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.
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.
The value of numerical aperture is determined from the refractive index profile of a symmetric geometry single-mode fiber, or two refractive index profiles for HiBi fiber.
Two index profiles are used because HiBi is birefringent - so the core has two distinct values of refractive index. There is approximately 10% difference between these two indices - the lower corresponding to the ‘fast’ polarization axis and the higher to the ‘slow’ polarization axis.