What is optical fiber’s mode field diameter?
Although most light travels inside an optical fiber’s core, the light actually spreads through a slightly larger volume including the inner edge of the fiber cladding. This effective area is called the fiber’s mode field diameter or MFD. Mode field diameter is a measure of the spatial extend of the fundamental mode and it is only important for single mode fibers. Its effect is so small in multimode fibers that it really doesn’t matter any more. Mode field diameter plays an important role in estimating splice losses, source to fiber coupler losses, macro bending and micro bending losses, etc. For single mode fibers manufacturing, MFD is used as a rather more important parameter than fiber’s core size.
