Method for coating optical waveguide fiber

Claim 1

In a method for applying a protective organic coating to a glass optical fiber wherein the glass fiber as drawn from a glass preform is transported through a liquid coating die having a fiber inlet, a fiber outlet, and a liquid reservoir containing an organic coating liquid, the liquid coating die providing a liquid coating on the fiber, wherein the liquid coating is thereafter cured to provide the protective organic coating, wherein the coating die is operated under a condition such that a downwardly drawn meniscus is formed in the surface of the coating material at the point of entry of the optical fiber into the coating liquid, and wherein bubble inclusions in the coating are reduced by reducing air entrainment into the coating liquid by the fiber, the improvement wherein: (a) the step of reducing air entrainment into the coating liquid comprises the step of passing the fiber through a fiber conditioning chamber positioned adjacent to and connected with the fiber inlet to the liquid reservoir, the chamber comprising a cylindrical channel through which the fiber is downwardly drawn into the reservoir, the chamber further comprising a gas inlet into which a purge gas in introduced, said gas being caused to flow upwardly in the cylindrical channel to provide countercurrent gas flow with respect to the downward direction of fiber draw; and (b) the purge gas is carbon dioxide.