Coherent Optical Phase Transfer across Turbulent Open Air Paths with 10^-17 instability at 1- second
Laura C. Sinclair, Hugo Bergeron, William C. Swann, Jean-Daniel Deschenes, Nathan R. Newbury
We demonstrate carrier-phase optical two-way time-frequency transfer (carrier-phase OTWTFT) across a turbulent air path. Through the two-way exchange of optical pulse trains from phase- stabilized frequency combs, carrier-phase OTWTFT continuously tracks the relative phase between two distant optical oscillators, i.e. cavity-stabilized lasers, operated at different carrier frequencies with a phase/timing precision of 9 mrad (7 attoseconds) at = 1 second averaging periods despite atmospheric fading and atmospheric phase noise across the 4-km free space link. The corresponding modified Allan deviation is 1.2x10^-17/t_avg^(3/2) from 1 ms to ~ 3 s, reaching 6x10^-20 at 850 seconds. This performance at one second is a 20-fold improvement over previous OTWTF that did not exploit the additional information in the carrier phase. At this level, carrier-phase OTWTF can distribute the stability of the next-generation optical clocks/oscillators without penalty. Moreover, the measurement effectively extends the sub- second mutual coherence time of the cavity-stabilized lasers to arbitrarily long coherence times, enabling future correlated spectroscopy between distant atomic clock samples for high precision clock comparisons.