Mode Field Diameter (MFD) is a measure of light intensity in the core of a single mode fiber. It is traditionally defined as the width where intensity falls to 1/e of its peak value, but standards now define it via the Petermann II integral of the far-field intensity distribution. MFD represents the effective area of light propagation in both the core and cladding. It provides important information about a cable's performance and impacts from bending or improper source-fiber coupling that could lead to excessive loss. MFD is tested using an optical time domain reflectometer to obtain the far-field profile and calculate the Petermann II integral to determine the MFD value.