Multi-wavelength coherent optical transmission using higher order modulation formats is an important focus area in optical communication systems research and development. To fulfill the ever increasing demand for higher transport capacity at reasonable implementation complexity Kemal et. al. propose a concept that uses optical frequency combs both at the transmitter and at the receiver. At the transmitter, the frequency comb acts as a precise multi-wavelength source. At the receiver, however, the frequency comb acts as a multi-wavelength optical local oscillator for coherent detection. In this detailed contribution, the authors describe the setup of the frequency comb and the synchronization method. Both frequency combs are based on gain switched injection locking of distributed feedback lasers where the receiver frequency comb is synchronized to the transmitter with respect to center frequency and free spectral range. A successful synchronization concept is essential for this scheme if it should be transferred to commercial systems. Furthermore, the authors describe a proof of concept transmission experiment. They could achieve an intradyne frequency below 15 MHz and a data rate above 1Tbit/s in 13 wavelength channels. Modulation formats of 16 QAM and QPSK have been used leading to a spectral efficiency of 5.16 Bit/s·Hz.
Besides the demonstration of the scientific progress, a sound discussion of implementation issues and a critical view on their own work emphasizes the high quality of this paper. Even if the presented transmission distance of 10km is rather short, we see a very important demonstration of frequency combs in high capacity coherent transmission systems having the potential to facilitate signal processing in coherent receivers. Thus, the concept has a high potential to be found in future commercial systems.
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