JIMS-TEC, Greater Noida
Engineering Physics Lab Manual
AIM: To determine numerical aperture of an optical fiber.
APPARATUS: A fiber optic, 20X microscopic objective, He-Ne laser, screen, graph paper and
THEORY: Numerical aperture accounts for the ligth gathering ability of the fiber and it amounts the light
accepted by the fiber. NA refers to the maximum angle at which the light incident on the fiber end is
totally internally reflected and is transmitted properly along the fiber. NA depends only on the refractive
indices of the material of the core and the cladding. Mathematically, NA is defined as a sine of angle of
acceptance. Thus if θo is the angle of acceptance then:
NA = sin (θ0)
NA = (n12 – n22)1/2/n0
NA is also called the figure of merit of optical fiber. The cone formed by the rotation of angle of
acceptance along the axis of the fiber is the cone of acceptance of the fiber. The light should strike the
fiber end within its cone of acceptance else it is refracted out of the fiber.
For short length of straight fiber, light ray incident at angle θ at the input end and must come out at same
angle from out put end. So, at the output end of the fiber a cone of the same angle appears. Hence NA can
be determined by making measurements on the cone at the output end of fiber.
NA = D / (4L2 + D2)1/2
where D = diameter of circular spot formed on the screen
L = distance between output end of the fiber and screen
Applied Physics Lab 1