  We theoretically show that an externally driven dipole placed inside a
cylindrical hollow waveguide can generate a train of ultrashort and
ultrafocused electromagnetic pulses. The waveguide encloses vacuum with perfect
electric conducting walls. A dipole driven by a single short pulse, which is
properly engineered to exploit the linear spectral filtering of the cylindrical
hollow waveguide, excites longitudinal waveguide modes that are coherently
re-focused at some particular instances of time. A dipole driven by a pulse
with a lower-bounded temporal width can thus generate, in principle, a finite
train of arbitrarily short and focused electromagnetic pulses. We numerically
show that such ultrafocused pulses persist outside the cylindrical waveguide at
distances comparable to its radius.
