A transportable interrogation laser system with an instability of $\mathrm{mod}\,\sigma_{\rm y} = 3\times10^{-16}$.

We present an interrogation laser system for a transportable strontium lattice clock operating at 698 nm, which is based on an ultra-low-expansion glass reference cavity. Transportability is achieved by implementing a rigid, compact, and vibration insensitive mounting of the 12 cm-long reference cavity, sustaining shocks of up to 50 g. The cavity is mounted at optimized support points that independently constrain all degrees of freedom. This mounting concept is especially beneficial for cavities with a ratio of length $L$ over diameter $D$ $L/D>1$. Generally large $L$ helps to reduce thermal noise-induced laser frequency instability while small $D$ leads to small cavity volume. The frequency instability was evaluated, reaching its thermal noise floor of $\mathrm{mod}\,\sigma_{\rm y} \approx 3 \times 10^{-16}$ for averaging times between 0.5 s and 10 s. The laser system was successfully operated during several field studies.