Define intermodal dispersion?
Dispersion of the transmitted optical signal causes distortion for both digital and analog transmitted signal. The dispersion causes broadening of the transmitted light pulse as they travel along the fibre. Ie, every pulse broadens and overlaps along with neighbors, eventually become indistinguishable at the receiver. This effect is called as inter symbol interference (ISI). The dispersion is separated within two:
• Intermodal dispersion
• Intra modal dispersion
Intermodal dispersion is caused through number of modes guided through the fibre. It is therefore prominent in multimode fibres, as different modes that constitute a pulse in multimode fibre travels along the channel at various group velocities, the pulse width at the output is depends upon the transmission times of the slowest and fastest modes.
The multimode step index fibre exhibit a huge amount of intermodal dispersion that results in greater pulse broadening. Thus the intermodal dispersion for the multimode fibres might be decreased through the adoption of an optimum refractive index profile which is given through the near parabolic profile of most graded index fibres. A purely single mode fibre has no intermodal dispersion and thus pulse broadening is due solely because of the intra modal dispersion. Thus the transmission bandwidth of single mode fibre is in GHz range, whereas the transmission through multimode step index fibre is commonly limited to bandwidth of a few tens of MHz.