ABSTRACT
Chapter 7 has presented the detection of optical signals by direct conversion of optical signal power into electronic current in the photodiode (PD) and then electronic preampli—cation.This chapter describes the fundamentals of coherent receivers including the detection of optical signals by mixing optical —elds with that of alocal oscillator (LO), a higher amplitude laser of the same or different frequency so that their beating product would result in demodulated signals preserving the signals in both phase and amplitude characteristics in the electronic domain. When the frequencies of the signal and LO are the same, then the reception is ahomodyne type. If they are different, then the process is aheterodyne type. Under practical homodyne reception, there would be some small frequencydifference and this is normally termed as intradyne reception. Optical preampli-—cationfor coherent detection is treated in Chapter 9. The electrical output of the coherent reception stage is then converted into the digital domain via an analog-to-digital converter (ADC) and then processed in digital signal processing (DSP) systems. This type of optical receiver is termed as the Digital Coherent Optical Receiver.
