  We propose a new type of experiment that compares the frequency of a clock
(an ultra-stable optical cavity in this case) at time $t$ to its own frequency
some time $t-T$ earlier, by "storing" the output signal (photons) in a fibre
delay line. In ultra-light oscillating dark matter (DM) models, such an
experiment is sensitive to coupling of DM to the standard model fields, through
oscillations of the cavity and fibre lengths and of the fibre refractive index.
Additionally, the sensitivity is significantly enhanced around the mechanical
resonances of the cavity. We present experimental result of such an experiment
and report no evidence of DM for masses in the [$4.1\times 10^{-11}$,
$8.3\times 10^{-10}$]~eV region. In addition, we improve constraints on the
involved coupling constants by one order of magnitude in a standard galactic DM
model, at the mass corresponding to the resonant frequency of our cavity.
Furthermore, in the model of relaxion DM, we improve on existing constraints
over the whole DM mass range by about one order of magnitude, and up to six
orders of magnitude at resonance.
