METHODS AND APPARATUS FOR PRODUCING OPTICAL FIBER

Abstract: Filament in tube and stick in tube processes of forming optical fiber are described. A solid or monolithic core feedstock (110) is disposed in a hollow cladding structure (112) to form a loosely filled cladding structure. The filled cladding structure is heated to a draw temperature approximately equal to the softening temperature of the cladding structure. The feedstock (110) melts and fills the heated portion of the cladding structure forming a filled core which can then be drawn into optical fiber or to an optical can which can then be further overclad consolidated and drawn into fiber. Feedstock (110) and cladding structures (112) having widely varying coefficients of expansion may be employed. The resulting fiber can be readily designed to be fused to existing installed fibers.

Description: METHODS AND APPARATUS FOR PRODUCING OPTICAL FIBER FIELD OF THE INVENTION The present invention relates generally to improvements in optical fiber waveguides and their manufacture. More particularly, the present invention relates to novel methods and apparatus for forming optical fiber waveguides via filament in tube and stick in tube methods of fiberization. BACKGROUND OF THE INVENTION Optical fiber waveguides have come to play an increasingly important role in communications. A range of optical fiber types with regard to size, index profiles, operating wavelengths, materials, etc., must be available in order to fulfill many different system applications. Further, there is an increasing need for active devices, such as amplifiers, lasers, switches and dispersion compensators. Additionally, optical fiber cables must be spliced together without excessive practical difficulties. It is important that these splicing techniques may be applied with ease in field locations where cable connection takes place. It is particularly important in many applications that a new fiber may be readily spliced to an existing fiber already in place. Put otherwise, removing all the existing fiber and replacing it with new fiber having different characteristics is often not an option. Various techniques are used to make optical fibers. In one procedure (see U.S. Patent No. 3,659,915), a rod of core material is placed within a tube of lower refractive index cladding material, forming a tight, concentric fit. The core material must be uniform in cross section and have a smooth surface. The temperature is then raised, and the rod and tube are drawn to the desired cross-sectional area. The resultant optical fiber by this process might not be ideal for communication because of excessive losses and dispersion. Another method (see U.S Patent No. 5,651 ,083) involves the insertion of a core material into a molten cladding material to create a preform. The core insertion is performed rapidly so that the core does not soften or dissolve during the procedure. The resultant preform is then drawn into optical fiber. Flouhde glasses, such as ZBLAN, manufactured in this fashion are not fusion spliceable to silica fibers, are prone to devitrification and have poor durability. One of the more important methods employed in making soot used in the manufacture of low loss optical fiber is the chemical vapor deposition (CVD) process. In one embodiment of the CVD process, relatively pure chemicals (such as silicon tetrachloride), are passed into a manifold with oxygen. They are then mixed and fed into a burner which is moved beneath a rapidly rotating bait rod or high purity fused silica tube. The result is that the silicon is oxidized to silica on the bait rod or silica tube. The deposit may be doped with a variety of materials. A resulting preform is typically consolidated and then drawn into optical fiber. This process is an outside CVD or OVD process. An inside or MCVD process is also a known CVD process. Current CVD methods for optical fiber fabrication are limited to compositions consisting almost entirely of silica. Only modest amounts of rare earth elements can be incorporated without clustering or crystallization. Volatile components such as alkalis and halogens cannot be readily introduced because of their tendency to vaporize during lay down. Other important glass modifiers such as alkaline earths cannot be incorporated due to lack of high vapor pressure CVD precursors. Even if glass soot can be deposited by CVD it must subsequently be consolidated which can lead to crystallization or to loss of glass components with high vapor pressures. Another fabrication technique known as the cullet in tube method has recently been developed. That approach is described in U.S. Patent Application Serial No. 08/944,932 filed October 2, 1997 which is assigned to the assignee of the present invention, and incorporated by reference herein in its entirety. In the cullet in tube process, a core cullet feedstock (having a particle size typically in the range of 100 - 5,000 urn) is introduced into a cladding structure. The end of the core/cladding structure is heated in a furnace to near the softening temperature of the cladding and drawn into optical fiber. This method overcomes some of the disadvantages of typical CVD processes, allowing the cladding composition to consist of pure Si0 and the core composition to consist of multicomponent glasses. A need, however, exists for methods and apparatus for making optical fiber from a variety of glass and glass-ceramic compositions that overcomes the disadvantages of the known methods, and that is more practical, efficient, and economical than conventional methods. By way of example, disadvantages of various prior art CVD techniques include the very limited compositions which can be fabricated using current CVD methods. Only modest amounts of rare earth elements can be incorporated without clustering. Volatile components, such as alkalis and halogens, cannot be introduced in significant quantities because of their tendency to vaporize during lay down. Other important glass modifiers such as the alkaline earths are difficult to incorporate due to lack of high vapor pressure CVD precursors. Even if a glass soot can be deposited by CVD, it must subsequently be consolidated which can also lead to a loss of glass components with high vapor pressures or crystallization. Disadvantages of typical rod-in-tube techniques include the requirement that the core and clad be highly similar. Both the coefficient of expansion and viscosity temperature profiles need to be similar, otherwise, the end product will be subject to cracking or breakage upon cooling. The present invention also relates to an optical fiber having a numerical aperture greater than about .35. Such fibers which comprise a core and clad region which are glass, and are doped with a rare earth element which is selected from the group consisting of ytterbium, neodymium, and erbium, can be used to fiber lasers. Such fiber lasers can be made by coupling a pump source coupled to the high NA fiber. Using the methods disclosed herein, numerical apertures of about .40 or greater (e.g. .45) have been achieved. SUMMARY OF THE INVENTION The present invention provides methods and apparatus for producing a wide variety of optical fibers via filament in tube or stick in tube methods of fibe zation. In one aspect, the present invention comprises the steps of filling a glass tube with a glass filament or stick of the desired material and subsequent drawing or elongation of the glass tube at elevated temperatures. The material within the tube melts at the draw temperature and fills the tube to form a continuous core. The loose fitting feedstock can be automatically fed or melted down by gravity to maintain a constant depth of molten feedstock, yielding a homogeneous and reproducible product. The feedstock can be comprised of a core material or a core/clad material. Likewise, the tube can be comprised of additional core material (e.g., which could be used to form the outer core region), core/clad material, or a cladding material. The present invention can be used to draw optical fiber directly (filament in tube) or can be used to make a core cane or a core clad cane which is then overcladded with additional material before being drawn into optical fiber (stick in tube). The present invention allows almost any glass that can be produced by chemical (sol-gel, vapor deposition, etc.) or physical (batch and melt) techniques to be economically fabricated in the form of a continuous clad filament. The rapid quenching permitted by this technique allows for previously unstable glasses and glass-ceramics to be formed as stable fibers. A more complete understanding of the present invention, as well as further features and advantages, will be apparent from the following Detailed Description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional drawing of suitable apparatus for performing the filament in tube method of drawing optical fiber in accordance with the present invention; FIG. 2 is a cross sectional drawing of suitable apparatus for performing the stick in tube method of drawing optical fiber in accordance with the present invention; FIG. 3 illustrates suitable apparatus for overcladding an optical cane formed in accordance with the present invention which may then be drawn into optical fiber in accordance with the present invention; FIG. 4 is a graph showing loss as a function of wavelength for a 5 meter span of optical fiber produced in accordance with a filament-in-tube method of the present invention; FIG. 5 is a graph showing the refractive index profile of a core clad cane produced in accordance with the present invention; FIG. 6 is a graph showing loss as a function of wavelength for a 5 meter span of optical fiber produced in accordance with a stick-in-tube-method of the present invention; and FIG. 7 is a graph showing the loss and mode field diameter as a function of fiber length for an optical fiber produced in accordance with the present invention. DETAILED DESCRIPTION The present invention provides methods and apparatus for producing a wide variety of optical fibers via filament in tube and stick in tube methods of fiberization as more fully discussed below. Before addressing the present invention in detail in connection with the drawings, various general aspects and advantages will first be generally addressed. First, a glass or crystalline stick of the desired core composition should be obtained. It does not matter if the stick has a round, square, or triangular, or some other different cross section, it need only fit within a cladding tube with which it is to be utilized. Unlike the well known rod-in-tube method, the inventive method does not require the core to fit tightly and concentrically within the cladding tube, since the core filament melts to conform to the cladding walls. Likewise, the tube bore need not be circular, but can be of a rectangular, elliptical, or other non-circular shape to enable formation of a fiber having a non-circular core. Because, in the present invention, the core glass or stick conforms to the shapes of the internal diameter of the tube, when a rectangular shaped tube bore is employed, the core glass will deform to the shape of the tube, thereby forming a core region upon solidification which is rectangular in shape. A fiber having a generally rectangular core can be made by employing a tube having a rectangular shaped inside diameter. After drawing of the fiber, the core remains of a substantially rectangular shape in cross-section (with some rounding of the corners of the rectangle). Fibers having a rectangular shaped core have been made, using the methods of the present invention disclosed herein, which exhibited a numerical aperture (NA) of greater than about .35, in particular we have achieved numerical apertures of about .45. Such high NA fibers cannot generally be made using CVD techniques, because the large amount of modifier needed for such refractive index changes causes crystallization, cracking, or sagging of the core material during manufacture. Such a fiber having rectangular shaped core can be used for efficiently coupling the light from a stripe laser diode. For instance, a typical high powered stripe laser diode emits a beam having essentially a 100 μm x 1 μm rectangular geometry. Accordingly, a beam having this geometry is more efficiently captured by a fiber formed in accordance with the present invention to have a substantially similar core geometry to that of the laser beam. Other non-circular shaped tube bores could also be employed, including elliptical, which could be employed to form polarization maintaining fibers. Likewise, tubes having an outer periphery that is non-circular in cross-section can also be utilized, to manufacture fibers whose outer periphery is non- circular. In each of these embodiments in which a non-circular ID or OD is to be maintained, high drawing viscosity is preferable to facilitate the fiber or core cane at least substantially retaining the shape of the ID or OD, or both. In particular, in one preferred embodiment used to make a fiber or core cane having either non-circular core or a non-circular outer fiber diameter, or both, the draw rate and temperature of the preform is maintained so that the resultant fiber or core cane at least substantially retains the shape of the inside and outside of the tube. Such a result is facilitated by maintaining the draw or redraw (in the case of making a core cane) temperature so that the tube viscosity is maintained at greater than about 10 7 Poise. The inventive method also does not require the core stick to be uniform in cross section and have a smooth surface, unlike prior rod-in-tube technology. The core stick can be fabricated by conventional crucible melting and casting, drawing, sol-gel, or some other technique. The stick is then loaded into the cladding tube. The composition of the tube which becomes the fiber cladding is not limited and can range from pure SiO 2 to multicomponent glasses. The only requirement is that the core glass melt at or below the softening point of the cladding tube and that the thermal expansion difference between the core and clad not be so large as to shatter the resultant fiber upon cooling as addressed in greater detail below. After the cladding tube is filled, it can be drawn down into fiber or canes for overcladding. The filled tube is heated to soften the cladding glass for elongation. As the cladding tube softens during the draw, the core stick will melt, fine (removal of bubbles), and conform to the walls of the cladding tube forming an interface determined by the inner surface of the cladding tube. The ratio of the outer diameter (OD) to the inner diameter (ID) of the tube will be roughly the same as the fiber or cane OD/ID ratio although it can be controlled by the pressure (positive or negative) applied over the molten core relative to outside the cladding tube. The draw temperature can also be used to control the core diameter as higher draw temperatures will lead to smaller core diameters for the same given fiber OD. This control represents a substantial advantage over conventional preforms where this ratio is fixed once the blank is fabricated. The higher temperatures used to draw the cladding tube (2000°C for the case of pure Si0 2 cladding) serve to homogenize the core melt and drive off detrimental water in the glass. In addition, a vacuum can be applied to a centerline to enhance water removal and fining. Utilizing an open centerline during the first draw step allows for atmospheric control of the melt at the drawing temperature. Oxidizing as well as reducing atmospheres can be introduced above the melt to control the redox state of the core material or maintain reduced metallic cores or superconductors in a dielectric cladding. Multiple concentric or parallel cores may also be made by this method where one core may carry optical information and the other electrical information. For example, stress rods (e.g. B 2 O 3 doped SiO 2 glass rods) can be placed within bores that are drilled in the walls of the tube. Such stress rods could be made via CVD and consolidated into glass, then placed within the bores drilled into the side walls of the tube. Alternatively, the stress rods could be comprised of a non-CVD formed glass (e.g. a melt glass) and placed within the bores of the side walls of the tube. In either case, after the glass stress rods were positioned within the bores of the sidewalls of the tube on opposite sides of the tube, the core glass feedstock could be fed into the inside diameter of the tube as described further herein. The resultant preform could be subsequently drawn to make a polarization maintaining fiber. Alternatively, two electrically conductive wires could be used in place of the stress rods (and likewise inserted into holes bored into the sidewalls of the tube) to make a fiber which may be used as an electro-optic switch (e.g., enabling application a voltage between the two wires to change the refractive index of the core). The pressure within the tube above the core can also be controlled to regulate the core diameter. This type of process control is not available with any of the current preform fiberization methods, and in the present invention, such control greatly facilitates the formation of certain combinations of tube and core glasses. For example, with thicker walled tubes, such vacuum assistance might not be important. However, with some thinner walled tubes, or in cases where the core glass has a larger melt depth within the tube, such vacuum application within the tube from above the core glass can be used to compensate for the forces of the melt glass pushing outwardly on the tube walls, thereby helping to maintain the internal shape of the tube walls. Controlled glass composition and thermal history can also be used to generate graded index profiles. Since the core is molten and the cladding is softening, diffusional processes are relatively fast, so graded index profiles can be created in situ. With appropriate choice of cladding material, the fibers produced can be fusion spliced to conventional fibers making them quite practical in existing fiber networks and easing device manufacture. The stick-in-tube method allows for complicated index profiles. For example, the first cladding tube could have a refractive index in between that of the core and the overclad tube to control the numerical aperture of the fiber or it could contain refractive index moats and rings inserted to engineer the dispersion and mode field diameter of the fiber. The first draw reduces the radial dimensions of the index profile by a factor of 6-8, and the second draw, reduces them down again by a factor of 400-500, so very fine structures can be achieved. The present invention will now be described more fully below with reference to the accompanying drawings, in which several presently preferred embodiments of the invention are shown. This invention may, however, be embodied in various forms and should not be construed as limited to the presently preferred embodiments set forth herein. Rather, applicants provide these embodiments so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art who will be readily able to adapt these teachings to a wide range of embodiments and applications. For example, though the present invention will be principally described in terms of waveguiding optical fiber, those skilled in the art will appreciate that the optical articles contemplated by the invention may also include, but are not limited to, planar amplifiers, couplers, fiber lasers, Faraday rotators, filters, optical isolators, and nonlinear waveguiding fibers. Moreover, the fabrication of continuous clad filaments for conductive conduit is contemplated, resulting in superconducting wire. Electro-optical and photonic crystal composites are also envisaged. FIG. 1 is a cross sectional drawing of an apparatus 100 which may suitably be used for implementing the filament in tube method of drawing optical fiber in accordance with the present invention. First, a cladding tube 112, having a 57 mm OD and a 2 mm ID in a preferred embodiment, with an inner wall 118 is purged with a drying gas, for example chlorine (Cl 2 ) or chlorine mixed with an inert gas, to remove unwanted moisture. A core feedstock or filament 1 10, having a 1.5 mm diameter in a preferred embodiment, is disposed within the cladding tube 112. This feedstock or filament 110 is preferably an elongated monolithic rod of material, however, a plurality of elongated rods can be stacked one atop the other within the cladding tube 112 to form the feedstock. Using a plurality of rods is particularly well suited for the production of dispersion managed fiber. The cladding tube 112 and the core filament 110 form a filled cladding tube with an open centerline 122 which is heated by a furnace 114, as described further below. The furnace 114 is operated at a draw temperature which is at or above the melting temperature of the core filament 110, but only causes the cladding tube 112 to soften. As the cladding tube 112 softens, the core filament 110 will melt at the draw temperature forming a core melt 120 contained within the cladding tube 112. It is presently preferred that the draw temperature is at or above the liquidus temperature of the core filament to eliminate crystals in core melt 120. As used herein, melt means that the core filament 110 flows and fills or deforms to the interior of the cladding tube 112 so that a filled cladding structure results. In accordance with one preferred embodiment of the present invention, during the step in which the core melts and deforms to the interior of the tube, the core preferably exhibits a viscosity of less than 10 6 poise, more preferably 10 4 poise, most preferably 1000 poise or less, and the cladding structure maintains a viscosity sufficient for the cladding to substantially retain its internal shape. Most preferably, the cladding tube 112 exhibits a viscosity greater than 10 7'6 poise at this temperature. This distinguishes the present invention from the more conventional methods (e.g. rod and tube or CVD) of the prior art, in which the viscosity of the core and clad were typically matched so that both the rod and tube had a viscosity that differed by less than a factor of about 10 at the temperature at which the fiber is drawn or cane is drawn. An optical fiber 1 16 is then drawn. While melting, the core filament 110 will preferably fine and conform to the interior wall 118 of the cladding tube 112, forming an interface determined by the inner surface 118, and completely filling the interior of the cladding tube 112. At its softening point, a glass cladding material 112 has a viscosity of about 10 7 6 poise. For some types of SiO 2 , this occurs at a temperature of about 2000°C. The cladding material should be selected so that at the temperature at which the core material is filling the interior of the cladding tube it has a viscosity greater than 10 7 poise, and preferably greater than 10 7 6 , and most preferably greater than 10 8 . However, at this same temperature, a core 120, such as 69.86 mole % silica (Si0 2 ), 18.63 mole % aluminum oxide (AI 2 O 3 ), 4.66 mole % sodium oxide (Na 2 O), and 6.85 mole % lanthanum fluoride (La 2 F 6 ) will have a viscosity of approximately 10 poise, seven orders of magnitude less than the cladding 112. The core 120 might suitably have a viscosity less than or equal to about 10 4 5 poise. In contrast, a typical rod in tube process would typically employ both core and cladding material having substantially the same viscosity. One significant advantage of the present invention is that the core filament 110 can be produced in any shape (round, square, triangular, etc.) and via any method (conventional crucible melting and casting, drawing, sol-gel, etc.). The only physical requirement is that the core filament 110 fit within the inner walls of the cladding tube 112. Thus, less rigid process controls are required during the manufacture of the core filament 110. Moreover, the loose fitting core filament 110 may be automatically fed down or dropped down as its bottom is melted to maintain a constant depth of molten core 120, yielding a homogeneous and reproducible optical fiber 116. The core filament 110 has a melting temperature, as defined above, which is below the softening temperature of the cladding 112, and the thermal expansion difference between the core filament 110 and the cladding 112 is not so large as to shatter the fiber 116 when it is cooled. The composition of the cladding 112 is preferably silicate glass, but it will be appreciated by those skilled in the art, that the composition of cladding 112 is essentially not limited and can range from pure SiO 2 to multicomponent glasses. FIG. 2 is a cross sectional drawing of an apparatus 200 which may suitably be employed for performing the stick in tube method of drawing optical fiber in accordance with the present invention. A one meter long Si0 2 cladding tube 212 (55 mm in outer diameter and 6 mm in inner diameter) is purged with drying gas to remove unwanted moisture. A 5 mm diameter core stick 210 is disposed or placed within the cladding tube 212 to form a filled cladding tube. The filled cladding tube is heated by a furnace 214 to 1700° C to soften the cladding tube 212 in preparation for elongation. As the cladding tube 212 softens, the core stick 210 melts, and a 6 mm outer diameter optical core cane 216 is then drawn in a standard manner. By "cane" or "core cane" as used herein, we mean an optical fiber precursor element comprising a core glass material, to which additional cladding must be added to the core cane prior to its being drawn into an optical fiber. Such additional cladding may be applied, for example, by inserting the cane into a glass cladding sleeve, or depositing additional core and/or cladding glass via outside vapor deposition or other methods. While melting, the core stick 210 will fine and conform to the interior walls of the cladding tube 212, forming an interface determined by the inner surface of the cladding tube 212. In this embodiment, wherein the core stock and tube are first redrawn into a core cane, the cladding material 212 preferably has a viscosity of approximately 10 8 poise at the draw temperature (e.g. about 1700°C for some forms of silica) and the core stick 210 will have a viscosity of approximately 10 4 poise or less at the draw temperature. In an embodiment of the present invention shown in FIG. 2, the resulting core cane 216 is then placed within an overcladding tube 220. The filled overcladding tube 220 is heated by a furnace 222 to soften the overcladding tube 220 in preparation for elongation. As the overcladding tube 220 softens, the cane 216 will soften, and an optical fiber 224 is drawn. In an exemplary method for forming fiber utilizing the stick in tube process of the present invention, a core glass of molar composition 70.0 SiO 2 - 11.25 AI 2 O 3 - 7.5 Ta 2 O 5 - 10 CaO - 2 CaF 2 - .05 Er 2 0 3 was batched from high purity powders, mixed, calcined at 400°C for 12 hours to dry the batch, and then melted in a covered high purity silica crucible at 1650°C for 4 hours. The melt was stirred with a fused silica rod to promote homogeneity, then cooled to 1500°C and drawn up into a 4-5 mm diameter stick from the melt. The 5 mm diameter stick of core glass was then inserted into a meter long 55 mm outer diameter (OD), consolidated, Si0 2 blank previously manufactured using the outside vapor deposition process with a 6 mm inner diameter (ID). The tube was purged with dry He gas to remove unwanted moisture and heated to 1800°C to soften the SiO 2 blank and drawn down into a 6 mm diameter core/clad cane which was flame cut into 1 meter long pieces. A 1 meter long piece was then mounted on a CVD lathe and overclad with SiO 2 soot to obtain the desired clad diameter/core diameter ratio of 32:1. The overclad cane was then consolidated between 1440 and 1500°C to form a monolithic SiO 2 blank with a core. This blank was then heated to 1950-2000°C in a graphite resistance furnace and drawn into standard 125 micron diameter fiber at a rate of 2 m/s. The resultant fiber having an Er-doped core is suitable for use as an optical amplifier. FIG. 3 is a drawing of an apparatus 300 used for overcladding an optical cane via an alternative CVD process and then drawing an optical fiber in accordance with the present invention. A cane 216, produced by the embodiment of the present invention shown in FIG. 2, is cut into 1 meter long pieces. The cut cane 216 is then mounted on a CVD lathe 332 and overclad with SiO 2 to obtain the desired ratio between the clad diameter and the core diameter, forming an overclad optical cane 330. The overclad cane 330 is then consolidated at a temperature between 1400° C and 1500° C to form a monolithic SiO 2 blank 336. An end of the monolithic blank 336 is then heated in a furnace 338 to a draw temperature of 1950-2000° C and drawn into standard 125 micron diameter optical fiber 340. FIG. 4 is a graph 400 showing loss as a function of wavelength for a 5 meter span of an optical fiber produced in accordance with the present invention. The low loss optical fiber (0.07 dB/m at 1310 nm) exhibits the same loss per meter, beginning to end, over a 2000 meter span. The core of the optical fiber was successfully doped with erbium ions, Er 3+ , as evidenced by the adsorption bands at 980 and 1500 nm. Additionally, Er 3+ fluorescence was observed from the optical fiber when 980 nm laser light was pumped into the fiber. Fig. 4 illustrates that, using the methods of the present invention, fiber can be made having a rare earth dopant therein which exhibit a background attenuation of less than 2 dB/m. FIG. 5 is a graph 500 showing the refractive index profile of a core cane produced in accordance with the present invention using the core glass composition described above with respect to Fig. 2. The core cane has a drawn diameter of 2.74 mm, the core of the cane having a diameter of .21 mm, for a core/clad thickness ratio of about .077. As can be seen in Fig. 5, the core exhibited a refractive index delta of about .11 (with respect to the silica core), or a delta percent of about 6.76 percent (again with respect to the undoped silica cladding). The observed maximum delta of 6.76 percent is significantly higher than that seen for typical CVD produced fiber. The cane was subsequently overclad and drawn into 10,000 m of homogeneous optical fiber. The total core diameter variance over the 10,000 m span was ± 0.25 μm, as compared to the cullet in tube method which would yield a variance of ± 4 μm. Thus, fiber manufactured in accordance with the present invention shows an improvement of at least an order of magnitude. Additionally, utilizing SiO 2 as the cladding material allowed the resultant optical fiber to be fusion spliced using conventional fusion splicers. Splice losses of less than 0.5 dB have been achieved when splicing the fibers made in accordance with the invention to SMF-28 optical fiber, and splice losses of less than 0.2 dB have been made when fibers in accordance with the invention were spliced to CS-980 optical fiber. FIG. 6 is a graph 600 showing loss as a function of wavelength for a 5 meter span of optical fiber produced in accordance with the present invention. A tube was made by depositing pure silica, followed by germania doped silica, followed by a pure silica cladding region. The resultant soot preform was consolidated to form a tube. Then, the same type of core glass rod described above with respect to Fig. 2 was inserted into the glass tube and drawn into a fiber. The resultant multi-component core in this case was comprised a central high index region surrounded by a silica moat, which was in turn surrounded by a ring of SiO 2 doped with germanium oxide (GeO 2 ). This demonstrates that complex index of refraction profiles can be made with less than 0.5 dB/m background attenuation FIG. 7 is a graph 700 showing the loss and mode field diameter as a function of fiber length for an optical fiber produced in accordance with the present invention. Fig. 7 demonstrates that mode field diameters can be expanded by employing a raised index ring outside the central high index region of the core, to thereby expand the mode field diameter beyond that of a what would be achieved using a single raised index core. Minimal variations in loss and mode field diameter for varying lengths are achievable, as illustrated by Fig. 7. The method of the present invention has a variety of advantages. The method of the invention opens up a large range of compositions for fiberization that have not previously been attainable through conventional CVD techniques which have been employed to make optical fiber. New compositions with high rare earth solubility, improved gain flatness and improved optical properties can be readily fabricated into fiber form. The method also accommodates large differences in thermal expansion between the core filament 110 or core stick 210, and cladding material 112 or cladding material 212, since the core 110, 210 is not rigidly bonded to the clad 112, 212 until the core filament 110 or core stick 210 is in fiber or cane form when the stress due to thermal expansion mismatch are much smaller than in a rigid monolithic preform of greater size, as these stress forces vary inversely with the square of the radius of the fiber, preform or the like. Accordingly, very large numerical aperture fibers for use as efficient couplers and lasers can be produced by the method of the present invention. The method also allows for atmospheric control of the core melt 120, 220 at the drawing temperature. Either oxidizing, reducing or chemically reactive atmospheres can be introduced utilizing the open centerline to control the redox state. The pressure above the core filament 110 or core stick 210 can be controlled to regulate the core diameter, as can the draw temperature. Higher draw temperatures will lead to smaller core diameters for the same given fiber outer diameter (OD), in contrast to conventional preforms where this ratio is fixed once the blank is fabricated. For example, these factors can be used to modulate core diameter by plus or minus 50% utilizing the present invention. The ratio of the OD to the inner diameter (ID) of the tube will be roughly the same as the optical fiber OD to ID ratio although, as stated, it can be controlled by positive or negative pressure applied over the molten core 120, 220 relative to outside the cladding tube 112 or cladding tube 212, respectively. Additionally, the high temperatures used to draw the optical fiber 116, 216 serve to homogenize the core melt 120, 220 and drive off detrimental water present in the core melt 120, 220. While the foregoing description includes detail which will enable those skilled in the art to practice the invention, it should be recognized that the description is illustrative in nature and that many modifications and variations thereof will be apparent to those skilled in the art having the benefit of these teachings. By way of example, while it is presently preferred that a core feedstock, such as core feedstock 110, be a solid rod, the core feedstock could conceivably be hollow, or be divided into several large blocks. Further, the term feedstock is intended to encompass a thin filament, a thicker stick, a plurality of elongated filaments bundled for insertion into the tube, or elongated filaments or sticks stacked axially one on top of the other for insertion into the tube, or the like, which will properly feed down upon melting. On the other hand, feedstock as defined herein preferably is not powder or cullet. Moreover, the feedstock can be formed from a core material alone or from a core material having a cladding material disposed thereon. Either of these embodiments can then be disposed within a tube formed from cladding material. Similarly, the tube can be formed from core material or cladding material. Thus, it is conceivable to manufacture a preform having a plurality of concentric rings of core material and cladding material, each ring having the same or different optical characteristics as other rings within the preform. In addition, a preform might be formed, cooled, stored and then later reheated and drawn although this is not presently preferred. Further, as appropriate, the term optical fiber should be construed as encompassing any fiber or fiber component employed in applications including but not limited to optical waveguides, single mode fibers, multi-mode fibers, amplifiers, electro-optical fibers, couplers, lasers, or the like. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or the scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims: We claim: 1. A method for producing an optical fiber, said method comprising the steps of: positioning a solid, elongated feedstock within a hollow tube; and heating at least a portion of said tube and feedstock to a temperature sufficient to cause said feedstock to deform to the shape of said tube, and reducing the outside diameter of the tube, wherein the feedstock comprises a softening point which is less than the softening point of said tube. 2. The method as claimed in claim 1 wherein said tube comprises a cladding structure. 3. The method as claimed in claim 1 wherein said hollow tube comprises a core material. 4. The method as claimed in claim 1 wherein said feedstock comprises a substantially continuous feedstock which is a core material. 5. The method as claimed in claim 4 wherein said feedstock comprises a plurality of feedstocks. 6. The method as claimed in claim 3, wherein said reducing the outside diameter step comprises forming a core cane, and said method further comprises the steps of overcladding said core cane with cladding material to form a core-clad preform, and drawing said core-clad preform into an optical fiber. 7. The method as claimed in claim 1 wherein said feedstock comprises a core feedstock, and wherein said tube comprises a cladding structure. 8. The method as claimed in claim 1 wherein said reducing the outside diameter step comprises drawing said tube and feedstock directly into an optical fiber. 9. The method as claimed in claim 1 wherein a difference in the softening point of said core feedstock and the softening point of said cladding structure is at least 100°C. 10. The method as claimed in claim 8 wherein said feedstock exhibits a viscosity of less than 10 6 poise at a temperature at which said tube exhibits a viscosity of 10 7'6 poise. 11. The method as claimed in claim 1 wherein said feedstock exhibits a viscosity of less than 10 6 poise at a temperature at which said tube exhibits a viscosity of 10 7'6 poise. 12. The method as claimed in claim 11 wherein said feedstock exhibits a viscosity of less than 10 4 poise when said tube exhibits a viscosity of 10 7 6 poise. 13. The method as claimed in claim 12 wherein said feedstock exhibits a viscosity of less than 1000 poise when said tube exhibits a viscosity of 10 7 6 poise. 14. The method as claimed in claim 1 wherein the coefficient of thermal expansion of said core feedstock is greater than the coefficient of thermal expansion of said tube. 15. The method as claimed in claim 7 wherein said cladding structure is essentially solely silica. 16. The method as claimed in claim 7 wherein said cladding structure is at least 90 weight percent silica made by a chemical vapor deposition process. 17. The method as claimed in claim 16 wherein said chemical vapor deposition process comprises an outside chemical vapor deposition process. 18. The method as claimed in claim 8 wherein said feedstock is fed at a faster rate than said tube. 19. The method as claimed in claim 7 wherein said cladding structure comprises a plurality of bores passing longitudinally therethrough, and wherein the method further comprises the steps of: positioning a metal within at least one of the plurality of bores defined by said cladding structure; and drawing said preform into an electro-optical fiber. 20. The method as claimed in claim 7 wherein said cladding structure comprises a plurality of bores passing longitudinally therethrough, and wherein the method further comprises the steps of: positioning a glass rod having a composition which differs from that of said cladding into at least one of the plurality of bores defined by said cladding structure; and drawing said preform into a polarization maintaining fiber. 21. The method as claimed in claim 8 further comprising the step of doping said feedstock with a rare earth element. 22. The method as claimed in claim 21 wherein said rare earth element is selected from the group consisting of ytterbium, erbium, praseodymium, and neodymium. 23. A method of making an amplifier using the fiber made in accordance with the method of claim 22 further comprising the step of coupling said optical fiber to a wavelength division multiplexer in optical communication with a pump laser and a signal source to form a fiber amplifier. 24. The method as claimed in claim 21 wherein said rare earth element is selected from the group consisting of ytterbium, neodymium, and erbium. 25. A method of making a fiber laser using the fiber made in accordance with the method of claim 24 further comprising the step of coupling said optical fiber to a pump source to form the fiber laser. 26. An optical fiber formed by the process of: positioning an elongated substantially continuous feedstock within a hollow tube; and heating at least a portion of said tube to a temperature sufficient to cause said feedstock to deform to the shape of said tube, thereby forming a preform. 27. The method of claim 1 , wherein the internal bore of said tube is non-circular. 28. The method of claim 27, wherein the internal bore of said tube is rectangular. 29. The method of claim 27, wherein the internal bore of said tube is elliptical 30. An optical fiber comprising a numerical aperture of about .35 or greater. 31. The optical fiber of claim 30, wherein said fiber comprises a core and clad region which are comprised of glass. 32. The optical fiber of claim 31 , wherein the core of said fiber is doped with a rare earth element which is selected from the group consisting of ytterbium, neodymium, and erbium. 33. A fiber laser comprising a pump source coupled to the fiber of claim 32. 34. The optical fiber of claim 32, wherein said fiber comprises a numerical aperture of about .40 or greater.

Inventors: CHU POLLY W, DAHLMAN REBECCA V H, DEJNEKA MATTHEW J, SOLOSKY JOHN W, WILSON OTIS L JR, YOST KEVIN J, CHU, POLLY, W., DAHLMAN, REBECCA, V., H., DEJNEKA, MATTHEW, J., SOLOSKY, JOHN, W., WILSON, OTIS, L., JR., YOST, KEVIN, J.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1999
Main classification: C03C37
IPC codes: C03B37, G02B6