Patent Application
20050097923
The invention relates to a mandrel assembly for forming a fiber optic sleeve tube. More particularly, the invention relates to a mandrel assembly for forming a quartz fiber optic sleeve tube, wherein the mandrel assembly includes an inner support rod.
Optical fibers are widely used in a variety of applications, such as medicine, industrial inspection, and communications. The outer layers of such fibers are typically drawn from quartz fiber optic sleeve tubes. Quartz fiber optic sleeve tubes are formed by first depositing a layer of silica soot onto a quartz mandrel tube via a flame hydrolysis reaction. The soot layer and quartz mandrel tube are then consolidated, or sintered, at high temperature to form the quartz fiber optic sleeve tube.
Dimensional control is very important for such fiber optic sleeve tubes. The inner diameter of the quartz mandrel tube may vary along the length of the quartz mandrel tube if the silica soot layer is non-uniform. Shrinkage and sagging due to creep occurring during consolidation may result in a fiber optic sleeve tube having a non-circular cross-section, which renders the sleeve tube unacceptable for its intended purpose. In addition, the quartz fiber optic sleeve tube must be substantially straight; any bowing of the part due to either non-uniform soot deposition or non-straight mandrels makes the sleeve tube unsuitable for its intended purpose.
Shrinkage and bowing problems have been previously addressed either through tight control of the soot deposition process or by finishing and machining the quartz sleeve tube after sintering. The degree of control that is required for uniform soot deposition is frequently difficult to achieve and results in a low yield of acceptable parts. Additional machining of the sintered quartz sleeve tube, on the other hand, is expensive and requires additional cleaning steps.
Currently, quartz fiber optic sleeve tubes are highly susceptible to dimensional variations that result in unacceptability for their intended use. Therefore, what is needed is a mandrel assembly that will reduce shrinkage and bowing of the quartz mandrel and the resulting sleeve tube. What is also needed is a support rod that will provide adequate support for a quartz mandrel during consolidation. What is further needed is a method of making a quartz fiber optic sleeve tube that meets the dimensional tolerances required for fiber optic manufacture.
The present invention meets these and other needs by providing a mandrel assembly that supports the quartz mandrel during consolidation of the soot layer and quartz mandrel. In addition, the invention provides a cylindrical support rod for the mandrel assembly and a method of making a quartz fiber optic sleeve tube using the mandrel assembly.
Accordingly, one aspect of the invention is to provide a system for sintering a quartz tube. The quartz tube has a cylindrical wall defining an annular space and an outer layer of silica soot deposited on its outer surface. The system comprises: a furnace for heating the quartz tube to a temperature of at least 1400° C. in a controlled atmosphere, the furnace having a heating zone in which the quartz tube is sintered; a support rod assembly disposable in the annular space; and means for positioning the quartz tube and a portion of the support rod assembly within the heating zone. The support rod assembly comprises: (i) a cylindrical support rod having a central portion, the central portion having a surface roughness of from about 0.1 micron to about 4 microns, wherein the central portion has an ovality of up to about 0.5 mm and a bow of up to about 0.7 mm/m along a longitudinal axis of the support rod assembly, wherein the cylindrical support rod has a coefficient of thermal expansion that is greater than a coefficient of thermal expansion of the quartz tube, wherein the support rod assembly is substantially chemically inert with respect to silica in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof at temperatures of at least 1400° C., and wherein the support rod assembly straightens and supports the quartz tube and prevents tapering of the inner diameter due to creep; and (ii) at least one retaining portion coupled to the at least one end of the cylindrical support rod for preventing slippage of the quartz tube from the support rod assembly.
A second aspect of the invention is to provide a support rod assembly for supporting a quartz tube during sintering. The support rod assembly comprises: a cylindrical support rod having a central portion, the central portion comprising a carbonaceous material and having a surface roughness of from about 0.1 micron to about 4 microns, wherein the central portion has an ovality of up to about 0.5 mm and a bow of up to about 0.7 mm/m along a longitudinal axis of the support rod assembly, wherein the central portion straightens and supports the quartz tube and prevents tapering of the inner diameter due to creep, and wherein the cylindrical support rod has a coefficient of thermal expansion that is greater than a coefficient of thermal expansion of the quartz tube and is substantially chemically inert with respect to silica in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof at temperatures of at least 1400° C.; and at least one retaining portion coupled to at least one end of the cylindrical support rod for preventing slippage of the quartz tube from the support rod assembly.
A third aspect of the invention is to provide a system for sintering a quartz fiber optic sleeve tube. The system comprises: a furnace for heating the quartz tube to a temperature of at least 1400° C. in a controlled atmosphere, the furnace having a heating zone in which the quartz tube is sintered; a support rod assembly for supporting the quartz tube during sintering; and means for positioning the support rod assembly and the quartz tube within the heating zone. The support rod assembly comprises: (i) a cylindrical support rod having a central portion, the central portion comprising a carbonaceous material and having a surface roughness of from about 0.1 micron to about 4 microns, wherein the central portion has an ovality of up to about 0.5 mm and a bow of up to about 0.7 mm/m along a longitudinal axis of the support rod assembly, wherein the central portion straightens and supports the quartz tube and prevents tapering of the inner diameter due to creep, and wherein the cylindrical support rod has a coefficient of thermal expansion that is greater than a coefficient of thermal expansion of the quartz tube and is substantially chemically inert with respect to silica in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof at temperatures of at least 1400° C.; and (ii) at least one retaining portion coupled to at least one end of the cylindrical support rod for preventing slippage of the quartz tube from the support rod assembly.
A fourth aspect of the invention is to provide a method of making a quartz fiber optic sleeve tube. The method comprises the steps of: providing a quartz tube, the quartz tube having a cylindrical wall defining an annular space and an outer layer of silica soot deposited on an outer surface thereof; providing a cylindrical support rod, wherein the cylindrical support rod has an ovality of up to about 0.5 mm and has a bow of less than about of up to about 0.7 mm/m along its longitudinal axis, and wherein the cylindrical support rod has a coefficient of thermal expansion greater than a coefficient of thermal expansion of the quartz tube, and has a surface roughness of from about 0.1 micron to about 4 microns; disposing the cylindrical support rod in the annular space of the quartz tube; and consolidating the outer layer of silica soot and the quartz tube at a first temperature, wherein the cylindrical support rod supports the quartz tube at the first temperature, and wherein consolidating the outer layer of silica soot and the quartz tube at the first temperature forms the fiber optic sleeve tube.
A fifth aspect of the invention is to provide a quartz fiber optic sleeve tube. The quartz fiber optic sleeve tube comprises a cylindrical wall defining an annular space therein. The cylindrical wall is substantially parallel to a longitudinal axis. The annular space has an ovality of up to about 0.5 mm and the quartz fiber optic sleeve tube has a bow of less than about of up to about 0.7 mm/m along the longitudinal axis. The quartz fiber optic sleeve tube is formed by: providing a quartz tube, the quartz tube having a cylindrical wall defining an annular space and an outer layer of silica soot deposited on an outer surface thereof; providing a cylindrical support rod, wherein the cylindrical support rod has a coefficient of thermal expansion that is greater than a coefficient of thermal expansion of the quartz tube, is substantially chemically inert with respect to silica in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof at temperatures of at least about 1400° C., has a surface roughness of from about 0.1 micron to about 4 microns, and has a bow of less than about of up to about 0.7 mm/m along the longitudinal axis; disposing the cylindrical support rod in the annular space of the quartz tube; and consolidating the outer layer of silica soot and the quartz tube at a first temperature, wherein the cylindrical support rod supports the quartz tube at the first temperature, and wherein and consolidating the outer layer of silica soot and the quartz tube at the first temperature forms the fiber optic sleeve tube.
A sixth aspect of the invention is to provide a mandrel assembly for fabricating a quartz fiber optic sleeve tube. The mandrel assembly comprises: a quartz tube, the quartz tube having a cylindrical wall defining an annular space and an outer layer of silica soot deposited on an outer surface thereof; and a cylindrical support rod disposed in the annular space. The cylindrical support rod has a coefficient of thermal expansion that is greater than a coefficient of thermal expansion of the quartz tube. The cylindrical support rod is substantially chemically inert with respect to silica in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof at temperatures of at least 1400° C., and has a surface roughness of from about 0.1 micron to about 4 microns. The cylindrical support rod straightens and supports the quartz tube and prevents tapering of the inner diameter due to creep.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms.
Dimensional control is important for such fiber optic sleeve tubes. Quartz mandrel tube 110 defines an inner annular space 116 having a circular cross-section and an inner diameter 112.
Previously, shrinkage and bowing problems have been addressed either through tight control of the soot deposition process or through further finishing and machining steps after the quartz sleeve tube has been sintered. However, the degree of control that is required for uniform soot deposition is frequently difficult to achieve and results in a low yield of acceptable parts. Additional machining of the sintered quartz sleeve tube, on the other hand, is expensive and requires additional cleaning steps.
In addition to preventing shrinkage and bowing during sintering, retention of the quartz mandrel tube during the sintering/consolidation process also presents a problem. The quartz mandrel tube is usually suspended vertically in a sintering furnace. The quartz mandrel tube may have a flared upper end that engages a coupling means that permits the quartz mandrel tube to be suspended in the sintering furnace. Due to thermal stresses encountered during consolidation, the quartz mandrel tube tends to fracture a short distance below the coupling means, causing the remainder of the quartz mandrel tube to fall and break.
The present invention addresses the problems of shrinkage and bowing of the quartz mandrel tube during sintering as well as retention of the quartz mandrel tube by providing a system, shown in
Furnace 310 may be any furnace known in the art that has a heating zone 312 that is capable of heating quartz tube 340 to a sintering temperature of at least 1400° C. in a controlled atmosphere. A non-limiting example of such a furnace is a resistively heated furnace. Alternatively, furnace 310 may also be a gas-fired hydrogen or methane furnace or an inductively heated furnace. The controlled atmosphere with heating zone 312 of furnace 310 comprises at least one inert or nonreactive gas, such as, but not limited to helium, argon, nitrogen, combinations thereof, and the like. Helium is preferably used to produce a quartz sleeve tube of optimum optical quality.
Means for positioning 330 serve to position quartz tube 340 and a portion of support rod assembly 320 within heating zone 312 of furnace 310. In one embodiment, shown in
Support rod assembly 320 engages a portion of quartz tube 340 to retain and support quartz tube 340 during sintering. Support rod assembly 320 comprises a cylindrical support rod 322 that is disposable in the annular space of quartz tube 340 and at least one retaining portion 326 for retaining and preventing slippage of quartz tube 340. The at least one retaining portion 326 is coupled to at least one of an end portion of quartz tube 340 and an end portion of cylindrical support rod 322.
As seen in
Cylindrical support rod 322 has a coefficient of thermal expansion that is greater than that of quartz tube 340. Support rod assembly 320 is substantially chemically inert with respect to silica at temperatures from about 1400° C. to about 1600° C. in an atmosphere comprising an inert gas and at least one of fluorine, chlorine, and combinations thereof; i.e., under the above conditions, reaction is limited to a surface region of cylindrical support rod 322 and the structural integrity of cylindrical support rod 322 is unaffected. In one embodiment, support rod assembly 320 is substantially chemically inert with respect to silica at temperatures from about 1400° C. to about 1600° C. in an atmosphere comprising helium and chlorine.
In one embodiment, central portion 324 is formed from a carbonaceous material, such as, but not limited to, graphite. Graphite may optionally be previously purified in the presence of chlorine gas at a predetermined temperature and have an ash content of less than 100 parts per million (ppm).
In another embodiment, central portion 324 further includes an outer coating (not shown) disposed on an outer surface of the central portion 324. Such an outer coating comprises at least one of graphite deposited by chemical vapor deposition, amorphous carbon, and boron nitride. Outer coating preferably has a surface roughness from about 0.1 micron to about 4 microns. In one embodiment, the coating is deposited on a central portion 324 comprising an alumina core or tube.
Cylindrical support rod 322 may be a single piece that incorporates central portion 324. Alternatively, cylindrical support rod 322 may comprise several pieces or segments that may be matingly joined together. In one non-limiting example, cylindrical support rod 322 comprises a first end, central portion 324, and a second end, wherein the three pieces are joined together by joining means, such as, but not limited to, threaded rods or the like, that are widely known in the art. At least one of the first end and second end may be adapted to mate or engage with the at least one retaining portion 326. In one non-limiting example, a first end is flared so as to engage the at least one retaining portion 326. Both cylindrical support rod 322 and central portion 324 are formed from either at least one solid piece of material, at least one tubular piece of material, or combinations thereof. In one embodiment, cylindrical support rod 322 and central portion comprise a solid graphite rod.
At least one retaining portion 326 secures quartz tube 340 and cylindrical support rod 322 by coupling to at least one of quartz tube 340 and cylindrical support rod 322. In addition, the at least one retaining portion 326 couples support rod assembly 320 and quartz tube 340 to means for positioning 330.
In one embodiment, shown in
In another embodiment, shown in
At least one retaining portion 326 also engages positioning means 330, and thus couples quartz rod 340 and cylindrical support rod 322 to positioning means 330. As seen in
Other means that are known in the art, such as, but not limited to, threaded rods, set screws, interference fits, and the like, may also be incorporated into the at least one retaining portion to engage and support positioning means 330, quartz tube 340, and cylindrical support rod 322.
In one embodiment, shown in
The present invention also provides a mandrel assembly for fabricating a quartz fiber optic sleeve tube. A mandrel assembly of the present invention is shown in
A cylindrical support rod 220 of the present invention is shown in
In one embodiment, cylindrical support rod 220 comprises a carbonaceous material. In a preferred embodiment, cylindrical support rod 220 comprises graphite. The graphite is preferably heat treated in the presence of chlorine gas at about 2500° C. with a hold time of about 5 hours prior to use in cylindrical support rod 220, and has an ash content of less than about 100 parts per million (ppm). In one embodiment, cylindrical support rod 220 may further include a coating 224 disposed on outer surface 222. Outer coating 224 comprises at least one of graphite, amorphous carbon, and boron nitride. Outer coating 224 may be deposited by means known in the art, such as, but not limited to, chemical vapor deposition, and application of a slurry by either painting or spraying. In another embodiment, cylindrical support rod 220 comprises a cylindrical alumina core having a coating 224 disposed on an outer surface 222 of the cylindrical alumina core, where outer coating 224 comprises at least one of graphite, amorphous carbon, and boron nitride. In one embodiment, cylindrical support rod 220 is formed from a solid rod. Alternatively, cylindrical support rod 220 may be formed from a tubular piece of material, or from any combination of tubular and solid portions.
Outer layer 214 of silica soot comprises a central portion 213 having an outer surface 215 that is substantially parallel to cylindrical wall 212 of quartz tube 210 and a central portion length 217. Following sintering, in which the outer layer 214 of silica soot and the cylindrical wall 212 of quartz tube 210 are consolidated, central portion 213 will ultimately be used as a fiber optic sleeve tube. Cylindrical support rod provides support to quartz tube 210 along central portion length 217. In order to provide sufficient support for central portion 213, cylindrical support rod 220 has a rod length 221 that is a least as great as central portion length 217. Cylindrical support rod 220 should extend beyond both ends of central portion 213.
Quartz tube 210 has an inner diameter ranging from about 15 mm to about 50 mm. In one embodiment, quartz tube 210 has an inner diameter from about 18 mm to about 30 mm. Inner support tube 220 has an outer diameter that differs from inner diameter of quartz tube 210 by up to about 0.1 mm over the length of quartz tube 210.
Mandrel assembly 200 is assembled by inserting cylindrical support rod 220 into annular space 218 of quartz tube 210, onto which an outer layer 214 of silica soot has been previously deposited. Cylindrical support rod 220 has been previously machined to minimize bow over its length. Materials, such as, for example, graphite, that are used to form cylindrical support rod 220 are easily machined to higher tolerances than those required for ovality specifications of quartz fiber optic sleeve tubes. A fiber optic sleeve tube (202 in
During consolidation, the silica soot sinters, and outer layer 214 of silica soot shrinks down onto the cylindrical wall 212 of quartz tube 210. Quartz tube 210 and outer layer 214 of silica soot shrink radially during consolidation, causing the inner diameter of cylindrical wall 214 to decrease while, at the same time, cylindrical support rod 220 expands with temperature. The initial diameter of cylindrical support rod 220 is chosen such that cylindrical support rod 220 has the desired diameter of the quartz fiber optic sleeve tube 202 at the sintering temperature. The quartz mandrel shrinks onto the expanded cylindrical support rod 220 and conforms to the shape and size of cylindrical support rod 220 at the sintering temperature, which, in one embodiment, is approximately 1500° C. Since the quartz conforms to the shape of cylindrical support rod 220 during sintering, the quartz fiber optic sleeve tube 202 produced by the present invention has less ovality than quartz fiber optic sleeve tubes 102 that are sintered without using cylindrical support rod 220.
At the sintering or consolidation temperature, cylindrical support rod 220 is stiff relative to quartz tube 210 and does not creep, and the amount of bow that can occur in the sintered sleeve tube is therefore minimized. When the sintering step is complete, the mandrel assembly 200 is removed from the furnace. Upon cooling, cylindrical support rod 220, which has a higher coefficient of thermal expansion than quartz, shrinks away from the consolidated, or sintered, outer layer 214 of silica soot and cylindrical wall 212 of quartz tube 210, allowing cylindrical support rod 220 to be easily removed from annular space 218. The end portions of consolidated outer layer 214 of silica soot and cylindrical wall 212 of quartz tube 210 are then removed, leaving consolidated central portion 213 intact as the fiber optic sleeve tube 202. The placement of cylindrical support rod 220 in annular space 218 consistently produces quartz fiber optic sleeve tubes having the same inner diameter.
Because quartz fiber optic sleeve tube 202 shrinks onto cylindrical support rod 220, fiber optic sleeve tube 202 experiences almost no axial shrinkage, whereas a fiber optic sleeve tube 202 produced without a cylindrical support rod 220 will experience axial shrinkage. In addition, using the cylindrical support rod 220 of the present invention during sintering allows an increased central portion length 217 of central portion 213 of outer layer 214 to be obtained on quartz tube 210. Thus, the size of usable central portion 213—and, ultimately, fiber optic sleeve tube 202—that can be formed in existing consolidation furnaces is increased.
A schematic representation of a quartz fiber optic sleeve tube 202 formed from mandrel assembly 200 is shown in
The following example illustrates the advantages and various features of the present invention.
Quartz soot was deposited on the outer surface of quartz tubes using flame hydrolysis. Thirty-three such tubes having an outer layer of quartz soot deposited thereon were prepared. A cylindrical graphite support rod of the present invention was inserted into the annular space of each of eight tubes to form a mandrel assembly of the present invention. Each of the eight mandrel assemblies was sintered at a temperature in a range from about 1400° C. to about 1500° C. The remaining twenty-five tubes were sintered at temperatures in a range from about 1400° C. to about 1500° C. without inserting the cylindrical support rod of the present invention. Dimensions, including cross-sectional area, inner diameter of the sintered tube, outer diameter taper of the sintered tube, bow, and ovality, of the sintered tubes were measured.
The variation of cross-sectional area (expressed in percent variation) measured for the quartz tubes that were sintered without inserting the cylindrical support rod of the present invention is plotted in
The sintered quartz tubes that were incorporated into mandrel assemblies (i.e., the tubes having a support rod inserted into the annular space) exhibited a significant reduction in variation of cross-sectional area. The mean percent variation was 2.0%, with a standard deviation of 0.71%. The percent variation obtained for these quartz tubes is plotted in
Values measured for bow for the quartz tubes that were sintered without inserting the cylindrical support rod of the present invention are plotted in
The sintered quartz tubes that were incorporated into mandrel assemblies exhibited a significant reduction in bow. The mean value of bow was 0.32 mm, with a standard deviation of 0.15 mm. The bow values obtained for these quartz tubes are also plotted in
Ovality of the sintered quartz tubes was also improved when a support rod of the present invention was inserted in the annular space of a quartz tube prior to sintering. The average ovality of tubes that were sintered using the support rod was about 0.05 mm, whereas the ovality of tubes that were sintered using the support rod had an average value of about 0.2 mm.
Thus, the insertion of the support rod of the present invention into the annular space of quartz tubes prior to sintering provides a significant improvement in dimensional control over the current practice of sintering without such a support rod. Bow and ovality of the sintered tube are reduced, and the degree of variation of cross-sectional area is reduced as well. In addition, insertion of the support rod yields a sintered quartz tube having a more consistent inner diameter over its length and a reduced degree of creep.
While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.
