The picture below shows a diagram of digital optical receiver.This picture comes from Fiber-Optic Communication Systems,P149.
In the diagram,the combination of main-amplifier and filter is called linear channel.But main-amplifier may be limit-amplifier,and the filter can be nonlinear,too.So,it seems it is of no sense.
One interpretation is that it is linear to digital signal.But i don't know if it is correct.
Can tell your understanding?
Thx...
Penghan
In most systems the received optical signal level is very low. The AGC amplifier gain is adjusted so that the output is at a desired level. Therefore the left and center thirds are usually operated in the linear region.