  We propose an efficient method for spatial filtering of light beams by
propagating them through 2D (also 3D) longitudinally chirped photonic crystals,
i.e. through the photonic structures with fixed transverse lattice period and
with the longitudinal lattice period varying along the direction of the beam
propagation. We prove the proposed idea by numerically solving the paraxial
propagation equation in refraction index-modulated media, and we evaluate the
efficiency of the process by plane-wave-expansion analysis. The technique can
be applied to filter (to clean) the packages of atomic waves (Bose
condensates), as well improve the directionality of acoustic and mechanical
waves.
