  We present the design and experimental proof of principle of a low threshold
optical parametric oscillator (OPO) that continuously oscillates over a large
bandwidth allowed by phase matching. The large oscillation bandwidth is
achieved with a selective two-photon loss that suppresses the inherent mode
competition, which tends to narrow the bandwidth in conventional OPOs. Our
design performs pairwise mode-locking of many frequency pairs, in direct
equivalence to passive mode-locking of ultrashort pulsed lasers. The ability to
obtain high powers of continuous \textit{and} broadband down-converted light
enables the optimal exploitation of the correlations within the down-converted
spectrum, thereby strongly affecting two-photon interactions even at
classically high power levels, and opening new venues for applications such as
two-photon spectroscopy and microscopy and optical spread spectrum
communication.
