  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 made of Invar
(FeNi36) which minimizes the thermal contraction. High coupling efficiency is
essential in quantum optomechanical experiments.
