Proton irradiation robustness of dielectric mirrors for high-finesse Fabry-P\'erot resonators in the near-infrared spectral range

We demonstrate that a proton irradiation with fluences of $3.6\times {10}^{10}$/cm${}^2$ at low energy ($<$ 36 MeV) and $1.46 \times 10^{10}$\mathrm{/}cm$^{2}$ at high energy (40 MeV and 90 MeV combined) on the dielectric mirrors of Fabry-P'erot cavities with a finesse of about 700 000 causes less than 5% change in the finesse. Furthermore, no influence on the coupling efficiency to the cavities was observed, the efficiency being approximately 70%. The irradiation was carried out with a spectrum approximating the proton energy spectrum of a highly elliptic Earth orbit with duration of 5 years, proposed for the Space-Time Explorer and Quantum Equivalence Space Test (STE-QUEST) mission [\url{http://sci.esa.int/ste-quest/}].