The core diameter of a multimode step index fibre is 50 μm. The numerical aperture is 0.25. calculate the number of guided modes at an operating wavelength of 0.75 μm.

The numerical aperture(NA) is defined as the sine of the acceptance angle.It determines the light gathering ability of the fibre. It is a measure of the amount of light that can be accepted by a fibre.

where α is the maximum angle of incidence for which total internal reflection can occur. n1 = refraction index of core. n2 = refractive index of cladding.

Use of optical fibre in temperature sensor:

If the fiber is subjected to heating the temperature causes a change in the refractive index of the fiber. As temperature increases the difference between the refractive index of core and cladding reduces leading to the leakage of light into the cladding.

A simple thermometer can be built by using LED as a light source,a coil of fiber as heat sensing element and a photo detector to measure the intensity of light temperature in the range from 80 to 700 deg celcius are measured using such a thermometer.