  The possibility to detect cosmic strings -- topological defects of early
Universe, by means of wave effects in gravitational lensing is discussed. To
find the optimal observation conditions, we define the hyperbolic-shaped
Fresnel observation zones associated with the diffraction maxima and analyse
the frequency patterns of wave amplification corresponding to different
alignments. In particular, we show that diffraction of gravitational waves by
the string may lead to significant amplification at cosmological distances. The
wave properties we found are quite different from what one would expect, for
instance, from light scattered off a thin wire or slit, since a cosmic string,
as a topological defect, gives no shadow at all.
