24 January 2011 Advances in coherent optical modems and 16-QAM transmission with feedforward carrier recovery
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Abstract
Polarization multiplexing and quadrature phase shift keying (QPSK) both double spectral efficiency. Combined with synchronous coherent polarization diverse intradyne receivers this modulation format is ultra-robust and cost-efficient. A feedforward carrier recovery is required in order to tolerate phase noise of normal DFB lasers. Signal processing in the digital domain permits compensation of at least chromatic and polarization mode dispersion. Some companies have products on the market, others are working on them. For 100 GbE transmission, 50 GHz channel spacing is sufficient. 16ary quadrature amplitude modulation (16-QAM) is attractive to double capacity once more, possibly in a modulation format flexible transponder which is switched down to QPSK only if system margin is too low. For 16-QAM the phase noise problem is sharply increased. However, also here a feedforward carrier recovery has been implemented. A number of carrier phase angles is tested in parallel, and the recovered data is selected for that phase angle where squared distance of recovered data to the nearest constellation point, averaged over a number of symbols, is minimum. An intradyne/selfhomodyne synchronous coherent 16-QAM experiment (2.5 Gb/s, 81 km) is presented.
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Reinhold Noé, Reinhold Noé, Sebastian Hoffmann, Sebastian Hoffmann, Christian Wördehoff, Christian Wördehoff, Ali Al-Bermani, Ali Al-Bermani, Mohamed El-Darawy, Mohamed El-Darawy, } "Advances in coherent optical modems and 16-QAM transmission with feedforward carrier recovery", Proc. SPIE 7960, Coherent Optical Communication: Components, Subsystems, and Systems, 79600L (24 January 2011); doi: 10.1117/12.881542; https://doi.org/10.1117/12.881542
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