Nonlinear coupling of linearly uncoupled resonators through a Mach–Zehnder interferometer

Abstract

Optical nonlinear processes in linearly uncoupled resonators are being actively studied as a convenient way to engineer and control the generation of non-classical light. In these structures, one can take advantage of the independent combs of resonances of two linearly uncoupled ring resonators for field enhancement, with the phase-matching condition being significantly relaxed compared to a single resonator. However, previous implementations of this approach have shown a limited operational bandwidth along with a significant reduction of the generation efficiency. Here, we experimentally demonstrate that a Mach–Zehnder interferometer can be used to effectively linearly uncouple two resonators and, at the same time, allows for their efficient nonlinear coupling. We demonstrate that this structure can lead to an unprecedented control over the rings' interaction and can operate over more than 160 nm, covering the S-, C-, and L-telecom bands. In addition, we show that the photon pair generation efficiency is increased by a factor of four with respect to previous implementations.

Generation of photon pairs by spontaneous four-wave mixing in linearly uncoupled resonators

“…The DC approach is simple and can be realized in compact structures [23], but it has two main drawbacks: first, its properties can be considered frequency independent only over a limited bandwidth of typically a few tens of nanometers at telecom wavelengths; second, the fields inside the coupler oscillate, reducing the nonlinear interaction efficiency. On the other hand, the MZI approach guarantees linear uncoupling isolation over a much larger bandwidth (hundreds of nanometers [28], thanks to the much lower frequency-sensitivity of the interference mechanism, and offers a higher conversion efficiency, given that the slowlyvarying envelope functions of the fields do not oscillate. The situation where the fields are at very distant frequencies, and thus the coupling ratios cannot be considered frequency-independent, will be addressed in future work.…”

“…The first is that the performance of the MZI coupler is less affected than that of the DC by the frequency dependence of the coupling coefficients, and thus of the splitting ratio, since perfect linear uncoupling holds for the MZI coupler with identical PCs. The fabrication of a device with sufficiently identical couplers was discussed [28], where linear uncoupling over a bandwidth of the order of hundreds of nanometers was achieved in the telecom band. The second advantage is a higher photon conversion efficiency for the MZI coupler than for the DC; in the MZI the field distribution in each arm of the interferometer is the same as that of an isolated channel, and thus the slowly varying envelope function component is not oscillating.…”