General techniques for the quality control of long lengths of optical fiber for telecommunications applications have been developed during the past 10 years in concert with ongoing handling scenarios that involved cabled versions of the tested fiber. In the present work, we discuss the same quality control techniques, but apply the results to stand-alone fiber (primary or secondary buffer only). Furthermore, by modifying the normal quality control techniques, we have produced a stand-alone fiber that exhibits superior mechanical performance for missile payout applications when compared with standard telecommunications fiber.
High quality optical fibers, which were produced to more exacting standards than long-haul telecommunications fibers, were subsequently exposed to high-stress bend tests along multikilometer lengths of the fiber. Mechanical failures during these tests could occur at stresses lower than the required 200 ksi proof test. The fracture surfaces for these failures were examined by normal fractographic techniques. These surfaces clearly showed that mechanical failure occurred at stresses below the proof stress. Furthermore, an appreciable number of these failures were accompanied by damage to the buffer, presumably preceding the mechanical failure.
To correct the situation, we introduced changes in both the process and the testing for these fibers. During processing, the fibers were prepared with high quality silica substrate tubing, and all processing was carried out in a clean room environment. Proof testing subsequent to production was changed from a single test at 200 ksi to multiple testing at 100 ksi increments. Testing was further modified to include preinspection of the entire buffer surface for flaws of a serious enough nature to lead to mechanical damage to the underlying glass.
The interaction of all of the previously noted changes helped to increase the reliability of the optical fiber for missile payout applications. Fractographs of the failures, proof test data, and identification of the buffer flaws, independent of fiber manufacturer or manufacturing process, will be discussed.