Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 05

Years: MAY 2012

The key parameters for describing the signal transmission in single mode fiber and multimode fiber are as follows:

1. Core diameter:

$\Rightarrow$ A single mode or mono mode step index fiber allows the propagation of only one traverse electromagnetic mode and hence the core diameter must be of the order of 2 µm to 10µm.

$\Rightarrow$ Multimode step index fiber with a core diameter of around 50 µm or greater allows the propagation of a finite number of guided modes along the channel.

1. Source required:

$\Rightarrow$ Single mode step index fiber requires highly directional source such as LASER diode because of small core diameter.

$\Rightarrow$ For multimode fiber LED can be used as light source.

1. Cut off frequency:

$\Rightarrow$ It is the shortest wavelength at which a fiber can support single mode operation. It is defined as:

$λ= \dfrac {2πa }{V}.NA$

There is cutoff wavelength $λ_c$ when $V=2.405 \\ ∴ λ_c= \dfrac {2πa}{2.405}.NA$

$\Rightarrow$ For single mode step index fiber $V ≥ 2.405,$ Modes are cutoff after this wavelength.

1. Mode field diameter (MFD)

$\Rightarrow$ MFD determines the size of light carrying portion of thefiber.

$\Rightarrow$ For single mode fiber MFD is very important since the light is not confined to the core diameter as in case of multimode fibers.

1. Numerical Aperture

$\Rightarrow$ For optical technology dimensions of core cladding are very small and hence it is difficult to measure acceptance angle, critical angle. Therefore all these parameters are integrated into a single parameter called as numerical aperture (NA).

1. V number

$\Rightarrow$ It determines the number of guided modes which can propagate through the fiber.