This document defines the method for determining the mode field diameter, 2wo, of a single-mode fiber by measuring the far field output distribution through a series of transmitting apertures of various size.

This test applies to single-mode fibers, Classes IVa and IVb (in accordance with EIA-4920000-A), for both the 1300 and 1550 nm wavelength ranges.

The mode field diameter measurement of a single-mode fiber is a two step process. First, the two-dimensional far field pattern is measured as the power passing through apertures of various size. Second, a mathematical procedure is used to calculate the mode field diameter from the far field data. A typical test setup is illustrated in Figure 1.