WO2007059336A1

WO2007059336A1 - Method and apparatus for manufacturing water-free optical fiber preforms

Info

Publication number: WO2007059336A1
Application number: PCT/US2006/044852
Authority: WO
Inventors: Amab Sarkar; Bedros Orchanian
Original assignee: Nextrom Oy
Priority date: 2005-11-18
Filing date: 2006-11-16
Publication date: 2007-05-24

Abstract

A method and apparatus are described for manufacturing substantially water-free optical fiber preforms by an outside vapor deposition (OVD) process, using a multi-functional apparatus that combines a vertical glass lathe with an OVD sintering system in one machine frame. The method includes steps of flame-sealing a quartz sintering tube to the handle of a porous soot preform made by an OVD process, after its deposition mandrel has been removed. The sintering tube is held in a rotatable chuck and used to feed the preform into a sintering muffle and oven. The open end of this sintering tube is connected through a rotary union above the chuck to a valve assembly that can be used to feed dehydration and sintering gases into the sintering muffle and also to draw a vacuum after sintering has been completed. The apparatus thereafter is used to seal the sintered preform's central aperture by collapsing a section of the sintering tube outside the sintering environment, while the aperture still is under vacuum, without allowing the central aperture to be exposed to ambient atmosphere. The resulting tubular preform thereafter can be processed further by a conventional steps of elongation to form a solid core preform, adding clad glass, and drawing into optical fiber.

Description

Mitll))iJl#aMji|j»i®tus for Manufacturing Water-Free Optical Fiber Preforms Background of the Invention

1. Field of the Invention

This invention relates generally to methods and apparatus for manufacturing optical fiber preforms suitable to be drawn into fibers for optical communication, and, more particularly, to methods and apparatus for manufacturing optical fiber preforms using outside vapor deposition (OVD).

2. Description of the Prior Art

Outside vapor deposition (OVD) is described in detail in chapter 2 of a book entitled "Optical Communications, Volume 1, Fiber Fabrication," edited by Tingye Li (1985). In the basic OVD process, vapors of glass-forming materials are fed through a water-generating flame, whereupon the vapors react to form small particles of glass, called soot, which are deposited onto a tapered, high-purity, ceramic mandrel. This forms a porous cylindrical body, with a tubular handle projecting from one end. The glass-forming materials are controlled such that both a core and part of a cladding are deposited. After such deposition has been completed, the mandrel is removed and the remaining porous tubular body is dehydrated and sintered to a dense glass tube. In a subsequent step, the two ends of the dense glass tube are sealed under vacuum, and the sealed tube then is elongated, to form a cylindrical core rod. Additional clad glass is added to the core rod by any of a variety of processes, to complete the manufacture of an optical fiber preform. This preform then can be drawn into optical fibers.

For commercial viability, optical fiber preforms must now have very low concentrations of water, or hydroxyl ions (OH). This is because the presence of OH in the final optical fiber can lead to a distinct absorption peak at a wavelength of about 1385 run. Low water content optical fibers were first reported in "Electronic Letters, 16 " pp 699-700 (1980), and such fibers have been produced commercially since the early 1980s. However, the OH content of these OVD fibers has been higher than is now required for commercial viability. This high OH content has occurred primarily because of difficulties in preventing re-hydration of the inner surface of the dehydrated and sintered core preform, particularly because of leakages in the region of the tubular handle, prior to sealing of its two ends. For this reason, substantial efforts have been made in recent years to adapt the basic OVD process to yield optical fiber preforms V≠MftM^ΪMΦbBSSS^' &vf OH content. The results of those efforts have not been entirely satisfactory.

U.S. Patent No. 6,477,305 Bl to Berkey et al. (the Berkey '305 patent) describes several approaches for producing low-water OVD optical fibers. In one disclosed approach, the process steps of dehydration and sintering are combined with the process step of sealing the tubular preform' s two ends under vacuum inside the sintering environment, without exposing the inner surface of the sintered preform to atmospheric air.

Persons skilled in the art will appreciate that sealing the two ends of the tubular preform inside or just above the sintering furnace, as taught in the Berkey '305 patent, is a complex procedure. Such persons also will appreciate that this procedure still yields optical fibers having sufficient OH content to cause a significant absorption peak at a wavelength of 1385 nm, as is evident from an inspection of the attenuation spectrum 102 depicted in Fig. 10 of the patent. In contrast, state-of-the-art fibers produced by a vapor axial deposition (VAD) process lack such an absorption peak, because preforms produced by a VAD process lack a center hole and, thus, do not encounter the problem of re-hydration of an inner surface.

It should, therefore, be appreciated that a need remains for an improved method and apparatus for producing OVD optical fiber preforms that are substantially water-free. The present invention satisfies this need.

Summary of the Invention The present invention resides in an improved method and apparatus for producing

OVD optical fiber preforms that are substantially water-free. In accordance with the method of the invention, a porous, tubular body produced by outside vapor deposition is initially provided, wherein the tubular body includes an upper end, a lower end, and an inner surface that defines a central aperture, and wherein a tubular handle projects from the upper end of the tubular body, in communication with the central aperture. The lower end of the central aperture is initially sealed, and a sintering tube is attached to the upper end of the tubular handle. The tubular body then is dehydrated and sintered into a dense glass tube, still including the central aperture. The central aperture then is evacuated, and a section of the sintering tube then is collapsed while the central aperture of the dense glass tube remains evacuated. This forms a tubular glass preform having its two ends sealed and its central aperture under vacuum, and further having substantially no re-hydration of its inner surface. ..^■ifiPfiϋϊΘ'iϊailed feature of the method of the invention, the step of collapsing includes a step of heating a section of the sintering tube sufficiently to cause it to collapse on itself and close its central aperture. Further, the step of dehydrating and sintering the tubular body includes a step of connecting a source of sintering gases to the tubular body via the sintering tube and the tubular handle, and the step of evacuating the central aperture of the dense glass tube includes a step of connecting a source of vacuum to the dense glass via the sintering tube and the tubular handle.

In other more detailed features of the invention, the step of dehydrating and sintering the tubular body occurs in a furnace, and the step of collapsing a section of the sintering tube occurs outside the furnace, using a lathe. The step of sealing the lower end of the central aperture of the tubular body includes a step of placing a plug in the central aperture, and the step of collapsing a section of the sintering tube occurs without the use of a separate plug to seal the upper end of the central aperture.

This method can be carried out using an apparatus that includes upper and lower feed-through chucks for controllably grasping and supporting the respective sintering tube and tubular handle during various steps of the process. These two chucks, as well as the lathe are mounted on a set of horizontal and vertical guide bars, and these guide bars are conditioned to controllably position these components during the process.

Other features and advantages of the present invention should become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

Brief Description of the Drawings

Figure 1 is a cross-sectional view of an optical fiber preform formed by an outside vapor deposition (OVD) process, after a ceramic mandrel has been removed and the tip of the preform has been plugged with an insert, as practiced according to the prior art.

Figure 2 is a cross-sectional view of an OVD optical fiber preform in accordance with the invention, after its attached handle has been sealed to a sintering tube.

Figure 3 is a cross-sectional view of the OVD optical fiber preform of Figure 2 as it is being inserted into a sintering muffle, ready to be dehydrated and sintered. ■'^•flglMiSsiil schematic elevational view of a dual-function apparatus used for sintering and sealing both ends of the optical fiber preform of Figure 2, without exposing the preform' s central aperture to ambient air.

Figure 5 is a cross-sectional view of the sintered glass preform produced by the apparatus of Figure 4, with both of its ends sealed and with its central aperture under vacuum.

Detailed Description of the Preferred Embodiment and Method

With reference now to the illustrative drawings, and particularly to Figures 2 and 3, there is shown a porous optical fiber preform 10 produced by an outside vapor deposition (OVD) process, which deposits soot onto a mandrel (not shown) and tubular quartz handle 12. As depicted, the mandrel has been removed, leaving an elongated central aperture 14 extending over the preform' s entire length, with the handle remaining secured to the preform' s upper end. A suitable quartz plug 16 has been inserted into the aperture's lower end, to seal that end of the aperture.

Figure 2 depicts the optical fiber preform 10 after it has been attached and hermetically sealed to a quartz sintering tube 18. This is accomplished, in the preferred embodiment, by flame-sealing the preform and sintering tube using a vertical glass lathe 20 that is a part of the apparatus shown in Figure 4. As shown in Figure 3, the sintering tube is a long, straightened tube having sufficient cross-sectional size to enable it to carry the load of both the core preform and the handle 12. The sintering tube's upper end is supported by a leak-free rotating union 22, with the rotating union's lower, rotatable end attached to the tube, and with the rotating union's upper, stationary end connected to two pipes. One pipe 24 is connected via a valve 26 to a vacuum pump (not shown), and the other pipe 28 is connected via a valve 30 to a supply of dehydration and sintering gases(likewise, not shown). An upper feed-through chuck 32 rotatably couples to the sintering tube 18 at a location just below the rotating union 22.

With continued reference to Figure 3, the optical fiber preform 10 is shown inserted into a sintering muffle defined by a quartz muffle tube 34. A dynamic seal 36 is located at the muffle tube's upper end, for sealing the muffle's interior environment while allowing the preform to be moved up and down, and to be rotated, within it. The muffle is located within a suitable furnace 38, which includes heating elements 40 for providing a desired temperature profile within the muffle. The furnace temperature can reach at least 1500 ⁰C, which is the temperature generally required to sinter the porous preform 10 into dense glass. Those skilled in fψeigiiiMJlβpϊirfefettέliaiiiitlhe sintering muffle, alternatively, could include an additional gas inlet at its bottom end and a suitable control system, for feeding desired gases into the muffle's interior and for controlling the pressure of such gases to a desired level, to maintain the muffle tube's geometry at elevated temperatures. It should also be appreciated that the furnace could have either a single-zone design or a dual-zone design, which would allow dehydration and sintering to occur sequentially, in two down-strokes of the preform 10, or in a single down- stroke.

Figure 4 is schematic diagram of the apparatus for use in dehydrating and sintering the optical fiber preform 10. The apparatus incorporates a machine frame that includes left, middle, and right fixed vertical guide bars 42a, 42b, and 42c, respectively, and upper, middle, and lower movable horizontal guide bars 44a, 44b, and 44c, respectively. The upper horizontal guide bar 44a is mounted for limited movement upward and downward on the left and right vertical guide bars 42a and 42c. Similarly, the middle and lower horizontal guide bars 44b and 44c both are mounted for limited movement upward and downward on the left and middle vertical guide bars 42a and 42b. Persons skilled in the art will understand various suitable- configurations for these guide bars and for mechanisms for operating them.

The upper feed-through chuck 32 is mounted on the upper horizontal guide bar 44a. This chuck is controllably movable leftward and rightward along the upper horizontal guide bar, and the upper horizontal guide bar, in turn, is movable upward and downward along the left and right vertical guide bars 42a and 42c, respectively, as the preform is moved during successive steps of the process.

Further, the glass lathe burner 20 is mounted on the middle horizontal guide bar 44b, and a lower feed-through chuck 46 is mounted on the lower horizontal guide bar 44c. These two components both are controllably movable left and right along their associated horizontal guide bars, and the two horizontal guide bars, in turn are controllably movable upward and downward on the left and middle vertical guide bars 42a and 42b, respectively.

Three processing positions are depicted in Figure 4: Position A, which is for loading the porous optical fiber preform 10; Position B, which is for dehydrating and sintering the preform; and Position C, which is for unloading the preform.

More particularly, in a first stage of operation, the porous optical fiber preform 10 is located at Position A. At this time, the deposition mandrel (not shown) has been removed

P^lug 16 has been inserted into the lower end of the central aperture 14. The preform is initially supported by the lower chuck 46, which grips the quartz handle 12. The sintering tube 18 then is inserted into the rotating union 22 and held in the upper chuck 32. Thereafter, the lower horizontal guide rail 44c is raised on the left and middle vertical guide rails 42a and 42b, respectively, to bring the upper end of the handle 12 into proximity with the lower end of the sintering tube 18. The middle horizontal guide rail 44b then is controllably moved vertically relative to the left and middle vertical guide rails, to bring the glass lathe burner 20 into alignment with the juncture of the handle and the sintering tube. In this position, the burner is operated to heat the confronting ends of the handle and the sintering tube, to seal the two components together. This kind of flame sealing procedure is conventionally performed using glass lathes of this kind. This forms a preform assembly.

Thereafter, in a second stage of operation, the preform assembly, which includes the porous optical fiber preform 10, the handle 12, and the sintering tube 18, is transferred to the site of the sintering muffle 34, i.e., Position B, for dehydration and sintering. This is accomplished by releasing the handle from the lower feed-through chuck 46, such that the assembly remains supported only by the upper feed-through chuck 32. The upper feed-through chuck and the upper horizontal guide bar 44a then are controlled to bring the preform assembly into a position above the sintering muffle 34. Thereafter, the upper horizontal guide bar 44a is lowered on the left and right vertical guide bars 42a and 42c, to lower the preform assembly into the muffle via the dynamic seal 36 at the muffle's upper end. At this time, the preform assembly is being continuously rotated by the upper feed-through chuck 32.

While positioned within the muffle 34, the optical fiber preform 10 is dehydrated and consolidated into a dense bubble-free glass tube, with its bottom end sealed. During this time, the valve 30 is opened, to admit dehydration and sintering gases via the pipe 28 and the sintering tube 18 into the preform's central aperture 14. After sintering has been completed, the valve 30 is closed, to terminate gas flow, and the valve 26 is opened to evacuate the preform's central aperture.

After dehydration and sintering have been completed, the sintered preform 10, along with its attached handle 12 and sintering tube 18, is raised out of the sintering muffle 34 while still under vacuum. This assembly then is moved horizontally to Position C (which actually is the same as Position A). At this time, the assembly no longer is being rotated by the upper feed-through chuck 32. In the Position C, the lower feed-through chuck 46 is made to "FgώllMigigii9.ric^}"lfi>|Mi® preform handle 12. In addition, the glass lathe burner 20 is controllably moved on the middle horizontal guide bar 44b to a position adjacent to a portion of the sintering tube 18 located immediately above the handle. While the upper and lower chucks then controllably rotate in synchronism, the burner 20 is operated so as to collapse that portion of the sintering tube, indicated by the reference numeral 48 (see Figure 5), and seal the upper end of the preform's aperture 14. This forms a sintered glass preform having both of its ends sealed and held under vacuum, as shown in Figure 5.

The sintering tube 18 then can be cut immediately above the portion that has been collapsed, and the sintered glass preform 10 removed for further processing. This cutting can conveniently be accomplished using a handheld diamond scribe (not shown). The process then can be repeated using the portion of the sintering tube 18 that remains, i.e., the portion located above the cut. This repeated use can continue several times, until the remaining portion of the sintering tube is too short for further use.

It is important to note that, throughout this procedure, the preform's central aperture 14 never is exposed to ambient air. Consequently the preform's inner surface never is exposed to water vapor, which otherwise could lead to the undesired presence of hydroxyl ions. The resulting sintered preform then can be elongated and conventionally processed to form optical fibers substantially free of water.

Although the invention has been described in detail with reference only to the preferred embodiment, those skilled in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims.

Claims

1. A method of manufacturing a substantially water-free optical fiber preform comprising: providing a porous tubular body produced by outside vapor deposition, wherein the porous tubular body includes an upper end, a lower end, and an inner surface that defines a central aperture, and wherein a tubular handle projects from the upper end of the porous tubular body, in communication with the central aperture; sealing the central aperture at the lower end of the porous tubular body; attaching a sintering tube to the tubular handle; dehydrating and sintering the porous tubular body into a dense glass tubular body, still including the central aperture; evacuating the central aperture of the dense glass tubular body; and collapsing a section of the sintering tube while the central aperture of the dense glass tubular body is evacuated, to form a tubular glass preform having its two ends sealed and its central aperture under vacuum, without exposing the central aperture to ambient conditions.

2. The method of claim 1, wherein the step of collapsing a section of the sintering tube includes a step of heating a section of the sintering tube sufficiently to cause it to collapse on itself and close its central aperture.

3. The method of claim 1, wherein: the step of dehydrating and sintering the porous tubular body includes a preliminary step of connecting a source of sintering gases to the porous tubular body via the sintering tube and the tubular handle; and the step of evacuating the central aperture of the dense glass tubular body includes a step of connecting a source of vacuum to the dense glass tubular body via the sintering tube and the tubular handle.

4. The method of claim 1, wherein: the step of dehydrating and sintering the porous tubular body occurs in a muffle tube and furnace; and _t the step of collapsing a section of the sintering tube occurs outside the muffle tube and furnace, using a glass lathe.

5. The method of claim 1, wherein: the step of sealing the lower end of the central aperture of the porous tubular body i§emmi$$WP.M≠ΦU mΦ>S m the central aperture; and the step of collapsing a section of the sintering tube occurs without the use of a separate plug to seal the upper end of the central aperture.

6. An apparatus for manufacturing a dense, glass, substantially water-free, optical fiber preform from a porous tubular body produced by outside vapor deposition, wherein the porous tubular body includes an upper end, a lower end, and an inner surface that defines a central aperture extending along the length of the porous tubular body, and wherein a plug seals the lower end of the central aperture and a tubular handle projects from the upper end of the porous tubular body, in communication with the central aperture, the apparatus comprising: means for attaching and hermetically sealing a sintering tube to the upper end of the tubular handle; a source of sintering gases to be fed through the sintering tube and the tubular handle to the central aperture of the porous tubular body; a muffle tube and furnace sized to received the porous tubular body and to dehydrate and sinter the porous tubular body into a dense glass tubular body, still including the central aperture; and a vacuum source to evacuate the central aperture of the dense glass tubular body; wherein the means for attaching and hermetically sealing also is configured for collapsing a section of the sintering tube while the central aperture of the dense glass tubular body is evacuated, to form a tubular glass preform having its two ends sealed and its central aperture under vacuum, without exposing the central aperture to ambient conditions.

7. The apparatus as defined in claim 6, wherein the means for attaching and hermetically sealing comprises a glass lathe.

8. The apparatus as defined in claim 6, and further comprising: a lower feed-through chuck configured to grasp and support the tubular handle and its attached porous tubular body; and an upper feed-through chuck configured to grasp and support the sintering tube and to support the sintering tube and its attached tubular handle and porous tubular body.

9. The apparatus as defined in claim 8, wherein and further comprising a plurality of guide rails configured to support and controUably position the means for attaching and hermetically

chuck, and the upper feed-through chuck during operation of the apparatus.

10. The apparatus as defined in claim 8, wherein the means for attaching and hermetically sealing is operable to collapse a section of the sintering tube while the upper feed-through chuck supports the sintering tube, the tubular handle, and the dense glass tubular body in a position outside the muffle tube and furnace.