Recent Patents on Engineering 2011, 5, 1-16 1 1872-2121/11 $100.00+.00 © 2011 Bentham Science Publishers Ltd. Coherent Optical Systems: Principles, Contemporary Implementations and Future Challenges for High Data Rate Communication Abid Munir*, Xin Xiangjun, Liu Bo, Ashiq Hussain, Abdul Latif and Aftab Hussain School of Electronics, Beijing University of Posts and Telecommunications, Beijing, China Received: August 18, 2010; Accepted: December 8, 2010; Revised: December 7, 2010 Abstract: This paper is a tutorial review for coherent optical communication with emphasis on digital coherent receivers because digital coherent optical receivers are promising solution for next generation optical communication systems. In this paper, principles of coherent communication has been reviewed along with its advantages and prospective modulation formats to achieve 111 Gbps. Digital coherent receivers and their signal processing modules have been reviewed in the light of recent publications and patents. In the end, realization challenges have been addressed from circuit and signal processing point of view. Keywords: Polarization diversity, phase diversity, polarization multiplexing, chromatic dispersion, polarization mode disper- sion, adaptive equalization, photonic integrated circuits. SECTION I: INTRODUCTION TO COHERENT COMMUNICATION SYSTEMS Due to ever increasing demand of higher data rates and more sophisticated transmission networks, research in opti- cal transmission networks is highly dedicated towards coher- ent optical systems. In comparison with legacy IM/DD (in- tensity modulation/direct detection) systems, coherent opti- cal systems promise better receiver sensitivity with inherent capability of multilevel modulations to increase data rates by many folds. Although realization of coherent systems is a tougher challenge, recent advances in optical and electronics devices and signal processing techniques made it viable to realize the systems on practical basis. In this paper, we will go through coherent communication principles in perspective of its advantages, recent reported achievements and chal- lenges to field deployment. This paper is organized in five sections. Section I gives an introduction which comprises principles behind coherent optical systems with some vari- ants of this mode of transmission. Section II presents a brief analysis of advantages of coherent systems with prospective modulation techniques to achieve higher data rates. Section III focuses on digital coherent receivers and an overview of recent works. Section IV covers the challenges to meet higher data rates of 100Gbps and beyond with conclusion in section V. a) Principle of Coherent Optical Communication Fundamental principle of coherent communication is mixing of a local signal with incoming received signal to translate all the modulation information from optical signal to electrical domain. Coherent transmission system is equipped with a light source at receiver to add light in the *Address correspondence to this author at the School of Electronics. Beijing University of Posts and Telecommunications, Beijing, China; Fax: +8615010116372; E-mail: abid.muneer@googlemail.com incoming received signal before detection to increase the signal strength, hence improves the sensitivity of the re- ceiver. Appropriate tuning of local oscillator power can de- crease the impact of white noise to improve the efficiency of receiver [1]. This section describes the basic principle of coherent receivers. In single ended receiver (Fig. 1), received electric field signal is written as in following equation E rcv (t ) = [ A s (t )e j s ( t ) + n x (t )]e j c t ) x + n y (t )e j c t ) y (1) Where A s (t ) and  s (t ) are the modulated amplitude and phase of the optical signal and n x (t ) and n y (t ) are noise components in x and y polarizations of the signal. Similarly we write the local oscillator signal which is E LO (t ) = [ A LO + n L (t )]e j LO t ) x (2) LO A is the continuous wave amplitude of local oscillator and n L (t ) is the noise of local oscillator which includes rela- tive intensity noise of the laser or amplifier noise. For sim- plicity, considering the local oscillator is aligned to the re- ceived signal in x polarization. Noise in orthogonal polariza- tion may be filtered out by a polarizer [2]. By using a 3dB Fig. (1). Single ended receiver.