Engineering Physics

11.2 Principle of optical fibre, acceptance angle and acceptance cone

Principle: Once light rays enter into core, they propagate by means of multiple total internal reflections at the core-cladding interface, so that the rays travel from one end of the optical fibre to the other. The phenomenon of total internal reflection in a straight optical fibre is explained in the following way. Let the refractive index of the core is n₁ and that of the cladding is n₂ such that n₁ > n₂. As shown in Fig. 11.2, a ray of light AO is incident at ‘O’ on the end face of core; let this ray makes an angle of incidence θ₀ with the axis of the fibre. This ray is refracted into the core and passes along OB, the angle of refraction in the core is, say θ₁. The ray OB ...