optical fibres

Step index fibre

A step index fibre is a type of optical fibre where the refractive index changes abruptly (in a ”step”) between the core and the cladding.

Refractive indexes of core vs cladding

The refractive index of the core is higher than the cladding

What are step index fibres used for

• medical instruments

• Sensors

• Data transmission

Function of the core

• central region which light travels through

• Transmits light signals

• Carries data encoded as light

Function of cladding

• keeps light inside the core

• Protects core from scratches

• Prevents signal loss

• Provides mechanical strength

Modal dispersion

the spreading out of a light pulse as it travels down an optical fibre because different light rays (modes) take different paths and therefore arrive at different times
 

Pulse broadening

• an optical pulse becomes wider in time as it travels through an optical fibre or another dispersive medium.

• The pulse may come out the other end stretched out, taking a longer time duration than when it started.

• In model dispersion different propagation modes take different paths with different travel times.

• Some parts of the pulse arrive earlier, others later this combined affect is that the originally sharp pulse becomes more Spread out.

How to limit modal dispersion

have refractive index of core and cladding close together - less difference in step index

Eg. Refractive index of core - 1.5

Refractive index of cladding - 1.4