Context :

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 can not 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 % .

Conclusions :