  We have developed a new method based on two cavities containing $\chi^{(2)}$
media to reshape optical pulses by an all-optical technique. The system is
entirely passive \emph{i.e.}, all the energy is brought by the incoming pulse
and uses two successive optical cavities with independent thresholds. The
output pulse is close to a rectangular shape. We show that this technique could
be extended to high bit rates and telecommunication wavelength using very small
cavities containing current nonlinear materials.
