Measurement of the core diameter of multimode graded-index fibers: A comparison of transmitted near-field and index profiling techniques

The core diameters of six graded-index fiber from four different fiber manufacturers were compared using the transmitted near-field (TNF), the refracted near-field (RNF), and the transverse-interferometric (TI) measurement methods. This study was part of an effort to develop a standardized, industry-wide definition of core diameter and to determine the precision of interlaboratory core-diameter measurements using different measurement techniques. For fibers with smooth index-of-refraction profiles, all three methods were in good agreement (
< 1.0-\mu
m difference). Substantial differences between the transmitted near field and the two profiling methods (RNF and TI) were observed for fibers having step structure near the core-cladding boundary. In an attempt to resolve these differences, splice-loss measurements were used as an indicator of diameter differences. These experiments suggested that curve-fitting routines should be applied to the two profiling methods. A comparison of the curve-fitted profile data with measured transmitted near-field data at points 2 percent above the baseline produced values for the diameters which agreed to within 1 μm for all of the fibers measured.