  We consider $N$ two-level atoms in a ring cavity interacting with a broadband
squeezed vacuum centered at frequency $\omega_{s}$ and an input monochromatic
driving field at frequency $\omega $. We show that, besides the central mode
(at $\o$), many other {\em sideband modes} are produced at the output, with
frequencies shifted from $\omega $ by multiples of $ 2(\omega -\omega_{s})$.
Here we analyze the optical bistability of the two nearest sideband modes, one
red-shifted and the other blue-shifted.
