Versatile low-complex 16-QAM optical transmitter driven by binary signals

We propose a low-complex architecture for a 16-QAM optical transmitter. The scheme is based on a single dual-drive IQ modulator with customized 80/20 splitting ratio of the input splitter of each nested MZM. In the manuscript is explained, through numerical simulations, how such kind of transmitter is able to generate offset-free optical 16-QAM signals. The transmitter is driven by four equal-amplitude binary signals only. It can be easily realized in an integrated format, such as by exploiting Silicon Photonics technology. The solution includes the possibility to finely tune the above-mentioned 80/20 splitting ratios so as to optimize the output constellation by compensating for device fabrication variabilities. Tunability of splitting ratios can be extended to all the splitters present in the scheme so as to enable, with the same transmitter, generation of more efficient 16-points constellations, such as the hexagonal 16-QAM. In order to make feasible the fabrication of the proposed transmitter, a tunable splitter design which exploits Silicon Photonics technology is included in the manuscript.