Experimental Demonstration of Flexible Modulation Formats for Optical Data Center Interconnects

Besides a high data rate and the required reach, one focus of optical transport systems for data center interconnects is a high cost efficiency. In order to achieve this, two different system concepts are presented: on the one hand, single-sideband transmission with direct detection (DD) and wavelength division multiplexing and on the other hand transmission with polarization multiplexing (PolMux) and coherent detection.

An optimization of the trade-off between reach and capacity, which is present in all communications systems, can only be achieved to a limited extent with conventional modulation formats such as pulse amplitude modulation (PAM) or quadrature amplitude modulation (QAM) due to their limited resolution. To allow non-integer rational values of bits per symbol, different flexible modulation formats are investigated: the multicarrier technique discrete multi-tone, time-domain hybrid PAM (TDH-PAM) and multidimensional PAM (MM-PAM).

The fine-granular variation of the data rate at a constant symbol rate and also the variation of the symbol rate at a fixed data rate are investigated in several experiments. For the systems with DD and optical SSB-filtering, different algorithms for the digital signal processing (DSP) are employed. Especially the compensation of nonlinear mixing terms that are caused by DD, can significantly improve the transmission quality. The Kramers-Kronig-algorithm, which allows the reconstruction of the optical field under certain conditions, has shown to be a promising approach.

A concept for the implementation of multidimensional PAM in coherent systems with PolMux is also derived and realized in several experiments. In addition, an adaptive DSP-chain is designed, which allows the compensation of different disturbances.