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The mode field diameter (MFD) is an essential parameter for understanding the transmission characteristics of optical fibers. A variable aperture (VA) technique is a standard test method for measuring the MFD of single-mode fibers. Unfortunately, the conventional one cannot measure the MFDs of some higher-order modes correctly because its measurement procedure is standardized for measuring the MFD of the fundamental mode. The conventional VA technique aligns the position of the optical fiber through a peak intensity search on the premise that the fundamental mode, whose intensity is maximum at the beam center, is measured. How to align the position of the optical fiber when measuring the MFD of higher-order modes has not yet been investigated. In this paper, we describe the principle and formulas based on the VA technique for measuring the MFD of a given higher-order mode in few-mode fibers (FMFs). Then, we propose a novel procedure for measuring the MFDs of FMFs. The test apparatus of our technique is almost the same as that of the conventional VA technique except for the use of a mode selective coupler. One can measure the MFDs of arbitrary modes in FMFs correctly by adjusting the position of the fiber under test using the fundamental mode before starting the measurement. Proof-of-principle experiments were performed for three kinds of FMFs, and the experimental results showed that our procedure is useful for characterizing the MFD of each mode.

Variable Aperture Technique for Measuring Mode Field Diameters Corresponding to Laguerre Gaussian Spot Size of Higher-Order Modes of Few-Mode Fibers