We present a cryogenic setup where an optical Fabry–Perot resonator is coupled to a single-
mode optical fiber with coupling efficiency above 90% at mK temperatures without
realignment during cooling down. The setup is prealigned at room temperature to
compensate for the thermal contraction and change of the refractive index of the optical
components during cooling down. The high coupling efficiency is achieved by keeping the
setup rotation-symmetric around the optical axis. The majority of the setup components are …