Failure of optical fibers with thin hard coatings

The strength of optical fibers with a thin brittle coating is examined. The inert strength of optical fibers coated with carbon (hermetic fibers) has been observed to be less than that of standard optical fibers. A fracture mechanics model was developed to explain the mechanism of this strength reduction. The model, based on the cracking of thin films in residual tension, will be used to predict growth of flaws from the carbon film and penetrating into the substrate. The model can be applied to all brittle coatings where delamination of the coating is not observed. Conditions under which cracks in the carbon film propagate into the substrate were investigated using a recently developed superposition scheme. The failure stress of the fiber was predicted using the fracture mechanics model and experimentally measured values of the properties of the film. The predicted values of strength are compared with the measured values of the strength and the difference is less than 10%.