A. Cingoz, A. Kowligy, J. Roslund, J. Abo-Shaeer, M. Boyd, W. Lunden, F. Roller, Vector Atomic; E. Caldwell, N. Newbury, L. Sinclair, B. Stuhl, W. Swann, National Institute of Standards and Technology; Jean-Daniel Deschenes, Octosig Consulting

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Over the past 20 years GPS has emerged as a global utility, underpinning much of our modern infrastructure. Transportation, telecommunications, and financial sectors fundamentally rely on GPS for accurate location and timing services; GPS’s radio frequency (RF) signal provides 10 ns-level synchronization globally. However, with the emergence of optical atomic clocks, our ability to generate time has greatly outpaced our ability to distribute it. Indeed, optical clocks provide sub-femtosecond timing jitter over 1 s, projecting to sub-100 ps jitter for >1,000 days [i, ii]. To address this performance gap, NIST has developed a free-space optical time-transfer technique that provides fs-level synchronization over 100 km links with pW of received power [iii, iv]. NIST’s scheme relies on the coherent exchange of optical pulse trains between frequency combs. The combination of the femtosecond timing precision possible with frequency combs, high bandwidth (1 THz) optical pulses, and low atmospheric dispersion has led to 1000× improvement over RF time transfer. Operationalizing the frequency comb-based time transfer system for the real world requires improvements in integration, ruggedization, and manufacturability of key subcomponents, including the optical frequency comb and optical heterodyne module. Recently, Vector Atomic has developed compact optical transceivers enabled by integrated photonics [v]. Each transceiver is packaged in a 35 L rackmount chassis that includes two optical-frequency combs, associated pump electronics, and a PIC-based optical heterodyne module. A pair of transceivers were recently evaluated over the NIST 30-km Table Mesa link in Boulder, Colorado. The modified Allan deviation of the link instability is 5E-15 at 1 second, limited by turbulence, and integrates down to 2E-18 at 2000 seconds. The corresponding timing deviation is below 10 fs for the duration of the evaluation. i. N. Hinkley, J.A. Sherman, N.B. Phillips, M. Schioppo, N.D. Lemke, K. Beloy, M. Pizzocaro, C.W. Oates, A.D. Ludlow, An atomic clock with 10E-18 instability, Science, 1240420 (2013) ii. B.J. Bloom, T.L. Nicholson, J.R. Williams, S.L. Campbell, M. Bishof, X. Zhang, W. Zhang, S.L. Bromley and J. Ye”, An optical lattice clock with accuracy and stability at the 10E-18 level,” Nature 506, 71, (2014) iii. Giorgetta, F. , Swann, W. , Sinclair, L. , Baumann, E. , Coddington, I. and Newbury, N. (2013), Optical two-way time and frequency transfer over free space, Nature Photonics, https://doi.org/10.1038/nphoton.2013.69 iv. Emily D. Caldwell, Laura C. Sinclair, William C. Swann, Jean-Daniel Deschenes, Benjamin K. Stuhl , and Nathan R. Newbury, Photon Efficient Optical Time Transfer, 2022 Joint Conference of the European Frequency and Time Forum & the IEEE International Frequency Control Symposium, Paris, France (2022) v. US Patent # 11063740 and # 11387914