METHOD MAKING A FIBER PREFORM USING COOLED SPINDLE

Abstract: A method of making a preform for an optical fiber. A bait rod or cane member (2) is supported between its two ends, at least one of the ends having a chuck (11) and spindle assembly (13), wherein the chuck (11) and spindle assembly (13) are cooled via forced air cooling.

Description: METHOD OF MAKING A FIBER PREFORM USING COOLED SPINDLE Field of the Invention The present invention relates to a method of making preforms for drawing optical fiber, more particularly to a method which involves utilizing a lathe in which a substrate is supported at two ends, and rotated, and at least one of the supporting members and/or the rotating members are cooled. Background of the Invention In the manufacture of an optical fiber, a preform is typically made using a chemical vapor deposition (CVD) process such as outside vapor deposition (OVD), vapor axial deposition (VAD) or modified chemical vapor deposition (MCVD) techniques. The resultant preforms are then dehydrated and consolidated to form a solid glass blank while MCVD preforms are collapsed to form a solid glass blank. The glass blanks are then drawn into fiber. Alternatively, the glass blanks can be drawn into core cane which is then overclad, for example, using OVD or rod-in-tube techniques to form an overclad preform. The overclad preform can subsequently be dehydrated and consolidated to form an overclad glass blank which is then drawn into fiber. One parameter of interest in the drawn fiber is the core/clad concentricity (hereinafter referred to as concentricity). Concentricity is a measure of how well the core of a fiber is centered with respect to the overall center of the fiber. It is believed that, for an overclad preform, the concentricity is largely determined during soot deposition. To provide higher throughput and equipment utilization, the size (both length and diameter) of an optical fiber preform has been increased in recent years. As the length has increased, the potential for problems with wobble has also increased substantially. This is believed to be primarily caused either from cane assemblies which are not perfectly straight or from changes in stresses or deposition rates along the cane assembly during the soot deposition process. Because of the heat generated by the CVD process, the chuck and/or spindle heat up to such a temperature that the spindle expands, oftentimes unevenly, due to the heat. During such operations, the chuck and/or spindle typically can reach temperatures as high as about 150°C. It is applicant's belief that this orbit or wobble by the chuck or spindle impacts negatively on the concentricity of the preform, and thus the resultant fiber, which is drawn from the preform. The result is often an unacceptable amount of orbit or wobble in the spindle which is transformed to the bait rod for the preform. This heating effect caused by the CVD process can be accentuated by the fact that the CVD burners are typically located on one side of the axis of the spindle shaft and chuck assembly. Consequently, this side of the spindle shaft can experience greater heating than the other side. As a result, it is more likely that the bearings which support the spindle shaft and which are located on the side of the CVD burners can heat to a higher temperature than the bearings opposite the CVD burners. Consequently, there is a likelihood that, if such heating is not controlled, the bearings on one side might expand to the point where they too would contribute significantly to the wobble imparted to the shaft, and therefore the chuck and bait rod. Summary of the Invention One aspect of the present invention relates to a method of making a preform for an optical fiber, in which a rotating spindle shaft is connected to a bait rod or cane. Simultaneous to said rotating step, soot is deposited via chemical vapor deposition onto the bait rod or cane, while at least a portion of the spindle is cooled via forced air cooling. In a preferred embodiment, the bait rod or cane is connected to the spindle via a chuck, and the chuck is also cooled by the forced air cooling. In one embodiment, forced air is blown along the spindle shaft relatively uniformly around the periphery of the spindle shaft. Such uniform application of cooling air may be facilitated, for example, by blowing forced air through a plurality of air exit orifices which are positioned around the spindle shaft. The cooling via forced air can further be facilitated by flowing said forced air through a protective housing which surrounds the spindle shaft and protects the region of the spindle shaft which is to be cooled from the heat generated in the CVD chamber. The forced air cooling in accordance with the invention is preferably sufficient to maintain the chuck, as well as the spindle shaft, at a temperature which is less than 120°C, preferably less than 100°C, and most preferably less than 90°C. The spindle shaft in many cases is maintained at less than 75°C. Another aspect of the invention relates to a method of drawing a fiber from a preform made in accordance with the invention, i.e., in which the soot for the preform is deposited via chemical vapor deposition onto a bait rod or cane which is connected to a rotating spindle while at least a portion of the spindle is cooled via forced air cooling. Utilizing the cooled spindle and chuck assembly of the present invention, applicants have been able to achieve greatly improved fiber concentricity compared to fibers made on identical processes not employing such a cooled spindle and chuck assembly. The invention is thus particularly useful in CVD processes for making glass preforms, particularly those which employ a rotating spindle which may be attached to a bait rod or cane substrate, e.g., outside vapor deposition (OVD) and vapor axial deposition (VAD). In addition, by enclosing the bearings which support the spindle shaft and imparting a flow of cooling air through the enclosure that protects these bearings, any non-uniform heating which might otherwise occur by the CVD burners being located on one side of the spindle shaft can be greatly mitigated. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention as described in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed, and are in no way intended to limit the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one or more embodiment(s) of the invention and together with the description serve to explain the principles of the invention. Brief Description of the Drawings Fig. 1 illustrates a cane assembly used in a soot deposition process to form an optical waveguide soot preform. Fig. 2 is a schematic of a typical apparatus used in a soot deposition process in accordance with the invention. Fig. 3 illustrates an enlarged view of the upper spindle/chuck assembly of the process illustrated in Fig. 2. Fig. 4 is a blown up sectional view of the cooling ring illustrated in Figs 2 and 3. Fig. 5 is a blown up sectional view of the cooling plate illustrated in Fig. 3. Detailed Description of the Invention Fig. 1 depicts a cane assembly 1 in which a bait rod or glass cane 2 is attached to an upper cane handle 3 and a lower cane handle 4. If a cane assembly as shown if Fig. 1 is not made and maintained to be nearly perfectly straight, it will wobble when rotated during the soot deposition process. This wobble will result in uneven deposition of soot around the cane due to the relative movement of the cane assembly with respect to the soot stream. Even if a cane assembly is made to be perfectly straight, wobble can still develop due to changes in stresses in the cane assembly or unequal deposition rates which result from uneven heating of the cane assembly during the soot deposition process. Fig. 2 depicts an apparatus for depositing soot onto a cane assembly. As used herein, cane refers to any glass or soot preform onto which additional soot is to be deposited. Such canes can be, for example, core canes or other canes made in a separate preform making process. For example such glass canes can be produced by depositing soot onto a bait rod, then removing the bait rod and consolidating the soot into glass and optionally drawing the resultant glass preform into a narrower diameter glass cane. However, the invention is not limited in application to further deposition of soot onto canes, but instead is equally applicable to deposition of soot onto bait rods. For example, such bait rods can be in the form of alumina or other refractory substrates employed for the initial deposition of soot in an outside vapor deposition (OVD) process to form a glass preform for making optical fiber or for making glass canes which can in turn be employed to make preforms for optical fiber. As used herein, however, bait rod refers to any form of refractory target, or mandrel which acts as a receiving substrate for the deposition of soot, including but not limited to soot targets which are employed in axial vapor deposition processes. Likewise, the invention is not limited to the OVD process illustrated in Fig. 2. The invention could also be employed on axial deposition processes, or any other process which employs a rotating spindle attached to a substrate for CVD deposition of soot. In the embodiment illustrated, cane assembly 1 is placed in upper chuck 11. Lower chuck 12 is attached to the lower end of cane assembly 1. Such chucks are further described in U.S. Patent No. 5,658,365, the specification of which is hereby incorporated by reference. Upper chuck 11 is attached to upper spindle 13. Upper spindle 13 is also attached to spindle motor 14 which is in turn attached to weighing device 15. Lower chuck 12 is attached to lower spindle 16. Spindle motor 14 is used to rotate upper spindle 13, upper chuck 11 , cane assembly 1 , lower chuck 12 and lower spindle 16, thereby allowing soot deposition to occur about the outer surface of cane assembly 1. Burner 21 represents a burner used in OVD processes, such as that disclosed in Backer et al. U.S. Patent No. 5,067,975, the specification of which is hereby incorporated by reference. Air bearings 17, 18, 19 and 20 are essentially frictionless in the direction of the axis of cane assembly 1. This allows the weight of the cane assembly to be accurately determined by weighing device 15 during the soot deposition process. By taring or zeroing, the initial weight of the cane assembly and components of the apparatus prior to deposited on cane assembly 1 can be accurately determined. This allows a control system (not shown) to determine, based on the weight of soot deposited, when to change process parameters, including the composition of the soot being deposited, and when to shut down the soot deposition process upon completion thereof. In the embodiment illustrated in Fig. 2, upper spindle shaft 13 and chuck 11 are retained within a protective structure 24. This protective structure and the upper spindle assembly are shown with more detail in Fig. 3. The protective housing 24 in Fig. 2 is comprised of lower housing 30 which surrounds upper chuck assembly 11 , and upper housing 32 which surrounds spindle shaft 13. Together, upper housing 32 and lower housing 30 facilitate cooling of the spindle shaft 13 and chuck assembly 11 by shielding heat from CVD chamber 10. The remainder of spindle shaft 13 which extends outside of CVD chamber 10 is not exposed to as much heat, and therefore does not require as much shielding therefrom. Cooling ring 34 is located directly above upper housing 32 and surrounds spindle shaft 13. The cooling ring 34 is shown in more detail in Figs. 3 and 4. As can be seen in Fig. 4, cooling ring 34 includes a plurality of air holes 36 through which air is injected through the cooling ring and directed downwardly into the upper housing 32 and relatively uniformly towards the outer periphery of spindle shaft 13. Because the air holes 36 of cooling ring 34 are disposed around the entire periphery of spindle shaft 13, cooling air is distributed evenly around the circumference of spindle shaft 13, thereby cooling it uniformly and preventing non-uniform heating which might otherwise cause wobble or orbit of the spindle shaft 13. As illustrated in Fig. 3, upper housing 32 and ceiling 52 separate the CVD chamber 10 from the upper end of spindle shaft 13. Cooling plate 50 is located at the bottom of upper housing 32, near the vicinity where upper housing 32 joints lower housing 30. As illustrated in Fig. 5, cooling plate 50 is comprised of four passageways 52 through which air is directed from upper housing 32 into lower housing 30. As can be seen in Figs. 3 and 5, the air is directed directly at spindle shaft 13. Consequently, in the embodiment illustrated, there are a plurality (namely, cooling ring 34, and cooling plate 50) of means for directing a flow of air towards and along the spindle shaft 13 to achieve a beneficial cooling effect. As can be seen in Fig. 3, the path of cooling air (illustrated generally by the path 40) is through cooling ring 34 out through air holes 36 into the upper housing 32, then into the lower housing 30, and out of the lower housing 30 into the deposition chamber 10 of the CVD manufacturing process. This cooling air thus maintains a flow of cooling air along the spindle shaft towards and into the CVD chamber. Another advantage of the present invention is that the bearings 18 which are closest to the heat generated by the CVD operation are enclosed in upper housing 32, which is further insulated by the protective effect provided by lower housing 30. In this manner, the bearings 18 can be maintained at a relatively low temperature, e.g., less than 100°C, more preferably less than 75°C. Illustrated below in Table 1 is the core/clad concentricity performance of several lathes both before and after forced air cooling in accordance with the invention was employed. The data illustrated is for fiber that was drawn using preforms made on the lathe both before and after forced air cooling in accordance with the present invention was implemented on the lathes. The performance of the forced spindle cooling system was analyzed on each lathe as it was installed. For each overclad lathe, core clad concentricity data was collected for 400 reels of finished fiber before and 400 reels of finished fiber after the installation. A mean and standard deviation were calculated for the before and after datasets and compared. As shown in each case there was a significant improvement in the core clad concentricity performance. Prior to implementing the cooling procedures of the present invention, the temperature of the upper chuck 11 was greater than 150°C for each of the lathes, and the shaft was a temperature of greater than 100°C at a distance of about one (1 ) inch (or less) from the chuck 11. However, after the forced air cooling was implemented, the chuck on each of the lathes was maintained at a temperature of 75°C, and the shaft was maintained at a temperature of about 46°C at a distance of about one (1 ) inch from the chuck. Table 1 Before After Blank Mean Sigma Mean Sigma A 0.136 0.077 0.100 0.054 B 0.157 0.077 0.126 0.074 C 0.154 0.074 0.137 0.061 As the data illustrates, the mean core/clad concentricity value greatly improved after forced air cooling was implemented. Although the invention has been described in detail for the purpose of illustration, it is understood that such detail is solely for that purpose and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A method of making a preform for an optical fiber comprising: rotating a spindle shaft, said spindle shaft connected to a substrate for deposition of soot; and simultaneous to said rotating step, depositing soot via chemical vapor deposition onto said substrate and cooling said support member and said spindle shaft via forced air cooling. 2. The method of claim 1 , wherein said substrate comprises a bait rod or glass cane. 3. The method of claim 1 , wherein said substrate comprises a glass core cane. 4. The method of claim 2, wherein said spindle shaft is connected to said bait rod or cane via a chuck. 5. The method of claim 1 , wherein said cooling step comprises blowing forced air along said spindle shaft. 6. The method of claim 5, wherein said cooling step comprises blowing said forced air relatively uniformly around the periphery of said spindle shaft. 7. The method of claim 6, wherein said cooling step further comprises blowing said forced air through a plurality of air exit orifices positioned around said spindle shaft. 8. The method of claim 1 , in which said cooling step is sufficient to maintain the entire length of said spindle shaft at a temperature which is less than 100°C. 9. The method of claim 7, in which said cooling step is sufficient to maintain the entire length of said spindle shaft at a temperature which is less than 80°C. 10. The method of claim 1 , in which the said cooling step is sufficient to maintain the entire length of said spindle shaft at a temperature which is less than 70°C. 11. The method of claim 4, in which the said cooling step is sufficient to maintain said chuck at a temperature which is less than 120°C. 12. The method of claim 1 , wherein said cooling step comprises: flowing said forced air through a protective housing which surrounds said spindle shaft. 13. The method of claim 9, wherein said cooling step further comprises disposing a cooling ring around said spindle shaft and positioned cooperatively to enable application of forced air through said housing, said cooling ring comprising a plurality of orifices therein disposed around the periphery of the spindle shaft. 14. A method of making a preform for an optical fiber comprising: rotating a spindle shaft, said spindle shaft connected to a bait rod or cane by a chuck; and simultaneous to said rotating step, depositing soot in a chemical vapor deposition chamber onto said bait rod or cane and cooling said spindle shaft and chuck via forced air cooling, wherein at least a portion of said spindle shaft and said chuck are protected by a protective housing surrounding said spindle shaft and chuck, and said forced air cooling comprises blowing said forced air through said protective housing and into said chemical vapor deposition chamber. 15. The method of claim 14, wherein said cooling comprises blowing said forced air through a plurality of air exit orifices positioned around said spindle shaft. 16. The method of claim 14, wherein said cooling is sufficient to maintain said spindle shaft at a temperature which is less than 80°C. 17. The method of claim 15, in which the said cooling step is sufficient to maintain said spindle shaft at a temperature which is less than 70°C.

Inventors: HEFNER DONALD L, LYNCH BRIAN C, WILLIAMS RICHARD R, HEFNER, DONALD, L., LYNCH, BRIAN, C., WILLIAMS, RICHARD, R.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1999
Main classification: C03B37
IPC codes: C03B19, C03B37, G02B6