  We propose an efficient broadband frequency generation technique for two
collinear optical parametric processes $\omega_3=\omega_1+\omega_2$ and
$\omega_4=\omega_1-\omega_2$. It exploits chirped quasi-phase-matched gratings,
which in the undepleted pump approximation regime perform population transfer
that is analogous to adiabatic population transfer in a three-state ``vee''
quantum system. The energy of the input fields is transferred adiabatically
either into $\omega_3$ or $\omega_4$ field, depending on which of the two phase
matchings occurs first by the local modulation period in the crystal. One can
switch the output between $\omega_3$ and $\omega_4$ by inverting the direction
of the local modulation sweep, which corresponds to a rotation of the crystal
by angle $\pi$
