Numerical Aperture (NA): NA is the light gathering ability or capacity of an optical fiber. More the NA. the more efficient will be fiber. It is also known as figure of merit.

NA is related to refractive index of core (n1), cladding (n2) and outside medium (n0) as

NA = √n1² – n2²/n0

If the medium is air then n0 =1, then

NA = √n1² – n2²

Acceptance angle (θ): It is the maximum angle made by the light ray with the fiber axis, so that light can propagate through the fiber after total internal reflection.

Relation NA and acceptance angle:

NA = Sin θ

Acceptance cone: It is the cone in which the light incident at acceptance angle or less than the acceptance angle and then the light can propagate through the fiber after total internal reflection.

Fractional Refractive index change (Δ)= Δ = n1 – n2/n1

Relation NA and Δ:

NA = n1√2 Δ

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