The processing of analog microwave-frequency signals using optical means becomes increasingly important as part of advanced cellular networks. Chip-level integration of microwave photonic filters, particularly in silicon, is considered necessary for their large-scale deployment.Discrete-time, delay-and-sum filters are widely used to select narrowspectral bands out of broad optical bandwidths. However, the long delays that are required to obtain narrowband filters are difficult to accommodate in integrated optic waveguide paths. In this work, we report discrete-time, integrated microwave photonic filters on standard silicon-on-insulator. Long delays are realized through the conversion of incoming radiofrequency modulation to the formof slow-moving surface acoustic waves. Conversion relies on thermo-elastic expansion of metallic gratings and does not involve piezoelectricity. Information is recovered in the optical domain via photoelastic modulation of probe light in a resonator waveguide. The resonator is patterned to support multiple delayed modulation events. Filters having up to 12 taps are demonstrated, with 175 ns-long delays and passbands as narrow as 5 MHz. The magnitude and radio-frequency phase of each filter tap are designed arbitrarily, independent of those of all others. The coherent summation of delayed waveform replicas is free of environmental phase drifts. Surface acoustic wave modulation of a compact, defect grating waveguide is demonstrated as well. Surface acoustic wave devices can significantly extend the signal-processing capabilities of silicon photonics.
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