Why MA & Filter be called linear channel?

[penghan]

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

 

[flatulent]

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.