Question

(a) What is the structure of an optical fiber?
(b) What is the principle used for transmitting audio and video signals using optical fiber? Explain the principle.

Answer 1

Hint: Propagation of signals through optical fiber depends on the total internal reflection property of light. Structure of optical fiber is made in such a way that any light within some acceptance angle passes through the fiber.

Complete step by step answer:
Part a
Optical fiber is a flexible transparent substance made up using silica glass or plastic. They have diameter in millimeter range and they are slightly thicker than human hair. Typically, an optical fiber consists of three major parts. They are (a) the core surrounded by (b) the cladding and (c) the outer coating to protect the cladding from external damage.

Part b
While travelling from a denser to rarer medium if light ray falls(i.e the angle of incidence) at an angle larger than the critical angle at the boundary then instead of transmitting to the rarer medium light ray completely reflects back in the same medium without being absorbed or transmitted, and this phenomenon is known as total internal reflection of light. In case of optical fiber, the refractive index(RI) of cladding used is always less than that of the core used. The critical angle for a particular optical fiber is given by ${\theta }_c={\sin }^{-1}\left({\displaystyle \frac{n_{cl}}{n_{co}}}\right)$,where $n_{cl}$and $n_{co}$ are the RI of cladding and core respectively. So, for any ray entering inside the fiber core at a lesser than critical angle starts reflecting multiple times inside the fiber core from the core-cladding boundary( shown in the figure) till it reaches the other end of the fiber. Hence, using this total internal reflection property of light audio and video signals are transmitted.

Note: A student might think that there is no difference between normal reflection and total internal reflection. Although they both follow the laws of reflection, they are fundamentally completely different phenomena. In normal reflection the reflecting surface absorbs some energy but in case of total internal reflection that never happens because the reflection happens at the boundary of two mediums. That’s why the term ‘total’ is used to describe it.