Re: Dispersion in an optical fibre

brobigi@xxxxxxxxx wrote:
Hello experts,

In one article, I have read that dispersion in optical fibre can cause
"time stretch" of a signal that is transmitted. I wonder how this is
possible. I know that dispersion is a phenomenon that causing phase
velocity to depends on frequency of a signal. Because of that I assume
that signals with different frequencies will have different velocities.
Also, if one signal is transmitted, dispersion will cause different
spectral components to propagate withe different velocities.
I must admit I don't really understand how signal is time stretched. Can
you give me any simple explanations?

Thank you

Here are four possible reasons, put simply as requested; but unfortunately there are more than four types of dispersion.

First, different wavelengths travel at different speeds because the refractive index of fibre is not constant with respect to wavelength (the speed of light in a medium of refractive index I is c/I).

Second, photons which enter eg a straight fibre slightly off-axis will bounce off the sides of the fibre more often than photons of the same wavelength which enter on-axis, and will have further to travel.

Third, larger wavelengths will have more of their waveforms travelling partly in the outer part of the fiber, where it travels faster than slower wavelengths.

Fourth, light waves traveling in a fibre self-reinforce, forming static waves a bit like the sound waves in an organ pipe. The bursts of waves travelling in different patterns tend to travel at different speeds.



I have no idea whether any of these has anything to do with dispersion in reality though, just a SWAG. :)



-- Peter Fairbrother
.

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