METHOD OF MANUFACTURING AN OPTICAL FIBER PREFORM

Abstract: A method of manufacturing an optical fiber preform including providing a first tubular core layer having a silica-containing glass or soot, depositing a doped silica soot onto the first tubular layer to form a second tubular core layer, consolidating at least the second tubular core layer, and etching away at least part of the first tubular core layer by exposing the layer to an etchant. Preferably less than all of the first tubular layer is etched away. According to one embodiment, the second core layer includes a rare earth element such as Er, Tm, Nd, Pm, Yb, or Sm, for example.

Description: METHOD OF MANUFACTURING AN OPTICAL FIBER PREFORM Field of the Invention The present invention relates generally to methods for manufacturing an optical fiber preform, and more particularly to a method of manufacturing an Erbium doped preform. Background of the Invention Processing of core cane traditionally includes depositing glass soot upon a rotating substrate to produce a soot tube. The soot tube is dried, consolidated and then drawn into a slender core cane. During the drawing process, the centerline hole is generally closed. Introducing various gases in predetermined amounts and times into a burner flame produces the desired refractive index profile of the core cane. This introduction produces oxides that may include, for example silicon oxide and germanium oxide. These oxides deposit on the rotating mandrel until the appropriate diameter of the core portion is reached. Typically, silica soot is deposited as the last step in the core cane making process. This layer of silica soot is termed "overcladding" and typically comprises about 95% of the volume of glass present in the soot blank. Once the core soot tube is formed, it is removed from the OVD lathe. The mandrel is removed from the soot tube thereby leaving an aperture extending along its axial length and positioned at the blank's centerline (hereinafter referred to as the "aperture" or the "centerline aperture"). The centerline aperture has a silica plug-like member inserted in its lower end to restrict gas flow therethrough. Subsequently, the soot tube is inserted into and held in a consolidation furnace. Chlorine and helium gas are included within the furnace, passed through the centerline aperture, and held at a temperature between about 800 °C -1200 °C to aid in hydrogen removal from the soot preform. In particular, chlorine permeates the interstices of the soot preform and flushes out any OH, H 2 or H 2 O contained therein. The cleaned and dried blank is then heated to a higher temperature, generally in the range of between about 1450 °C to about 1600 °C, (depending upon preform composition) until the deposited soot blank consolidates and transforms into a solid, high-purity glass having superior optical properties. Typically, the blank is subjected to gradient consolidation, a technique taught whereby the bottom tip is consolidated first; the consolidation continuing along the preform until completed. As mentioned above, once the preform is consolidated, it is removed from the furnace and transferred to another furnace where the preform is drawn, under a vacuum, to close the centerline aperture and, in a controlled fashion, stretch the preform into a core cane of a desired diameter. The core cane is then cut into segments, each of which is then again overclad with a significant amount of Si0 2 soot to an appropriate diameter and again consolidated thereby resulting in a glass preform which has no aperture and includes the desired amount of cladding surrounding the core. The resulting preform is then transferred to a drawing furnace where it is drawn into optical fiber. Certain fibers encounter problems when drawn in accordance with the above-mentioned method of manufacturing preforms. For example, in fibers for amplifier applications, adding certain dopants such as aluminum oxide and erbia oxide to the soot results in problems during centerline aperture closure, such as defects and non-uniformity. Furthermore, there is a need for a cost- effective method of producing rare earth doped fibers in greater quantities. Summary of the Invention In accordance with a first embodiment of the invention, a method of manufacturing an optical fiber preform with lower centerline defects is provided. The method in accordance with the invention comprises providing a first tubular core layer including silica-containing glass or soot. Outside of the first tubular layer is deposited a doped silica soot thereby forming a second tubular core layer. At least the second tubular core layer is then consolidated (and preferably the first and second tubular layers), and at least part of the first tubular core layer is removed, preferably by exposing the layer to an etchant. The first core layer may be, for example, a thin deposited layer of silica soot, germania-doped silica soot, or a silica soot doped with any other glass former. Optionally, the first layer may be a silica-glass containing tube, for example. The tube may also include suitable doping with a glass former. In the embodiment where the layer is deposited soot, the soot may be deposited by any known deposition method, such as OVD for example. The second core layer preferably includes a rare earth dopant such as Er, Tm, Nd, Pm, Yb, or Sm, for example. Preferably also, the second tubular core layer is doped with alumina in an amount of about 1% to15% by weight. The second core layer is preferably deposited by an OVD method or the like on the outside of the first layer formed. Following deposition, at least the second layer is dried and consolidated through conventional. The etchant used to remove at least part of the first core layer is preferably SF 6 or NF 3 . Although other suitable etchants may be employed. The etchant is preferably provided by flowing it through the centerline aperture of the preform. Etching takes place at a temperature between about 1150 °C and 1400 °C. Upon completion of the etching step, preferably less than all of the first tubular layer is etched. away. Thus, it should be recognized that, in accordance with a preferred embodiment a small amount of the first core layer is left unetched. This small amount of sacrificial layer remaining prevents, for example, puddling of the rare earth dopants upon closure of the aperture when drawing the consolidated preform into a core cane due to a lesser reactivity of rare earth metals with fluorine. Puddling was found by the inventors to occur if the etching step were allowed to remove all the first core layer and some of the second layer. Puddling of such rare earth metals, such as Erbium, may be detrimental to the optical properties of the resultant fiber drawn from the optical preform. Therefore, it should be recognized that because of the lack of puddling and other centerline defects, fibers produced from preforms manufactured in accordance with the present invention have excellent properties for use in amplifiers and other optical components. According to a preferred embodiment of the invention, a method and apparatus is provided wherein greater than 50% of the quantity of glass in the core cane comprises core material. According to the method, following the step of drawing the consolidated core blank into a core cane, greater than 50% of the quantity of glass in the core cane comprises core material. This is accomplished by depositing substantially less silica overcladding prior to the step of drawing into a core cane. More preferably yet, greater than 80% of the quantity of glass in the core cane comprises core material. Even more preferably, greater than 90% comprises core material. Advantageously, the first core layer acts as a sacrificial layer and prevents any alumina, preferably present in the second core layer, from coming into direct contact with the alumina bait rod, thus preventing the formation of any defect sites containing alumina resulting when the mandrel is removed. According to another embodiment of the invention, an optical fiber core cane is provided having body of silica-containing glass material including a rare earth element having a length greater than 0.5 m and a diameter of greater than 7 mm and that includes greater than 50% core material by volume. This advantageously allows significantly more fiber to be drawn therefrom as compared to the prior art. Other advantages and features of the invention will be understood with reference to the following detailed description, claims and appended drawings. Brief Description of the Figures Fig. 1 illustrates a perspective view of a first core soot segment formed on a bait rod. Fig. 2 illustrates a perspective view of a second core soot segment deposited on the first core segment. Fig. 3 is a cross-sectional end view of a soot preform in accordance with the invention taken along line 3-3 of Fig. 2. Fig. 4 illustrates a side cross-sectional view of a soot preform with the bait rod removed and the end plugged. Fig. 5 illustrates a partial cross-sectional side view of a soot preform during the drying step. Fig. 6 illustrates a partial cross-sectional side view of a vitrified preform during the consolidation step. Fig. 7 illustrates a partial cross-sectional side view of a vitrified preform with the bottom portion of the plug removed and whereby the etchant is provided through the centerline aperture. Fig. 8 illustrates a cross-sectional end view of an etched preform illustrating an unetched portion of the first layer still remaining. Fig. 9 illustrates a partial cross-sectional side view of a draw apparatus for producing a core cane. Fig. 10 illustrates a cross-sectional side view illustrating an alternate tubular silica-containing glass tube forming the first core layer. Fig. 11 illustrates a side view illustrating the step of depositing of silica- containing soot onto a core cane. Fig. 12 illustrates a partial cross-sectional side view of a vitrified preform formed during a consolidation step. Detailed Description of the Invention Reference will now be made in detail to the present preferred embodiments of the invention with reference to the drawings. Wherever possible, the same reference numerals shall be used throughout to refer to the same or like parts. As shown in Fig. 1 , a first step in the method of manufacturing an optical fiber preform in accordance with the present invention is to provide a first tubular core layer 32 including silica-containing glass or silica-containing soot. In accordance with a first embodiment, a slightly tapered alumina bait rod 22 is rotated within a laydown apparatus including a lathe (not shown) for rotating the bait rod and one or more burners 24 (only one being shown). A handle 25 is typically provided on one end of the bait rod 22 to facilitate handling in later manufacturing processes to be discussed further herein. As should be understood, the handle and/or rod may be grasped by support and chuck portions of the lathe during laydown. The burners 24 are provided, for example, with a source 23 of fuel and combustion supporting gas, such as methane and oxygen, respectively. The furl is ignited in the presence of oxygen to form a flame 30. A first silicon-containing precursor 28 in liquid or vapor form is delivered to the burner 24 and introduced into the flame 30 to form silica-containing soot 26. The first precursor 28 may be mixed with other gasses, such as oxygen, or it may be separately provided to the burner 24 if a liquid-type delivery burner is utilized. Thus, it should be recognized that the first tubular core layer 32 in this embodiment is deposited onto the bait rod 22 by an Outside Vapor Deposition (OVD) process. The first tubular core layer 32 is preferably less than about 3 mm thick and more preferably less than about 1.0 mm thick. Most preferably, the first layer is doped with a glass former, such as germania, such that the viscosity of the fist core layer is reduced to aid in hole closure. According to another embodiment, the first tubular core layer 32 may be alternately formed from a vitrified tubular silica-containing glass tube such as illustrated in Fig. 10. Preferably, the tube comprises a germania doped silica glass. In the next step, as best illustrated in Fig. 2, a doped silica soot 34 is deposited onto the outside of the first tubular core layer 32 to form a second tubular core layer 36. Similar to the first layer, fuel 23 is provided to burner 24 and ignited. Precursor 29 is provided to the flame 30 and oxidizes to form the doped silica soot 34. The composition of the second tubular core layer 36 is different from the first core layer 32. Most preferably, the doped silica soot of the second tubular core layer 36 is doped with a rare earth element. Thus, precursor 29 includes a rare earth element. Preferably, the rare earth element is selected from a group consisting of Er, Tm, Nd, Pm, Yb, and Sm. In particular, Erbia in concentrations of between about 0.001% and 1.0% are desired in the soot, and more preferably between about 0.01 and 0.5%. Additionally, Alumina in concentrations of 0%- 5% by weight is preferred, and more preferably between about 1.0% and 10% by weight. Alumina is added to the second core layer 36 to increase the solubility of the rare earths in glass thereby reducing clustering and also improving the spectroscopy of the rare earth ion. Germania is also preferably present in amounts ranging from about 0% to 25% by weight and more preferably in an amount between about 12% to 22% by weight. Figs. 2, 3 and 4, respectively, illustrate a cross-sectional views of the soot blank 38 manufactured in accordance with the invention. The blank 38 includes the first tubular core layer 32, the second tubular core layer 36 and the handle 25. Prior to the next step in the blank processing, the bait rod 22 is removed, as shown in the Fig. 4, and a plug 40 has been inserted in the end of the blank 38. The plug 40 is vitrified silica and includes a larger diameter recessed portion that is inserted into the centerline aperture 42 of the blank. The plug 40 also includes a smaller capillary tube 43 at the farthest outward end of the plug. The capillary tube 43 allows limited gas flow in the next step as will be described herein. In the illustrated embodiment of Fig. 5, the soot blank 38 is installed in a furnace 44. The furnace typically includes a muffle tube 46 manufactured from a ultra-pure silica material, insulation 48, and a heat source 50 such as induction coils shown. The blank 38 is first dried at a temperature between about 875°C and 950°C for about 2 hours in the presence of an atmosphere of drying gas 52 such as a mixture of helium and chlorine gasses. Preferably, the gasses are present in about 94%-100% helium and 0%-5% chlorine. The gasses are passed into the centerline aperture 42 where they disperse out through the soot interstices. Additionally, the drying gasses may be provided between the soot blank 38 in the muffle tube 46 thereby surrounding the blank, if desired. Once the blank 38 is suitably dried, it is consolidated into a solid vitrified glass mass; preferably in the same furnace 44 as shown in Fig. 5. During consolidation, at least the second tubular core layer 36 (Fig. 4) of the blank 38 is consolidated. However, most preferably, both the first and second tubular layers 32, 36 are consolidated in the furnace 44 as shown in Fig. 6. Consolidation takes place at about between 1350 °C and 1500 °C, and most preferably at about 1430 °C for about 4 hours in the presence of an inert consolidation gas 54 such as helium or argon. Consolidation is preferably carried out by a gradient consolidation method where the blank is lowered through a hot zone. The now consolidated preform 55 is quickly withdrawn from the furnace 44 and the outermost end of the plug 40 is knocked off by an operator thereby leaving plug portion 40a. This leaves a larger diameter through hole and removes the capillary tube on the lower end of the preform 55 for better etchant flow and access as shown in Fig. 7. The preform 55 is quickly lowered back into the furnace 44 and then a supply of etchant 56 is directed into and through the centerline aperture 42 as shown by the arrows labeled "A" to expose the surface of the first tubular layer 32 to the etchant. The etchant 56 is preferably a gas with a high degree of reactivity with oxide based glasses. The etchant 56 most preferably is a gas selected from a group consisting of SF 6 and NF 3 . The most preferred etchant is SFβ and is supplied at the centerline at a flow rate of about 75 standard cubic centimeters per minute. However, it should be recognized that any other suitable etchant may be employed. In a preferred embodiment, the etchant gas is included with a carrier gas 58, such as helium. Preferably, the etchant 56 is included in a range of amounts between about 5.0 % and about 25%, and most preferably about 18%, and helium is provided in a range between about 75 % to about 95%, and most preferably in about 82% by volume. The etchant 56 performs the function of etching away at least part of the first tubular core layer 32. In a preferred embodiment, after consolidation, the layer is about 40 microns thick. It is preferable to etch away between about 80% and 95% of the first tubular layer 32, i.e., less than the entire layer 32 upon completion of the etching step. Thus, it should be recognized that part of the first core layer preferably remains after etching. Thus, when the first layer 32 includes germania, a portion of a germania doped glass of the first .tubular core layer remains after etching. This remaining layer prevents inconsistent etching of the constituents in the second tubular core layer. In particular, because of the preferential removal of non-rare earth dopants due to their higher reactivity, localized increases in the amount of rare earth dopants, which may lead to clustering or other negative spectrascopic effects is avoided. The step of etching preferably occurs at an etching temperature between about 1150 °C and 1400 °C, and most preferably between about 1150 °C and 1250 °C within the furnace 44. Fig. 8 illustrates a cross section of the consolidated preform 55, subsequent to the etching step. Shown are the first core layer 32 (partially etched away), the second core layer 36, and the plug portion 40a. According to a further embodiment of the method invention, as best illustrated in Fig. 9, the consolidated core blank 55 is transferred to a draw furnace 64 wherein a step of drawing the consolidated core blank 55 into a core cane 62 takes place. Drawing takes place at a temperature between about 1600 °C and 1800 °C in an atmosphere of an inert gas, such as helium. A vacuum may be applied while drawing, preferably about 100 Torr, to close the centerline aperture 42. Tension capstans 67, one or both of which are motor driven (motor not shown), provide a suitable tension force to draw from the core preform 55, a core cane to a desired dimension "d" of about 9 mm. Suitable controls 72 receive a signal indicative of the diameter "d" via a non- contact sensor 68 and control the capstan rate and down feed rate 70 of the core blank 55 such that the desired diameter is maintained. Once the cane 62 is formed, a cutter mechanism 72, such as a flame burner, cuts the canes to the desired length. According to a preferred embodiment of the invention, following the step of drawing the consolidated core blank into a core cane, greater than 50% of the quantity of glass in the core cane comprises core material. This is accomplished by depositing significantly less silica overcladding prior to the step of drawing into a core cane. By the term "core material", what is referred to herein is the physical core, i.e., that portion of the core cane that, when drawn into a fiber, will be the light carrying region. More preferably yet, greater than 80% of the quantity of glass in the core cane comprises core material and most preferably, greater than 90% of the quantity of glass in the core cane comprises core material. According to another embodiment of the invention, a method of manufacturing an optical fiber preform is provided, comprising the steps of forming a body of silica-containing soot 38 including a tubular core layer 36 of doped silica soot, the tubular core layer being doped with a rare earth element, the body having a centerline aperture 42, consolidating at least the tubular core layer to form a consolidated core blank 55, and drawing the consolidated core blank and closing the centerline aperture to form a core cane 62 that includes greater than 50% core material by volume. According to another embodiment of the invention, an optical fiber core cane is provided that comprises a body of silica containing glass material including a rare earth element. The length of the core cane 62 is greater than 0.5 m and has a diameter of greater than 7.0 mm. The resultant core cane includes greater than 50% core material by volume. In accordance with another preferred step in accordance with an embodiment of the invention, as best shown in Fig. 11 , silica-containing cladding soot 76 is formed by passing a precursor 82, such as SiCI , into a flame of a burner 24. The oxidized soot 76 is deposited onto the core cane 62 to form a soot preform 80 having a cladding layer 78. A handle 25 may be included on one end of the preform 80 as heretofore described. Following deposition of the cladding soot onto the core cane, the soot is preferably dried as described with reference to the method of Fig. 5 and an additional step of consolidating the soot preform 80 preferably takes place. As illustrated in Fig. 12, the preferably previously dried soot preform is placed into a consolidation furnace 44. Consolidation gas 54, such as helium, is provided to the soot preform. The temperature inside the furnace is increased to between about 1400 °C and 1550 °C for about 4 hours. After this period, the soot has vitrified into a high-purity glassy mass including the core 62 and cladding 78 and collectively referred to herein as the consolidated preform 84. This preform 84 is ultimately placed in a draw furnace and optical fiber is drawn therefrom. It will be apparent to those of ordinary skill in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.

Claims: Claims What is claimed is: 1. A method of manufacturing an optical fiber preform, comprising the steps of: providing a first tubular core layer including silica-containing glass or silica-containing soot, depositing doped silica soot onto the first tubular core layer to form a second tubular core layer, consolidating at least the second tubular core layer to form a consolidated core blank, and removing less than all of the first tubular core layer upon completion of removing. 2. The method of claim 1 wherein the step of removing comprises a step of etching wherein the first tubular core layer is exposed to an etchant. 3. The method of claim 2 wherein the step of etching occurs at an etching temperature between about 1150 ° C and 1400 ° C. 4. The method of claim 2 wherein the etching temperature is between about 1150 ° C and 1250 ° C. 5. The method of claim 2 wherein the etchant is selected from a group consisting of SFβ and NF 3 . 6. The method of claim 1 further comprising a step of doping the first tubular core layer with a glass former. 7. The method of claim 6 wherein the glass former comprises germania. 8. The method of claim 1 further comprising a step of leaving at least 5% of the first tubular core layer remaining following the step of removing. 9. The method of claim 1 further comprising a step of drawing the consolidated core blank into a core cane. 10. The method of claim 9 further comprising a step of depositing silica-containing soot onto the core cane to form a soot preform. 11. The method of claim 10 further comprising a step of consolidating the soot preform. 12. The method of claim 1 further comprising a step of leaving a portion of a germania doped glass of the first tubular core layer upon the completion of the step of removing. 13. The method of claim 1 further comprising a step of doping the doped silica soot with a rare earth element. 14. The method of claim 13 wherein the rare earth element is selected from a group consisting of Er, Tm, Nd, Pm, Yb, and Sm. 15. The method of claim 1 further comprising a step of forming the first tubular core layer by depositing the silica-containing soot onto a rotating bait rod. 16. The method of claim 1 further comprising a step of forming the first tubular core layer from a vitrified tubular silica-containing tube. 17. The method of claim 1 further comprising a step of doping the second tubular core layer with alumina in an amount of about 1 % to about 15% by weight. 18. The method of claim 1 further comprising a step of drawing the consolidated core blank into a core cane and wherein greater than 50% of the quantity of glass in the core cane comprises core material. 19. The method of claim 18 wherein greater than 80% of the quantity of glass in the core cane comprises core material. 20. The method of claim 18 wherein greater than 90% of the quantity of glass in the core cane comprises core material. 21. A method of manufacturing an optical fiber preform, comprising the steps of: providing a germania doped, silica-containing, first tubular core layer, depositing an erbium doped, silica soot onto the first tubular layer to form a second tubular core layer, consolidating at least the second tubular core layer, and etching away only part of the first tubular core layer by exposing the layer to an SF 6 etchant at an etching temperature between about 1150 °C and 1400 °C. 22. A method of manufacturing an optical fiber preform, comprising the steps of: providing a silica-containing, first tubular core layer having a centerline aperture, depositing a rare earth doped silica soot to form a second tubular core layer, consolidating at least the second tubular core layer, etching away only first portion of the first tubular core layer through the centerline aperture and upon completion of the etching step leaving a second portion of the first tubular core layer unetched, and closing any remaining aperture to form a rod. 23. A method of manufacturing an optical fiber preform, comprising the steps of: providing a silica and glass-former containing, first tubular core layer having a centerline aperture, depositing a doped silica soot including a rare earth element and alumina onto the first tubular core layer to form a second tubular core layer, consolidating at least the second tubular core layer, etching away only a first portion of the first tubular core layer through the centerline aperture wherein upon completion of the etching step, a second portion of the first tubular core layer remains unetched, and closing any remaining aperture to form a rod. 24. A method of manufacturing an optical fiber preform, comprising the steps of: forming a body of silica-containing soot including a tubular core layer of doped silica soot, the tubular core layer being doped with a rare earth element, the body having a centerline aperture, consolidating at least the tubular core layer to form a consolidated core blank, drawing the consolidated core blank and closing the centerline aperture to form a core cane that includes greater than 50% core material by volume. 25. The method of claim 24 further comprising greater than 80% core material by volume. 26. The method of claim 24 further comprising greater than 90% core material by volume. 27. The method of claim 24 comprising forming in the body of silica- containing soot a first tubular core layer through the centerline aperture formed therethrough. 28. The method of claim 25 further comprising a step of etching away only a first portion of the first tubular core layer through the centerline aperture wherein upon completion of the etching step, a second portion of the first tubular core layer remains unetched. 29. A method of manufacturing an optical fiber preform, comprising the steps of: forming a body of silica-containing glass material including a rare earth element, drawing the body of glass material to form a core cane that includes greater than 50% core material by volume. 30. An optical fiber core cane, comprising: a body of silica-containing glass material including a rare earth element having a length greater than 0.5 m and a diameter of greater than 7 mm and that includes greater than 50% core material by volume.

Inventors: CROSSLAND CARL E, LACOE JASON A, CROSSLAND, CARL E, LACOE, JASON A
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2001
Main classification: C03B37
IPC codes: C03B37