Optical Fiber Characterization

NBS Special Publication 637, Optical Fiber Characterization [1], is a two-volume compilation of previously published NBS Technical Notes concerning the characterization of optical fibers used for telecommunications. The Technical Notes appeared in the late 1970s and early 1980s, which was the period of commercial infancy of optical telecommunications fiber. During that period, the commercially viable fibers were of the multimode type rather than single mode; that is, they supported the transmission along the fiber of a number of electromagnetic modes having different combinations of electric field and magnetic field configurations. Consequently, SP 637 (as this publication is commonly called) is mainly concerned with the characterization of multimode fiber. While single mode fibers could be fabricated, the technology to exploit them was not readily available at the time. In the earliest fiber, large concentrations of the highly polar hydroxyl radical (OH) in the glass caused high loss at the long wavelengths effective for single-mode operation. Also, the small core size of single-mode fiber requires connectors, splices, and special sources to be aligned with high dimensional accuracy, and it took years to develop such components that could be manufactured at an acceptable cost. Multimode fibers with core diameters in the 50 m to 100 m range could be easily joined with low loss, and light from laser diodes with large emitting areas could be readily launched into the large cores in a simple manner. The large core size also allowed the use of LED (light emitting diodes) sources in designing practical systems. There were no standard measurement procedures available in the early period of development of optical fiber. An effort was started by the military to draft “mil standards” for fiber. It was a useful start, and a standards document was produced. However, the technical challenges were difficult, and rapid changes were occurring in the industry. A larger United States standards effort, principally among manufacturers, started in the Electronic Industries Association (EIA). NBS staff attended the first EIA organizational meetings where attendees determined the important technical areas and established a network of committees. It was soon evident that there was a role for NBS. Manufacturers had developed their own measurement methods, and in many instances different technical