Patent Application
20120160167
The present invention relates to optical preform fabrication utilizing a plasma chemical vapor deposition (PCVD) process and, more particularly, to an apparatus providing external heating of the preform substrate during the deposition process.
One way of manufacturing an optical fiber preform is known in the art as the Plasma Chemical Vapor Deposition (PCVD) process. According to this process, one or more doped or undoped glass layers are deposited onto the interior of a substrate tube using low-pressure plasma in the glass substrate tube. After the glass layers have been deposited onto the interior of the glass substrate tube, the glass substrate tube is subsequently contracted by heating into a solid rod. In one embodiment, the solid rod may be externally provided with an additional amount of glass (i.e., by means of an external vapor deposition process), or by using one or more preformed glass tubes, thereby obtaining a composite preform. From this preform, one end is heated and drawn down in diameter to produce optical fibers.
The plasma is created by an RF field generator that surrounds a glass substrate tube. The RF generator couples the high-frequency energy into the plasma. At one end of the substrate tube, the doped or undoped reactive gases are supplied, after which a reaction takes place under the influence of the plasma, and doped or undoped glass layers are deposited onto the interior of the substrate tube. The other end of the substrate tube is connected to a vacuum pump so that a reduced pressure (e.g., between 5 and 50 mbar) is provided in the interior of the substrate. The generator is moved reciprocally in the longitudinal direction of the substrate tube so that a thin glass layer is deposited onto the interior of the substrate tube with every transversal.
It has been found that to optimize the deposited material characteristics of the deposited glass using the PCVD method, the substrate needs to be heated to (and preferably maintained at) a constant temperature in the range of the glass transition temperature of the deposited glass during the deposition process. In particular, the uniform substrate heating ensures smooth, bubble-free deposition of the glass as it is being fused to the inner substrate wall.
Several prior art methods are available for providing the substrate heating. For example, the entire assembly—substrate and work coil—can be placed in a temperature-controlled furnace that is maintained at the desired temperature. While this method provides the desired substrate heating, problems arise with the need to also expose the work coil to the elevated temperature. That is, the work coil assembly needs to be insulated and cooled so that it is not destroyed. Additionally, the need to locate the entire assembly in a furnace makes it difficult for an individual to observe the substrate during the deposition process.
Alternatively, an external burner (such as an oxyhydrogen torch) may be added to the arrangement and caused to traverse the substrate tube in advance of the work coil to heat the substrate. This heating may not be as uniform as desired (or as efficient), and requires additional energy and expense to deploy.
It is also possible to utilize substrate tubes with relatively large wall thicknesses to provide sufficient thermal mass to retain the heat from previous passes of the work coil. However, this approach is also problematic since it unduly restricts the tube sizes that can be used (since the initial wall thickness will increase significantly throughout the deposition process).
The needs remaining in the prior art are addressed by the present invention which relates to optical fiber preform fabrication utilizing a plasma chemical vapor deposition (PCVD) process and, more particularly, to a PCVD apparatus including a insulative covering disposed to surround at least a portion of the substrate tube and provide external heating of the substrate tube during the deposition process.
A standard PCVD process is known to generate a considerable amount of thermal energy; the plasma fireball itself is a direct source of heat, while the substrate tube is an indirect source (convective and radiative) of heat. In accordance with the present invention, therefore, an insulative covering functions to capture and retain this thermal energy. As a result, the areas of the substrate tube that are removed from the current location of the plasma will absorb this captured thermal energy from the insulative covering. After a short, gradual heating period prior to deposition where the plasma is traversed reciprocally, the substrate remains at an essentially constant temperature until the next pass of the work coil.
The insulative covering is configured to only partially surround the substrate tube so that operating personnel are able to directly observe the deposition process. The covering may be in the form of a single piece of insulative material disposed along the length of the substrate tube (not necessarily along the entire length of the tube, as a matter of design choice). Alternatively, the insulative covering may comprise two or more separate longitudinal segments, with a separation between segments sufficient to allow for observation of the deposition process. The insulative covering may maintain a fixed relationship with the substrate tube, or may be adjustable (in terms of separation between the covering and the substrate, for example) if need be. In one embodiment, the insulative covering may be at least in part a transparent material that allows for unobstructed observation of both the plasma and the substrate.
The separation distance between the insulative covering and the substrate is also controlled to provide the desired degree of substrate heating. The material used to form the insulative covering is also a design consideration and will factor into the determination of the proper separation distance. Supplemental heating or cooling of the insulative covering itself may be utilized to assist in maintaining a uniform substrate tube temperature.
Other and further aspects and advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
Referring now to the drawings,
Apparatus 10 further comprises a chemical delivery system 14 to deliver one or more chemical reactants (such as GeCl4, SiCl4, C2F6, SiF4 and O2) into substrate tube 12 through a first rotating seal 16 formed within a first end of tube 12. Although not shown in
As shown in
The creation of the plasma generates heat both within and outside of substrate tube 12. Heretofore, the thermal energy external to substrate tube 12 was generally lost and, as a result, the tube would exhibit fluctuations in its temperature.
In accordance with the present invention, the inclusion of an insulative covering 20 around at least a portion of substrate tube 12 functions to capture the heat and retain the external thermal energy, thus maintaining substrate tube 12 at a relatively constant temperature. By virtue of maintaining substrate tube 12 at a relatively constant temperature (preferably, the glass transition temperature of the material being deposited), the deposition will be smooth and bubble-free.
It is important that insulating covering 20 be formed of an appropriate material (such as a refractory material) that is able to withstand the temperatures associated with the PCVD process and provide the desired heat capture/reflection function. Materials such as, for example, silica, alumina, magnesia, zirconia or mullite may be used.
Indeed, it is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments that can represent applications of the principles of the present invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the present invention as defined by the claims appended hereto.
