Coherent systems are regaining interest due to the availability of digital signal processing [1] and low-priced components as well as the partly relaxed receiver requirements at high data rates. Coherent detection provides several advantages. The preservation of the temporal phase enables new methods for adaptive electronic compensation of chromatic dispersion. When concerning WDM systems coherent receivers offer tunability and allow channel separation via steep electrical filtering. Furthermore, only coherent detection permits to reach the ultimate limits of spectral efficiency [2]. To converge to these limits further development of optical multi-level modulation systems is needed. Concerning multi-level modulation, coherent systems are very beneficial because all the information of the optical field is available in the electrical domain and the demodulation can be performed electrically, avoiding complex optical interferometric demodulation needed in direct detection (DD) systems. Considering spectral efficiency for WDM and practical feasibility for high data rates, homodyne receivers are superior to their heterodyne counterpart and seem to be the right choice for future networks.However, homodyne receivers are sensitive to laser phase noise. So far used methods to handle the phase noise are phase diversity (not applicable for high-order modulation formats) or the implementation of an optical phase locked loop (stringent requirements on laser linewidth and loop delay). Based on the model of electrical and wireless systems and due to the recent availability of high speed signal processing, a new option arises to cope with the phase noise in optical homodyne receivers. That is digital phase estimation. From the samples of the received in-phase and quadrature signals the phase error can be calculated and the constellation diagram can be rotated into the right position. Phase Estimation leads to more relaxed linewidth requirements, which was already proposed in [1] and [3] for QPSK modulation.This paper investigates an optical coherent system with Square-16-QAM modulation. Such a system is innovative for optical systems and is one step ahead to the just investigated DQPSK-DD and 8-DPSK-DD modulation formats. Different optical transmitters for Square-16-QAM are compared with respect to optical and electrical complexity and concerning the properties of the optical output signals. Furthermore the homodyne receiver is illustrated, with special attention to the algorithm for Square-16-QAM phase estimation. Challenges for the practical realization, as the electrical driving of the modulators in the transmitter as well as the feasibility of the necessary 2x4-hybrid and the implementation of digital phase estimation in hardware at the receiver side, are discussed. This work covers theoretical considerations, numerical simulations and experimental results which are used to estimate the performance of those systems.References:[1] D.-S. Ly-Gagnon et al., "Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal", OFC 2005, Anaheim, paper OTuL4.[2] J.M. Kahn, K.P. Ho, "Spectral Efficiency Limits and Modulation/Detection Techniques for DWDM Systems", IEEE Journal of Selected Topics in Quantum Electronics, Vol. 10, No. 2, pp. 259-272, 2004[3] R. Noé, "PLL-Free Synchronous QPSK Receiver Concept with Digital I&Q Baseband Processing", in Proc. 30th European Conference on Optical Communication ECOC 2004, paper We4.P.120

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