The adaptive control using BP neural networks for a nonlinear servo-motor

The servo-motor possesses a strongly nonlinear property due to the effect of the stimulating input voltage, load-torque and environmental operating conditions. So it is rather difficult to derive a traditional mathematical model which is capable of expressing both its dynamics and steady-state characteristics. A neural network-based adaptive control strategy is proposed in this paper. In this method, two neural networks have been adopted for system identification （NNI） and control （NNC）, respectively. Then, the commonly-used specialized learning has been modified, by taking the NNI output as the approximation output of the servo-motor during the weights training to get sensitivity information. Moreover, the rule for choosing the learning rate is given on the basis of the analysis of Lyapunov stability. Finally, an example of applying the proposed control strategy on a servo-motor is presented to show its effectiveness.

Characteristic Analysis of White Gaussian Noise in S-Transformation Domain

The characteristic property of white Gaussian noise (WGN) is derived in S-transformation domain. The results show that the distribution of normalized S-spectrum of WGN follows X2?distribution with two degrees of freedom. The conclusion has been confirmed through both theoretical derivations and numerical simulations. Combined with different criteria, an effective signal detection in S-transformation can be realized.

Time-space multiplexed photonic-electronic digital multiplier

Optical computing has shown immense application prospects in the post-Moore era.However,as a crucial component of logic computing,the digital multiplier can only be realized on a small scale in optics,restrained by the limited functionalities and inevitable loss of optical nonlinearity.In this paper,we propose a time-space multiplexed architecture to realize large-scale photonic-electronic digital multiplication.

103 GHz germanium-on-silicon photodiode enabled by an optimized U-shaped electrode

High-performance germanium photodiodes are crucial components in silicon photonic integrated circuits for large-capacity data communication.However,the bandwidths of most germanium photodiodes are limited by the intractable resistance–capacitance parasitic effect.Here,we introduce a unique U-shaped electrode to alleviate this issue,reducing the parasitic effect by 36%without compromising any other performance.Experimentally,a large bandwidth of 103 GHz,an optical responsivity of 0.95 A/W at 1550 nm,and a dark current as low as 1.3 nA are achieved,leading to a record high specific detectivity.This is the first breakthrough to 100 GHz bandwidth among all vertical germanium photodiodes,to the best of our knowledge.Open eye diagrams of 120 Gb/s on-off keying and 200 Gb/s four-level pulse amplitude signals are well received.This work provides a promising solution for chip-based ultra-fast photodetection.

Transient long-range distance measurement by a Vernier spectral interferometry

Rapid and long-range distance measurements are essential in various industrial and scientific applications,and among them,the dual-comb ranging system attracts great attention due to its high precision.However,the tem-poral asynchronous sampling results in the tradeoff between frame rate and ranging precision,and the non-ambiguity range(NAR)is also limited by the comb cycle,which hinders the further advancement of the dual-comb ranging system.Given this constraint,we introduce a Vernier spectral interferometry to improve the frame rate and NAR of the ranging system.First,leveraging the dispersive time-stretch technology,the dual-comb interferometry becomes spectral interferometry.Thus,the asynchronous time step is unlimited,and the frame rate is improved to 100 kHz.Second,dual-wavelength bands are introduced to implement a Vernier spectral interferometry,whose NAR is enlarged from 1.5 m to 1.5 km.Moreover,this fast and long-range system also demonstrated high precision,with a 22.91-nm Allan deviation over 10-ms averaging time.As a result,the proposed Vernier spectral interferometry ranging system is promising for diverse applications that necessitate rapid and extensive distance measurement.

Enhanced terahertz vibrational absorption spectroscopy using an integrated high-Q resonator

The terahertz(THz) absorption spectrum is a powerful method to identify substances. The improvement focuses on sensitivity and recovery ability. Here, we demonstrate enhanced THz vibrational absorption spectroscopy based on an on-chip THz whispering gallery mode resonator(THz-WGMR). A THz-WGMR with high Q can store energy and enhance the interaction between the THz waves and the target substances to capture the unique absorption fingerprint information. Therefore, it possesses significant sensitivity to identify trace amounts of substances. As a proof of concept, lactose powder and glucose powder are applied to demonstrate the effectiveness of our approach in recovering fingerprint absorption spectroscopy. Compared with a straight waveguide, the high sensitivity of the THz-WGMR is illustrated. The change of the transmissivity caused by the lactose reaches 7.8 d B around 532 GHz for the THz-WGMR, while only 1.4 d B for the straight waveguide, demonstrating the state-ofthe-art sensing performance in fingerprint absorption recovery. We believe the proposed integrated THz-WGMR will promote the THz identification of tiny fingerprint substances.

Source-configured symmetry-broken hyperbolic polaritons

Polaritons are quasi-particles that combine light with matter,enabling precise control of light at deep subwavelength scales.The excitation and propagation of polaritons are closely linked to the structural symmetries of the host materials,resulting in symmetrical polariton propagation in high-symmetry materials.However,in low-symmetry crystals,symmetry-broken polaritons exist,exhibiting enhanced directionality of polariton propagation for nanoscale light manipulation and steering.Here,we theoretically propose and experimentally demonstrate the existence of symmetry-broken polaritons,with hyperbolic dispersion,in a high-symmetry crystal.We show that an optical disk-antenna positioned on the crystal surface can act as an in-plane polarized excitation source,enabling dynamic tailoring of the asymmetry of hyperbolic polariton propagation in the high-symmetry crystal over a broad frequency range.Additionally,we provide an intuitive analysis model that predicts the condition under which the asymmetric polaritonic behavior is maximized,which is corroborated by our simulations and experiments.Our results demonstrate that the directionality of polariton propagation can be conveniently configured,independent of the structure symmetry of crystals,providing a tuning knob for the polaritonic response and in-plane anisotropy in nanophotonic applications.

χ(^(2)) nonlinear photonics in integrated microresonators

Second-order(χ^((2))) optical nonlinearity is one of the most common mechanisms for modulating and generating coherent light in photonic devices.Due to strong photon confnement and long photon lifetime,integrated microresonators have emerged as an ideal platform for investigation of nonlinear optical efects.However,existing silicon-based materials lack a χ^((2)) response due to their centrosymmetric structures.A variety of novel material platforms possessing χ^((2)) nonlinearity have been developed over the past two decades.This review comprehensively summarizes the progress of second-order nonlinear optical efects in integrated microresonators.First,the basic principles of χ^((2)) nonlinear efects are introduced.Afterward,we highlight the commonly used χ^((2)) nonlinear optical materials,including their material properties and respective functional devices.We also discuss the prospects and challenges of utilizing χ^((2)) nonlinearity in the feld of integrated microcavity photonics.

Self-calibrating microring synapse with dual-wavelength synchronization

As a resonator-based optical hardware in analog optical computing, a microring synapse can be straightforwardly configured to simulate the connection weights between neurons, but it faces challenges in precision and stability due to cross talk and environmental perturbations. Here, we propose and demonstrate a self-calibration scheme with dual-wavelength synchronization to monitor and calibrate the synaptic weights without interrupting the computation tasks. We design and fabricate an integrated 4 × 4 microring synapse and deploy our self-calibration scheme to validate its effectiveness. The precision and robustness are evaluated in the experiments with favorable performance, achieving 2-bit precision improvement and excellent robustness to environmental temperature fluctuations(the weights can be corrected within 1 s after temperature changes 0.5°C). Moreover, we demonstrate matrix inversion tasks based on Newton iterations beyond 7-bit precision using this microring synapse. Our scheme provides an accurate and real-time weight calibration independently parallel from computations and opens up new perspectives for precision boost solutions to large-scale analog optical computing.

On-chip photonic spatial-temporal descrambler

As an indispensable part to compensate for the signal crosstalk in fiber communication systems,conventional digital multi-input multi-output(MIMO)signal processor is facing the challenges of high com-putational complexity,high power consumption and relatively low processing speed.The optical MIMOenables the best use of light and has been proposed to remedy this limitation.However,the currently existing optical MIMO methods are all restricted to the spatial di-mension,while the temporal dimension is neglected.Here,an on-chip spatial-temporal descrambler with four channels were devised and its MIMO functions were experimentally verified simultaneously in both spatial and temporal dimensions.The spatial crosstalk of single-channel descrambler and four-channel descrambler is respectively less than-21 dB and-18 dB,and the time delay is simultaneously com-pensated successfully.Moreover,a more universal model extended to mode-dependent loss and gain(MDL)compensation was further de-veloped,which is capable of being cascaded for the real optical trans-mission system.The first attempt at photonic spatial-temporal de-scrambler enriched the varieties of optical MIMO,and the proposed scheme provided a new opportunity for all-optical MIMO signal pro-cessing.

Photonic generation of ASK microwave signals with SSB format

Optical beating is the usual approach to generation of microwave signals.However,the highest frequency achievable for microwave signals is limited by the bandwidths of optoelectronic devices.To maximize the microwave frequency with a limited bandwidth of a photodetector(PD)and relieve the bandwidth bottleneck,we propose to generate microwave signals with the single sideband(SSB)format by beating a continuous wave(CW)light with an optical SSB signal.By simply adjusting the frequency diference between the CW light and the carrier of the optical SSB signal,the frequency of the generated microwave SSB signal is changed correspondingly.In the experiment,amplitude shift keying(ASK)microwave signals with the SSB format are successfully generated with diferent carrier frequencies and coding bit rates,and the recovered coding information agrees well with the original pseudo random binary sequence(PRBS)of 2^(7)−1 bits.The proposed approach can signifcantly relieve the bandwidth restriction set by optoelectronic devices in high-speed microwave communication systems.

On-chip programmable pulse processor employing cascaded MZI-MRR structure

Optical pulse processor meets the urgent demand for high-speed,ultra wideband devices,which can avoid electrical confinements in various fields,e.g.,alloptical communication,optical computing technology,cohcrcnt control and microwave fields.To date,great efforts have been madc particularly in on-chip programmable pulse proccessing.Here,we experimentally demonstrate a programmable pulse proceesor employing 16cascaded Mach-Zehnder interferometer coupled microring resonator(MZI-MRR)structure based on silicon-oninsulator wafer.With micro-heaters loaded to the device,both amplitude and frequency tunings can be realized in each MZI-MRR unit.Thanks to its reconfigurability and integration ability,First,it can serve as a fractional differentiator whose tuning range is 0.51-2.23 with deviation no more than 7%.Second,the device can be tuned into a programmable optical filter whose bandwidth varies from 0.15to 0.97nm.The optical filter is also shape tunable.Especially,15-channel wavelength selective switches are generated.

Photonic matrix multiplication lights up photonicaccelerator and beyond

Matrix computation,as a fundamental building block of information processing in science and technology,contributes most of the computational overheads in modern signal processing and artificial intelligence algorithms.Photonic accelerators are designed to accelerate specific categories of computing in the optical domain,especially matrix multiplication,to address the growing demand for computing resources and capacity.Photonic matrix multiplication has much potential to expand the domain of telecommunication,and artificial intelligence benefiting from its superior performance.Recent research in photonic matrix multiplication has flourished and may provide opportunities to develop applications that are unachievable at present by conventional electronic processors.In this review,we first introduce the methods of photonic matrix multiplication,mainly including the plane light conversion method,Mach–Zehnder interferometer method and wavelength division multiplexing method.We also summarize the developmental milestones of photonic matrix multiplication and the related applications.Then,we review their detailed advances in applications to optical signal processing and artificial neural networks in recent years.Finally,we comment on the challenges and perspectives of photonic matrix multiplication and photonic acceleration.

Performance of integrated optical switches based on 2D materials and beyond

Applications of optical switches,such as signal routing and data-intensive computing,are critical in optical interconnects and optical computing.Integrated optical switches enabled by two-dimensional(2D)materials and beyond,such as graphene and black phosphorus,have demonstrated many advantages in terms of speed and energy consumption compared to their conventional silicon-based counterparts.Here we review the state-of-the-art of optical switches enabled by 2D materials and beyond and organize them into several tables.The performance tables and future projections show the frontiers of optical switches fabricated from 2D materials and beyond,providing researchers with an overview of this field and enabling them to identify existing challenges and predict promising research directions.

Simultaneous ultraviolet,visible,and near-infrared continuous-wave lasing in a rare-earth-doped microcavity

Microlaser with multiple lasing bands is critical in various applications,such as full-color display,optical communications,and computing.Here,we propose a simple and efficient method for homogeneously doping rare earth elements into a silica whispering-gallery microcavity.By this method,an Er-Yb co-doped silica microsphere cavity with the highest quality(Q)factor(exceeding 108)among the rare-earth-doped microcavities is fabricated to demonstrate simultaneous and stable lasing covering ultraviolet,visible,and near-infrared bands under room temperature and a continuous-wave pump.The thresholds of all the lasing bands are estimated to be at the submilliwatt level,where both the ultraviolet and violet continuous wave upconversion lasing from rare earth elements has not been separately demonstrated under room temperature until this work.This ultrahigh-Q doped microcavity is an excellent platform for highperformance multiband microlasers,ultrahigh-precision sensors,optical memories,and cavity-enhanced light–matter interaction studies.

Compact and broadband multimode waveguide bend by shape-optimizing with transformation optics

Multimode waveguide bend is one of the key components for realizing high-density mode-division multiplexing systems on chip.However,the reported multimode waveguide bends are either large,bandwidth-limited or fabrication-complicated,which hinders their applications in future high-density multimode photonic circuits.Here we propose a compact multimode waveguide bend supporting four TE modes simply by shape-optimizing with transformation optics.The shape of the waveguide is optimized in the virtual space with gradient distribution of the refractive index,so that the scattering loss and intermode cross talk are well suppressed.After conformal mapping back into the physical space,a compact(effective radius of 17μm)multimode bending waveguide is obtained.Simulations show that the proposed multimode waveguide bend has little loss(<0.1 dB)and low cross talk(<−20 dB)throughout an ultrabroad wavelength range of 1.16–1.66μm.We also fabricated the shape-optimized multimode bending waveguide on a silicon-on-insulator wafer.At 1550 nm wavelength,the measured excess losses for the four lowest-order TE modes are less than 0.6 dB,and the intermode cross talks are all below−17 dB.Our study paves the way for realizing high-density and large-scale multimode integrated optical circuits for optical interconnect.

Broadband on-chip integrator based on silicon photonic phase-shifted Bragg grating

All-optical integrators are key devices for the realization of ultra-fast passive photonic networks, and, despite their broad applicability range(e.g., photonic bit counting, optical memory units, analogue computing, etc.), their realization in an integrated form is still a challenge. In this work, an all-optical integrator based on a silicon photonic phase-shifted Bragg grating is proposed and experimentally demonstrated, which shows a wide operation bandwidth of 750 GHz and integration time window of 9 ps. The integral operation for single pulse, inphase pulses, and π-shifted pulses with different delays has been successfully achieved.

A small microring array that performs large complex-valued matrix-vector multiplication

As an important computing operation,photonic matrix-vector multiplication is widely used in photonic neutral networks and signal processing.However,conventional incoherent matrix-vector multiplication focuses on real-valued operations,which cannot work well in complex-valued neural networks and discrete Fourier transform.In this paper,we propose a systematic solution to extend the matrix computation of microring arrays from the real-valued field to the complex-valued field,and from small-scale(i.e.,4×4)to large-scale matrix computation(i.e.,16×16).Combining matrix decomposition and matrix partition,our photonic complex matrix-vector multiplier chip can support arbitrary large-scale and complex-valued matrix computation.We further demonstrate Walsh-Hardmard transform,discrete cosine transform,discrete Fourier transform,and image convolutional processing.Our scheme provides a path towards breaking the limits of complex-valued computing accelerator in conventional incoherent optical architecture.More importantly,our results reveal that an integrated photonic platform is of huge potential for large-scale,complex-valued,artificial intelligence computing and signal processing.

Linear all-optical signal processing using silicon micro-ring resonators

Silicon micro-ring resonators （MRRs） are compact and versatile devices whose periodic frequency response can be exploited for a wide range of applications. In this paper, we review our recent work on linear all-optical signal processing applications using silicon MRRs as passive filters. We focus on applications such as modulation format conversion, differential phase-shift keying （DPSK） demodulation, modulation speed enhancement of directly modulated lasers （DMLs）, and monocycle pulse generation. The possibility to implement polarization diversity circuits, which reduce the polarization dependence of standard silicon MRRs, is illustrated on the particular example of DPSK demodulation.

Silicon-on-insulator-based microwave photonic filter with widely adjustable bandwidth

We demonstrate a silicon-based microwave photonic filter(MPF) with flattop passband and adjustable bandwidth. The proposed MPF is realized by using a 10 th-order microring resonator(MRR) and a photodetector,both of which are integrated on a photonic chip. The full width at half-maximum(FWHM) bandwidth of the optical filter achieved at the drop port of the 10 th-order MRR is 21.6 GHz. The ripple of the passband is less than 0.3 dB, while the rejection ratio is 32 dB. By adjusting the deviation of the optical carrier wavelength from the center wavelength of the optical bandpass filter, the bandwidth of the MPF can be greatly changed. In the experiment, the FWHM bandwidth of the proposed MPF is tuned from 5.3 to 19.5 GHz, and the rejection ratio is higher than 30 dB.