This invention relates to methods of and apparatus for overcladding a glass rod. More particularly, this invention relates to methods and apparatus for causing a glass tube to be collapsed onto a glass rod to provide an optical fiber preform.
Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc. has been found to be one of the most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, reactant-containing gases, such as SiCl4 and GeCl4 are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a preform tube. Once a sufficient number of layers have been deposited, the preform tube is then heated to cause it to be collapsed to yield a preform or preform rod as it is often called.
One way in which the productivity of the MCVD process can be increased is first to produce an undercladded preform, having a larger than desired core-to-cladding mass ratio. The preform is inserted into a glass tube which is referred to as an overcladding tube and which is then collapsed onto the preform. This process is referred to as the rod in tube technique, or RIT.
At the present time, production methods of manufacturing overclad preforms are usually performed on a vertical lathe, with a vacuum arrangement attached to the lower headstock of the lathe. The lower headstock of the lathe has a chuck mounted thereon attached to a spindle which provides rotary motion. There is no relative motion between the chuck and the vacuum arrangement. A heating member, such as an oxyhydrogen torch, is made to travel along the length of the rod with the overcladding tube surrounding it to soften the tube and collapse it about the rod. The vacuum arrangement creates a low pressure or vacuum to provide a pressure bias to the tube to aid in its collapse, and, also, to remove possible contaminants that may have been interjected into the space between the rod and the tube prior to the collapse of the latter. Such a vacuum arrangement, including the apparatus for generating the vacuum, severely limits the length of the overclad preform product, thereby inhibiting the use of enhanced production techniques which require, for example, longer preforms.
One widely used vacuum apparatus comprises a hollow circular cavity concentric with the spindle centerline of rotation, the bottom of the cavity being perpendicular to the centerline or axis of rotation. The top of the cavity is open to the atmosphere. The cavity diameter may be appropriately sized to allow for various tube diameters to be inserted. The closed end of the cavity has an axial bore therein to link to a rotary union member which is attached to the vacuum structure. As a consequence, a vacuum source may be supplied in the stationary reference frame of the lathe and linked to the cavity which rotates when the headstock or tailstock spindle is rotating. A cylindrical insert is placed inside of the cavity, having a diameter roughly equal to the cavity interior diameter. This insert has a lower surface parallel to the bottom of the cavity and rests on the bottom thereof. The upper surface is also parallel to the bottom of the cavity and has a washer of refractory material mounted thereon. Both the insert and the washer have a central or axial hole extending therethrough which allow routing of the vacuum source to the upper surface, while the lower surface of the insert forms a mechanical seal with the cavity. There may be some leakage in this sealing arrangement, however, it is generally insufficient to necessitate aborting the operation.
The end of the overclad tube, which has been prepared, as by grinding, to be perpendicular to its cylindrical axis, rests on the refractory washer surface, forming a seal therewith, which may have some leakage. The washer thus functions as an axial position spacer and as a face seal between the overclad tube and the insert. The vacuum source, with this arrangement, is thus routed to the interior of the overclad tube. Once the tube has been positioned with its own weight exerting the sealing force on the refractory washer, the jaws of the lathe chuck are closed so that the tube is clamped rotationally and axially in position and the vacuum is transferred through the rotary union to the interior of the overclad tube. The tube cannot be moved axially for repositioning without breaking the seal. Thus, the span between the headstock and the tailstock of the lathe, at maximum separation, imposes a limit on the combined length of the rod to be inserted in the tube and the tube itself. This, in turn, limits the length of the preform product, thereby limiting the production process and preventing enhancement thereof or any increase in preform length. In addition, the heating element can only apply the desired heat to a limited length of rod-in-tube which represents a further limitation on the length of the finished preform. Any axial adjustments or relative movement of the rod and the tube are also prevented, or, at least, limited.
It is desirable, therefore, that such limitation on the length of the preform be eliminated, or at least reduced so that, in accordance with enhanced production, longer preforms may be produced.
The present invention is a sealing arrangement that, in effect, eliminates dependency upon the sealing interface between the heretofore used insert and the bottom of the cavity and the sealing interface between the end of the tube and the refractory washer by eliminating both the insert and the washer, and thereby making axial adjustment feasible. As a consequence, as will be seen more clearly hereinafter, the length of the tube within the rod portion can be measurably increased with the net result that longer preforms are created.
In greater detail, the longitudinal stem of the rotary union, at its distal end, has a disposable seal mounting hub interface. A disposable resilient sealing member of suitable material, such as Teflon(copyright) or hard rubber, for example, is fastened to the distal end of the stem at the hub interface. The sealing member has an outside diameter that is a sealing fit within the internal diameter of the tube. By xe2x80x9csealing fitxe2x80x9d is meant a fit sufficiently tight to constitute a substantially hermetic seal but not so tight that the tube cannot be axially moved relative thereto. Some leakage at the seal may occur, but as the vacuum or pressure differential increases, so too does the sealing capability of the seal.
As the overclad tube is lowered into the cavity, the seal system is engaged and vacuum application can start as soon as the engagement of the sealing member, with the ID (inner diameter) of the tube takes place. The tube can then be axially positioned as required without engagement for sealing purposes of the end thereof. If the tailstock (or headstock) of the lathe has a pass through bore, it can be appreciated that a tube of greater length than has heretofore been possible may be used, which may be moved axially to bring more tube length into the heating region without destroying the vacuum seal. The chuck of the lathe headstock or tailstock is used to clamp the tube in place, and may be loosened to allow moving of the tube axially and then reclamped. The rotary union, which has its rotating portion connected to the sealing member and its stationary portion connected to the vacuum apparatus, is dependent upon the friction between the sealing member and the tube for driving the rotating portion as the tube is rotated. With a tight seal, the rotation of the tube is transmitted to the rotary union substantially without slippage and the seal remains intact. Alternatively, the tightness of the seal can be such that the tube rotates with respect to the sealing member without compromising the seal. Adjustments can be made to select any combination between these two extremes, such as stationary (seal within the tube) wherein the rotary portion rotates, combination seal/rotary union rotation, or seal only rotation (relative to the tube). The amount of leakage and vacuum properties of the seal will necessarily vary depending upon which option is chosen, or which is caused by environmental or production line conditions, without destroying the integrity of the seal.
With the structure of the invention as hereinbefore described, axial movements and adjustments between the rod and the seal can be made without destroying the vacuum seal, thereby making possible considerable production enhancement, such as much longer preforms, than heretofore achievable.