IAN LANG ELECTRONICS

Optoelectronics

Distortion of Signals.

No signal is ever a perfect copy of the original audio that created it because of the limits of technology but some come very close. The further from perfect it is, the more distorted it is said to be. In fiber optic transmissions the distortion is caused by dispersion.
 Dispersion can be intermodal or intramodal. The first case comes from the fact that we may send two pulses of light down a core at the same time. If both are equally coherent (i.e travelling in a narrow concentrated way) and we send one as an axial ray (that is travelling down the centre of the core) and one as a meridional (being reflected from cladding to core) we have a situation like the one in fig 1 below:

Fig 1: shows graphically the paths of two rays of light down the core and cladding of optical fiber cable. Ray A is the meridional and Ray B the axial.

It becomes obvious that although both may move at the same speed they are not at the same velocity as the direction of Ray A changes frequently. Consequently , as Ray B is travelling a shorter distance in the same time, the effect is that the pulses of light will "spread" as Ray A is overtaken by Ray B.
Eventually they will blend together and the information will be lost. The "spreading" is more formally known as dispersion and because this is between two modes (a mode meaning nothing more than a method) the dispersion is intermodal. Any fibre carrying more than one mode is termed "multimodal".
It is important to realise that what we have just looked at is a graphic model to aid comprehension. In reality the light is a pattern of electrical and magnetic fields propagated along the fibre and the blending comes from mutual interference between the two.It is also important to realise that here we have ignored the effects of intramodal dispersion altogether.

We can counter intermodal dispersion simply by propagating only one mode, the axial, or by leaving longer pauses between transmission modes, or by using a graded index fibre. We will study this more deeply in Q9: Aspects of Optical Transmission.