Towards optical frequency geopotential difference measurements via a flying drone.

Geopotential and orthometric height differences between distant points can be measured via timescale comparisons between atomic clocks. Modern optical atomic clocks achieve statistical uncertainties on the order of 10, allowing height differences of around 1 cm to be measured. Frequency transfer via free-space optical links will be needed for measurements where linking the clocks via optical fiber is not possible, but requires line of sight between the clock locations, which is not always practical due to local terrain or over long distances.

Demonstration of 100 Gbps coherent free-space optical communications at LEO tracking rates.

Free-space optical communications are poised to alleviate the data-flow bottleneck experienced by spacecraft as traditional radio frequencies reach their practical limit. While enabling orders-of-magnitude gains in data rates, optical signals impose much stricter pointing requirements and are strongly affected by atmospheric turbulence. Coherent detection methods, which capitalize fully on the available degrees of freedom to maximize data capacity, have the added complication of needing to couple the received signal into single-mode fiber.

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

Corner cube retroreflectors are commonly used as cooperative targets in free-space laser applications. The previous literature suggests that due to path reciprocity, a retroreflected beam is self-corrected across a turbulent atmosphere and should show no angle-of-arrival variability in the near field. This is at odds with recent experiments that rely on angle-of-arrival measurements in retroreflected beams for effective tip/tilt correction.

Point-to-point stabilized optical frequency transfer with active optics.

Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied sciences. However, atmospheric turbulence creates phase noise and beam wander that degrade the measurement precision. Here we report on phase-stabilized optical frequency transfer over a 265 m horizontal point-to-point free-space link between optical terminals with active tip-tilt mirrors to suppress beam wander, in a compact, human-portable set-up.