A measure of the diameter of the area in which an optical signal propagates through a single-mode fiber (SMF). Although most of the signal is confined to the core, some of the signal travels in the cladding, so the mode field diameter is of greater significance than the core diameter. See also cladding, core, optical fiber, and SMF. Cladding Normal (Transverse) Light Source Critical Angle Cladding Core High-Order Modes Low-Order Modes