  We investigate a new mode-locking regime in the singly-resonant OPO employing
simultaneous amplitude- and frequency-modulation of the intracavity field. This
OPO exhibits deterministic, "turn-key" formation of a stable, broadband,
chirped frequency comb with high conversion efficiency. Comb-forming dynamics
follow a simple phase-space dynamical model, governed by cavity dispersion and
modulator chirp, which agrees closely with full numerical simulations. The comb
exhibits fast, mode-hop-free tuning over the full gain window of the OPA
crystal, controlled by the modulator frequency. Conditions for comb stability,
and techniques to enhance comb bandwidth through intentional phase-mismatch and
chirping, are investigated.
