Simultaneous Measurement of the Core Diameter and the Numerical Aperture in Dual-Mode Step-Index Optical Fibers

We present a simple and precise method for the simultaneous measurement of the core diameter and the numerical aperture of step-index fibers. In contrast with conventional near-field and far-field methods proposed in the literature, this method is based on the measurement of the difference between the propagation constants of the modes LP₀₁ and LP₁₁ near the cutoff wavelength. We experimentally validate and highlight the high level of precision of this method with the characterization of commercial fiber samples.