Towards optical frequency geopotential difference measurements via a flying drone

Free space optical link to a tethered balloon for frequency transfer and chronometric geodesy

We present the results of an optical link to a corner cube on board a tethered balloon at 300 m altitude including a Tip/Tilt compensation for the balloon tracking. Our experiment measures the carrier phase of a 1542 nm laser, which is the useful signal for frequency comparison of distant clocks. An active phase noise compensation of the carrier is implemented, demonstrating a fractional frequency stability of 8 × 10-19 after 16 s averaging, which slightly (factor ∼ 3) improves on best previous links via an airborne platform. This state-of-the-art result is obtained with a transportable set-up that enables a fast field deployment.

Atmospheric turbulence characterization with simultaneous measurement of phase, angle of arrival, and intensity in a retroreflected optical link

Free-space optical transmission through the Earth's atmosphere is applicable to high-speed data transmission and optical clock comparisons, among other uses. Fluctuations in the refractive index of the atmosphere limit the performance of atmospheric optical transmission by inducing phase noise, angle-of-arrival variation, and scintillation. The statistics of these deleterious effects are predicted by models for the spatial spectrum of the atmospheric refractive index structure. We present measurements of phase fluctuations, angle-of-arrival variations, and scintillation, taken concurrently and compared with models for the atmospheric refractive index structure. The measurements are also cross-compared by deriving independent estimates of the turbulence structure constant $C_n^2$. We find agreement within an order of magnitude for derived $C_n^2$ values for all three metrics.

Stabilized free space optical frequency transfer using digitally enhanced heterodyne interferometry

Free-space continuous-wave laser interferometry using folded links has applications in precision measurement for velocimetry, vibrometry, optical communications, and verification of frequency transfer for metrology. However, prompt reflections from the transceiver optics degrade the performance of these systems, especially when the power of the returning signal is equal to or less than the power of the prompt reflections. We demonstrate phase stabilized free-space continuous-wave optical frequency transfer that exploits the auto-correlation properties of pseudo-random binary sequences to filter out prompt reflections. We show that this system significantly improves the stability and robustness of optical frequency transfer over a 750 m turbulent free-space channel, achieving a best fractional frequency stability of 8 × 10^-20 at an integration time of τ = 512 s, and cycle-slip-free periods up to 162 min.