  Aims. This article shows the first evidence for gravitational lensing
phenomena in high energy gamma-rays. This evidence comes from the observation
of a gravitational lens induced echo in the light curve of the distant blazar
PKS 1830-211. Methods. Traditional methods for the estimation of time delays in
gravitational lensing systems rely on the cross-correlation of the light curves
of the individual images. In this paper, we use 300 MeV-30 GeV photons detected
by the Fermi-LAT instrument. The Fermi-LAT instrument cannot separate the
images of known lenses. The observed light curve is thus the superposition of
individual image light curves. The Fermi-LAT instrument has the advantage of
providing long, evenly spaced, time series. In addition, the photon noise level
is very low. This allows to use directly Fourier transform methods. Results. A
time delay between the two compact images of PKS 1830-211 has been searched for
both by the autocorrelation method and the "double power spectrum" method. The
double power spectrum shows a 3 {\sigma} evidence for a time delay of 27.5
$\pm$ 1.3 days, consistent with the result from Lovell et al. (1998). The
relative uncertainty on the time delay estimation is reduced from 20% to 5%.
