  The creation of high-performance narrowband filters is of great interest for
many RF-signal processing applications. To this end, numerous schemes for
electronic, MEMS-based, and microwave photonic filters have been demonstrated.
Filtering schemes based on microwave photonic systems offer superior
flexibility and tunability to traditional RF filters. However, these
optical-based filters are typically limited to GHz-widths and often have large
RF insertion losses, posing challenges for integration into high-fidelity
radiofrequency circuits. In this article, we demonstrate a novel type of
microwave filter that combines the attractive features of microwave photonic
filters with high-Q phononic signal processing using a photonic-phononic
emit-receive process. Through this process, a RF signal encoded on a guided
optical wave is transduced onto a GHz-frequency acoustic wave, where it may be
filtered through shaping of acoustic transfer functions before being re-encoded
onto a spatially separate optical probe. This emit-receive functionality,
realized in an integrated silicon waveguide, produces MHz-bandwidth band-pass
filtering while supporting low RF insertion losses necessary for high dynamic
range in a microwave photonic link. We also demonstrate record-high internal
efficiency for emit-receive operations of this type, and show that the
emit-receive operation is uniquely suitable for the creation of serial filter
banks with minimal loss of fidelity. This photonic-phononic emitter-receiver
represents a new method for low-distortion signal-processing in an integrated
all-silicon device.
