Triply-resonant coupled-cavity electro-optic modulators for on-chip RF photonic systems

Abstract

We propose a triply-resonant electro-optic modulator architecture which maximizes modulation efficiency using simultaneous resonant enhancement of the RF drive signal, the optical pump, and the generated optical sideband. Optical enhancement of the optical pump and the sideband is achieved using resonant supermodes of two coupled optical resonators, and the RF enhancement is achieved with LC circuits formed by the capacitances of the optical resonators and inductors which can be implemented using CMOS technology. In the proposed configuration, the photon lifetime determines the bandwidth of the RF response but not its center frequency, which is set by the coupling strength between the two resonators and is not subject to the photon lifetime constraint inherent to conventional single-mode resonant modulators. This enables efficient operation at high RF carrier frequencies without a reduction in efficiency commonly associated with the photon lifetime limit. Two optical configurations of the modulator are proposed: a "basic" configuration with equal Q-factors in both supermodes, most suitable for narrowband RF signals, and a "generalized" configuration with independently tailored Q-factors of the two supermodes, which makes it possible to broaden the RF bandwidth without sacrificing the resonant enhancement of the optical pump and paying a penalty in modulation efficiency associated with doing so. Additionally, a significant gain in modulation efficiency is expected from RF signal enhancement by LC resonant matching. This results in a modulator which is compact, efficient, and capable of modulation at high RF carrier frequencies. The proposed modulator architecture optimally engineers the interaction of the device with each of the three signals and between them, can be applied to any modulator cavity design or modulation mechanism and promises to enable complex RF-photonic systems on chip.