Hybrid QAM transmission techniques for single-carrier ultra-dense WDM systems

We demonstrate a hybrid QAM transmission technique for single-carrier ultra-dense wavelength division multiplexing (UD-WDM) systems. With 112-Gb/s per channel on 25-GHz grid, we found the hybrid 4 and 8QAM technique shows a much better performance than the pure 4QAM and exhibits an ~0.9-dB OSNR benefit over the pure 8QAM.