Applications of orthogonal frequency division multiplexing for long-haul optical transmission systems

This work focuses on various applications of OFDM in the optical communication area. OFDM attracted attention as a modulation candidate for high capacity fiber-optic transmission systems as it is ideally suited for flexi-rate transponder technology. Due to its well-defined spectral shape it is ideal for passive colorless multiplexing. In this thesis various aspects of OFDM for fiber optic transmission systems are analyzed. The first project discussed in this thesis is a real-time transmitter. A 93.8 Gb/s real-time optical OFDM transmitter with 1024-point IFFT using polarization multiplexing and 4-QAM modulation is demonstrated. Around 80% of the resources of the FPGA are used due to the high IFFT size but still a stable transmitter performance was achieved. The limited resources of FPGAs make it challenging to place and route such a high size IFFT. Next, a novel method is proposed for mitigation of nonlinear effects. The main advantage of the proposed RF-pilot tone based method is that both SPM and XPM-induced impairments are compensated for without the need of direct knowledge from co-propagating channels. Unlike common nonlinearity mitigation methods, it does not add any additional complexity. Finally, the applications of OFDM with few mode fibers are analyzed. Single mode fiber is the standard for optical transmission, but the explosively increasing capacity demand makes the researchers to look into technologies to scale beyond the capacity limits of the single mode fiber. Few mode fibers are a promising candidate for the future. Here, OFDM proves to have the least complexity among some of the well-known equalizers as a result of the analytical work. Experimental results of OFDM transmission over strongly coupled few mode fibers are demonstrated as well. 337.5-Gb/s is achieved over 130 km strongly coupled FMF with inline multi-mode amplifier.