The differential signalling only helps if the noise offsets both signals in the same direction though. If it just attenuates (which fog for example will do to o

The differential signalling only helps if the noise offsets both signals in the same direction though. If it just attenuates (which fog for example will do to optical signals), it won’t help you. And the other method you described could work (depending on the SNR of course, at some point effectively no signal will come through the fog) but will lower data rate.