Optimization of Multi-Carrier Modulation Techniques for Spectral Efficient Transmission Under Strong Optical Filtering. By exploiting and optimizing the concept of multi-carrier modulation technique, it is possible to reduce the impact of cascaded ROADMs on subcarrier multiplexing, which consists in strong optical filtering effects on the trasmitted signal. This thesis shows that by optimizing the number of subcarriers and by using Frequency Domain Hybrid Modulation Formats (FDHMFs) , i.e. assigning a different 2^N-QAM (N integer) modulation format to each subcarrier, and some more advanced techniques, like offline bit and/or power loading, these filtering effects can be efficiently predicted and near-optimally counteracted. Considering a rate constraint of 200G and up to 7 cascaded ROADMs, the obtained simulation results have been validated through experimental measurements..