Advanced modulation formats for short-range optical communications

Licentiatavhandling, 2011

The demand for faster short range optical links is increasing and simple modulation techniques, such as on-o keying do not scale well to higher data rates, because of the limited bandwidth-distance product of the multimode ber. The industry has responded to this problem by aggregating multiple parallel links operating at lower bit-rate. Another possible way of improving the short range optical interconnect is to used better, more advanced modulation formats. Advanced modulation formats in the context of short range optical communication are the topic of this thesis. A broad spectrum of modulation formats have been studied, starting with pulse amplitude modulation, through conventional subcarrier modulation, to a new class of three-dimensional subcarrier modulation. Although these modulation formats are very diverse, they also have one aspect in common; they can be implemented in systems using intensity modulation and direct detection. The modulation formats were investigated experimentally in an optical link using a directly modulated vertical cavity surface emitting laser as a transmitter, photodiode for direct detection in the receiver and multimode ber as the transmission medium. The relatively simple pulse amplitude modulation was the easiest one to implement at high bit-rates; 30 Gbps error free real time transmission was demonstrated. The subacarrier modulation is more complex and required o-line signal processing, however operation at bit-rates as high as 37 Gbps were demonstrated. Finally, modulation formats for IM/DD links with improved sensitivity were investigated, showing 0.6 dB better sensitivity and the same spectral efficiency as OOK. More complex format with 8 levels and 2 dB better sensitivity than subcarrier 8-QAM was also demonstrated.