Efficient wideband tunable radio frequency–optical signal conversion via triply resonant active–passive photonic molecules

Electro-optic (EO) transduction of weak radio-frequency (RF) and millimeter-wave signals, such as those received by an antenna, onto laser sidebands for processing in the optical domain requires efficient EO modulators. Microrings offer spatial density and efficiency advantages over Mach-Zehnder modulators, but conventional single-ring modulators suffer a fundamental trade-off between resonantly-enhanced conversion efficiency and the RF carrier frequency that it can accommodate. Dual-cavity ``photonic molecule'' modulators resolve this trade-off, allowing high efficiency independent of the RF carrier frequency by providing separate resonant supermodes to enhance the laser local oscillator (LO) and the narrowband RF-detuned sideband. However, the RF frequency is fixed at design time by geometry, with efficiency dropping quickly for RF carriers away from the design value. We propose a novel, triple-cavity configuration with an off-resonant middle ring acting as an effective tunable coupler between two active modulator cavities. This configuration provides wideband tunability of the target RF carrier while maintaining efficient sideband conversion. When the middle ring is passive (high Q), this configuration provides wide RF tunability with no efficiency penalty over the fixed dual-cavity case, and could become an important building block for future RF/mm-wave photonic integrated circuits.