[Jenda_]

> because optical attenuation rises exponentially with length

I believe the attenuation is stated in dB/km, therefore rises linearly (or even logarithmically if you look at it from the uncommon energy-wise point of view) with distance. Why should exponential be the case?

[retrac]

Imagine a length of fibre that transmits 90% of the light put in. Take the output, and pass it through another equal length. At the output of the second fibre length, we have 90% of 90%.