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/m2) 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.
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—Henry David Thoreau (1817–1862)
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