A variety of measurements are to be carried out on optical fibers and the components connected to the optical fiber. Some important measurements are given in the following.

The numerical aperture (NA) is generally measured for a multi-mode optical fiber. Since the NA is related to the acceptance angle of the optical fiber, the NA can be obtained from the angle of the emerging light cone from the fiber.

A fiber is excited with a source like LED or a visible laser. The light emerging from the other end of the fiber is projected on a screen as shown in Fig.

The spot size and the distance of the spot from the tip of the fiber is measured go get the NA of the fiber.

From the Fig we get,

Generally, near the input end of the optical fiber a large number of non-propagating modes are excited, which eventually die down along the length of the fiber. Very close to the input end of the fiber, the NA is higher compared to the true NA (some time called the saturated NA of the fiber).

To get a correct estimate of the NA of the fiber, the length of the fiber should be sufficiently long. Alternatively, the fiber can be passed through a mode scrambler so that the non-propagating modes quickly radiate their power, leaving only propagating modes inside the fiber.