OPTOELECTRONICS LETTERS Vol.14 No.5, 1 September 2018 Coherent quadrature phase shift keying optical com- munication systems Fady I. El-Nahal* Institute for Communications Engineering, Technical University of Munich, Munich 80290, Germany 1 (Received 4 March 2018; Revised 9 April 2018) ©Tianjin University of Technology and Springer-Verlag GmbH Germany, part of Springer Nature 2018 2 Coherent optical fiber communications for data rates of 100 Gbit/s and beyond have recently been studied extensively because high sensitivity of coherent receivers could extend the transmission distance. Spectrally efficient modulation techniques such as M-ary phase shift keying (PSK) can be employed for coherent optical links. The integration of mul- ti-level modulation formats based on coherent technologies with wavelength-division multiplexed (WDM) systems is vital to meet the aggregate bandwidth demand. This paper reviews coherent quadrature PSK (QPSK) systems to scale the network capacity and maximum reach of coherent optical communication systems to accommodate traffic growth. Document code: A Article ID: 1673-1905(2018)05-0372-4 DOI https://doi.org/10.1007/s11801-018-8032-y * E-mail: fady20@gmail.com The increase in the transmission capacity of wave- length-division multiplexed (WDM) systems has resulted great interest in multi-level modulation formats based on coherent technologies to meet the ever-increasing band- width demand [1-4] . Coherent optical communication sys- tems have great prospective for upgrading the capacity of currently deployed optical fiber links. The demonstration of digital carrier phase estimation in coherent receivers has resulted in great interest in coherent optical commu- nications [2] . Spectrally efficient modulation techniques known from wired or wireless communication systems such as M-ary phase shift keying (PSK), quadrature am- plitude modulation (QAM) and coherent optical or- thogonal frequency division multiplexing (CO-OFDM) can be employed for coherent optical links [5,6] . Modulation formats with k bits of information per symbol can achieve a spectral efficiency of up to k bit/ s/Hz/polarization compared with 1 bit/s/Hz/polarization for binary modulation formats. For instance, modulation formats with 2 bits of information per symbol such as QPSK can realize up to 2 bit/s/Hz/polarization of spec- tral efficiency using half the symbol rate while keeping the bit rate. The reduced symbol rate offers numerous gains with regard to tolerance to chromatic dispersion and polarization-mode dispersion (PMD). Moreover, since the phase information is preserved after detection, linear equalization methods can be used to compensate linear optical impairments, such as chromatic dispersion and PMD. In addition, advanced forward error correction (FEC) techniques can be applied to increase reach and robustness of optical communication systems. These aforementioned advantages of coherent links have a great potential to revolutionize current optical communication systems [3,7] . Phase and polarization diverse digital coherent receiv- ers have the potential to increase the capacity of current optical fiber networks, where all four optical carrier di- mensions (the in-phase and quadrature-phase compo- nents of two orthogonal polarizations) are used for mod- ulation [8,9] . Quadrature phase-shift keying (QPSK) mod- ulation/demodulation research employing optical IQ modulation (IQM) and optical delay detection has been demonstrated, where the bit rate was doubled while maintaining the symbol rate [10] . 100-Gbit/s transmission systems employing QPSK modulation, polariza- tion-division multiplexing at a symbol rate of 25 GBd, have recently been demonstrated and deployed in com- mercial networks [11] . The recent development of high-speed digital signal processing (DSP) has offered a simple and efficient means for estimating the optical carrier phase by retriev- ing the IQ components of the complex amplitude of the carrier from the homodyne detected signal [12,13] . This concept was demonstrated in Ref.[14], where a 20-Gbit/s QPSK signal was demodulated with a phase-diversity homodyne receiver followed by digital carrier-phase estimation. DSP improves system stability significantly as compared with the optical phased locked loop (OPLL) scheme [1] . Thus the integration of coherent detection and DSP has become a key part of the next generation of optical communication systems. The post signal-processing feature of the digital co- herent receiver where the IQ demodulation is a linear process is a major advantage. All the information on the complex amplitude of the optical signal is maintained