Hello Sasha,
We know that in fiber optics , the mode field diameter (MFD) is an expression of distribution of the irradiance, i.e., the optical power per unit area, across the end face of a single- mode fiber.
For a Gaussian intensity (i.e., power density, W/m 2 ) distribution in a single-mode optical fiber , the mode field diameter is that at which the
electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power density is reduced to 1/e 2 of the maximum power density, because the power density is proportional to the square of the field strength.
I will suggest you to go through this tutorial link of which I am posting below. I hope this would help to some extent.
Regards