The single mode fibre non-linear effects is a major source of losses in the optical millimeter wave RoF systems. In a typical OMMW-RoF system which operates at a single wavelengths (means XPM and FWM don't have any effects) which of the two SPM and SBS becomes dominant in the system and why?
SBS will become dominant and will impose stringent power limit and propagation distance along the fiber.
The effects are quite different. SBS leads to the reflection of optical power above a certain threshold. The threshold depends on the fibre and the source. If the source is modulated at a frequency much higher than the SBS linewidth then the threshold is much larger as compared to a perfect zero-linewidth source. Typical numbers might be a few mW for the (zero-linewidth) threshold and 100MHz for the SBS linewidth. So SBS in most high bit rate systems is not seen.
If you do not see SBS, then SPM is all that is left. This will be significant when the power times the effective length (i.e. the length adjusted for the effect of loss) is about 1.
I am not familiar with RoF technology, but I would guess that the fiber lengths are of metropolitan magnitude, meaning a few tens of Km at most. I would also guess that the power levels used would not be very high, also because of the relatively short distances the signal has to travel. If the above are true, SPM would not be a significant factor. On the other hand, if I am not mistaken, SBS would be rather important for a CW or quasi-CW carrier wave. In that case the SBS effect would be the dominant one.
sbs WILL RESULT in the moduulation of the waves with envelop of around 100MHz - SPM forces a phase change on the lightwaves which carry the modulated signals. This in turn will results in the change of the change of the group velocity and hence distortion of teh modulated envelop.
Thank you very much Prof. Blow for your comment. There are a few points I have noticed that if you may explain.
1) You mentioned it depends on the fibre and the source. How does it depends to the source or the fibre? you mean multimode mode fibre for instance might be different from single mode?
2) Does external or direct modulation makes any difference in SBS effect?
3) Can you please introduce me any references or books on this matter?
You advice is very helpful.
Many thanks
It depends on the fibre through properties like the mode area and the SBS line width. These things will determine the exact value of the SBS threshold. The formula can be found here: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CDsQFjAB&url=http%3A%2F%2Fwww.jdsu.com%2Fproductliterature%2Fbrillouinscattering_tn_cms_ae_0306.pdf&ei=5rz3UZ3JEIqs0QXkw4C4Bw&usg=AFQjCNGpXx_9Fo8yVU_wSwcnxDhly7zw2w&sig2=rb-1Iukn5EeymuzSOBMnpA&bvm=bv.49967636,d.d2k
How the modulation is applied is not important, the thing that matters is the ratio of the modulation bandwidth to the SBS line width.
Most books on nonlinear optics will cover SBS. Try "The Elements of Nonlinear Optics" by Butcher and Cotter.
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