Photonic Frequency Conversion of OFDM Microwave Signals in a Wavelength-Scale Optomechanical Cavity

Optomechanical (OM) cavities enable coupling of near-infrared light and GHz-frequency acoustic waves in wavelength-scale volumes. When driven in the phonon lasing regime, an OM cavity can perform simultaneously as a nonlinear mixer and a local oscillator -- at integer multiples of the mechanical resonance frequency -- in the optical domain. In this work, we use this property to demonstrate all-optical frequency down- and up-conversion of MHz-bandwidth orthogonal frequency division multiplexed signals compliant with the IEEE 802.16e WiMAX wireless standard at microwave frequencies. To this end, we employ a silicon OM crystal cavity (OMCC), supporting a breathing-like mechanical resonance at $f_m\approx$ 3.9 GHz and having a foot-print $\approx$ 10 $\mu$m$^{2}$, which yields frequency conversion efficiencies better than -17 dB in both down- and up-conversion processes at mW-scale driving power. This work paves the way towards the application of OMCCs in low-power all-photonic processing of digitally-modulated microwave signals in miniaturized silicon photonics chips.