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

I. Core diameter:

• A single mode or monomode 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.

• 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.

II. Source required:

• Single mode step index fiber requires highly directional source such as LASER diode because of small core diameter.
• For multimode fiber LED can be used as light source.

III. Cut off frequency:

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

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

IV. Mode field diameter (MFD)

• MFD determines the size of light carrying portion of the fiber.

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

V. Numerical Aperture

• 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).

VI. V number

• It determines the number of guided modes which can propagate through the fiber.