  We propose the use of modulated spectra of astronomical sources due to
gravitational lensing to probe Ellis wormholes. The modulation factor due to
gravitational lensing by the Ellis wormhole is calculated. Within the
geometrical optics approximation, the normal point mass lens and the Ellis
wormhole are indistinguishable unless we know the source's unlensed luminosity.
This degeneracy is resolved with the significant wave effect in the low
frequency domain if we take the deviation from the geometrical optics into
account. We can roughly estimate the upper bound for the number density of
Ellis wormholes as $n\lesssim 10^{-9}\{AU}^{-3}$ with throat radius $a\sim1\cm$
from the existing femto-lensing analysis for compact objects.
