A transportable clock laser system with an instability of $1.6 \times 10^{-16}$

TL;DR

This paper reports a transportable ultra-stable clock laser system with extremely low instability and acceleration sensitivity, suitable for precise timekeeping and frequency standards in mobile environments.

Contribution

The development of a transportable clock laser with unprecedented low acceleration sensitivity and a stability of 1.6×10⁻¹⁶ at one second, using advanced crystalline mirror coatings and a specialized cavity design.

Findings

Achieved a fractional frequency instability of 1.6×10⁻¹⁶ at one second.

Demonstrated the lowest published acceleration sensitivities for transportable systems.

Built a cavity with a predicted thermal noise floor of 7×10⁻¹⁷.

Abstract

We present a transportable ultra-stable clock laser system based on a Fabry-P\'erot cavity with crystalline Al$_{0.92}$Ga$_{0.08}$As/GaAs mirror coatings, fused silica (FS) mirror substrates and a 20~cm-long ultra-low expansion (ULE\textsuperscript{\textregistered}) glass spacer with a predicted thermal noise floor of $\mathrm{mod}\,\sigma_\mathrm{y} = 7 \times 10^{-17}$ in modified Allan deviation at one second averaging time. The cavity has a cylindrical shape and is mounted at ten points. Its measured sensitivity of the fractional frequency to acceleration for the three Cartesian directions are $2(1) \times 10^{-12}$/(ms$^{-2}$), $3(3) \times 10^{-12}$/(ms$^{-2}$) and $3(1) \times 10^{-12}$/(ms$^{-2}$), which belong to the lowest acceleration sensitivities published for transportable systems. The laser system's instability reaches down to $\mathrm{mod}\,\sigma_\mathrm{y} = 1.6 \times 10^{-16}$