Mode field diameter, MFD
Mode-field diameter (MFD) is important because, as a measure of the size of the optical field itself, it enables you to design launch-optics to match the mode of the source to that of the fiber and so maximize launching-efficiency. MFD is determined by the numerical aperture (NA) and cut-off wavelength of the fiber and is related to the diameter of the fiber core. In general, MFD is greater than the physical diameter of the fiber core - which means that some optical power is always guided by the fiber cladding.
MFD is typically defined as the radial position where intensity falls to e-2 of the peak intensity. MFD for any step-index fiber may be estimated from NA and cut-off information to calculate the V-value, which is directly related to the MFD by the following approximation:
V = V-value of fiber
a = core radius
The MFD is typically larger than the core diameter, though of course varies with the interplay of parameters that affect V-value. In telecoms fiber operated above cut-off, the core diameter might be around 9 µm, and the MFD is around 10.4 µm. With very high NA fiber, up around 0.2 or 0.3, the core diameter is just a few microns while MFD might be around 5 µm. The ratio of MFD to core diameter is largely driven by how close to cut-off the fiber is operated, and at any given wavelength the fraction of mode field in the cladding will be independent of the NA, though the actual values of MFD and core diameter are very sensitive to cut-off and operating wavelength.
From these equations, you can see that MFD increases as cut-off wavelength increases (as does core-size). Also, for a fixed cut-off, the further above cut-off you transmit the larger the MFD becomes - that is the more power is transmitted in the cladding.
Be aware that this approximation is most valid for estimating MFD in germanosilicate core singlemode and HiBi fibers. For rare-earth doped fibers it can be useful to estimate mode field to evaluate overlap with the core and model amplifier or laser performance.