Pairwise mode-locking in dynamically-coupled parametric oscillators

We demonstrate a novel source of bright, broadband, parametric radiation with unique coherence properties. Despite being broadband, the emission is not pulsed and lacks first-order coherence, yet possesses a very high degree of second-order coherence, somewhat similar to broadband two-mode squeezed vacuum, but with classical power levels. Our configuration is comprised of two coupled parametric oscillators within identical multimode cavities, where the coupling between the oscillators is modulated in time at the repetition rate of the cavity modes, with some analogy to active mode-locking in lasers. We therefore term our configuration ``pairwise mode-locking'', which we demonstrate in a radio-frequency (RF) experiment, covering over an octave of bandwidth with approximately $20$ resonant mode-locked pairs, filling most of the available bandwidth between DC and the pump frequency. We accompany our experiment with an analytic model that accounts for the properties of the coupled parametric oscillators near threshold.