Leveraging the time programmable frequency comb, recent demonstrations of free-space optical time transfer show femtosecond time transfer and sub 10 -18 -level frequency transfer across 300 km of air at losses expected for ground-to-space optical links.

We describe a general design for a compact frequency comb-based optical time transfer and ranging node with a volume of 14L, a mass of 10 kg, and a power consumption of 46 W. We assess the residual noise from the comb-based system by making both ranging and time transfer measurements using these compact nodes over a 4.4 km free-space testbed. We demonstrate that this node design has the potential to support sub-femtosecond clock comparisons and sub-micron range measurements at averaging intervals of 1 s with a mean received power of 20 nW. This is more than sufficient to support future space-based distributed coherent sensing at observing frequencies beyond 1 THz.

Microwave atomic clocks can easily be synchronized across long distances using RF methods. Their more precise optical cousins require a subtler approach.

With the demonstration of quantum-limited optical time transfer capable of tolerating the losses associated with long ground-to-space links, two future applications of free-space time transfer have emerged: intercontinental clock comparisons for time dissemination and coherence transfer for future distributed sensing in the mm-wave region. In this paper, we estimated the projected performance of these two applications using quantum-limited optical time transfer and assuming existing low-size, low-weight, and low-power hardware. In both cases, we limit the discussion to the simplest case of a single geosynchronous satellite linked to either one or two ground stations. One important consideration for such future space-based operations is the choice of reference oscillator onboard the satellite. We find that with a modestly performing optical reference oscillator and low-power fiber-based frequency combs, quantum-limited time transfer could support intercontinental clock comparisons through a common-view node in geostationary orbit with a modified Allan deviation at the 10−16 level at 10-s averaging time, limited primarily by residual turbulence piston noise. In the second application of coherence transfer from ground-to-geosynchronous orbit, we find the system should support high short-term coherence with ∼10 millirad phase noise on a 300 GHz carrier at essentially unlimited integration times.

We present a concept for a high-precision optical atomic clock (OAC) operating on an Earth-orbiting space station. This pathfinder science mission will compare the space-based OAC with one or more ultra-stable terrestrial OACs to search for space-time-dependent signatures of dark scalar fields that manifest as anomalies in the relative frequencies of station-based and ground-based clocks. This opens the possibility of probing models of new physics that are inaccessible to purely ground-based OAC experiments where a dark scalar field may potentially be strongly screened near Earth's surface. This unique enhancement of sensitivity to potential dark matter candidates harnesses the potential of space-based OACs.

SummaryWe demonstrate free-space time transfer across a 300 km atmospheric link with greater than 100 dB of loss. With received powers of only a few hundred femtowatts we synchronize two optical oscillators to 300 attoseconds and reach a fractional frequency instability of 3.1×10 -19 , all without the use of large telescope apertures or adaptive optics. This work demonstrates a viable path for future time transfer between ground and satellite nodes in a global optical clock network with performance commensurate with state-of-the-art optical clocks and a low power-aperture product design. Work of the US government, not subject to copyright.

Frequency-comb based optical ranging and synchronization has the potential to support future space-based distributed sensing. However, comb-based systems cannot yet meet the requirements on size/weight and power (SWaP) needed for space-based operation even though the performance of these systems can achieve extremely high precision and accuracy. Here, we describe a ground-based experiment that demonstrates progress on utilizing low size/weight and power frequency combs, data processing and optical terminals for precision ranging measurements over free space. The small platforms and techniques described here are a first step towards future integration onto a spacecraft platform.

Frequency comb based optical time transfer can provide femtosecond-level timing which will support future clock networks. However, for long-distance terrestrial links, nonreciprocal atmospheric turbulence induces a timing penalty. Here, we quantify this penalty.

A pair of optical frequency comb-based transceivers employing photonic-chip based heterodyne detection were evaluated for two-way time transfer over a 30-km freespace link, demonstrating frequency instability < 3 × 10 -18 at 2×10 3 s with ∼6 pW of received power.

The combination of optical time transfer and optical clocks opens up the possibility of large-scale free-space networks that connect both ground-based optical clocks and future space-based optical clocks. Such networks promise better tests of general relativity1-3, dark-matter searches4 and gravitational-wave detection5. The ability to connect optical clocks to a distant satellite could enable space-based very long baseline interferometry6,7, advanced satellite navigation8, clock-based geodesy2,9,10 and thousandfold improvements in intercontinental time dissemination11,12. Thus far, only optical clocks have pushed towards quantum-limited performance13. By contrast, optical time transfer has not operated at the analogous quantum limit set by the number of received photons. Here we demonstrate time transfer with near quantum-limited acquisition and timing at 10,000 times lower received power than previous approaches14-24. Over 300 km between mountaintops in Hawaii with launched powers as low as 40 µW, distant sites are synchronized to 320 attoseconds. This nearly quantum-limited operation is critical for long-distance free-space links in which photons are few and amplification costly: at 4.0 mW transmit power, this approach can support 102 dB link loss, more than sufficient for future time transfer to geosynchronous orbits.

Recent advances in optical atomic clocks and optical time transfer have enabled new possibilities in precision metrology for both tests of fundamental physics and timing applications. Here we describe a space mission concept that would place a state-of-the-art optical atomic clock in an eccentric orbit around Earth. A high stability laser link would connect the relative time, range, and velocity of the orbiting spacecraft to earthbound stations. The primary goal for this mission would be to test the gravitational redshift, a classical test of general relativity, with a sensitivity 30,000 times beyond current limits. Additional science objectives include other tests of relativity, enhanced searches for dark matter and drifts in fundamental constants, and establishing a high accuracy international time/geodesic reference.

The classic self-referenced frequency comb acts as an unrivaled ruler for precision optical metrology in both time and frequency. Two decades after its invention, the frequency comb is now used in numerous active sensing applications. Many of these applications, however, are limited by the tradeoffs inherent in the rigidity of the comb output and operate far from quantum-limited sensitivity. Here we demonstrate an agile programmable frequency comb where the pulse time and phase are digitally controlled with +/- 2 attosecond accuracy. This agility enables quantum-limited sensitivity in sensing applications since the programmable comb can be configured to coherently track weak returning pulse trains at the shot-noise limit. To highlight its capabilities, we use this programmable comb in a ranging system, reducing the detection threshold by 5,000-fold to enable nearly quantum-limited ranging at mean pulse photon number of 1/77 while retaining the full accuracy and precision of a rigid frequency comb. Beyond ranging and imaging, applications in time/frequency metrology, comb-based spectroscopy, pump-probe experiments, and compressive sensing should benefit from coherent control of the comb-pulse time and phase.

Two decades after its invention, the classic self-referenced frequency comb laser is an unrivalled ruler for frequency, time and distance metrology owing to the rigid spacing of its optical output 1 , 2 . As a consequence, it is now used in numerous sensing applications that require a combination of high bandwidth and high precision 3 – 5 . Many of these applications, however, are limited by the trade-offs inherent in the rigidity of the comb output and operate far from quantum-limited sensitivity. Here we demonstrate an agile programmable frequency comb where the pulse time and phase are digitally controlled with ±2-attosecond accuracy. This agility enables quantum-limited sensitivity in sensing applications as the programmable comb can be configured to coherently track weak returning pulse trains at the shot-noise limit. To highlight its capabilities, we use this programmable comb in a ranging system, reducing the required power to reach a given precision by about 5,000-fold compared with a conventional dual-comb system. This enables ranging at a mean photon per pulse number of 1/77 while retaining the full accuracy and precision of a rigid frequency comb. Beyond ranging and imaging 6 – 12 , applications in time and frequency metrology 1 , 2 , 5 , 13 – 23 , comb-based spectroscopy 24 – 32 , pump–probe experiments 33 and compressive sensing 34 , 35 should benefit from coherent control of the comb-pulse time and phase.

1Department of Physics, University of Nevada, Reno, Nevada 89557, USA 2Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 3Department of Physics, HEPL, Stanford University, 452 Lomita Mall, Stanford, California 94305 USA 4National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA 5Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA [Dated December 20, 2021]

Frequency-comb-based optical two-way time and frequency transfer (O-TWTFT) can support future ultra-precise clock networks over free-space links. However, demonstrations have thus far only operated at one corner of a complex parameter space, which balances received optical power, timing performance, and update rate. Here, we analyze the performance of O-TWTFT across this parameter space, with a specific focus on extending the link distance at constant launch power and telescope aperture. We perform experiments across a three-node network spanning 28 km of turbulent air, and successfully demonstrate a more than 2000\ifmmode\times\else\texttimes\fi{} reduction in the required received optical power corresponding to a potential 45\ifmmode\times\else\texttimes\fi{} increase in distance. This distance increase does come with an associated reduction in timing precision, with the uncertainty increasing from 60 as to 20 fs at 10 s averaging times. However, this level of precision is still more than adequate for most applications. In addition, it comes with a reduction in sampling rate, which potentially limits this approach to static links. Interestingly, because this system optimization does not require any hardware modifications, O-TWTFT links could be dynamically tuned to support future long-distance optical clock networks over a range of conditions and applications.

We demonstrate optical two-way time-frequency transfer (O-TWTFT) across a three-node free-space network. The network spans 28 km of outdoor air and relays time and frequency from a master node, through an intermediate fielded node, to an end node. Despite strong atmospheric turbulence along the close-to-ground free-space links and large ambient temperature swings at the fielded node, the network transfers time with relative timing offsets below 5 fs. Additionally, the network transfers frequency with instability below 10(-18) at averaging times greater than 200 s-i.e. below the level needed to compare state-of-the-art optical atomic clocks.

Optical two-way time-frequency transfer, based on the exchange of frequency comb pulses, can support femtosecond-level time transfer and 10-18-level frequency comparison. Here, we present recent results with a focus on the impact of atmospheric turbulence.

A major design goal for femtosecond fiber lasers is to increase the output power but not at the cost of increasing the noise level or narrowing the bandwidth. Here, we perform a computational study to optimize the cavity design of a femtosecond fiber laser that is passively modelocked with a semiconductor saturable absorbing mirror (SESAM). We use dynamical methods that are more than a thousand times faster than standard evolutionary methods. We show that we can obtain higher pulse energies and hence higher output powers by simultaneously increasing the output coupling ratio, the gain, and the anomalous group delay dispersion. We can obtain output pulses that are from 5 to 15 times the energy of the pulse in the current experimental design with no penalty in the noise level or bandwidth.