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

We present a transportable ultra-stable clock laser system based on a Fabry-Pérot 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 $\mathrm{m}\mathrm{o}\mathrm{d}{\sigma }_{\mathrm{y}}=7\times {10}^{-17}$ 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 $\mathrm{m}\mathrm{o}\mathrm{d}{\sigma }_{\mathrm{y}}=1.6\times {10}^{-16}$