicon compounds dissociate and a fluorine doped silica is formed and deposited as a soot onto the surface of a pure fused silica rod. The Fluorine dopant lowers the refractive index of silica. The rod and deposited soot are then heated to consolidate the soot into a composite glass preform and fiber is drawn from -the preform. The -Fiber has an undoped fused silica core and a fluorine doped cladding of lower refractive index than the core which is a necessary characteristic for a fiber to func-tion as an optical waveguide.
Using this method, it is not possible to incorporate sufficien-t fluorine to achieve a refractive index depression greater than 0.003. This is the level required for a fiber with an undoped silica core to function as a single mode optical waveguide in the 1.3 micron wavelength range. To achieve the required refractive index difference between core and cladding, an index raising dopant such as germania must be added to the core. ~lowever a dopan-t within the core increases Rayleigh scattering and has a negative impact on lifetime.
Any process which can be used to make silica based optical waveguide ~1~
with lit-tle or no core dopant is intrinsically superior.
Fluorine in glass lowers the refractive index while most other dopants such as germanium, phosphorus and aluminum raise the index. To make a single mode waveguide with a difference in index between core and cladding of 0.003 or greater, some combination oF
core and cladding doping is generally used. In fact, a waveguide can be made having a pure silica core and a fluorine doped cladding, a pure silica cladding and a doped core or any proportion in between these two. The only modification that is necessary in the process is that one mus-t be able to control the relative flow of fluorine doping ma-terial during cladding sintering as well as that of the core dopant during core deposition. While a pure silica core structure is preferred for low loss and environmental stability, it has an outer cladding (i.e. non-waveguide) of pure silica which can strip the light from the lower index cladding. This design therefore requires a larger proportion of waveguide cladding, than a pure silica cladding (matched) cladding design. This extra cladding is expensive to make (as much as twice the price) and therefore structures such as pure silica cladding have cost advantages over pure silica core designs which made them attractive for applications in which loss and environmental hardiness are not crucial.
Our copending Canadian Patent Application Serial number ~76,84~, filed on 19 March 1985 in the name of Pe-ter P. Pilon and enti-tled OPTICAL WAVEGUIDE MANUFACTURE, describes an alternative method of fabricating a fluorine doped silica clad fiber. A silica cylinder is made by depositing particulate core silica onto the outside of a fused silica support tube, drying the core silica in chlorine, heating the silica to densify it and removing the support tube by etching. Further particulate silica is deposited and is then heated in a fluorine-containing gas to dry, fluorine diFfuse, and sinter the porous outer part of the silica. The resulting tubular silica preForm is heated to collapse the tubular preform into a rod from which optical waveguide is drawn, the waveguide having a fluorine doped silica cladding. The par-ticulate silica can be subjected to a chlorine drying step before fluorine drying, diffusion and sintering.
This technique is relatively time consuming and it has been found difficult to obtain a clean interface between the glass core and cladding soot. Also the process is not easily applicable to the manuFacture of single mode fiber due to the large cladding to core ratio needed.
A fabrication method is now proposed which overcomes these disadvantages of known processes for making fluorine doped clad silica core fiber. The method is readily modified to make doped core, undoped silica clad fiber or fiber having both core and cladding dopants. The process is amenable to scaling to large pre-Form sizes which can have a significant cost lowering effect on the production oF
such waveguide.
According to the invention, there is provided a method of manufacturing optical waveguide comprising depositing a layer of particulate silica on an outer surFace of a fused silica cylindrical support tube, drying the deposited silica and difFusing fluorine into it, sintering the deposited silica to cause -Fusion thereof, etching away the support tube, depositing a layer of particulate silica on the inside sur-Face of -the resulting tube, collapsing the composite tubular preform obtained into a rod preform, heating the rod preform to a drawing temperature and drawing op-tical waveguide from the rod pre-form, such waveguide having a core and a cladding derived from the deposited silica, the cladding being derived from the fluorine diffused region thereof.
The deposited silica is preFerably dried by subjecting the soot to a mixture of chlorine and helium. Preferably the support tube is etched away by passing a gaseous mix-ture containing sulphur hexafluoride through the center of the suppor-t tube. Advantageously the support tube is itself, supported by a graphite rod so that the tube is not distor-ted when the fluorine doped silica is sin-tered. Any residual graphite within the tube can be remo.ed prior to the step of etching away the support tube by subjecting the center of the -tube to a wet etch, for example within a mixture of nitric and hydrochloric acids.
One embodiment of the invention will now be described with re-Ference to the accompanying drawings in which:-Figure 1 shows the end part of an optical waveguidemade by a method according to the invention, the Figure also illus-trating a refractive index profile across the fiber, and Figures 2 to 9 are schematic views of stages in the manufacture of opt-ical waveguide by one method according to the invention.
Referring -to Figure 1~ an optical fiber has a core 10 of high purity Fused silica, a cladding 12 of fluorine doped silica, and a silica jacket 14. The single mode optical fiber has an outer diameter of 125 microns with a core diameter of about 9 microns. The fluorine is present in an amount sufficient that the refract;ve index of the cladding region is 1.444 or less compared -to 1.447 for the core region at a wavelength of 1300 mm.
To make a fiber having the structure and composi-tion shown in Figure 1, a tube is made from particulate silica. The tube is dried and fluorine is diFfused into it. The tube is then sintered into a fused silica tube and a layer cf pure fused silica is deposited onto the interior surface of the tube. The resulting tube is collapsed and then drawn to produce fiber having a relatively low refractive index cladding corresponding to the fluorine doped region.
Referring particularly to Figures 2 to 8, Figure 2 shows a tubular fused silica support tube 16, 120 centimeters in length with an internal diameter of 5 millimeters and an external diameter of 7 millimeters. The ends of the silica support tube are fixed in the chucks 18 of a glass working lathe. A silica soot producing burner 20 is mounted to direct a flame at the support tube 16. Silicon tetrachloride entrained within a stream of oxygen by bubbling the oxygen through silicon tetrachloride is fed to a central tubular chamber within the burner 200 Argon, which separates the silicon tetrachloride vapour from the burner gases within the burner itself is fed to a second surrounding annular chamber, hydrogen is fed to a third annular chamber, and a mixture of argon and oxygen is fed to an outer burner chamber. The flow rates are 2 to 4 1 itres per minute of oxygen to the first chamber, 2 litres per minute of argon to the second chamber, 10 to 20 litres per minute of hydrogen to the third chamber and 15 to 30 litres per minute of argon with 3 litres per minute of oxygen to the outer chamber. The burner is moved along the length of the support tube at 8 cen-timeters per minute and the support tube 16 is ro-tated at 30 revolutions per minute.
Particulate cladding silica 24 is deposited onto one end of the support tube 16 to a diameter of 4.0 centimeters, length of 40 centimeters and a density of 0.30 grams cm~3. The rate of growth depends on the diameter of the support tube 16 as supplemented by previously deposited particulate silica. The deposited silica 24 has a very high moisture content which is untenable if the silica is to become part of an optical waveguide since the moisture results in a large absorption peak near 1400 nanometers. This reduces the transmission at 1300 and 1550 nanometers which are the output wavelengths of long wavelength light emitting devices of interest in fiber optic communications systems.
To remove this OH moisture absorption peak, the particula-te silica 24 is dried in a chlorine-containing atmosphere at high temperature. As shown in Figure 3, the support -tube 15 toge-ther with the deposited silica 24 is mounted within a 100 millimeter internal diameter silica tube 26 which sits in a vertical furnace 28 having a heating element 42 that is shorter than the 40 cm long soot 20 boule 24. The furnace temperature is held at 1300C while the soot boule is slowly lowered through the heat zone 42 -to cause drying of the soot. A mixture of chlorine (150 cc/min), and oxygen (3000 cc/min) is fed -through the tube 26. Hydrogen contained within the porous silica as -the hydroxyl species reacts with the chlorine -to produce volatile hydrogen chloride which is entrained within -the gas stream and removed. Removal of hydroxyl species renders subsequently formed fused silica very highly -transmissive.
r7 In a subsequent sin-tering or consolidation s-tep (Figure 4), the Furnace 2~ is heated to a tempera-ture of 1580C and the soot is driven through the hea-t zone 42 at a rate o-F 0.2 cm/min -to sin-ter the soot 24 into clear glass 43. After a three hour furnace traversal period in an atmosphere of 2 (3000 cc/min); Cl2 (150 cc/min); He (1500 cc/min) and SF6 (150 cc/min) the soot is consolidated to a fused silica -tube 32 about 30 centimeters in length having an external diameter of 1.~ centimeters.
The refractory rod 22 is removed and the support tube 16 is then etched away. Initially, a wet etchant, 1:2 nitric acid:hydrochloric acid, is used to remove carbon and outgassing contaminants from the inside of the tube 16 (Figure 5).
The composition of the support tube 16 can be silica doped with the OH group, for example, to improve etchability. However care must be taken to ensure that such an impurity is not absorbed into the cladding silica 24 while it is porous. Moreover any impurity used should not cause a marked change in the coefficient of thermal expansion of -the silica since otherwise there will be untenable stress introduced at the support tube/cladding silica interface during temperature fluctuations associated with processing.
This is followed by a gas phase etching step (Figure 6). Tubular silica handles 34 are attached to both ends of the cladding and support tube structure such that a continuous center hole runs down the axis of the systern. This is placed in a glass working lathe and a 1:1:1 mixture of helium, oxygen and sulphur hexafluoride is fed through the center hole. At the same time, the preform is subjected to a number of passes of the localized zone furnace 44 a-t a rate of 1.0 cm/min to es-tablish a temperature at the radial center of the preform of about 1600C. At this temperature the silica of the initial support tube 16 is etched away. The amount of material removed from the tube is determined by the temperature and relative -Flow rate of the SF6.
Referring to Figure 7, further fused silica 36 is deposited to a depth of 0.01 cm using a modified chemical vapour deposition (~lCVD) process described in MacChesney et al. A furnace 44 heats the tube 32 to a temperature of 1600 to 1700C and downwardly traverses the tube at a ra-te of 5 cm/min as a mixture of SiC14 (60 cc/min) He (1000 cc/min) and 2 (1000 cc/min) is passed down the tube and the tube is rotated at 30 revolutions/min. As the gaseous mixture passes through the hot zone~ the SiC14 dissociates and reacts with the oxygen to form particulate silica. The silica soot 46 settles on the inside surface of the tube 32 and is subsequently consolidated as the hot zone reaches it.
In an alternative method, the gas mixture and the hot zone are directed in opposite directions and the silica ls deposited as silica soot to a depth of 2 to 3 mm with a density of approximately 0.1 g/cm3. A drying gas mixture of chlorine and oxygen ls then passed through the tube and the tube is heated to approximately 1200C
for 1 hour to reduce the OH moisture content. The silica tube is then placed in a furnace at a temperature 1600 to 1700C to sinter the centrally deposited silica.
The composite tubular preform obtained is then collapsed (Figure 8) by heating to a temperature in the range 1850 to 2200C and traversing the burner or furnace at 1 cm/min towards the inlet encl while maintaining the Cl2/He atmosphere in central bore 38.
In another alternative approach, fluorine alone can be used both to dry and dope. By using the chlorine and fluorine drying techniques at various stages in the fabrication process, a mois-ture level oF less than 0.1 parts per million in the fused silica is achieved.
The diameter of the tubular rod is then increased from about 1.6 cm to about 2.5 cm by adding a silica jacket 14. To do this a silica tube 40 is heated to collapse it down onto -the outside of the fluorine doped silica 32 (Figure 8).
Referring to Figure 9, the composite rod preform finally obtained is subsequently placed in a vertical orientation drawing tower with a furnace zone at which -the preform temperature is raised to about 2000C which is higher than the silica softening point. Fiber 42 is pulled from the lower end oF the preform by a drum onto which the fiber is wound after being cooled and coated with a protective acrylate or silicone layer. The fiber has a high purity silica core and a relatively lower refractive index fluorine doped silica cladding.
The waveguide produced using the dimensions indicated is single mode fiber having a core diameter of about 9 microns, a cladding diameter of about 80 microns, and an overall diameter o-f 125 microns. As previously indicated the refractive index difference is 25 approximately 0.0045.
A refractive index difference oF the cladding relative to that of pure silica is controlled by the ratio of helium to SF6 in the mixture used during the fluorine doping stage of Figure 5 assuming that the chlorine and oxygen levels are held constant. The amount of Fluorine necessary to lower the refractive index of silica to 1.439 at a wavelength of 1300 mm is about the limiting level at which fluorine can be incorporated into silica using this method. To obtain a refractive index difference larger than OnO08 for a silica based fiber, the refractive index oF the core can be increased above the value of that of the pure silica. Most dopants increase the refractive index of silica, so by incorporation of the dopant material into the silica internally deposited within the tube, the refractive index of the resulting waveguide core is increased above that of pure silica.
Germanium can be included within the MCVD deposited silica by entraining germanium tetrachloride with the silicon tetrachloride injected into the tube.
Claims (18)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of manufacturing a single mode optical waveguide comprising depositing a first layer of particulate silica on an outer surface of a fused silica cylindrical support tube, then drying and diffusing fluorine into the deposited silica, then sintering the deposited silica to cause fusion thereof, then etching away the support tube, then directly depositing a second layer of silica on the inside surface of the resulting tube, collapsing the composite tubular preform obtained into a rod preform, heating the rod preform to a drawing temperature and drawing a single mode optical waveguide from the rod preform, such single mode optical waveguide having a core and a cladding derived from the deposited silica, said core derived from said second layer of deposited silica and the cladding being derived from the first layer of deposited silica.
2. A method as claimed in claim 1 in which prior to etching away the support tube, an internal surface thereof is cleaned by immersing in acid.
3. A method as claimed in claim 2 wherein the acid is a mixture of hydrochloric and nitric acids.
4. A method as claimed in claim 1 in which the support tube is etched using fluorine.
5. A method as claimed in claim 4 in which the fluorine is used in the form of sulphur hexafluoride.
6. A method as claimed in claim 5 wherein an etchant gas containing a fluorine compound is fed through the support tube as a localized hot zone furnace traverses the tube.
7. A method as claimed in claim 6 wherein the fluorine containing compound is mixed with oxygen and helium.
8. A method as claimed in claim 1 wherein the second layer of silica is deposited as a layer of particulate silica and is dried prior to collapsing the composite tubular preform.
9. A method as claimed in claim 1 wherein the fluorine is diffused into the first layer of particulate silica from a fluorine containing compound.
10. A method as claimed in claim 9 wherein the fluorine containing compound is sulphur hexafluoride.
11. A method as claimed in claim 8 wherein a mixture of gases is used simultaneously to dry and fluorine dope the first layer of particulate silica.
12. A method as claimed in claim 11 wherein the mixture of gases is selected from the group consisting of chlorine, oxygen, helium and sulphur hexafluoride.
13. A method as claimed in claim 12 wherein the support tube is etched by passing the etchant gas along a central bore within the support tube.
14. A method as claimed in claim 1 in which the support tube is made of fused silica doped with a material which renders the silicon more easily etchable than pure silica.
15. A method as claimed in claim 1 in which etching is continued after removal of the support tube so as to etch away a portion of the initially deposited layer of silica.
16. A method as claimed in claim 15 in which the support tube is etched away after the deposited particulate silica of said first layer has been sintered to cause fusion thereof.
17. A method as claimed in claim 15 in which the support tube is mounted between spaced chucks during deposition of the first layer of particulate silica thereon.
18. A method as claimed in claim 15 in which during drying and doping of the first layer of particulate silica a refractory liner rod is positioned within the support tube.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000522402A CA1261127A (en) | 1986-11-06 | 1986-11-06 | Optical waveguide manufacture |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000522402A CA1261127A (en) | 1986-11-06 | 1986-11-06 | Optical waveguide manufacture |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1261127A true CA1261127A (en) | 1989-09-26 |
Family
ID=4134306
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000522402A Expired CA1261127A (en) | 1986-11-06 | 1986-11-06 | Optical waveguide manufacture |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1261127A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1705157A1 (en) * | 2005-03-22 | 2006-09-27 | The Furukawa Electric Co., Ltd. | Method of manufacturing microstructured optical fiber |
| US7653317B2 (en) | 2003-09-26 | 2010-01-26 | The Furukawa Electric Co., Ltd. | Semiconductor laser device |
| CN115557692A (en) * | 2022-09-07 | 2023-01-03 | 武汉长盈通光电技术股份有限公司 | Preparation method of large-numerical-aperture preform |
-
1986
- 1986-11-06 CA CA000522402A patent/CA1261127A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7653317B2 (en) | 2003-09-26 | 2010-01-26 | The Furukawa Electric Co., Ltd. | Semiconductor laser device |
| EP1705157A1 (en) * | 2005-03-22 | 2006-09-27 | The Furukawa Electric Co., Ltd. | Method of manufacturing microstructured optical fiber |
| CN115557692A (en) * | 2022-09-07 | 2023-01-03 | 武汉长盈通光电技术股份有限公司 | Preparation method of large-numerical-aperture preform |
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