1. Field of the Invention
The present invention generally relates to the field of optical fibers, and in particular coated optical fibers and optical fiber ribbons and methods for the fabrication thereof.
2. Discussion of Related Art
FIG. 1 depicts a prior art optical fiber ribbon 100, which includes a plurality of prior art coated optical fibers 110 and a matrix 118, which surrounds coated optical fibers 110.
Optical fiber ribbon 100 is generally useful for transmitting information. Each of coated optical fibers 110 includes an optical fiber 112, which can be made from glass or other optically transmitting material. Optical fiber 110 further includes a prior art fiber coating 111, which consists of a primary coating 114 formed on optical fiber 112 and a secondary coating 116 formed on primary coating 114.
Primary coating 114 is typically formulated to allow ease of removal from optical fiber 112. Secondary coating 116 is typically a layer distinct from primary coating 114 and is designed to provide properties, such as hardness that is greater than that of primary coating 114.
It is known in the art to deposit primary and secondary coatings 114 and 116 as separate, distinct layers. As a consequence, a thin interpenetration layer 117 can form at the interface between primary and secondary coatings 114 and 116. Interpenetration bonding layer 117 has a thickness that is much less than the overall thickness of fiber coating 111 and can be on the order of microns. Thin interpenetration layer 117 tends to deteriorate, and this deterioration can result in the separation of secondary coating 116 from primary coating 114.
It is also known in the art that matrix 118 should preferably allow ease of separation from coated optical fibers 110, while providing mechanical protection and reduced friction at its outer surfaces. For example, as further illustrated by FIG. 1, matrix 118 can be made from a primary matrix layer 120 and a secondary matrix layer 122. A thin interpenetration layer 123 exists between primary and secondary matrix layers 120 and 122, and can deteriorate to cause the undesireable separation of layers 120 and 122, similar to the separation described with reference to primary and secondary coatings 114 and 116 of coated optical fiber 110.
FIG. 2 is a cross-sectional view of prior art coated optical fiber 110 and includes graphical representations of the hardness and concentration of a hardening agent just prior to curing for prior art fiber coating 111. The hardening agent is a constituent added to make secondary coating 116 harder than primary coating 114. The concentration depicted is that just prior to curing since primary and secondary coatings are made from UV-curable formulations of oligomers and liquid monomers (with reactive usually acrylate functional groups), and photoinitiators. The photoinitiator absorbs energy radiated by a UV light source, fragmenting into reactive species, and then initiating a polymerization/hardening reaction of the monomer and oligomers. The result is a solid network of crosslinking between the monomers, oligomers, and other additives. Thus, individual constituents, such as hardening agents, are generally transformed during the curing step.
A graph 124 of hardness illustrates that the hardness of prior art fiber coating 111 changes sharply as a step-function at interpenetration layer 117 due to the fact that primary and secondary coatings 114 and 116 are applied as separate, distinct layers having distinct hardness properties. Specifically, the hardness of secondary coating 116 is generally uniform throughout and is greater than that of primary coating 114.
A graph 130 of concentration of a hardening agent illustrates that the concentration of a hardening agent just prior to curing also changes sharply as a step-function at interpenetration layer 117 due to the fact that primary and secondary coatings 114 and 116 are applied as separate, distinct layers having distinct compositions and that primary coating 114 is typically cured prior to the curing of secondary coating 116.
In view of the above, there exists a need for an improved coated optical fiber and an improved optical fiber ribbon that provide interior peelability and reduced exterior friction while reducing or eliminating the problem of layer separation at interpenetration layers.