Solution:

Mode Field Diameter (MFD):

Optical power (in single-mode fiber) travels in both the core and the cladding.

In many situations, not the least being when we join fiber, we need a number that will give us a measure of the extent of the region that carries the optical signal. In single-mode fiber, the core diameter is not sufficient.

We introduce the concept of the “mode field”. The mode field can be considered the effective core of the fiber although the real core size is typically somewhat smaller.

One important point is that there is no abrupt boundary that defines the the extent of the mode field.

Even though the signal decays exponentially there is no exact cutoff point.

Therefore we must choose an arbitrary point as the boundary. The figure shows the distribution of optical power across the diameter of a typical single-mode fiber. The definition of the mode field diameter is shown.

The MFD of standard SMF at 1550 nm is between 10.5 microns and 11 microns depending on the fiber. In the 1310 nm band, the MFD of standard fiber is 9.3 microns.