Numerical Aperture (NA)
The numerical aperture is a measure of the acceptance angle of the fiber. It is very important because it determines how strongly a fiber guides light, and so how resistant it is to bend-induced losses.
Numerical aperture can be defined by the acceptance angle of the fiber, though as this is highly diverging in space it is rather complicated to reach a simple definition.
It is most convenient to define the NA in terms of the relative indices of core and cladding glass forming the fiber waveguide:
NA may also be approximated as (2ncore.δn)½ where δn is the index difference between the core and cladding. An optical fiber with ‘high’ numerical aperture will confine light more strongly in the core, and so support guidance further above cut-off. This attribute has two important effects:
a) it will be single-mode over a greater range of wavelengths than is possible with a fiber with a ‘low’ numerical aperture fiber.
b) it will still guide a single-mode when coiled or bent to a smaller diameter than is possible with a ‘low’ numerical aperture fiber.