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.

We propose and demonstrate an high-speed single-end-access Brillouin optical correlation-domain sensor based on a lock-in-amplifier (LIA)-free detection scheme. In the system, the pump and the probe are injected into the same end of a polarization-maintaining fiber (PMF) with orthogonal polarization states, and a state-of-polarization (SOP)-selective reflection configuration installed at the other end of the PMF is used to reflect the probe but block the pump. Therefore, the forward pump interacts with the backward probe, and this situation is similar to the process in the conventional double-end-access Brillouin optical correlation domain analysis (BOCDA) system. In order to implement high-speed measurement, a voltage controlled oscillator (VCO), rather than a microwave synthesizer, is used to sweep the pump-probe frequency interval rapidly. By using a LIA-free detection scheme, an ultrahigh sampling rate of up to 200 kS/s at an arbitrary position is firstly achieved. As a proof of concept, dynamic strains with frequencies up to 10,000 Hz are measured at an arbitrary position with a centimeter-level spatial resolution. When distributed measurements are performed with 400 points, a repetition rate of 250 Hz with a measurement accuracy of 1.8 MHz (the sampling rate is set to be 100 kS/s) is verified by measuring dynamic strains with frequencies up to 40 Hz in a fully distributed manner.

In this paper, we propose and demonstrate a high-speed Brillouin optical correlation domain analysis (BOCDA) system by exploiting the slope of the Brillouin gain spectrum (BGS). In the slope-assisted (SA) scheme, the Brillouin frequency shift (BFS) change is obtained by recording the Brillouin power variation. In this case, the time-consuming frequency sweeping process is avoided, leading to an ultrahigh measurement speed for Brillouin sensors. Here, this SA concept is adopted into the BOCDA system to improve its measurement speed. Moreover, in order to achieve pump-power-independent and fiber-loss-insensitive measurements, a dual-slope-assisted (DSA) scheme is implemented by measuring the ratio between readings taken from both slopes of the BGS. By employing this new technique, a repetition rate of 625 Hz as well as a centimeter-level spatial resolution is achieved, which is verified by measuring dynamic strains with frequencies up to 100 Hz. Meanwhile, by using the proposed system, distributed and dynamic tracking of mechanical wave propagating along the fiber under test (FUT) is realized. Moreover, the limitations of DSA-BOCDA on measurement accuracy, spatial resolution, and measurement speed are also discussed in details.

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.

On-chip photonic generation of tunable wideband phase-coded linearly-chirped microwave waveforms (PC-LCMWs) is proposed and experimentally demonstrated. The on-chip generation system consists of two main parts. One part is a hybrid Fourier-domain mode-locked opto-electronic oscillator (FDML-OEO) for a wideband linearly-chirped microwave waveform (LCMW) generation; the other part is a high-speed microwave photonic phase shifter (MPPS) to perform the phase-coding. By synchronizing the two parts, a PC-LCMW is finally generated. In the proposed chip, a thermally-tunable ultrahigh-Q micro-ring resonator (MRR) is a key component which is incorporated in the FDML-OEO loop to select the oscillation frequency, and an electrically-tunable micro-disk resonator (MDR), as another key component, is used to code the phase of the generated microwave signal while maintaining the amplitude by controlling the free carrier concentration in the lateral PN junction of the disk. With the use of the two key component chips, an experiment is performed and different PC-LCMWs are generated with a tunable center frequency from 12 to 16 GHz, a bandwidth from 2 to 6 GHz, and phase code of 7-bit and 13-bit Barker code. In particular, a PC-LCMW with a maximum bandwidth as wide as 6 GHz and an ultra-large time-bandwidth product (TBWP) as large as 1.96 × 10 ⁶ is experimentally demonstrated. The proposed integrated PC-LCMW generation system holds great advantages including ultracompact configuration and fast tunability, which holds a high potential for applications in modern high-resolution multi-functional radar system.