Highest-Speed Modulators Enabling High-Capacity Free Space Optical Communications at Low SNR

Free-space optical satellite-earth links often operate at the SNR limit. We show that under low SNR conditions it is more favorable to operate at higher speeds with lower order modulation formats. This follows from the Shannon capacity limit formula where the transmission bandwidth has a linear impact, whereas the SNR has a logarithmic effect. In practice, bandwidth limitations from the hardware need to be considered making capacity maximization a joint optimization. Here we experimentally substantiate these findings by e.g., comparing two 128 Gbit/s signals - encoded either as DP 64 GBd 2 PAM or a DP 32 GBd 4 PAM signal. It is found that the 64 GBd 2 PAM performs better for the same bit-error rate. To facilitate highest speed operation we employ a packaged plasmonic modulator with a 3 dB bandwidth > 110 GHz. The plasmonic modulator also enabled us to send up to 160 GBd 2 PAM signals, achieving to the best of our knowledge the highest symbol rate in any free-space optical communication link. Reaching an achievable information rate of 276 Gbit/s. Even higher rates of 424 Gbit/s were achieved by employing a DP 128 GBd 4 PAM signal. This shows that once hardware bandwidth limitations come in place it is more favorable to increase the modulation complexity. Furthermore, we have shown that plasmonic organic hybrid modulators can withstand space radiation with only minor degradations. Lastly, the conditions during the experiment have been investigated and have been shown to constitute a worst-case scenario for earth-GEO feeder links.