Fiber numerical aperture

The base concept of numerical aperture is simple, it tells you the maximum incident angle at which a ray of light can effectively enter an optical system. Why don’t call it that way then? Simply because that angle depends on the geometry and also on the refractive index of the medium. These three are linked together by the formulas below.

When it’s about a fiber numerical aperture, the concept is the same, you ultimately are interested in the maximum incidence angle admissible. The calculations are a little different though. To have an electromagnetic wave confined inside of an optical fiber throughout its propagation, you need to create a condition called total internal reflection. This is obtained when a wave goes from a higher refractive index medium to a lower one at a certain angle. Optical fibers are built that way, with a core and a cladding made of different dielectric materials with different refractive indexes. Therefore, depending on these indexes, you can determine what’s the numerical aperture of the said fiber and the maximum incident angle of the system.