Coherent receivers for fiber optic communications

Bandwidth hungry applications like Internet of Things, edge computing, cloud computing, cloud storage, online video classes, online gaming, webinars, etc., are increasing, and the number of telecom/internet customers is growing each day. Largely with incoherent method of transmission, the legacy fiber optic networks have met the bandwidth requirement for over two decades. Due to the exponential rise in the bandwidth requirement, it has become imperative to explore other degrees of freedom of the optical carrier for modulation, so that spectral efficiency can be increased by replacing only the transceivers at the edges of the networks. Implementation of coherent communication systems was possible after overcoming the challenges both in hardware and software. Though research in the area of coherent optical communications started much earlier than that of EDFA, it faced challenges such as (i) stabilizing frequency difference between a transmitter and receiver within close tolerances (ii) lack of availability of an optical hybrid/mixer to combine the signal and the local oscillator and (iii) digital signal processing capability to compensate for channel and receiver impairments. Further setback was the commercial implementation of EDFA-based systems. We review the suitability of different types of receivers, requiring minimal or no DSP, depending on the modulation scheme and the geographical area of coverage of the networks.