• In step index fiber, the refractive index of core is uniform throughout and undergoes an abrupt change (step change) at the core cladding interface.

• The refractive index profile is defined as:

• Depending upon number of modes propagated by the fiber we have single mode and multimode step index fibers.

I. Single mode step index fiber.

• 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.

• It has high information carrying capacity.

• The single mode step index fiber has the distinct advantage of low intermodal dispersion, as only one mode is transmitted.

• It can be used for long distance communication

• It requires highly directional source such as LASER diode because of small core diameter, hence the cost is higher than multimode step index fiber.

II. Multimode step index fiber.

• 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.

• The total number of guided modes Ms for a step index fiber is given by

$$Ms = \frac{V^2}2$$

Where, V denotes the V-number which is a normalized frequency parameter.

• In multimode step index fiber dispersion is high due to the differing group velocities of the propagating modes.

• Due to high dispersion the information carrying capacity is less.

• It cannot be used for long distance communication as dispersion is a function of length.

• Simple to fabricate than single mode fiber and LED can be used as light source.