What is optical fiber’s numerical aperture?
Optical fiber’s capability to collect light is not only determined by fiber core size, but also by its acceptance angle. Acceptance angle is the range of angles over which a light ray can enter the fiber and be trapped in its core. The acceptance angle and numerical aperture are figures of merit used to describe the angles associated with light propagation in optical fibers. The sine of the half-angle of the acceptance angle is known as the numerical aperture – NA. These parameters can be related to the refractive indices of the fiber materials by using Snell’s law and a bit of geometry. The aforementioned formula can only be used for step-index fibers and numerical aperture is not calculated the same way in graded-index fibers. Actually NA varies across the core with the refractive index. However, you can measure NA for graded-index fibers by monitoring the divergence angle of light leaving a fiber core as show below. The light emerging from a multimode fiber spreads over an angle equ
