Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Low.

Construction of optical Fiber:

An optical fiber has in general three coaxial regions. The innermost region is the light guiding region known as core. It is surrounded by middle coaxial region cladding. The outer most region is called as Sheath. The refractive index of cladding is always less than core. The purpose of cladding is to make the light confined to the core. Light launched into the core and striking the core cladding interface at an angle greater than critical angle will be reflected back into the core. Since the angle of incidence and the angle of reflection are equal, light continue to rebound and propagate through the fiber. Sheath protect the cladding and core from contamination, moisture. It also gives mechanical strength to the fiber.

Total internal reflection:

When a ray passes from a denser medium to rarer medium. It bends away from the normal. As the angle of incidence increases, refracted ray bents more and more away from the normal. At a particular angle of incidence say i = ∅_c refracted ray glides the surface r = 90 degree

We know from Snell`s law sin i / sin⁡r = $n_2$ / $n_1$

Therefore $sin∅_c$ = $n_2$ / $n_1$

At any angle greater than $∅_c$, the ray is reflected back into same medium as if it had encountered a perfectly reflecting surface. Such a phenomenon in which light is totally reflected from denser medium to rarer medium is called as total internal reflection.

Numerical Aperture:

The main function of an optical fiber is to accept and transmit as much as light from the source as possible. The light gathering ability of fiber is called as Numerical aperture and it is determined by the acceptance angle. It is measure of amount of light that can be accepted by fiber N.A depends on R.I of core and cladding. N.A= √($n_1^2$ - $n_2^2$ )

Acceptance angle:

The maximum angle of incidence for which the ray propagates down the fiber is called as acceptance angle.

$i_m$ =$sin^{-1} ⁡√($n_1^2$ - $n_2^2$ )