  We introduce a general approach for generation of sets of three-dimensional
quasi-non-spreading wavepackets propagating in linear media, also referred to
as linear light bullets. The spectrum of rigorously non-spreading wavepackets
in media with anomalous group velocity dispersion is localized on the surface
of a sphere, thus drastically restricting the possible wavepacket shapes.
However, broadening slightly the spectrum affords the generation of a large
variety of quasi-non-spreading distributions featuring complex topologies and
shapes in space and time that are of interest in different areas, such as
biophysics or nanosurgery. Here we discuss the method and show several
illustrative examples of its potential.
