  In optical clocks, transitions of ions or neutral atoms are interrogated
using pulsed ultra-narrow laser fields. Systematic phase chirps of the laser or
changes of the optical path length during the measurement cause a shift of the
frequency seen by the interrogated atoms. While the stabilization of cw-optical
links is now a well established technique even on long distances, phase stable
links for pulsed light pose additional challanges and have not been
demonstrated so far. In addition to possible temperature or pressure drift of
the laboratory, which may lead to a Doppler shift by steadily changing the
optical path length, the pulsing of the clock laser light calls for short
settling times of stabilization locks. Our optical path length stabilization
uses retro-reflected light from a mirror that is fixed with respect to the
interrogated atoms and synthetic signals during the dark time. Length changes
and frequency chirps are compensated for by the switching AOM. For our
strontium optical lattice clock we have ensured that the shift introduced by
the fiber link including the pulsing acousto optic modulator is below $2\cdot
10^{-17}$.
