Multilevel modulation in short-range optical links

Doctoral thesis, 2013

Power and cost efficient short-range optical links are one of the technologies enabling large scale high-performance computers and data centers. Such links typically use directly modulated vertical cavity surface emitting lasers, multi-mode fiber, direct detection and on-off keying (OOK) modulation. In this work replacement of OOK with multilevel modulation formats is studied, with possible application in future optical interconnects. Subcarrier modulation and pulse amplitude modulation (PAM) were studied because they can be implemented in systems using intensity modulation and direct detection. Previously known subcarrier modulation formats were found to use the optical power inefficiently. In this thesis optimized subcarrier formats were demonstrated experimentally, in particular a three-dimensional four-level optimized subcarrier format called on-off phase-shift keying with 0.6~dB sensitivity improvement over OOK and 2~dB improvement over subcarrier quadrature phase shift-keying at the same bit rate. Similar sensitivity improvements were demonstrated for eight- and sixteen-level subcarrier formats. PAM offers a good trade-off between complexity and sensitivity. It was demonstrated in real-time at bit-rates of up to 60~Gbps with four levels and up to 35~Gbps with eight levels. Practical aspects of its implementation, such robustness to intersymbol interference and application of forward error correction (FEC) were investigated theoretically and experimentally. It was shown that 4-PAM with FEC can double the throughput relative to OOK in the same system, while retaining similar sensitivity.