A compact, robust, and transportable ultra-stable laser with a fractional frequency instability of $1\times {10}^{-15}$

We present a compact and robust transportable ultra-stable laser system with minimum fractional frequency instability of $1\times {10}^{-15}$ at integration times between 1 to 10 s. The system was conceived as a prototype of a subsystem of a microwave-optical local oscillator to be used on the satellite mission STE-QUEST (Space-Time Explorer and QUantum Equivalence Principle Space Test, http://sci.esa.int/ste-quest/). It was therefore designed to be compact, to sustain accelerations occurring during rocket launch, to exhibit low vibration sensitivity, and to reach a low frequency instability. Overall dimensions of the optical system are $40$\text{ cm}$\times20$\text{ cm}$\times30\textrm{ cm}$. The acceleration sensitivities of the optical frequency in the three directions were measured to be $1.7\times {10}^{-11}\mathrm{/}g$, $8.0\times {10}^{-11}\mathrm{/}g$, and $3.9\times {10}^{-10}\mathrm{/}g$, and the absolute frequency instability was determined via a three-cornered hat measurement. The design is also appropriate and useful for terrestrial applications.