Michael Cermak

Synchronization of Distant Optical Clocks at the Femtosecond Level

The use of optical clocks/oscillators in future ultra-precise navigation, gravitational sensing, coherent arrays, and relativity experiments will require time comparison and synchronization over terrestrial or satellite free-space links. Here we demonstrate full unambiguous synchronization of two optical timescales across a free-space link. The time deviation between synchronized timescales is below 1 fs over durations from 0.1 s to 6500 s, despite atmospheric turbulence and kilometer-scale path length variations. Over several days, the time wander is 40 fs peak-to-peak. Our approach relies on the two-way reciprocity of a single-spatial-mode optical link, valid to below 225 attoseconds across a turbulent 4-km path. This femtosecond level of time-frequency transfer should enable optical networks using state-of-the-art optical clocks/oscillators.

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

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.

Open-path dual-comb spectroscopy to an airborne retroreflector

We demonstrate a new technique for spatial mapping of multiple atmospheric gas species. This system is based on high-precision dual-comb spectroscopy to a retroreflector mounted on a flying multi-copter. We measure the atmospheric absorption over long open-air paths to the multi-copter with comb-tooth resolution over 1.57 to 1.66 pm, covering absorption bands of CO2, Cm, H2O and isotopologues. When combined with GPS-based path length measurements, a fit of the absorption spectra retrieves the dry mixing ratios versus position. Under well-mixed atmospheric conditions, retrievals from both horizontal and vertical paths show stable mixing ratios as expected. This approach can support future boundary layer studies as well as plume detection and source location.

Optical system design for femtosecond-level synchronization of clocks

Synchronization of optical clocks via optical two-way time-frequency transfer across free-space links can result in time offsets between the two clocks below tens of femtoseconds over many hours. The complex optical system necessary to support such synchronization is described in detail here.

Regional sensing with an open-path dual comb spectroscopy and a UAS (Conference Presentation)

The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here we show that this novel spectroscopy source can be employed for regional (~kilometer squared) monitoring using an array of stationed retros or in conjunction with an unmanned aerial systems (UAS). Both fixed and UAS systems combine the high-precision, multi-species detection capabilities of open-path DCS with the spatial scanning capabilities to enable spatial mapping of atmospheric gas concentrations. The DCS systems measure the atmospheric absorption over long, 100m to 1 km, open air paths with 0.007cm-1 resolution over 1.57 to 1.66 um, covering absorption bands of CO2, CH4, H2O and isotopologues.

Optical synchronization of clocks across a 12-km turbulent air path over a city

We demonstrate synchronization of two clocks to within femtoseconds across a 12 km air path over three days. We demonstrate adaptive-optics terminals can be used for improved link availability without degradation of the synchronization performance.

Free-space time and frequency transfer

The rapid advance in optical clocks and oscillators calls for similar advances in free-space frequency/time transfer. I will discuss our frequency-comb based system for coherent optical transfer of time and frequency over free-space links with femtosecond level stabilities and with robustness to atmospheric turbulence.