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

TL;DR

This paper demonstrates a phase stabilized optical frequency transfer over a 265-meter free-space link using active optics to suppress atmospheric turbulence effects, achieving stability surpassing that of the best optical atomic clocks.

Contribution

It introduces a compact, portable active optics system for stabilizing optical frequency transfer over turbulent free-space links, significantly reducing phase noise and enabling clock-level stability.

Findings

Achieved 80 dB suppression of atmospheric phase noise.

Attained fractional frequency stability of 1.6×10⁻¹⁹ after 40 seconds.

Demonstrated continuous coherent transmission for up to an hour.

Abstract

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.