Mode field diameter
In fiber optics, the mode field diameter 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 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/e2 of the maximum power density, because the power density is proportional to the square of the field strength. 1/e2 is 0.135 times the power or a loss of -8.68 dB.
'"Mode Field Diameter"' is also defined as the maximum area, where the light signal resides. It consists of the core and some part of the cladding. It is slightly larger than the Core as the light travels through the core and few of the cladding as well.
