# Numerical Aperture Definition

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The light-gathering ability of an optical fiber, as determined by the square root of the difference of the squares of the refractive indexes of the core (n 1 ) and the cladding (n 2 ), and as expressed in the equation: NA = (n 1 2 - n 2 2 ) 1 / 2 Fiber optic transmission systems (FOTS) are based on the principle of total internal reflection, meaning that all light injected into the fiber is retained in the fiber.The objective is to retain all components of the optical signal in the core. However, a light source naturally injects some light rays into the core at angles less than the critical angle, which is perpendicular to the plane of the core/cladding interface. At such severe angles, the incident light rays penetrate the interface and enter the cladding, where they may be lost. The numerical aperture essentially is an indication of how well an optical fiber accepts and propagates light. As illustrated in Figure N-2, optical fiber with a small NA (top) requires more directional, i.e., collimated, light, whereas fiber with a large NA (bottom) does not.The higher NA allows the fiber to accept more light and propagate more modes.The NA is mathematically equal to the sine of the angle of acceptance. Note: The NA is important in multimode fiber (MMF). It is not, however, a critical measurement in single-mode fiber (SMF), as the small core supports only a single mode of propagation and, therefore, the light is neither reflected nor refracted.The light-accepting ability can also be defined in terms of the cone of acceptance, which is the maximum angle at which the fiber will accept incident light, represented in three dimensional view. See also angle of acceptance, collimation, cone of acceptance, critical angle, MMF, SMF, and total internal reflection.

Webster's New World Telecom
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