# Mode field diameter calculator

## About this calculator

This calculator computes the mode-field diameter of the fundamental mode in a step-index fiber using Marcuse's equation:

$MFD = 2a\left(0.65 + \dfrac{1.619}{V^{3/2}} + \dfrac{2.879}{V^6}\right)$

where $V$ is the normalized frequency

$V = \dfrac{2\pi}{\lambda}a\mathrm{NA}$

and $\lambda$ is the signal wavelength, $a$ is the fiber core radius and $\mathrm{NA}$ is the core numerical aperture.