  Injection locking is a powerful technique for synchronization of oscillator
networks and controlling the phase and frequency of individual oscillators
using similar or other types of oscillators. Here, we present the first
demonstration of injection locking of a radiation-pressure driven
optomechanical oscillator (OMO) via acoustic waves. As opposed to previously
reported techniques (based on pump modulation or direct application of a
modulated electrostatic force), injection locking of OMO via acoustic waves
does not require optical power modulation or physical contact with the OMO and
it can easily be implemented on various platforms. Using this approach we have
locked the phase and frequency of two distinct modes of a microtoroidal silica
OMO to a piezoelectric transducer (PZT). We have characterized the behavior of
the injection locked OMO with three acoustic excitation configurations and
showed that even without proper acoustic impedance matching the OMO can be
locked to the PZT and tuned over 17 kHz with only -30 dBm of RF power fed to
the PZT. The high efficiency, simplicity and scalability of the proposed
approach paves the road toward a new class of photonic systems that rely on
synchronization of several OMOs to a single or multiple RF oscillators with
applications in optical communication, metrology and sensing. Beyond its
practical applications, injection locking via acoustic waves can be used in
fundamental studies in quantum optomechanics where thermal and optical
isolation of the OMO are critical.
