Numerical aperture (NA) of an optical fiber is a measure of how much light the fiber can accept and guide. In simple terms, it tells you the light- gathering ability of the fiber.

Quick Scoop

For a fiber in air, the numerical aperture is commonly written as:

NA=sin⁡(θa)\text{NA}=\sin(\theta_a)NA=sin(θa​)

where θa\theta_a θa​ is the acceptance angle of the fiber.

If the core refractive index is n1n_1n1​, the cladding refractive index is n2n_2n2​, and the outside medium has refractive index n0n_0n0​, then:

NA=n0sin⁡(θa)=n12−n22\text{NA}=n_0\sin(\theta_a)=\sqrt{n_1^2-n_2^2}NA=n0​sin(θa​)=n12​−n22​​

for the usual step-index fiber case in air, n0≈1n_0\approx 1n0​≈1.

Why it matters

  • Higher NA means the fiber can accept light over a wider angle.
  • It is often described as the fiber’s light-collecting power.
  • A larger NA generally makes coupling light into the fiber easier.

Simple intuition

Think of the fiber entrance like a funnel for light. A wide funnel accepts more incoming rays, while a narrow funnel accepts fewer. NA tells you how wide that funnel is for the fiber’s guided light cone.

If you want, I can also give you:

  1. the derivation of NA,
  2. a numerical example , or
  3. a one-line exam definition.