Optical two-way time and frequency transfer over free space

Abstract

The transfer of high-quality time-frequency signals between remote locations underpins a broad range of applications including precision navigation and timing, the new field of clock-based geodesy, long-baseline interferometry, coherent radar arrays, tests of general relativity and fundamental constants, and the future redefinition of the second [1-7]. However, present microwave-based time-frequency transfer [8-10] is inadequate for state-of-the-art optical clocks and oscillators [1,11-15] that have femtosecond-level timing jitter and accuracies below 1E-17; as such, commensurate optically-based transfer methods are needed. While fiber-based optical links have proven suitable [16,17], they are limited to comparisons between fixed sites connected by a specialized bidirectional fiber link. With the exception of tests of the fundamental constants, most applications instead require more flexible connections between remote and possibly portable optical clocks and oscillators. Here we demonstrate optical time-frequency transfer over free-space via a two-way exchange between coherent frequency combs, each phase-locked to the local optical clock or oscillator. We achieve femtosecond-scale timing deviation, a residual instability below 1E-18 at 1000 s and systematic offsets below 4E-19, despite frequent signal fading due to atmospheric turbulence or obstructions across the 2-km link. This free-space transfer would already enable terrestrial links to support clock-based geodesy. If combined with satellite-based free-space optical communications, it provides a path toward global-scale geodesy, high-accuracy time-frequency distribution, satellite-based relativity experiments, and "optical GPS" for precision navigation

Hybrid fiber links for accurate optical frequency comparison

“…The optical two-way time and frequency transfer technique was introduced in [22] over a 2-km-long free space link via two-way exchange between frequency combs using time-of-flight measurements. The two-way optical phase comparison technique using a fiber network was first introduced in [18], in which a Sagnac interferometer configuration was used to compare ultra-stable lasers through a 47-km-long fiber link.…”

The Application of Coherent Local Time for Optical Time Transfer and the Quantification of Systematic Errors in Satellite Laser Ranging

“…Giorgetta et al have demonstrated that fs-mode-locked lasers can be used to transfer time over a fixed 2 km free space distance with a precision of 0.2 fs at 1 second of integration (Giorgetta et al 2013). Deschenes et al have taken this concept further to a 4 km terrestrial distance and they could show that the long-term stability of the time transfer reached a value of 40 fs peak to peak over two days in the presence of a turbulent atmosphere (Deschênes et al 2016).…”

Repetition rate stabilization of an optical frequency comb based on solid-state laser technology with an intra-cavity electro-optic modulator

“…Highly stable optical frequency combs (OFCs) have led to numerous breakthroughs in the field of precision optical metrology and measurements [1,2] such as ultra-precise timing [3] and distance measurements [4,5] time and frequency transfer [6,7] generation of ultra-low phase noise microwaves [8][9][10][11] as well as optical spectroscopy [12] and even time-resolved spectroscopy in chemical reactions [13]. All these systems are mainly relying on modelocked laser technology while microresonator-based combs are currently intensively investigated [14].…”