Glass fibers with very low optical transmission loss now can be made routinely. Attention has now shifted to those physical characteristics of the fibers that are important to a practical glass fiber cabling technology. D. Gloge reports in The Bell System Technical Journal, Vol. 54, No. 2, p245, that glass fibers are surprisingly susceptible to mechanical distortions caused by very small anisotropic forces. These distortions can lead to unacceptable optical losses. The kinds of forces implicated are unavoidable in normal cable processing and installation. Gloge has also shown that the distortions can be reduced by providing the fiber with a protective coating. He suggests some guidelines for the choice of the coating material, especially in the selection of the hardness of the coating. The assumption is made here that fibers used in commercial transmission equipment will have applied to them some form of protective coating. This assumption is widely made in the art.
The study by Gloge of mechanical deformation of fibers emphasizes the need for a coating process which avoids similar mechanical abuse during the coating process. We have found that coating processes, such as wick coating, that involve mechanical contact with the fiber introduce mechanical infirmities in the fiber. These may cause optical losses and may even lead to fiber breaks. It is evident that the uncoated glass fiber is even more susceptible to mechanical damage than the coated fibers studied by Gloge. It is desirable therefore to coat the fiber as soon as the fiber is formed, before it encounters any mechanical guides or reels. It is also desirable that the coating process itself be designed to avoid any physical touching of the fiber other than by the coating material itself.
This invention is a technique and corresponding apparatus for coating a continuous fiber in accordance with the objectives just outlined. It involves passing the continuous fiber through the coating material, typically a liquid that later cures, with the coating material contained in a specially designed container. The entrance and exit openings for the fiber are capillaries. At least one of the capillaries is located at the base of the container (in a gravitational sense) so that it is continuously exposed to the coating liquid. As the fiber is drawn through this capillary the fiber will tend to remain centered within the capillary, away from the potentially hazardous walls, through the action of fluid dynamics. In principle the combination resembles a fluid bearing. The other capillary opening may be disposed also below the fluid level, in which case it behaves similarly to that just described. In one form of the invention one capillary resides above the fluid level. The fiber is centered within this capillary by providing a flow of gas so that it acts as a gas bearing. For the purposes of this invention a gas bearing is one having a pressure differential between the interior and exterior of the capillary. See also e.g., U.S. Pat. No. 3,480,340 dated Nov. 25, 1969. The gas flow serves another important role in accordance with the invention which is to control the flow of fluid through an immersed capillary. The control is effected most conveniently by creating a negative pressure within the container although other arrangements in the same spirit will be described. This aspect of the invention becomes especially useful when the coating liquid is very fluid and would tend otherwise to flow freely through the immersed capillary. We have found that coatings become uneven due to dripping when the flow is uncontrolled. If the fiber travels upward through the capillary the flow of fluid through the capillary is reduced but the dripping problem remains, this time as the fiber leaves the free surface of the fluid.
It is evident that however the capillaries are arranged they will have a common axis.