High performance integrated photonic circuit based on inverse design method

The basic indexes of all-optical integrated photonic circuits include high-density integration,ultrafast response and ultralow energy consumption.Traditional methods mainly adopt conventional micro/nano-structures.The overall size of the circuit is large,usually reaches hundreds of microns.Besides,it is difficult to balance the ultrafast response and ultra-low energy consumption problem,and the crosstalk between two traditional devices is difficult to overcome.Here,we propose and experimentally demonstrate an approach based on inverse design method to realize a high-density,ultrafast and ultra-low energy consumption integrated photonic circuit with two all-optical switches controlling the input states of an all-optical XOR logic gate.The feature size of the whole circuit is only 2.5μm×7μm,and that of a single device is 2μm×2μm.The distance between two adjacent devices is as small as 1.5μm,within wavelength magnitude scale.Theoretical response time of the circuit is 150 fs,and the threshold energy is within 10 fJ/bit.We have also considered the crosstalk problem.The circuit also realizes a function of identifying two-digit logic signal results.Our work provides a new idea for the design of ultrafast,ultra-low energy consumption all-optical devices and the implementation of high-density photonic integrated circuits.

Pixelated non-volatile programmable photonic integrated circuits with 20-level intermediate states

Multi-level programmable photonic integrated circuits(PICs)and optical metasurfaces have gained widespread attention in many fields,such as neuromorphic photonics,opticalcommunications,and quantum information.In this paper,we propose pixelated programmable Si_(3)N_(4)PICs with record-high 20-level intermediate states at 785 nm wavelength.Such flexibility in phase or amplitude modulation is achieved by a programmable Sb_(2)S_(3)matrix,the footprint of whose elements can be as small as 1.2μm,limited only by the optical diffraction limit of anin-house developed pulsed laser writing system.We believe our work lays the foundation for laser-writing ultra-high-level(20 levels and even more)programmable photonic systems and metasurfaces based on phase change materials,which could catalyze diverse applications such as programmable neuromorphic photonics,biosensing,optical computing,photonic quantum computing,and reconfigurable metasurfaces.

Two-photon lithography for integrated photonic packaging

Photonic integrated circuits(PICs)have long been considered as disruptive platforms that revolutionize optics.Building on the mature industrial foundry infrastructure for electronic integrated circuit fabrication,the manufacturing of PICs has made remarkable progress.However,the packaging of PICs has often become a major barrier impeding their scalable deployment owing to their tight optical alignment tolerance,and hence,the requirement for specialty packaging instruments.Two-photon lithography(TPL),a laser direct-write three-dimensional(3-D)patterning technique with deep subwavelength resolution,has emerged as a promising solution for integrated photonics packaging.This study provides an overview of the technology,emphasizing the latest advances in TPL-enabled packaging schemes and their prospects for adoption in the mainstream photonic industry.

Photonic integrated neuro-synaptic core for convolutional spiking neural network

Neuromorphic photonic computing has emerged as a competitive computing paradigm to overcome the bottlenecks of the von-Neumann architecture.Linear weighting and nonlinear spike activation are two fundamental functions of a photonic spiking neural network(PSNN).However,they are separately implemented with different photonic materials and devices,hindering the large-scale integration of PSNN.Here,we propose,fabricate and experimentally demonstrate a photonic neuro-synaptic chip enabling the simultaneous implementation of linear weighting and nonlinear spike activation based on a distributed feedback(DFB)laser with a saturable absorber(DFB-SA).A prototypical system is experimentally constructed to demonstrate the parallel weighted function and nonlinear spike activation.Furthermore,a fourchannel DFB-SA laser array is fabricated for realizing matrix convolution of a spiking convolutional neural network,achieving a recognition accuracy of 87%for the MNIST dataset.The fabricated neuro-synaptic chip offers a fundamental building block to construct the large-scale integrated PSNN chip.

Integrated photonic systems based on transformation optics enabled gradient index devices

Integrated photonics is expected to play an increasingly important role in optical communications,imaging,computing and sensing with the promise for significant reduction in the cost and weight of these systems.Future advancement of this technology is critically dependent on an ability to develop compact and reliable optical components and facilitate their integration on a common substrate.Here we reveal,with the utility of the emerging transformation optics technique,that functional components composed of planar gradient index materials can be designed and readily integrated into photonic circuits.The unprecedented design flexibility of transformation optics allows for the creation of a number of novel devices,such as a light source collimator,waveguide adapters and a waveguide crossing,which have broad applications in integrated photonic chips and are compatible with current fabrication technology.Using the finite-difference time-domain method,we perform full-wave numerical simulations to demonstrate their superior optical performance and efficient integration with other components in an on-chip photonic system.These components only require spatially-varying dielectric materials with no magnetic properties,facilitating low-loss,broadband operation in an integrated photonic environment.

Stochastic collocation for device-level variability analysis in integrated photonics

We demonstrate the use of stochastic collocation to assess the performance of photonic devices under the effect of uncertainty. This approach combines high accuracy and efficiency in analyzing device variability with the ease of implementation of sampling-based methods. Its flexibility makes it suitable to be applied to a large range of photonic devices. We compare the stochastic collocation method with a Monte Carlo technique on a numerical analysis of the variability in silicon directional couplers.

Robust and low-cost interrogation technique for integrated photonic biochemical sensors based on Mach–Zehnder interferometers

We describe and experimentally demonstrate a measuring technique for Mach–Zehnder interferometer(MZI)based integrated photonic biochemical sensors. Our technique is based on the direct measurement of phase changes between the arms of the MZI, achieved by signal modulation on one of the arms of the interferometer together with pseudoheterodyne detection, and it allows us to avoid the use of costly equipment such as tunable light sources or spectrum analyzers. The obtained output signal is intrinsically independent of wavelength, power variations, and global thermal variations, making it extremely robust and adequate for use in real conditions. Using a silicon-on-insulator MZI, we demonstrate the real-time monitoring of refractive index variations and achieve a detection limit of 4.1 × 10^(-6)refractive index units(RIU).

High-Q micro-ring resonators and grating couplers for silicon-on-insulator integrated photonic circuits

An ultra-small integrated photonic circuit has been proposed,which incorporates a high-quality-factor passive micro-ring resonator（MR） linked to a vertical grating coupler on a standard silicon-on-insulator（SOI） substrate.The experimental results demonstrate that the MR propagation loss is 0.532 dB/cm with a 10μm radius ring resonator,the intrinsic quality factor is as high as 202.000,the waveguide grating wavelength response curve is a 1 dB bandwidth of 40 nm at 1540 nm telecommunication wavelengths,and the measured fiber-to-fiber coupling loss is 10 dB.Furthermore,the resonator wavelength temperature dependence of the 450 nm wide micro-ring resonator is 54.1 pm/℃.Such vertical grating coupler and low loss MR-integrated components greatly promote a key element in biosensors and high-speed interconnect communication applications.

Nonlinear integrated photonics

The field of nonlinear photonics is in full development. This special issue of Photonics Research takes you through the current issues of this fast-growing field of research, drawing on the current state of the art and seeking, through a selection of articles, to outline some trends for the future.

Fundamentals and applications of photonic waveguides with bound states in the continuum

Photonic waveguides are the most fundamental element for photonic integrated circuits(PICs).Waveguide properties,such as propagation loss,modal areas,nonlinear coefficients,etc.,directly determine the functionalities and performance of PICs.Recently,the emerging waveguides with bound states in the continuum(BICs)have opened new opportunities for PICs because of their special properties in resonance and radiation.Here,we review the recent progress of PICs composed of waveguides with BICs.First,fundamentals including background physics and design rules of a BIC-based waveguide will be introduced.Next,two types of BIC-based waveguide structures,including shallowly etched dielectric and hybrid waveguides,will be presented.Lastly,the challenges and opportunities of PICs with BICs will be discussed.

Applications of thin-film lithium niobate in nonlinear integrated photonics

Photonics on thin-film lithium niobate(TFLN)has emerged as one of the most pursued disciplines within integrated optics.Ultracompact and low-loss optical waveguides and related devices on this modern material platform have rejuvenated the traditional and commercial applications of lithium niobate for optical modulators based on the electro-optic effect,as well as optical wavelength converters based on secondorder nonlinear effects,e.g.,second-harmonic,sum-,and difference-frequency generations.TFLN has also created vast opportunities for applications and integrated solutions for optical parametric amplification and oscillation,cascaded nonlinear effects,such as low-harmonic generation;third-order nonlinear effects,such as supercontinuum generation;optical frequency comb generation and stabilization;and nonclassical nonlinear effects,such as spontaneous parametric downconversion for quantum optics.Recent progress in nonlinear integrated photonics on TFLN for all these applications,their current trends,and future opportunities and challenges are reviewed.

Hybrid material integration in silicon photonic integrated circuits

Hybrid integration ofⅢ-Ⅴand ferroelectric materials is being broadly adopted to enhance functionalities in silicon photonic integrated circuits(PICs).Bonding and transfer printing have been the popular approaches for integration of III–V gain media with silicon PICs.Similar approaches are also being considered for ferroelectrics to enable larger RF modulation bandwidths,higher linearity,lower optical loss integrated optical modulators on chip.In this paper,we review existing integration strategies ofⅢ-Ⅴmaterials and present a route towards hybrid integration of bothⅢ-Ⅴand ferroelectrics on the same chip.We show that adiabatic transformation of the optical mode between hybrid ferroelectric and silicon sections enables efficient transfer of optical modal energies for maximum overlap of the optical mode with the ferroelectric media,similar to approaches adopted to maximize optical overlap with the gain section,thereby reducing lasing thresholds for hybridⅢ-Ⅴintegration with silicon PICs.Preliminary designs are presented to enable a foundry compatible hybrid integration route of diverse functionalities on silicon PICs.

Photoic crystal nanobeam cavity devices for on-chip integrated silicon photonics

Integrated circuit(IC)industry has fully considered the fact that the Moore’s Law is slowing down or ending.Alternative solutions are highly and urgently desired to break the physical size limits in the More-than-Moore era.Integrated silicon photonics technology exhibits distinguished potential to achieve faster operation speed,less power dissipation,and lower cost in IC industry,because their COMS compatibility,fast response,and high monolithic integration capability.Particularly,compared with other on-chip resonators(e.g.microrings,2D photonic crystal cavities)silicon-on-insulator(SOI)-based photonic crystal nanobeam cavity(PCNC)has emerged as a promising platform for on-chip integration,due to their attractive properties of ultra-high Q/V,ultra-compact footprints and convenient integration with silicon bus-waveguides.In this paper,we present a comprehensive review on recent progress of on-chip PCNC devices for lasing,modulation,switching/filting and label-free sensing,etc.

Science Letters:The Moore’s Law for photonic integrated circuits

We formulate a “Moore’s law” for photonic integrated circuits (PICs) and their spatial integration density using two methods. One is decomposing the integrated photonics devices of diverse types into equivalent basic elements, which makes a comparison with the generic elements of electronic integrated circuits more meaningful. The other is making a complex compo- nent equivalent to a series of basic elements of the same functionality, which is used to calculate the integration density for func- tional components realized with different structures. The results serve as a benchmark of the evolution of PICs and we can con- clude that the density of integration measured in this way roughly increases by a factor of 2 per year. The prospects for a continued increase of spatial integration density are discussed.

Low-loss chip-scale programmable silicon photonic processor

Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications,such as lidar,radar,and artificial intelligence.Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility,and thus makes it possible to develop large-scale programmable optical signal processors.The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors.In this paper,we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches.The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers(MZCs),four Ge/Si photodetectors,four channels of thermally-tunable optical delaylines.Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step.Particularly,these waveguide spirals used here are designed to be as wide as 2μm,enabling an ultralow propagation loss of 0.28 dB/cm.Meanwhile,these MZCs and MZSs are designed with 2-μm-wide arm waveguides,and thus the random phase errors in the MZC/MZS arms are negligible,in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly.Finally,this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities,including tunable time-delay,microwave photonic beamforming,arbitrary optical signal filtering,and arbitrary waveform generation.

Digital synthesis of programmable photonic integrated circuits

Programmable photonic waveguide meshes can be programmed into many different circuit topologies and thereby provide a variety of functions.Due to the complexity of the signal routing in a general mesh,a particular synthesis algorithm often only accounts for a specific function with a specific cell configuration.In this paper,we try to synthesize the programmable waveguide mesh to support multiple configurations with a more general digital signal processing platform.To show the feasibility of this technique,photonic waveguide meshes in different configurations(square,triangular and hexagonal meshes)are designed to realize optical signal interleaving with arbitrary duty cycles.The digital signal processing(DSP)approach offers an effective pathway for the establishment of a general design platform for the software-defined programmable photonic integrated circuits.The use of well-developed DSP techniques and algorithms establishes a link between optical and electrical signals and makes it convenient to realize the computer-aided design of optics–electronics hybrid systems.

Robust optical mode converter based on topological waveguide arrays

Optical mode converters are essential for enhancing the capacity of optical communication systems. However, fabrication errors restrict the further improvement of conventional mode converters. To address this challenge, we have designed an on-chip TE0–TE1mode converter based on topologically protected waveguide arrays. The simulation results demonstrate that the converter exhibits a mode coupling efficiency of 93.5% near 1550 nm and can tolerate a relative fabrication error of 30%. Our design approach can be extended to enhance the robustness for other integrated photonic devices, beneficial for future development of optical network systems.

High-intensity spatial-mode steerable frequency up-converter toward on-chip integration

Integrated photonic devices are essential for on-chip optical communication,optical-electronic systems,and quantum information sciences.To develop a high-fidelity interface between photonics in various frequency domains without disturbing their quantum properties,nonlinear frequency conversion,typically steered with the quadratic(χ2)process,should be considered.Furthermore,another degree of freedom in steering the spatial modes during theχ2 process,with unprecedent mode intensity is proposed here by modulating the lithium niobate(LN)waveguide-based inter-mode quasi-phasematching conditions with both temperature and wavelength parameters.Under high incident light intensities(25 and 27.8 dBm for the pump and the signal lights,respectively),mode conversion at the sum-frequency wavelength with sufficient high output power(−7–8 dBm)among the TM01,TM10,and TM00 modes is realized automatically with characterized broad temperature(ΔT≥8°C)and wavelength windows(Δλ≥1 nm),avoiding the previous efforts in carefully preparing the signal or pump modes.The results prove that high-intensity spatial modes can be prepared at arbitrary transparent wavelength of theχ2 media toward on-chip integration,which facilitates the development of chip-based communication and quantum information systems because spatial correlations can be applied to generate hyperentangled states and provide additional robustness in quantum error correction with the extended Hilbert space.

Photonic integrated optical phased arrays and their applications[Invited]

In recent years,optical phased arrays(OPAs)have attracted great interest for their potential applications in light detection and ranging(Li DAR),free-space optical communications(FSOs),holography,and so on.Photonic integrated circuits(PICs)provide solutions for further reducing the size,weight,power,and cost of OPAs.In this paper,we review the recent development of photonic integrated OPAs.We summarize the typical architecture of the integrated OPAs and their performance.We analyze the key components of OPAs and evaluate the figure of merit for OPAs.Various applications in Li DAR,FSO,imaging,biomedical sensing,and specialized beam generation are introduced.

Gigahertz-rate-switchable wavefront shaping through integration of metasurfaces with photonic integrated circuit

Achieving spatiotemporal control of light at high speeds presents immense possibilities for various applications in communication,computation,metrology,and sensing.The integration of subwavelength metasurfaces and optical waveguides offers a promising approach to manipulate light across multiple degrees of freedom at high speed in compact photonic integrated circuit(PIC)devices.Here,we demonstrate a gigahertz-rate-switchable wavefront shaping by integrating metasurface,lithium niobate on insulator photonic waveguides,and electrodes within a PIC device.As proofs of concept,we showcase the generation of a focus beam with reconfigurable arbitrary polarizations,switchable focusing with lateral focal positions and focal length,orbital angular momentum light beams as well as Bessel beams.Our measurements indicate modulation speeds of up to the gigahertz rate.This integrated platform offers a versatile and efficient means of controlling the light field at high speed within a compact system,paving the way for potential applications in optical communication,computation,sensing,and imaging.