Opto-mechanical interactions in planar photonic integrated circuits draw great interest in basic research and applications. However, opto-mechanics is practically absent in the most technologically significant photonics platform: silicon on insulator. Previous demonstrations required the under-etching and suspension of silicon structures. Here we present surface acoustic wave-photonic devices in silicon on insulator, up to 8 GHz frequency. Surface waves are launched through absorption of modulated pump light in metallic gratings and thermo-elastic expansion. The surface waves are detected through photo-elastic modulation of an optical probe in standard race-track resonators. Devices do not involve piezo-electric actuation, suspension of waveguides or hybrid material integration. Wavelength conversion of incident microwave signals and acoustic true time delays up to 40 ns are demonstrated on-chip. Lastly, discrete-time microwave-photonic filters with up to six taps and 20 MHz-wide passbands are realized using acoustic delays. The concept is suitable for integrated microwave-photonics signal processing.

Dense wavelength division multiplexers are key components of data communication networks. This paper presents a silicon-photonic eight-channel multiplexer device with a channel spacing of only 0.133 nm (17 GHz). Devices were fabricated in a commercial silicon foundry, in 8" silicon-on-insulator wafers. The device layout consists of seven unbalanced Mach-Zehnder interferometers in a cascaded tree topology, and each interferometer unit also includes a nested ring resonator element. The transfer function of each unit is that of a maximally flat, autoregressive, moving-average filter. The devices are characterized by uniform passbands, sharp spectral transitions between pass and stop bands, and strong out-of-band rejection. The worst-case optical power crosstalk is −22 dB. The proper function of the device requires careful control of optical phase delays over 14 distinct optical paths. Post-fabrication trimming of phase delays was performed through local illumination of a photo-sensitive upper cladding layer of chalcogenide glass. The de-multiplexing of three adjacent QAM-16, 40 Gbit/s wavelength-division channels was successfully demonstrated. The devices are applicable in data communication and in integrated-photonic processing of radio-over-fiber waveforms.

Gallium-nitride (GaN) is a promising material platform for integrated electro-optic devices due to its wide direct bandgap, pronounced nonlinearities and high optical damage threshold. Low-loss ridge waveguides in GaN layers were recently demonstrated. In this work we provide a first report of four-wave mixing in a GaN waveguide at telecommunication wavelengths, and observe comparatively high nonlinear propagation parameters. The nonlinear coefficient of the waveguide is measured as 1.6±0.45 [Wm] -1 , and the corresponding third-order nonlinear parameter of GaN is estimated as 3.4±1e-18 [m 2 /W]. The results suggest that GaN waveguides could be instrumental in nonlinear-optical signal processing applications.

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 narrow spectral 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 radio-frequency modulation to the form of 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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