Fiber Bragg gratings have been invented for over three decades and have been intensively employed in optical communications systems and sensor systems. The key limitation of a fiber Bragg grating is that once fabricated, it is hard to be tuned or with very limited tunability via thermal or mechanical tuning. Recently, great efforts have been directed into the study of realizing Bragg gratings on silicon-on-insulator (SOI) platform. The key advantage of Bragg gratings implemented based on SOI is that the gratings can be fast and electrically reconfigured with the use of free-carrier plasma dispersion effect in silicon, which can be employed for fast programmable photonic signal processing. In this paper, Bragg gratings based on SOI that are electrically programmable will be reviewed, and their use for advanced programmable optical and microwave signal processing will be discussed.

We propose and experimentally demonstrate an on-chip optical sensor based on a dual-passband microwave photonic filter (MPF) incorporating a silicon photonic integrated microdisk resonator (MDR). Two whispering gallery modes (WGMs) supported by the MDR that are experiencing different wavelength shifts are employed for simultaneous temperature and refractive index (RI) measurements. To increase the interrogation speed and resolution, the MDR is incorporated in an MPF to produce two microwave passbands. By applying a broadband linearly chirped microwave waveform (LCMW) to the MPF, two filtered microwave waveforms with their temporal locations or equivalently central frequencies corresponding to the wavelength shifts of the notches are generated. The measurement of the temporal locations or equivalently the central frequencies is performed at high speed and high resolution using a digital signal processor (DSP). To increase the signal-to-noise ratio (SNR) of the filtered microwave waveforms, a noise reduction algorithm based on phase-only filtering is proposed and employed. The on-chip optical sensing system is experimentally demonstrated. The sensing system can provide high temperature and RI interrogation resolutions of 2.4 ×10-5 °C and 9.1×10-8 RIU at a high interrogation speed of 1 MHz.

The realization of optical nonlinear activation functions (NAFs) is essential for integrated optical neural networks (ONNs). Here, we propose and experimentally demonstrate a photonic method to implement reconfigurable and low-threshold all-optical NAFs based on a compact and high-Q add-drop microring resonator (MRR) on silicon. In the experiment, four different NAFs including softplus, radial basis, clamped ReLU, and sigmoid functions are realized by exploiting the thermo-optical (TO) effect of the MRR. The threshold to implement NAFs is as low as 0.08 mW. As a demonstration, a handwritten digit classification benchmark task is simulated based on a convolutional neural network (CNN) using the obtained activation functions, where an accuracy of 98% is realized. Thanks to the unique advantages of ultra-compact footprint and ultralow threshold, the proposed nonlinear unit is promising to be widely used in large-scale integrated ONNs.

Phase‐structured light beams carrying orbital angular momentum (OAM) have a wide range of applications ranging from particle trapping to optical communication. Many techniques exist to generate and manipulate such beams but most suffer from bulky configurations. In contrast, silicon photonics enables the integration of various functional components on a monolithic platform, providing a way to miniaturize optical systems to chip level. Here, we propose a series of on‐chip subwavelength holographic waveguide structures that can convert the in‐plane guided modes into desired wavefronts and realize complex free‐space functions, including the generation of complex phase‐structured light beams, arbitrarily directed vortex beam emission and vortex beam focusing. We use a holographic approach to design subwavelength holographic surface gratings, and demonstrate broadband generation of Laguerre–Gaussian (LG) and linearly polarized (LP) modes. Moreover, by assigning appropriate geometric phase profiles to the spiral phase distribution, the off‐chip vortex beam manipulation including arbitrarily directed emission and beam focusing scenarios can be realized. In the experiment, directed vortex beam emission is realized by using a fabricated tilt subwavelength holographic fork grating. The proposed waveguide structures enrich the functionalities of dielectric meta‐waveguide structures, which can find potential applications in optical communication, optical trapping, nonlinear interaction and imaging.

Thanks to the large time bandwidth product (TBWP), linearly chirped microwave waveforms (LCMWs) are widely used in modern radar systems to achieve high-resolution detection and imaging. To overcome the challenge of small unmanned aerial vehicle detection and tracking, radar systems are required to have a higher resolution and multi-function operation, in which an ultra-wideband LCMW is highly preferred with a flexible tuning in the center frequency, instantaneous bandwidth, and multi-band operation. In this paper, we propose and experimentally demonstrate an approach to generating flexible ultra-wide LCMWs based on a Fourier-domain mode-locked optoelectronic oscillator (FDML-OEO) incorporating a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. In the DP-QPSK modulator, two dual-parallel Mach-Zehnder modulators (DP-MZMs) are integrated. With the use of the upper DP-MZM, an FDML-OEO is produced to generate a wideband LCMW with a tuning in the center frequency and instantaneous bandwidth. With the injection of the generated LCMW into the lower DP-MZM, an ultra-wideband LCMW is generated via microwave frequency multiplication, and multi-band waveform generation is enabled by controlling the bias condition of the lower DP-MZM. An experiment is performed and an LCMW with a maximum bandwidth as broad as 10.8 GHz is generated. By adjusting the driving signal applied to the FDML-OEO, the generated LCMW can be tuned in the center frequency from 16.2 to 23.2 GHz and the bandwidth from 3.6 to 10.8 GHz. By controlling the bias point of the lower DP-MZM, a dual-band LCMW is also experimentally demonstrated. Thanks to the ultra-wide bandwidth and strong flexibility of the generated LCMWs in terms of tunable center frequency, instantaneous bandwidth and multiband operation, the proposed approach offers a promising LCMW generator in the next-generation high-resolution radar systems.

Silicon-based wavelength-division-multiplexing (WDM) optical interconnection networks have recently been emerged as an effective solution in the datacenter to cope with the ever-increasing data traffic thanks to high data throughput and low power consumption. In a WDM network, a high-speed electro-optic modulator is a key component that encodes the signal to be transmitted on an optical carrier. However, the conventional modulators usually have a large size and relatively high power-consumption. For future high-density and large-scale integrated photonic circuits, small footprint and low power-consumption is significantly preferred. In this paper, we propose and design a low-loss ultra-compact silicon photonic micro-disk modulator (MDM). The proposed MDM has a specifically-designed slab waveguide to make the disk waveguide compatible with the lateral PN junction, which enables the MDM fabrication directly using the standard foundry multi-project wafer (MPW) run. An ultra-compact MDM with a radius as small as 3.7 μm is designed, fabricated and characterized. Its off-resonance insertion loss is measured to be less than 0.7 dB, and its measured electro-optic frequency response shows a modulation bandwidth as broad as 30.5 GHz. The performance of using the MDM to perform modulation of a 25 Gbit/s nonreturn-to-zero 2 ⁹ -1 pseudorandom binary sequence is also evaluated when the detuning wavelength is set to be 0.031 nm, corresponding to a modulation bandwidth of 14.5 GHz. The proposed MDM provides the above performance without altering a standard foundry MPW run and shares the advantages of MDM including ultra-compact footprint, low insertion loss, and wide modulation bandwidth, which can find extensive applications in large-scale WDM optical interconnection networks and microwave photonics.