Describe the structure of an optical fiber. Mention few conditions for propagation of light.

Optical Fiber :-

An optical fiber is a glass or plastic fiber designed to guide light along its length. Fiber optics is the overlap of applied science and engineering concerned. Optical fibers are widely used in fiber-optic communication which permits transmission over longer distances and at higher data rates than other forms of wired and wireless communications. Fibers are used instead of metal wires because signals propagate along them with less loss and they are immune to electromagnetic interferences. Optical fibers are also used to form sensors, and in a variety of other applications.

The optical fiber is simply a cylindrical waveguide system operating at optical frequencies. It consists of a core at the centre and a cladding outside the core The core is generally cylindrical dielectric glass with a refractive index and cladding is second dielectric sheath or cover usually of glass with a lower refractive index.

Structure of an Optical Fiber : -

The diagram shows the typical structure of a fiber used for communication links. It has an inner glass core with an outer cladding. This is covered with a protective buffer and outer jacket. This design of fiber is light and has a very low loss, making it ideal for the transmission of information over long distances.

Physically a fiber optics is a very thin flexible medium having a cylindrical shape that consists of three sections, core, cladding and protective jacket.

The core is the innermost section having diameter approximately 5 µm - 100 µm and is made up of glass or plastic. This core is surrounded by cladding of slightly lower refractive index with a diameter ~125 µm which is of glass or plastic. The outermost section is made of plastic polymer or some other suitable material and is used for the protection against moisture, crushing, abrasion or any other environmental dangers. A small impurity in the fiber optics cable cause the light pulse to lose some of the information that it is carrying. Because of this reason, ultrapure glass is required as it allows the light pulses to flow through the long strands of fiber optic cable without being distorted. Sometimes plastic fibers or plastic clad silica fibers are also used. Plastic fiber is inexpensive, has some limitations considering UV resistance, life expectancy, humidity resistance, heat, bending sensitivity with environment.

Propagation of light :-

In optical fibers, propagation of light takes place due to total internal reflection. When light falls one end of the optical fiber; it gets refracted into the fiber. The refracted ray of light falls on the interface separating the fiber and coating an angle which is greater than the critical angle. Thus the TIR can take place. The light travels the entire length of the fiber and arrives at other ends of the fiber without any loss in its intensity even if the fiber is rounded and curved.

It has two necessary conditions:

i) The light must travel from denser to rarer.

ii) The angle of incidence must be greater than the critical angle.

Note: There are two limitations that an optical fiber has. The first one is the actual loss of light as it travels through the fiber and the other one is a maximum limit of the bandwidth of the signals that can be carried.