  We analyze the phenomenon of nonlinear stochastic resonance (SR) in noisy
bistable systems driven by pulsed time periodic forces. The driving force
contains, within each period, two pulses of equal constant amplitude and
duration but opposite signs. Each pulse starts every half-period and its
duration is varied. For subthreshold amplitudes, we study the dependence of the
output signal-to-noise ratio (SNR) and the SR gain on the noise strength and
the relative duration of the pulses. We find that the SR gains can reach values
larger than unity, with maximum values showing a nonmonotonic dependence on the
duration of the pulses.
