Numerical aperture and acceptance angle

I have discussed about the principle of optical fiber in my earlier articles. Today let us discuss the terms related to fiber:

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

Step index and graded index fiber

Mode of the fiber: As we have discussed in the earlier article about the light propagation through an optical fiber after total internal reflection. The path followed by the light in a fiber is called the mode of the fiber.

The light can pass through one path ( that is mono mode or single mode fiber) or more than one path (that is called multi mode fiber). On the basis of this, the fiber is divided into two types:

1. Step index fiber

2. Graded index fiber

Let us discuss the difference between the two: Continue reading “Step index and graded index fiber”