Mode field diameter
Mode field diameter MFD
The mode field diameter MFD is the diameter of the beam profile on exiting the single-mode fiber. The MFD dependends on the wavelength and the effective numerical aperture NAe2 of the fiber according to:
{!{!{MFD=\frac{2\cdot \lambda}{\pi\cdot {\rm NA}_{e^2}}}!}!}
Both the MFD, and the effective NAe2 are given at the 13.5 % (1/e2)-level of the Gaussian profile. For both, singlemode and polarization-maintaining fibers, the MFD is of the same magnitude as the core diameter.When coupling high power levels, fibers with a larger MFD might be preferred. For fibers with a large MFD (small NAe2), the power density within the fiber is reduced and the Brillouin threshold Pcr is increased.
Additionally, in order to prevent the damage of the fiber end faces, fiber connectors with end caps may be needed.