Mumbai University > Electronics Engineering > Sem7 > Optical Fiber Communication

Marks: 10M

Year: May 2012

1. Numerical Aperture:

• It is defined as the light gathering capability of an optical fiber.

• It is the ability of fiber to collect the light from the source and save the light inside it by maintaining the condition of total internal reflection.

  The Numerical Aperture is given by:

  N.A= $n_1*(2∆)^{1/2}$

  or

  N.A = $\sqrt{n_1^2-n_2^2 } = sin θ_a$

  where $n_1$= refractive index of core.

  $n_2$= refractive index of cladding.

  $θ_a$= acceptance angle.

• The NA is always chosen so as to accept maximum incident light, satisfying other requirements.

2. Critical Angle:

• At a specific value of incidence angle the angle of refraction is 90°. This incidence angle is known as critical angle $θ_c$.

• At this angle of incidence the refracted ray emerges parallel to the interface between the dielectrics and this is the limiting case of refraction and the angle of incidence which is termed as critical angle.

• It is the angle above which total internal reflection takes place, which is the main principle behind transmission of light through the fiber optics.

• $θ_{c }= sin^{-1} (\frac{n_2}{n_1} )$

3. Propagating mode:

• Light is continuous wave outside any medium but when it interacts with the medium it changes its property and becomes discrete inside the fiber as shown in the figure. Single light ray split into number of rays these different rays are known as modes of fiber.

• Different values of m signify different possible launching angles which can successfully propagate in the fiber core.

• There may be N possible modes of propagation for which the rays successfully propagate through the fiber creating unique light intensity patterns around axis of core.

• The optical fiber also selects finite number of modes which satisfy the basic phase conditions. This further decreases the light accepting efficiency of fiber.

• The distribution of light in core and cladding region defines the two basic modes of propagation as bounded modes and hybrid modes.