Classifications

• • 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]
  • C03B37/01413—Reactant delivery systems
  • C03B37/0142—Reactant deposition burners
• • 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
  • C03B2201/00—Type of glass produced
  • C03B2201/02—Pure silica glass, e.g. pure fused quartz
• • 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
  • C03B2201/00—Type of glass produced
  • C03B2201/06—Doped silica-based glasses
  • C03B2201/30—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi
  • C03B2201/31—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi doped with germanium
• • 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
  • C03B2207/04—Multi-nested ports
  • C03B2207/06—Concentric circular ports
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2207/00—Glass deposition burners
  • C03B2207/04—Multi-nested ports
  • C03B2207/12—Nozzle or orifice plates
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2207/00—Glass deposition burners
  • C03B2207/50—Multiple burner arrangements
  • C03B2207/54—Multiple burner arrangements combined with means for heating the deposit, e.g. non-deposition burner
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2207/00—Glass deposition burners
  • C03B2207/60—Relationship between burner and deposit, e.g. position
  • C03B2207/64—Angle
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2207/00—Glass deposition burners
  • C03B2207/80—Feeding the burner or the burner-heated deposition site
  • C03B2207/85—Feeding the burner or the burner-heated deposition site with vapour generated from liquid glass precursors, e.g. directly by heating the liquid
  • C03B2207/87—Controlling the temperature
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/50—Glass production, e.g. reusing waste heat during processing or shaping
  • Y02P40/57—Improving the yield, e-g- reduction of reject rates

Definitions

• This invention relates to improved methods of making optical waveguides and optical waveguide preforms and products by vapor deposition techniques.
• the most commonly used techniques for optical waveguide manufacture are based on dissociation in a flame of glass forming constituents to build up a porous preform of vitreous particles called "soot.”
• the "soot" preform is converted to a glassy state by sintering at an elevated temperature.
• the final product is drawn from a combination of core and cladding layers drawn under given temperature and tension conditions.
• An early, widely used optical waveguide manufacturing process is the "inside vapor deposition,” process, also known as the modified chemical vapor deposition process (CVD) .
• CVD modified chemical vapor deposition
• MCVD a soot core is deposited within the interior of a hollow, silica tube. After vitrification, the core-tube body is compacted prior to or during the drawing of a preform.
• a plasma chemical vapor deposition (PCVD) process similarly employs a hollow silica tube to deposit glass soot material within the tube.
• PCVD plasma chemical vapor deposition
• Disadvantages of the MCVD and PCVD processes include that silica tubes are costly, and further that the size of the preform that can be made is restricted.
• ODD outside vapor deposition
• the OVD process is referred to as "radial" deposition because soot material is deposited radially around a pre-existing, rod shaped mandrel. Once deposition is complete, the mandrel is removed from the deposited material prior to further processing into a preform. The removal of the mandrel is a delicate procedure and therefore imposes a length restriction on the resulting preform.
• VAD vapor-phase axial deposition
• the cylindrical core resulting from a VAD process can be continuously sintered, if desired, to provide a glass start rod.
• additional soot cladding layers can be concurrently deposited over the surface of the cylindrical core with a separate soot stream directed radially toward the axially developed core.
• the diameter of the glass start rod therefore, may be readily increased, unlike the case with MCVD or PCVD processes.
• VAD process has become more apparent, as the usage of optical waveguides has increased, and the technical requirements by communication systems have become more demanding.
• signal attenuation of 0.4 dB per kilometer, low dispersion, and precise cut-off wavelength characteristics are often sought.
• optical waveguides have, in addition to very low impurities and a homogeneous microbubble-free structure, a reliable and predictable refractive index profile.
• production costs and yields for making glass preforms must be satisfactory for mass production quantities.
• VAD processes used heretofore are now less than satisfactory from one or more standpoints.
• HVD redeposition or "hybrid"
• 5,028,246 uses as a first step an axial VAD torch to deposit a soot core using a high velocity laminar soot stream at an angle greater than 60° relative to the axis of the core cylinder. Then, as a- subsequent step, a pure silica cladding is deposited with an OVD soot stream.
• One key difference between VAD and HVD processes is the tilt angle formed between the deposition torch and the core axis. The tilt angle for the VAD method is less than 60°, whereas the HVD process uses a tilt angle which is larger than 60°. The HVD process, however, may not provide the larger preform and reproducibility necessary for mass scale production of high quality optical fibers.
• the soot on soot on soot (SSS) method of making preform involves the following basic steps. First, a soot core is axially deposited with a soot stream provided by an axial torch. This soot core cylinder then acts as a mandrel for further soot deposition, which becomes integrated with the final core. Second, additional soot core material is deposited on the core cylinder using a radial deposition method. Thus, the completed soot core comprises the axial soot core and the additional soot core layers deposited in the second step. In accordance with the present invention, the resulting soot core cylinder from this second step is both large and uniform in diameter unlike cylinders that are made using prior art methods.
• a small amount of silica soot cladding is deposited.
• the soot boule is next purified and sintered to a glass core rod.
• This core rod is then stretched down to obtain a smaller diameter for further cladding.
• Any further cladding necessary for a proper b/a ratio, where b is the cladding radius, and a is the core radius, can be added either by providing additional soot cladding layer (s) or using a rod-in-tube, over-jacketing method.
• a method of making a preform for optical waveguide fibers having low attenuation comprising the steps of: depositing a core soot material along the axis of a rotating target by directing a first stream of core soot material towards the rotating target to develop a first soot core having predetermined length and substantially uniform density or a predetermined density profile; and depositing one or more additional layers of core soot material over the first soot core using a second stream of core soot material positioned radially at an angle substantially perpendicular to the axis of rotation.
• a method of making optical waveguide fibers having very low attenuation comprising the steps of: depositing a core soot material along the axis of a rotating target by directing a first stream of core soot material towards the rotating target to develop a first soot core having predetermined length and substantially uniform density or a predetermined density profile; depositing one or more additional layers of core soot material over the first soot core using a second stream of core soot material positioned radially at an angle substantially perpendicular to the axis of rotation, developing a substantially uniform density soot core cylinder having a substantially uniform diameter along its length; depositing cladding soot material on the outer surface of the soot core cylinder with a radial stream of cladding soot material to provide a cladding layer coextensive with the soot core cylinder; drying and sintering the product thus formed; and drawing the dried and sintered product into optical waveguide fibers.
• the present invention eliminates the need for synchronization of radial deposition with axial deposition and provides a low hydroxyl ion and void-free soot-soot interface.
• the soot core boule obtained in accordance with the present invention has a substantially uniform diameter and a refractive index uniform along its length, because the radial deposition layering of the final soot core used in accordance with the present invention reduces tapering and periodic diameter variations of the axially deposited first soot core.
• soot core preforms require no mandrels and can be made with high repeatability, at high deposition rate.
• FIG. 1 is a combined, simplified perspective and block diagram view of a system in accordance with the invention for fabricating optical waveguide cores and preforms;
• FIG. 2 is an enlarged side view of a torch and soot stream configuration used in core deposition.
• FIG. 3 is a block diagram of steps in a method of forming one type of optical waveguide in accordance with the present invention.
• FIG. 4 is a flow chart of steps for an SSS single- mode fiber manufacturing method in accordance with a specific embodiment of the present invention.
• FIG. 5 is a typical refractive index profile of an SSS core rod for making single mode fiber.
• FIG. 6 is a histogram of cutoff wavelengths in 1500 km SSS single-mode fibers.
• FIG. 7 is a histogram of mode field diameter in SSS single-mode fibers.
• FIG. 8 is a scatter plot of measured cutoff wavelengths along fiber length from the SSS core rod Z2019 with same cladding thickness.
• FIG. 9 is a bar plot of OTDR measured attenuation at 1310 nm on fibers made by SSS process.
• FIG. 10 is a bar plot of OTDR measured attenuation at 1550 nm on fibers made by SSS process.
• FIG. 11 is a histogram of attenuation at 1385 nm in
• FIG. 12 is a histogram of zero dispersion wavelength of SSS single-mode fibers.
• FIG. 13 is a histogram of dispersion slope data of SSS single-mode fibers.
• FIG. 14 is a scatter plot of the relationship between zero dispersion wavelength and the product of mode field diameter and cutoff wavelength.
• FIG. 15 is a scatter plot of dependence of dispersion slope on zero dispersion wavelength in SSS single-mode fibers.
• FIG. 16 is a histogram of cladding non-circularity of SSS single-mode fibers with tube over-jacketing.
• FIG. 17 is a histogram of core non-circularity of SSS single-mode fibers with tube over-jacketing.
• FIG. 18 is a histogram of core/clad offset of SSS single mode fibers with tube over-jacketing.
• Fig. 1 shows in block diagram form a system for practicing the method of the present invention.
• the system generally comprises an axial traverse mechanism 40, a deposition chamber 10, a radial cladding torch 50, an axial core torch 16, end burners 27, 26, a chemical delivery system 12, 14 and a control unit 42.
• the operative components of the system are mounted within a large enclosed deposition chamber 10.
• a first bubbler 12 which may be inside or outside the chamber 10, contains purified precursor materials, such as a silica compound like SiCl 4 .
• Silica glass precursor vapors are forced out of the first bubbler 12 by directing a carrier gas, preferably oxygen or other suitable media, under pressure into the bubbler 12.
• a second bubbler 14 contains purified precursor dopant materials, which in a preferred embodiment is a germania compound GeCl 4 .
• Dopant precursor vapors are forced out of the second bubbler 14 by the 0 2 carrier.
• a valve 15 in the 0 2 carrier line can be operated to shut off the second bubbler 14, when desired.
• the entrained glass forming compounds in the vapor stream are mixed and are dissociated, upon being fed into a first torch 16 held, in the illustrative embodiment shown in FIG. 1 in substantially fixed position relative to the deposition zone.
• the axial torch 16 generates a streamlined soot stream 17 that is directed toward the bait rod 34 upwardly, at an angle preferably greater than about 60° relative to the rotation axis of the bait rod 34.
• a narrow light beam from a laser 18 is directed off a mirror 19 and onto a photodetector 20.
• the beam of the mirror 19 is angled to intercept the axis at the geometric center of the deposited material.
• the soot stream 17 is, however, not centered on the geometric center but is offset both radially and axially by predetermined amounts, as known in the art.
• a relatively short length of silica bait rod 34 is centered along the reference axis on a chuck 36.
• the chuck 36 and bait rod 34 are rotated at a selected rate, preferably about 28 r.p.m., for core deposition, by a rotary drive 38 mounted on a linear traverse mechanism 40.
• a position controller 42 receiving signals from the photodetector 20 in the core deposition mode can run the traverse mechanism 40 unidirectionally, and at a desired rate.
• the traverse mechanism 40 can be made to reciprocate through any chosen length of travel at a desired rate by bypassing the position controller 42.
• the bait rod 34 is first reciprocated through a short distance, and thereafter moved unidirectionally under position control.
• the traverse mechanism 40 can also reciprocate through a substantial total length for deposition of subsequent soot core layers and cladding.
• a side, or radial, torch 50 within the deposition chamber 10 is spaced along the reference axis from the bait rod 34 and separately used for radially depositing additional soot core layers and, subsequently, cladding layers.
• the radial torch 50 is fed from the first bubbler 12 and the second bubbler 14, when valve 52 is open, with a particulate forming compound, preferably a mixture of pure SiCl 4 , GeCl 4 , and 0 2 carrier gas.
• a particulate forming compound preferably a mixture of pure SiCl 4 , GeCl 4 , and 0 2 carrier gas.
• Other forming compounds can be used, as known in the art.
• the chemical components in the soot stream 17 emanate at a velocity of preferably about 40 ft/sec. In a preferred embodiment, other gases flow out at velocities of about 25 ft/sec. Other velocities can be used in alternative embodiments, if necessary.
• An exhaust outtake 22 immediately above the target area collects gases and non-impinging particulates at a gas flow rate of approximately 300 ft/min.
• a first end burner 26 adjacent the axial torch 16 aids in bringing the temperature at the point of deposition up to a given level before deposition begins.
• a valve 28 can be turned on or off to control usage of the axial torch 16.
• the system used in a preferred embodiment of the present invention is described, for example, in U.S. Pat. Nos. 5,028,246; 5,364,430, and 5,558,693 the content of which is hereby incorporated for all purposes. Other systems may also be used in accordance with the principles of the present invention.
• -Formation of a first soot core for an optical waveguide preform is accomplished in a preferred embodiment using an axial deposition.
• the deposition chamber 10 is first cleaned, and the bait rod 34 is mounted in the chuck 36, and centered on the axis of rotation.
• the axial torch 16 is positioned with respect to the laser beam that defines the tip of the deposited material during position control.
• the axial torch 16 is then lit, the exhaust velocity is maintained, and conditions are allowed to stabilize before the bait rod 34 is rotated, preferably at about 28 r.p.m., by the drive 38, and advanced into the path of the first soot stream 17 containing core particulates.
• the soot stream 17 impinges on and about the leading end of the bait rod 34, which is oscillated back and forth preferably at a rate of about 45 cm/hr.
• a bulbous starter tip 60 develops over the free end of the bait rod 34, to a length of approximately 2 cm. When sufficient material has been deposited, in accordance with the present invention, this bulbous tip 60 forms an adequate base or anchor for development of a first soot core 62.
• Axial development of the first soot core 62 commences, in accordance with the preferred embodiment, with the position controller 42 initially providing a fixed withdrawal rate of about 12 cm/hr.
• the first soot stream 17 sprays the end of the bulbous starter tip 60, initializing the first soot core 62.
• the flow in the stream 17 used in accordance with the present invention is laminar, and the deposition rate is approximately 0.14 grams per minute.
• the high flow rate in an upward direction is accompanied by some overspray, but gases that bypass the first soot core 62 are exhausted in the system shown in FIG.
• the withdrawal rate of 12 cm/hr is slightly faster than the first soot core 62 growth rate, but enables growth to be equilibrated on the bulbous starter tip 60.
• the position controller 42 is then switched 5 to the servo mode, with the laser beam intercepting the geometric center of the free end of the first soot core 62.
• the position controller 42 responds to the signals from the photodetector 20 by withdrawing the core cylinder 62 so as to maintain its free end in a constant position as matter is
• the first soot core 62 continues to grow and is withdrawn by the traverse mechanism
• step 20 step is to form one or more additional soot core layers onto the first soot core 62.
• the torch 16 is turned off .
• the first soot core is withdrawn back to the left side of the end burner 27 to wait for
• the radial torch 50 and the two end burners, 26 and 27, are then turned on and allowed to stabilize.
• the traverse mechanism 40 is controlled to move back and forth so that the first soot core 62 will be in the path of the chemical stream 55 emitted from radial torch 50.
• a soot layer will be deposited on the surface of the first soot core 62 as it passes through the chemical stream 55.
• the two end burners 26 and 27 are used to pre- densify the previously deposited soot core layers before the
• Soot core layers are added using radial deposition until a selected diameter is achieved resulting in a final soot core cylinder 64.
• multiple soot streams may be simultaneously employed for forming additional soot core layers onto the first soot core 62 by placing another radial torch displaced at an angle or in such manner as to avoid soot stream interference .
• the use of multiple radial torches to increase deposition speed is also applicable to the radial cladding process discussed next.
• the next step is the cladding deposition.
• the radial torch 50 is now supplied with a suitable cladding agent, as known in the art, and allowed to stabilize.
• the traverse mechanism 40 is actuated to reciprocate the length of the core cylinder 64 in opposition to the side torch 50.
• the core cylinder 64 turned at about 20 r.p.m. in a specific embodiment by the rotary drive 38, the radial torch 50 is held at a substantially constant distance from the axis of rotation of the soot core cylinder 64.
• the soot core cylinder 64 is then traversed back and forth relative to the cladding chemical stream 55 by the traverse mechanism 40 at a rate of about 250 cm/hr.
• cladding deposition of a pure silica soot particulate takes place at an average rate of approximately 2.5 grams per minute derived by flame hydrolysis of the carrier borne vapor from the first bubbler 12.
• the deposition temperature at the surface of the soot core cylinder 64 is gradually raised to a normal operating level by incrementally increasing the gas/oxygen flow rates over the first fifteen minutes of operation, until a thin contact layer of approximately 5 mm has been deposited on the soot core cylinder 64.
• the resultant lower temperatures avoid boil-off of germania (Ge0 2 ) from within the soot core cylinder 64, but the initially deposited cladding particulate nonetheless firmly unites to the surface of the core soot cylinder 64.
• the interface between the soot core cylinder 64 and a fully developed outside cladding layer 65 is in the nature of a very thin transitional layer of substantially constant characteristics and very low moisture content, factors which are of substantial significance to the refractive index profile and hydroxyl ion content of the ultimate optical waveguide.
• cladding layer 65 Deposition of cladding layer 65 is continued until a final diameter is obtained, giving a ratio of cladding radius (b) to core radius (a) of preferably about 2:1 or greater. Lower ratios are possible in alternative embodiments.
• a ratio of cladding radius (b) to core radius (a) of preferably about 2:1 or greater.
• Lower ratios are possible in alternative embodiments.
• the surface velocity increases, although the rate of increase slows as the radius becomes larger.
• the burner temperature is increased in steps with time to correspond generally to surface velocity, so as to maintain density substantially constant. Additional aspects of this process are discussed, for example, in U.S. Pat. Nos. 5,028,246; 5,364,430 and 5,558,693 which are incorporated by reference .
• Drying and sintering is performed after all soot core layers and partial cladding are deposited.
• the preliminary glass preform rods are cladded with a soot process or an over- jacketing process to proper thickness before fiber draw.
• the glass preform rods are again rotated and reciprocated across the soot stream from radial torch 50, to deposit a further cladding soot layer of the desired thickness on the drawn rod.
• the redeposited rod is then dried and sintered as before to provide a final fiber preform.
• Such preforms are themselves commercial products because many manufacturers prefer to draw their own optical waveguides .
• the preform rods are drawn, in -conventional fashion, to a final iber diameter of 5 125 ⁇ m for operation at 1310 to 1550 nm wavelength.
• These optical waveguide fibers are single mode fibers having a b/a ratio of about 13, and attenuation typically about 0.33 dB/km, at 1310 nm and more typically 0.2 dB/km, at 1550 nm and dispersion of 0 less than 3.5 ps/nm/km in the wavelength range between 1280- 1330 nm.
• the angle between the core soot stream 5 and the rotation axis of the core can be kept the same, but the reference axis can be tilted relative to the horizontal, and the soot stream directed more nearly vertical, or purely vertical, with the exhaust outtake being reconfigured if needed to provide clearance about the core cylinder while 0 providing the function of extracting overspray.
• the vitrified rods can be cleaned in a variety of ways, including dry gas etching and high intensity laser beam polishing as well as fire polishing.
• a 5 further step of separately depositing additional cladding layers over the soot core using radial, multiple-pass deposition is performed.
• the final soot core with a chosen cladding thickness is then dried, sintered, and drawn to form optical waveguide fibers.
• the cladding may be fluorinated during zone sintering to lower the refractive index below that of a pure silica core.
• the deposition of core soot and cladding soot materials may be conducted while the soot core axis of rotation is positioned vertically rather than horizontally. While extended horizontally, the maximum deposition length is limited by the weight of the soot core and its ability to support itself.
• the position and flow of the soot streams may be varied to make waveguides that have specific refractive index profiles.
• the deposition of additional layers of soot core material may be conducted by a plurality of soot streams simultaneously.
• the cladding soot stream is of substantially uniform cross-section.
• the cladding soot stream velocity directed at the soot core may be increased during deposition so as to limit surface temperature until an initial cladding layer is built up on the soot core cylinder.
• the first torch 16 used for core deposition is shown, along with the relative geometries of the gas streams.
• the chemical soot stream 17 emanates from a central aperture 70 around which an inner ring of orifices 72 provides an inner shield of oxygen flow.
• An intermediate ring of flammable gas and oxygen orifices 74 provides a circular flame that converges slightly.
• a final outer shield of oxygen emanates from orifices 76 in an outer ring. This arrangement maintains the chemical soot stream with a substantially constant diameter which provides the needed flame for disassociation.
• FIG. 3 it shows one embodiment of the method in accordance with this invention, where optical waveguides of even lower attenuation (i.e., about 0.2 dB/km or less) are provided by using a pure silica core and a cladding of a lower index of refraction.
• a pure silica soot core is deposited and additional soot core layers are deposited, as depicted in FIG. 1.
• This body is then dried and sintered, and a cladding, also of silica soot, is deposited thereon to a desired thickness.
• This soot layer is then dried at about 1150 °C. in a hydrophilic atmosphere.
• the drying and sintering steps are carried out in a closed furnace into which gas flows are injected at controlled rates as the body is held at a controlled temperature.
• the furnace is heated to a temperature in the range of 800-1250°C over 20 minutes with a chlorine flow of 350 cc/min, helium flow of 7000 cc/min and oxygen flow of 140 cc/min, then held for 30 minutes at 1150 °C under the same conditions.
• the preform is withdrawn with the same flows maintained, the withdrawal being effected within 10 minutes.
• SSS method used in accordance with the present invention deposits more core material along with the axial core before the deposition of silica soot cladding. This modification provides at least the advantages listed below.
• An advantage of the present invention is that a large sized initial core rod decreases the relative diffused interface thickness between the core and the cladding in the final fibers, which improves the fibers' bending sensitivity.
• Another advantage of the present invention is that the resulting large soot core cylinder makes large scale production of high quality optical fibers feasible.
• the process described in the Example below yields over 360 km single-mode fibers per core preform compared to 120 km using a single step, axial deposition process.
• more core deposited with radial deposition increases process productivity by over 300 percent. Further increases in fiber output can be easily achieved by depositing more Ge0 2 doped silica layers with the radial torch.
• the combination of the axially deposited first soot core and the radially deposited additional soot core layers can provide another dimension to control the refractive index profile, so as to enable making, for example, step-index profiles, triangular profile and semi-parabolic profiles, by adjusting the position of the axial torch 16 and varying chemical flows to the radial torch 50.
• an axially deposited soot cylinder could have some diameter variations caused by an unstable chemical delivery system, such as bubbler temperature fluctuation and carrier gas flow fluctuation.
• the core cylinder diameter variation will result in mode field diameter variations in the resulting fibers.
• the diameter variations of the axially formed first soot core are reduced in the final soot core cylinder. Therefore, the effect of an unstable chemical delivery system on soot deposition is reduced to a very low level; repeatability of the process is increased, rendering the method of this invention much more suitable and practical for large scale, high quality optical fiber manufacturing.
• the large core rod size and excellent optical characteristics provided by the present invention makes it possible to produce one core rod that yields up to a thousand kilometers of a single-mode fiber. This presents a significant commercial advantage in large scale optical fiber production.
• FIG. 4 shows a flow chart of the partial OVD process for SSS single-mode fiber manufacturing.
• Axial deposition of the first soot core was performed using an axial torch. SiCl 4 and GeCl 4 flow rates during deposition are 119.4 and 44.5 cc/min respectively.
• the deposition temperature is maintained at 1150°C by an optical pyrometer aimed at the geometrical center of the soot core deposition.
• the rods with the partial cladding were radially deposited with additional soot layers or over-jacketed with 1400 mm 2 CSA synthetic silica glass tube and then drawn to a final fiber.
• Table 3 Process dimension parameters with partial OVD cladding/tube jacketing method.
• the example SSS single-mode fiber waveguide structure is a matched-cladding design.
• a typical refractive index profile is shown in FIG. 5.
• functional properties such as cutoff wavelength and mode field diameter, are mainly determined by fiber refractive index profiles, variations of cutoff wavelengths and mode field diameters were used to evaluate the process stability and capability for single-mode fiber manufacturing.
• the targeted parameters for this single-mode waveguide structure are listed in Table 4.
• FIGS. 6 and 7 show histograms of both cutoff wavelength and mode field diameters from 1500 km, SSS single-mode fibers. Cutoff wavelength variation is mainly caused by core diameter variations. A tighter cutoff wavelength has been achieved with all-soot cladding process to provide even more precise cladding thickness. Overall, more than ninety percent of the fibers produced met cutoff wavelength and mode field diameter specifications targeted in Table 4.
• HVD core rods have had significant core diameter variations, which consists of both core rod diameter tapering and periodic variation along the core.
• the core diameter taper is caused by soot cylinder sagging during deposition.
• the periodic diameter variation along the core is mainly caused by chemical bubbler temperature fluctuation. 5
• SSS process in accordance with the present invention more than half of the core materials is axially deposited.
• the multiple-pass deposition method overcomes, to some degree, high frequency variation in the core diameter caused by unstable deposition. Therefore, the overall axial
• FIG. 8 shows the measured cutoff wavelengths along 250 km single-mode fibers from one SSS core rod with the same cladding thickness. All measured cutoff wavelengths are near
• Fiber attenuation is low in the fibers made by SSS technique. Results show that fiber attenuation is peaked at 0.33dB/km at 1310 nm and 0.185dB/km at 1550 nm, respectively. FIGS. 9 and 10 show OTDR measured attenuation at both 1310 nm
• Zero dispersion wavelength and dispersion slope were measured for all fibers produced in SSS pilot production.
• Average zero dispersion wavelength was 1316 nm, with a standard deviation of 2.63 nm.
• Average dispersion slope was 0.0851 ps/nm 2 -km, with a standard deviation of 0.0025. Histograms of dispersion properties are shown in FIGS. 10 and 11. The relationship between measured zero dispersion wavelength and product of mode field diameter and cutoff wavelength is shown in FIG. 14.
• any variation in fiber zero dispersion or dispersion slope comes mainly from the variation of refractive index profile and core diameter. Any deviation from the step type reactive index profile may cause unwanted changes in the fiber's dispersion properties.
• the center zero dispersion wavelength stays at 1314 nm, with a cutoff wavelength and mode field diameter of 1270 nm and 9.4 ⁇ m, respectively, as seen in FIG. 12.
• the refractive index profile can be manipulated with varying chemical flows layer by layer during the radial deposition step.
• cladding non-circularity varies in value from 0.1 to 1.2 percent, with a peak near 0.5 percent. Core non-circularity depends mainly on the core making process itself. As shown in FIG. 17, 99 percent of fibers in accordance with the present invention have non-circularity of less than 5 percent, with most fibers having non-circularity of less than one percent. Core/clad offset is determined mainly by the over-jacketing process.
• FIG. 18 shows a core/clad offset histogram of SSS fibers. About five percent of SSS fibers have offsets more than one micrometer.
• Table 4 The overall performance of the SSS single-mode fibers manufactured in the pilot production is listed in Table 4. Table 4 shows that most of the fibers lost are due to core/clad offset (6.43%) and cutoff wavelength (8%). This loss has been dramatically reduced when an all-soot cladding process is used to make SSS single-mode fibers. With all- soot cladding, exact cladding thickness can be made to meet precise cutoff wavelength specifications. The core and cladding concentricity can also be improved when the target rod is well aligned during radial soot deposition.
• Standard single-mode fibers made with the SSS process have shown excellent performance. More than ninety percent of fibers have attenuation of less than 0.34dB/km at 1310 mn and 0.2 dB/km at 1550 nm.

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