Coherent optical FSK transmission systems

Abstract

Coherent optical systems are getting much closer to commercial realizations than expected only few years ago. Front‐running candidates for simplicity and effectiveness are FSK communication systems. Several reasons stand in favour of this system: most evident are direct modulation of semiconductor lasers, generation of two‐lobe spectrum or compact spectrum by simply setting the modulation index, possibility of envelope or discriminator detection, full compatibility with optical amplifiers due to constant optical envelope, direct application to subcar‐rier multiplexing with analog optical FM. In this paper we describe the basic FSK trasmission system, presenting the relative problems and illustrating possible different solutions for transmission and for the receiver, and concentrating on specific subsystems time by time, such as line coder, automatic frequency control, receiver balanced front‐end, post‐detection pre‐amplifier, discriminator and envelope detection, from both theoretical and implementation points of view.