Pairwise Mode-Locking of Coupled Parametric Oscillators

We present a novel configuration of two parametrically coupled oscillators that produces a bright, broadband squeezed oscillation. We implement a concept very similar to active mode-locking in lasers, but with one striking... [ view full abstract ]

We present a novel configuration of two parametrically coupled oscillators that produces a bright, broadband squeezed oscillation. We implement a concept very similar to active mode-locking in lasers, but with one striking difference - the produced oscillation is not pulsed and the different modes do not have trivial phase relations among them, in a way analogous to a frustrated spin-model. This source of bright, broadband squeezed light is unique, and could provide an important light source for a variety of applications from communication protocols to interferometry and quantum information. Our configuration involves two coupled parametric oscillators, similar to a system of two coupled pendula. This alone already shows unique steady-state coherent behavior that is unlike what observed in systems of coupled oscillators, where the coherent phenomena exists only as a transient. The core of our concept is that the coupling between the two parametric oscillators is not constant in time, but rather modulated at the repetition rate of the cavity. This actively couples adjacent frequency pairs - transferring energy coherently from one frequency pair to its neighbor, stimulating a bright, broadband oscillation around the carrier frequency, very similar to active mode-locking in lasers, yet with a very important difference - this oscillation is not pulsed. Unlike the situation in mode-locked lasers, the modulation of the coupling does not favor any one instance of time. In fact, since the gain medium only acts as a mediator between the different fields, without storing energy in the medium, the OPO has no 'memory' and energy that was not immediately extracted from the pump is lost. Pulse formation in time is therefore not only inefficient in a CW-pumped OPO, it is deleterious.

We verify our ideas with an experiment in the radio-frequency (RF) domain as well as numerical simulations. In our experiment we achieve pairwise mode-locking of $\sim 20$ resonant modes filling the entire bandwidth between DC and the pump frequency.