Photonic Integrated Circuits Research Articles

Variable optical attenuator using a multi-path interferometer embedded in an optical cavity

Variable optical attenuators (VOAs) used in photonic integrated circuits suffer from trade-off between dynamic range of attenuation and large footprint and introduce inadvertent changes in optical phase while attenuating the input signal. In this work, we propose a VOA architecture using micro-ring resonators which provides ≥60dB optical attenuation of selected wavelengths, while preserving the input optical phase, in a compact footprint. It enables tailoring of the dynamic range and insertion loss for different circuit applications in photonic signal processing, programmable photonics, high performance computing, and datacenter communication.

Broadband and easily fabricated double-tip edge coupler based on thin-film lithium niobate platform

Due to the significant difference in spot size between lithium-niobate-on-insulator (LNOI) waveguide and fiber, there is a substantial coupling loss between them, which limits the interconnection and application of LN photonic chips. Although some LNOI edge couplers have been designed and fabricated with low coupling loss between chip and fiber, they require high etching accuracy. In this paper, a low-loss, broadband and easily fabricated edge coupler is designed and fabricated on the LNOI platform. To reduce the etching requirement for tip width, a design has been adopted with symmetrical double tips at the edge. Additionally, a multimode interferometer (MMI) is introduced to ensure large fabrication tolerance. The proposed design only requires a single full etching step, significantly reducing stringent requirements for process with high precise. Experimental results demonstrate that the device exhibits a coupling loss less than 3.5 dB/facet in the wavelength range of 1500–1640 nm, while achieving a low coupling loss of 2.71 dB/facet at 1550 nm.

Plasmonic-based electro-absorption modulator integrated with different dielectric materials at 1.55 µm wavelength

In this paper an electro-absorption modulator (EAM) based on plasmonic material along with different dielectric materials were investigated. The study focuses on the performance matrices of the device based on four different dielectric materials such as hafnium dioxide (HfO2), silicon nitride (Si3N4), boron nitride (hBN), and silicon dioxide (SiO2). A finite element method (FEM) is used to carry out the investigated results like insertion loss (IL), extinction ratio (ER), and figure of merit (FOM). The device yielded a very high ER of 35.42 dB/µm and an IL of 0.02 dB/µm at 1.55 µm wavelength. A modulation frequency (f3dB) of 0.46 THz, 1.50 THz, 2.39 THz, and 4.24 THz was achieved for the EAMs with HfO2, Si3N4, hBN, and SiO2as the dielectric material, respectively. Also, the investigated EAM with different dielectric materials has the potential to be integrated in Photonic Integrated Circuits (PICs).

Glass interposer for heterogeneous integration of flip-chipped photonic and electronic integrated circuits

Glass interposer for heterogeneous integration of flip-chipped photonic and electronic integrated circuits

Fast topological pumps via quantum metric engineering on photonic chips.

Topological pumps have garnered substantial attention in physics. However, the requirement for slow evolution speed to satisfy adiabaticity greatly restricts their application in on-chip devices. Here, we discover a direct link between adiabaticity and quantum metric, the real part of quantum geometry that has been relatively less explored compared to its imaginary counterpart, the Berry curvature. We demonstrate that the evolution speed of topological pumps between nontrivial edge states can be increased by reducing the quantum metric via introduction of long-range coupling to the celebrated Rice-Mele model. This fast topological pump can occur without affecting the bulk state evolution, which challenges the common understanding. We experimentally confirm our findings by using a platform consisting of bilayer integrated silicon waveguides operating at telecommunication wavelengths. Our work provides possibilities for lifting topological pumps from the constraints of slow evolution and paves the way toward compact photonic integration.

Symmetric silicon microring resonator optical crossbar array for accelerated inference and training in deep learning

Photonic integrated circuits are emerging as a promising platform for accelerating matrix multiplications in deep learning, leveraging the inherent parallel nature of light. Although various schemes have been proposed and demonstrated to realize such photonic matrix accelerators, the in situ training of artificial neural networks using photonic accelerators remains challenging due to the difficulty of direct on-chip backpropagation on a photonic chip. In this work, we propose a silicon microring resonator (MRR) optical crossbar array with a symmetric structure that allows for simple on-chip backpropagation, potentially enabling the acceleration of both the inference and training phases of deep learning. We demonstrate a 4×4 circuit on a Si-on-insulator platform and use it to perform inference tasks of a simple neural network for classifying iris flowers, achieving a classification accuracy of 93.3%. Subsequently, we train the neural network using simulated on-chip backpropagation and achieve an accuracy of 91.1% in the same inference task after training. Furthermore, we simulate a convolutional neural network for handwritten digit recognition, using a 9×9 MRR crossbar array to perform the convolution operations. This work contributes to the realization of compact and energy-efficient photonic accelerators for deep learning.

Piezoelectrically tunable, narrow linewidth photonic integrated extended-DBR lasers

Recent advancements in ultra-low-loss silicon nitride (Si3N4)-based photonic integrated circuits have surpassed fiber lasers in coherence and frequency agility. However, high manufacturing costs of DFB and precise control requirements, as required for self-injection locking, hinder widespread adoption. Reflective semiconductor optical amplifiers (RSOAs) provide a cost-effective alternative solution but have not yet achieved similar performance in coherence or frequency agility, as required for frequency modulated continuous wave (FMCW) LiDAR, laser locking in frequency metrology, or wavelength modulation spectroscopy for gas sensing. Here, we overcome this challenge and demonstrate an RSOA-based and frequency-agile fully hybrid integrated extended distributed Bragg reflector (E-DBR) laser with high-speed tuning, good linearity, high optical output power, and turn-key operability. It outperforms Vernier and self-injection locked lasers, which require up to five precise operating parameters and have limitations in continuous tuning and actuation bandwidth. We maintain a small footprint by utilizing an ultra-low-loss 200 nm thin Si3N4 platform with monolithically integrated piezoelectric actuators. We co-integrate the DBR with a compact ultra-low-loss spiral resonator to further reduce the intrinsic optical linewidth of the laser to the Hertz-level—on par with the noise of a fiber laser—via self-injection locking. The photonic integrated E-DBR lasers operate at 1550 nm and feature up to 25 mW fiber-coupled output power in the free-running and up to 10.5 mW output power in the self-injection locked state. The intrinsic linewidth is 2.5 kHz in the free-running state and as low as 3.8 Hz in the self-injection locked state. In addition, we demonstrate the suitability for FMCW LiDAR by showing laser frequency tuning over 1.0 GHz at up to 100 kHz triangular chirp rate with a nonlinearity of less than 0.6% without linearization by modulating a Bragg grating using monolithically integrated aluminum nitride (AlN) piezoactuators.

Narrow nano-beam generation inside of silicon photonic chip with a hemi-cylinder

A photonic nanojet (PNJ) can be shrunk using a high index dielectric micro-particle to generate a narrow nano-beam. Considering silicon has a high refractive index and low absorption in near-infrared waves, here, we present a narrow nano-beam generation inside of a silicon photonic chip with a hemi-cylinder. The proposed design consists of a silicon hemi-cylinder on a silicon substrate with a micro-hole. At first, we investigate the narrow PNJ inside of the silicon substrate produced by a silicon hemi-cylinder on a silicon substrate. In order to make the PNJ inside of the silicon substrate usable, the properties of the PNJ influenced by the position of the added micro-hole filled with water are studied. A narrow nano-beam can be obtained when the position of the added micro-hole is well-chosen. Then, the narrow nano-beam affected by the silicon hemi-cylinder immersed different mediums is presented. Finally, we discuss the influence of the perturbation of illumination angle on the properties of the nano-beam. The proposed structure may have potential applications in enhanced Raman spectroscopy and optofluidic chips.

All-Optic Logical Operations Based on the Visible-Near Infrared Bipolar Optical Response.

The burgeoning need for extensive data processing has sparked enthusiasm for the development of a novel optical logic gate platform. In this study, junction field-effect phototransistors based on molybdenum disulfide/Germanium (MoS2/Ge) heterojunctions are constructed as optical logic units. This device demonstrates a positive photoresponse that is attributed to the photoconductivity effect occurring upon irradiation with visible (Vis) light. Under the illumination of near-infrared (NIR) optics with wavelengths within the communication band, the device shows a negative photoresponse, which is associated with the interlayer Coulomb interactions. The current state of the device can be effectively modulated as different logical states by precisely tuning the wavelength and power density of the optical. Within a 3×3 MoS2/Ge phototransistor array, five essentially all-optical logic gates ("AND," "OR," "NAND," "NOT," and "NOR") can be achieved in every signal unit. Furthermore, three complex all-optical logical operations are demonstrated by integrating two MoS2/Ge phototransistors in series. Compared to electronic designs, these all-optical logic devices offer a significant reduction in transistor number, with savings of 50-94% when implementing the above-mentioned functions. These results present opportunities for the development of photonic chips with low power consumption, high fidelity, and large volumes.

Ultra‐Low Loss Lithium Niobate Polarizer with Enhanced Anti Bound State in the Continuum

AbstractThin film lithium niobate (TFLN) has emerged as a promising platform for photonic integrated circuits (PICs). However, the polarization crosstalk is usually inevitable owing to its birefringence. Polarizers, as key polarization handling devices, are essential to ensure polarization purity in PICs. In this work, a novel ultra‐low loss polarizer is proposed and demonstrated based on the enhanced anti‐bound state in the continuum (anti‐BIC) on TFLN. With meticulously designed waveguide width, constructive interference occurs for the leaky modes, such that transverse magnetic (TM) bound mode can drastically couple into the continuum modes, leading to high propagation loss for TM polarization. Meanwhile transverse electric (TE) polarization remains bound with high optical confinement, hence low propagation loss. Furthermore, it have demonstrated that angled sidewalls can facilitate the mode conversion from bound mode into continuum mode in lithium niobate (LN), which can be an efficient method for controlling the bound mode coupling into the continuum. Ultra‐low loss of <≈0.04 dB and compact footprint of 46 µm are experimentally demonstrated from 1460 to 1600 nm for the fabricated polarizer.

An integrated atom array-nanophotonic chip platform with background-free imaging

Arrays of neutral atoms trapped in optical tweezers have emerged as a leading platform for quantum information processing and quantum simulation due to their scalability, reconfigurable connectivity, and high-fidelity operations. Individual atoms are promising candidates for quantum networking due to their capability to emit indistinguishable photons that are entangled with their internal atomic states. Integrating atom arrays with photonic interfaces would enable distributed architectures in which nodes hosting many processing qubits could be efficiently linked together via the distribution of remote entanglement. However, many atom array techniques cease to work in close proximity to photonic interfaces, with atom detection via standard fluorescence imaging presenting a major challenge due to scattering from nearby photonic devices. Here, we demonstrate an architecture that combines atom arrays with up to 64 optical tweezers and a millimeter-scale photonic chip hosting more than 100 nanophotonic cavities. We achieve high-fidelity ( ~ 99.2%), background-free imaging in close proximity to nanofabricated cavities using a multichromatic excitation and detection scheme. The atoms can be imaged while trapped a few hundred nanometers above the dielectric surface, which we verify using Stark shift measurements of the modified trapping potential. Finally, we rearrange atoms into defect-free arrays and load them simultaneously onto the same or multiple devices.

Recent Advances and Applications of Semiconductor Optical Amplifiers

This paper describes the recent advances in device designs and optical transmission applications of semiconductor optical amplifiers (SOA). The device advances described are quantum-dot-based SOA and photonic-integrated circuits using SOA. The use of nonlinear properties of SOAs in high-speed optical transmission is discussed.

Large-scale photonic network with squeezed vacuum states for molecular vibronic spectroscopy

Although molecular vibronic spectra generation is pivotal for chemical analysis, tackling such exponentially complex tasks on classical computers remains inefficient. Quantum simulation, though theoretically promising, faces technological challenges in experimentally extracting vibronic spectra for molecules with multiple modes. Here, we propose a nontrivial algorithm to generate the vibronic spectra using states with zero displacements (squeezed vacuum states) coupled to a linear optical network, offering ease of experimental implementation. We also fabricate an integrated quantum photonic microprocessor chip as a versatile simulation platform containing 16 modes of single-mode squeezed vacuum states and a fully programmable interferometer network. Molecular vibronic spectra of formic acid and thymine under the Condon approximation are simulated using the quantum microprocessor chip with high reconstructed fidelity ( > 92%). Furthermore, vibronic spectra of naphthalene, phenanthrene, and benzene under the non-Condon approximation are also experimentally simulated. Such demonstrations could pave the way for solving complicated quantum chemistry problems involving vibronic spectra and computational tasks beyond the reach of classical computers.

Simultaneous optical power insensitivity and non-volatile wavelength trimming using 2D In4/3P2Se6 integration in silicon photonics

In integrated photonic circuits, microring resonators are essential building blocks but are susceptible to phase errors due to fabrication imperfections and optical power fluctuations. Conventional active phase tuning methods are power-intensive and challenging to integrate into densely packed photonic chips. This study proposes a solution by integrating a thin 2D layer of In4/3P2Se6 (InPSe) onto silicon microring resonators (Si-MRR). This approach mitigates sensitivity to laser power and achieves non-volatile wavelength trimming. Under bias voltage, the device exhibits electro-optic behavior, offering a non-volatile phase trimming rate of −2.62 pm/V to −4.62 pm/V, corresponding to InPSe thicknesses of 45 nm to 120 nm. Low optical losses of 0.0091 to 0.0361 dB/μm were also measured, corresponding to thicknesses of 30 nm to 120 nm. The devices demonstrate stable in-situ resonance wavelength stabilization and bidirectional trimming, ensuring cyclic stability for non-volatile phase control. This advancement enhances the performance of silicon photonics across diverse applications, facilitating high-capacity, high-power operation in compact designs.

Photonics for Neuromorphic Computing: Fundamentals, Devices, and Opportunities.

In the dynamic landscape of Artificial Intelligence (AI), two notable phenomena are becoming predominant: the exponential growth of large AI model sizes and the explosion of massive amount of data. Meanwhile, scientific research such as quantum computing and protein synthesis increasingly demand higher computing capacities. As the Moore's Law approaches its terminus, there is an urgent need for alternative computing paradigms that satisfy this growing computing demand and break through the barrier of the von Neumann model. Neuromorphic computing, inspired by the mechanism and functionality of human brains, uses physical artificial neurons to do computations and is drawing widespread attention. This review studies the expansion of optoelectronic devices on photonic integration platforms that has led to significant growth in photonic computing, where photonic integrated circuits (PICs) have enabled ultrafast artificial neural networks (ANN) with sub-nanosecond latencies, low heat dissipation, and high parallelism. In particular, various technologies and devices employed in neuromorphic photonic AI accelerators, spanning from traditional optics to PCSEL lasers are examined. Lastly, it is recognized that existing neuromorphic technologies encounter obstacles in meeting the peta-level computing speed and energy efficiency threshold, and potential approaches in new devices, fabrication, materials, and integration to drive innovation are also explored. As the current challenges and barriers in cost, scalability, footprint, and computing capacity are resolved one-by-one, photonic neuromorphic systems are bound to co-exist with, if not replace, conventional electronic computers and transform the landscape of AI and scientific computing in the foreseeablefuture.

Frequency stabilization of C-band semiconductor lasers through a SiN photonic integrated circuit

Integrated semiconductor lasers represent essential building blocks for integrated optical components and circuits and their stability in frequency is fundamental for the development of numerous frontier applications and engineering tasks. When dense optical circuits are considered, the stability of integrated laser sources can be impaired by the thermal cross-talk generated by the action of neighboring components, leading to a deterioration of the long-term system performance (on the scale of seconds). In this work we show the design and the experimental characterization of a silicon nitride photonic integrated circuit (PIC) that is able to frequency stabilize 16 semiconductor lasers, simultaneously. A stabilized 50 GHz-spaced two-channel system is demonstrated through the detection of the related beating note and the stability of the resulting waveform is characterized via the use of artificially induced thermal cross-talk stimuli.

Vacuum deposition of χ(2) nonlinear organic single crystal films on silicon

Integrating second order nonlinear (χ(2)) optical materials on chip is an ongoing challenge for Si photonics. Noncentrosymmetric molecular crystals have the potential to deliver high χ(2) nonlinearity with good thermal stability, but so far have been limited to growth from solution or the melt, which are both difficult to control and scale up in manufacturing. Here, we show that large (>100 μm) single crystal domains of the nonlinear molecule 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene] malononitrile (OH1) can be grown monolithically on either glass or Si via vacuum evaporation, followed by a short thermal annealing step. The crystallites are tens of nanometer thick and exhibit strong second harmonic generation with their primary χ(2) tensor component lying predominantly in plane. Remarkably, we find that a single domain can grow uninterrupted through nearby channels etched on a Si wafer, which may provide a path to integrate OH1 on Si or Si3N4 waveguides for a broad range of χ(2)-based photonic integrated circuit functionality.

End-to-end simulations of photonic phase correctors for adaptive optics systems

Optical beams and starlight distorted by atmospheric turbulence can be corrected with adaptive optics systems to enable efficient coupling into single-mode fibers. Deformable mirrors, used to flatten the wavefront in astronomical telescopes, are costly, sensitive, and complex mechanical components that require careful calibration to enable high-quality imaging in astronomy, microscopy, and vision science. They are also impractical to deploy in large numbers for non-imaging applications like free-space optical communication. Here, we propose a photonic integrated circuit capable of spatially sampling the wavefront collected by the telescope and co-phasing the subapertures to maximize the flux delivered to an output single-mode fiber as the integrated photonic implementation of a deformable mirror. We present the results of end-to-end simulations to quantify the performance of the proposed photonic solution under varying atmospheric conditions toward realizing an adaptive optics system without a deformable mirror for free-space optical receivers.

A chip-integrated homodyne detection system with enhanced bandwidth performance for quantum applications

The rapid development of quantum technology has driven the need for high-performance quantum signal processing modules. Balanced homodyne detector (BHD) is one of the most promising options for practical quantum state measurement, providing substantial advantages of cost-effectiveness, no cooling requirement, and system compactness. However, due to the stringent requirements in BHD design, it typically suffers from a relatively small operating bandwidth which limits the overall speed of a quantum system. In this study, we propose comprehensive modelling for the BHD in quantum applications and enhance the performance of BHDs based on our modelling. Specifically, we utilise a photonic chip approach and optimise the electronic design to create the integrated BHD, which significantly boosts the 3 dB bandwidth to 4.75 GHz and achieves a shot-noise-limited bandwidth of 23 GHz. We demonstrate the capability of this setup to generate quantum random numbers at a rate of 240 Gbit s−1, highlighting its potential for ultra-high-speed quantum communication and quantum cryptography applications.

Low loss fiber-coupled volumetric interconnects fabricated via direct laser writing

Photonic integrated circuits (PICs) are vital for high-speed data transmission. However, optical routing is limited in PICs composed of only one or a few stacked planes. Further, coupling losses must be low in deployed systems. Previously, we developed the subsurface controllable refractive index via beam exposure (SCRIBE) technique to write accurate 3D gradient refractive index (GRIN) profiles within a mesoporous silica scaffold. Here, we apply SCRIBE to fabricate low loss, broadband, polarization insensitive, fiber-coupled, single-mode volumetric interconnects that include waveguides traversing arbitrary 3D paths. By seamlessly integrating mode-matching subsurface lenses and GRIN waveguide tapers, calibrating for positional writing errors, implementing multipass exposure, automating alignment, and switching to antireflection coated fibers, we reduced the insertion loss for a fiber-PIC-fiber interconnect from 50 to 2.14 dB, or 1.47 dB, excluding the fiber array’s loss. Further, we establish an upper bound of 0.45 dB loss per coupler. We report quality factors of 27,000 and 77,000 and bending losses of 6 and 3 dB/cm for 15 and 30 µm radii microrings, respectively. We also demonstrate Bézier escalators, polarization-rotating and polarization-splitting interconnects, and a seven-channel 25 µm pitch volumetric interconnect. The SCRIBE platform presents a clear path toward realizing 3D PICs with unique functionality.