Synchronization of clocks through 12 km of strongly turbulent air over a city

Abstract

We demonstrate real-time, femtosecond-level clock synchronization across a low-lying, strongly turbulent, 12-km horizontal air path by optical two-way time transfer. For this long horizontal free-space path, the integrated turbulence extends well into the strong turbulence regime corresponding to multiple scattering with a Rytov variance up to 7 and with the number of signal interruptions exceeding 100 per second. Nevertheless, optical two-way time transfer is used to synchronize a remote clock to a master clock with femtosecond-level agreement and with a relative time deviation dropping as low as a few hundred attoseconds. Synchronization is shown for a remote clock based on either an optical or microwave oscillator and using either tip-tilt or adaptive-optics free-space optical terminals. The performance is unaltered from optical two-way time transfer in weak turbulence across short links. These results confirm that the two-way reciprocity of the free-space time-of-flight is maintained both under strong turbulence and with the use of adaptive optics. The demonstrated robustness of optical two-way time transfer against strong turbulence and its compatibility with adaptive optics is encouraging for future femtosecond clock synchronization over very long distance ground-to-air free-space paths.

Femtosecond time synchronization of optical clocks off of a flying quadcopter

“…Assuming successful suppression of this velocity-induced bias and following similar analysis as in Ref. 14…”

“…Such networks will need to compare and synchronize clocks over free-space optical links between moving airborne or satellite-borne clocks. However, current comb-based optical two-way time-frequency transfer (O-TWTFT) [13][14][15] cannot support femtosecond clock synchronization in the presence of motion. Even modest closing velocities between clocks lead to many picoseconds of non-reciprocity in the twoway optical time-of-flight and correspondingly large time synchronization errors.…”

“…These turbulence-induced effects are far less for rf links, because of the longer wavelength, or for fiber-optic links, because of the stable medium. Previous comb-based O-TWTFT has nevertheless overcome these turbulence effects to achieve femtosecond synchronization [13][14][15] . Here, we focus on the second critical challenge.…”

Measurement of the impact of turbulence anisoplanatism on precision free-space optical time transfer

“…They must also rely heavily on the reciprocity, or equal time-of-flight, inherent in optical bi-directional singlemode links even across highly turbulent air [11] in order to cancel out variations in the time-offlight. Recent comb-based optical two-way time-frequency transfer (O-TWTFT) has verified this reciprocity down to levels of 100 attoseconds in time and levels of 10 -19 or below in fractional frequency [7,9,12]. However, if the clock platforms are moving, this reciprocity will break down.…”

Comparing Optical Oscillators across the Air to Milliradians in Phase and 10−17 in Frequency

“…This approach exploits the reciprocity (equality) in the time-of-flight for light to travel each direction across a single-mode link [36], just as in rf-based two-way satellite time-frequency transfer [37-39] and analogous fiber-optic demonstrations [23,[40][41][42]. In previous work, this OTWTFT approach used the arrival time of the frequency comb pulses to support frequency comparisons at residual instabilities of ~4×10 -16 at 1-second averaging times [29], and ultimately to enable sub-femtosecond time synchronization of distant optical and microwave-based clocks [32][33][34].Here, we demonstrate OTWTFT can exploit the carrier phase of the frequency comb pulses for much higher performance. While carrier-phase measurements are relatively straightforward across optical fiber because of the uninterrupted stable signal, the same is not true of a free-space link where atmospheric turbulence leads to strong phase noise and signal intermittency, in turn presenting a severe challenge to "unwrapping" the measured phase without catastrophic ±π phase errors.…”