High capacity coherent optical systems: Advanced modulation formats and margins for transmission impairments

Summary form only given. Laser phase noise from the transmitter and Local Oscillator lasers strongly limits the performance of high capacity, high constellation coherent optical transmission systems. Using Digital Signal Processing techniques it is possible to mitigate the influence with specific form of the DSP for different modulation formats. In this paper we will present a number of possible mitigation techniques, applicable for such modulation schemes as N-level PSK and N-level QAM. It will specifically be shown that the circular QAM configuration has inherent significant added phase noise tolerance compared to the classical square configuration. The research has also been performed considering the so-called Equalization Enhanced Phase Noise, which appears as an added phase noise based impairment for coherent transmission systems with optically induced dispersion and digital (electrical) dispersion compensation. A novel understanding of the origin of EEPN and of straightforward mitigation techniques will be presented.Novel detailed theoretical studies presented in [3] shows: 1) EEPN results from the non-linear inter-mixing between the sidebands of the dispersed signal and the noise sidebands of the local oscillator; 2) The transmission penalty is mainly due to the slow optical frequency fluctuations of the local oscillator; 3) The required cut-off frequency increases linearly with the white frequency noise level and hence the linewidth of the local oscillator laser, but is virtually independent of the symbol rate and the accumulated dispersion. Circular QAM allows straightforward Viterbi-Viterbi implementation of carrier phase extraction in the coherent Rx and is a good choice for systems impaired by phase noise. New fundamental result: EEPN mitigation can be fully obtained using slow low bandwidth digital coherent enhancement (DCE) implementation in the Rx or using electrical feedback of the LO - due to the inherent dependency of EEPN on only the low frequency part of the laser phase noise. Thus straightforward elimination of EEPN effects is possible in future “long distance” transmission of high capacity coherent systems.