Speckle-based cloud sensing for adaptive multi-band optical satellite downlinks

Laser links between satellites and the ground can carry far more data than radio, but they have a simple weakness: clouds. Water droplets scatter the light so many times that no direct beam survives, and only a faint, diffuse glow reaches the receiver. Today the usual response is to hand over to another ground station under clear sky, based largely on weather forecasts and sky cameras. How much capacity these cost, and how often the handover was unnecessary, is not well known.

Scattered light is not only a loss. Because laser light is coherent, light that has travelled along many different paths through the cloud interferes and forms a random pattern of bright and dark grains, called speckle. This pattern flickers as the droplets move, and the way it flickers reveals how fast the channel is changing and how long it stays faded. The light carries a second clue as well: water absorbs infrared light very differently from one wavelength band to the next, and over the long winding paths inside a cloud that difference builds up. Comparing widely separated bands therefore reveals how thick the cloud is and how large its droplets are.

Biomedical optics has relied on both clues for decades, to measure blood flow and tissue properties centimeters below the skin. A cloud above a ground station is, optically, a similar problem on a much larger scale.

This PhD will bring these methods to optical satellite communication. The goal is not to see through thick clouds. It is to measure the cloud from the communication signal itself and use that measurement in two ways: to keep the optical link working under conditions where it would otherwise be abandoned, and to judge accurately – and early enough to act — when the optical link no longer offers an advantage and traffic should move elsewhere.

You will build a model of light propagation through clouds that captures both the speckle fluctuations and the wavelength-dependent absorption, and use it to map out where the useful operating region actually lies. You will develop methods to estimate the state of the cloud, and to predict where it is heading, from very weak received light. You will design a receiver that divides the incoming light across space, wavelength and time so that these measurements can be made while data is being received, and test the approach in a laboratory scattering setup. Real clouds will be characterized from the ground, by measuring the statistics of light scattered back from the cloud base.

The work combines optical communication, diffuse optics and low-light detection, with supervision spanning photonic systems at imec and diffuse optics. If you are interested in working between these fields, we encourage you to apply!