Coatings for Optical Fibers

Optical fiber coatings provided the key element for the transition of fiber technology from the laboratory to the conduit. The history of the development of optical fiber coatings will be reviewed and the importance of coatings for fiber optic sensors will be discussed. Since coating requirements for telecommunication and sensor applications vary considerably; the similarities and differences of fiber coatings for these applications will be outlined and the particular requirements of fiber coatings for sensors will presented. The remainder of the presentation will concentrate on fiber coatings which respond to the presence of an external field. Such coatings, when combined with single mode fibers form a composite structure in which strains developed in the coating material are coupled to the fiber. The strains in the fiber produce a phase modulation of the guided light in response to the applied field. These strains can be measured using a fiber interferometer which coherently combines the output of the coated fiber with that of an insensitive reference fiber. Figure 1 illustrates the normalized fiber phase response as a function of coating thickness for several coating materials. These materials were chosen as examples in order to demonstrate sensitivity to a variety of external fields. Finally, the applicabi1ity of field sensitive coating materials for fiber sensors will be addressed.