Wavefront sensing for terrestrial, underwater, and space-borne free-space optical communications

We present several solutions to problems particular to adaptive optics for free-space laser-based communications. Specifically, for scenarios where strong scintillation is present, we have developed a digital, adaptable Shack-Hartmann wavefront sensor, as well as the modal holographic wavefront sensor based on the Karhunen-Loève modes. Additionally, using the same modal basis and optimization algorithms from deep learning, we have improved upon stochastic parallel gradient descent wavefront-sensorless approach. For underwater communications, we have set up a water tank and demonstrated real-time adaptive optics in the visible. For deep-space downlinks, we have investigated several wavefront sensing modalities with respect to their robustness to very low signal-to-background ratios expected during daytime. We also present results of data transmission experiments using coherent modulation over a 400-m double-pass horizontal link.