In fiber optics, the efficiency of optical power transfer between two optical components. Note 1: The transfer may take place (a) between an active component, such as an LED, and a passive component, such as an optical fiber, or (b) between two passive components such as two optical fibers. Note 2: Coupling efficiency is usually expressed as the ratio, converted to percent, of the input power, i.e., the available power from one component, to the power transferred to the other component.