A fiber optic receiver needs to convert the incoming optical signal to an electrical signal so it can be sampled. How is this achieved in practice?
I believe a photodiode is used, but would a photodiode not have the problem that the frequency of the incoming optical signal is too high? Such high frequencies cannot propagate through the conductors.
This is what Wikipedia says:
Coherent receivers use a local oscillator laser in combination with a pair of hybrid couplers and four photodetectors per polarization, followed by high speed ADCs and digital signal processing to recover data modulated with QPSK, QAM, or OFDM.
If I am not mistaken, this implies that you interfere the incoming signal with one of a plurality of coherent optical reference signals such that each of said reference signals is equal to one possible received signal. This way you can detect whether the input signal is one of the expected values given the interference pattern. Another possibility is to have each reference signal be such that it destructively interferes with the incoming signal. But wouldn’t that limit the modulation schemes you can use? Can you use frequency modulation over fiber optics?
Any references, block diagrams etc. will be greatly appreciated.