Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied science, from measurements of fundamental constants and searches for dark matter, to geophysics and environmental monitoring. However, turbulence in the atmosphere creates phase noise on the laser signal, greatly degrading the precision of the measurements, and also induces scintillation and beam wander which cause periodic deep fades and loss of signal. We demonstrate 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. A phase stabilized 715 m underground optical fiber link between the two terminals is used to measure the performance of the free-space link. The active optics terminals enabled continuous, coherent transmission over periods of up to an hour. We achieve an 80 dB suppression of atmospheric phase noise to $3\times10^{-6}$ rad$^{2}$Hz$^{-1}$ at 1 Hz, and an ultimate fractional frequency stability of $1.6\times10^{-19}$ after 40 s of integration. At high frequency this performance is limited by the residual atmospheric noise after compensation and the frequency noise of the laser seen through the unequal delays of the free space and fiber links. Our long term stability is limited by the thermal shielding of the phase stabilization system. We achieve residual instabilities below those of the best optical atomic clocks, ensuring clock-limited frequency comparison over turbulent free-space links.

/pdf/point-to-point-stabilised-optical-frequency-transfer-with-3ov2q4ibec.pdf

Point-to-Point Stabilised Optical Frequency Transfer with Active Optics

A global network of optical atomic clocks will enable unprecedented measurement precision in fields including tests of fundamental physics, dark matter searches, geodesy, and navigation. Free-space laser links through the turbulent atmosphere are needed to fully exploit this global network, by enabling comparisons to airborne and spaceborne clocks. We demonstrate frequency transfer over a 2.4 km atmospheric link with turbulence similar to that of a ground-to-space link, achieving a fractional frequency stability of 6.1E-21 in 300 s of integration time. We also show that clock comparison between ground and low Earth orbit will be limited by the stability of the clocks themselves after only a few seconds of integration. This significantly advances the technologies needed to realize a global timescale network of optical atomic clocks.

Ultra-stable Free-Space Laser Links for a Global Network of Optical Atomic Clocks

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock signal dissemination and physical constants measurement. Several demonstrations have been made successfully over fiber networks, but not much work has been done yet through the open air where atmospheric turbulence prevails. Here, via an 18 km outdoor link, we transmit multiple optical carriers that are extracted directly from a near-infrared frequency comb over a 4.2 THz spectral range in stabilization to a high-finesse cavity with a 1.5 Hz linewidth. Proof-of-concept experiments show that the comb-rooted optical carriers are transferred in parallel with collective suppression of atmospheric phase noise to -80 dBc/Hz. In addition, microwaves can also be delivered by pairing two separate optical carriers bound with inter-comb-mode coherence, e.g. a 10 GHz signal with phase noise of -105 dBc/Hz at 1 Hz offset. Further, an add-on demonstration is made on multi-channel coherent optical communications with the potential of multi-Tbps data transmission in free space.

/pdf/free-space-transfer-of-comb-rooted-optical-frequencies-over-4mq3eelijz.pdf

Free-space transfer of comb-rooted optical frequencies over an 18 km open-air link

We demonstrate frequency-comb-based optical two-way time-frequency transfer across a three-node clock network. A fielded, bidirectional relay node connects laboratory-based master and end nodes, allowing the network to span 28 km of turbulent outdoor air while keeping optical transmit powers below 5 mW. Despite the comparatively high instability of the free-running local oscillator at the relay node, the network transfers frequency with fractional precision below 10−18 at averaging times above 200 s and transfers time with a time deviation below 1 fs at averaging times between 1 s and 1 h. The successful operation of this network represents a promising step toward the operation of future free-space networks of optical atomic clocks.

Optical time-frequency transfer across a free-space, three-node network

Future free-space optical clock networks will require optical links for time and frequency transfer. In many potential realizations of these networks, these links will extend over long distances and will span moving platforms, e.g., ground-to-air or ground-to-satellite. In these cases, the transverse platform motion coupled with spatial variations in atmospheric optical turbulence will lead to a breakdown in the time-of-flight reciprocity upon which optical two-way time-frequency transfer is based. Here, we report experimental measurements of this effect by use of comb-based optical two-way time-frequency transfer over two spatially separated optical links. We find only a modest degradation in the time synchronization and frequency syntonization between two sites, in good agreement with theory. Based on this agreement, we can extrapolate this 2-km result to longer distances, finding only a few-fs timing noise increase due to turbulence for a link from ground to a midearth orbit satellite.

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

High-precision optical frequency transfer through free-space links will enable advances in fields ranging from coherent optical communication and satellite Doppler ranging, to tests of general relativity and fundamental physics. Previous work has focused on two-way time and frequency transfer techniques using optical frequency combs. Here the authors show that a continuous-wave coherent link can exceed the fractional frequency stability of frequency-comb methods by an order of magnitude. They also highlight and discuss the technical hurdles to implementing similar links over the distances required to reach low Earth orbit.

Stabilized Free-Space Optical Frequency Transfer

The transfer of high-precision optical frequency signals over free-space links, particularly between ground stations and satellites, will enable advances in fields ranging from coherent optical communications and satellite Doppler ranging to tests of General Relativity and fundamental physics. We present results for the actively stabilized coherent phase transfer of a 193 THz continuous wave optical frequency signal over horizontal free-space links 150 m and 600 m in length. Over the 600 m link we achieved a fractional frequency stability of 8.9e-18 at one second of integration time, improving to 1.3e-18 at an integration time of 64 s, suitable for transmission of optical atomic clock signals. The achievable transfer distance is limited by deep-fading of the transmitted signal due to atmospheric turbulence. We also estimate the expected additional degradation in stability performance for frequency transfer to Low Earth Orbit.

Stabilized free-space optical frequency transfer

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Stabilized Free-Space Optical Frequency Transfer

Stabilized free-space optical frequency transfer