Angle-of-arrival variability of retroreflected lasers despite atmospheric reciprocity

—by Shane Walsh and David Gozzard

Researchers in the Astrophonotics Group at UWA have shown how a retroreflected laser can be used to correct for turbulent beam wander of free-space laser links.

Laser links between spacecraft and ground stations are critical for future high-precision fundamental science experiments, high-bandwidth optical communications, quantum key distribution and other applications of quantum networks.  However, atmospheric turbulence causes the laser beam to wander and scintillate, which results in extreme amplitude noise and dropouts of the laser link.

The Astrophotonics Group at UWA are applying adaptive optics systems, and other technologies adapted from astronomy, to create robust ground-to-space laser links for science and communication.  Astronomical adaptive optics systems use lasers mounted to the telescope to create artificial ``guide stars’’ in the upper atmosphere as a reference to remove scintillation.  However, they still require a real guide star to sense and correct for beam wander, because the laser follows the same path up and down and its projected position appears fixed.

We showed that this is not the case for a laser retroreflected by a corner cube, and that a retroreflected laser can serve as its own beacon for first-order adaptive optics correction.  Our result is important for research using folded free-space laser links because it allows beam wander caused by atmospheric turbulence to be corrected without the need for a powered beacon at the remote site.  Some published experiments have utilised this result, but previous analytical studies and comparisons with laser guide stars suggest it shouldn’t have worked.  Our paper describes why it does.

We are currently using this technique for our ground-to-drone free-space links, with the retroreflected laser not only being used to correct for turbulent beam wander, but also maintaining pointing to the drone as it is buffeted by wind.  These links are a precursor to ground-to-space links that will revolutionise communications, geodesy and fundamental physics.

This work is making crucial contributions to the development of the Western Australian Optical Ground Station, the first optical space communications ground station in the Southern Hemisphere, currently being commissioned at UWA with the support of EQUS and Goonhilly Earth Station.

Shane Walsh is a Research Fellow in the UWA Astrophotonics Group, which is led by EQUS Associate Investigator Sascha Schediwy.  The group also includes EQUS Associate Investigators David Gozzard and Ben Dix-Matthews.

Major funding support

Australian Research Council

The Australian Research Council Centre of Excellence for Engineered Quantum Systems (EQUS) acknowledges the Traditional Owners of Country throughout Australia and their continuing connection to lands, waters and communities. We pay our respects to Aboriginal and Torres Strait Islander cultures and to Elders past and present.