Chip-scale blue light phased array

Optical multi-beam steering and communication using integrated acousto-optics arrays

Optical beam steering enables optical sensing, imaging, and long-range communication over free space. Despite the inherent speed of light, advanced applications increasingly require simultaneous steering of multiple, independently controlled beams, to enhance imaging throughput, boost communication bandwidth, and control qubit arrays for scalable quantum computing. However, precise multi-beam steering and control remain a significant challenge with current solid-state beam steering technologies, driving the need for integrated and scalable multi-beam steering solutions. Here, we report a scalable multi-beam steering system comprising an array of integrated acousto-optic beam steering channels on a thin-film lithium niobate platform. Each channel generates tens of individually controllable beams at 780 nm with sub-microsecond switching time by exciting acoustic waves using multi-tone microwave signals. We demonstrate the system's unique capabilities through multiple-input, multiple-output free-space communications, simultaneously transmitting to multiple receivers at megabits/sec data rates. This technology is readily scalable to steer hundreds of optical beams from a compact chip, potentially advancing many areas of optical technologies and enabling novel applications.

Four directional circular optical phased array for uniform two-dimensional beam steering at visible wavelength

Optical phased arrays (OPAs), owing to their remarkable capacity for rapid and precise beam steering, have emerged as a pivotal technology in various applications such as augmented reality, free-space optical communication, and optical imaging. In this paper, we present a 64-element two-dimensional (2D) circular OPA operating at a visible wavelength of 632.5 nm. To the best of our knowledge, this is the first time that 2D continuous beam steering with pure phase control has been realized at the visible wavelengths. We propose arrowhead antennas to reduce residual energy while maintaining the integrity of the far-field pattern. Furthermore, a four-directional array arrangement has been introduced to expand the field of view and enhance the uniformity of the side lobe suppression ratio (SLSR) at different azimuth angles. The measurement for the fabricated OPA shows 2D beam steering with an elevation angle of 9° and an azimuth angle of 360° with an SLSR of over 3 dB.

Visible light thermo-optic switches using an Nb2O5 waveguide

Visible-light waveguides and thermo-optic switches were fabricated and demonstrated using Nb2O5, a material with a high refractive index in the visible spectrum, to investigate its material properties and switching performance. To the best of our knowledge, this study is the first to explore these characteristics. The thermo-optic coefficient of Nb2O5 was determined to be 2.27 × 10-5 K-1, while the fabricated switch exhibited an extinction ratio of 6.5 dB and a response time of 77 μs. These results suggest that Nb2O5 is a promising material for advancing the miniaturization of visible-light devices.

Implantable silicon neural probes with nanophotonic phased arrays for single-lobe beam steering

In brain activity mapping with optogenetics, patterned illumination is crucial for targeted neural stimulation. However, due to optical scattering in brain tissue, light-emitting implants are needed to bring patterned illumination to deep brain regions. A promising solution is silicon neural probes with integrated nanophotonic circuits that form tailored beam patterns without lenses. Here we propose neural probes with grating-based light emitters that generate a single steerable beam. The light emitters, optimized for blue or amber light, combine end-fire optical phased arrays with slab gratings to suppress higher-order sidelobes. In vivo experiments in mice demonstrated that the optical phased array provided sufficient power for optogenetic stimulation. While beam steering performance in tissue reveals challenges, including beam broadening from scattering and the need for a wider steering range, this proof-of-concept demonstration illustrates the design principles for realizing compact optical phased arrays capable of continuous single-beam scanning, laying the groundwork for advancing optical phased arrays toward targeted optogenetic stimulation.

Development of wafer-scale multifunctional nanophotonic neural probes for brain activity mapping

Optical techniques, such as optogenetic stimulation and functional fluorescence imaging, have been revolutionary for neuroscience by enabling neural circuit analysis with cell-type specificity. To probe deep brain regions, implantable light sources are crucial. Silicon photonics, commonly used for data communications, shows great promise in creating implantable devices with complex optical systems in a compact form factor compatible with high volume manufacturing practices. This article reviews recent developments of wafer-scale multifunctional nanophotonic neural probes. The probes can be realized on 200 or 300 mm wafers in commercial foundries and integrate light emitters for photostimulation, microelectrodes for electrophysiological recording, and microfluidic channels for chemical delivery and sampling. By integrating active optical devices to the probes, denser emitter arrays, enhanced on-chip biosensing, and increased ease of use may be realized. Silicon photonics technology makes possible highly versatile implantable neural probes that can transform neuroscience experiments.

Angularly offset multiline dispersive optical phased array enabling large field of view and plateau envelope

We propose and demonstrate an angularly offset multiline (AOML) dispersive silicon nitride optical phased array (OPA) that enables efficient line beam scanning with an expanded field of view (FOV) and plateau envelope. The suggested AOML OPA incorporates multiline OPA units, which were seamlessly integrated with a 45° angular offset through a thermo-optic switch based on a multimode interference coupler, resulting in a wide FOV that combines three consecutive scanning ranges. Simultaneously, a periodic diffraction envelope rendered by the multiline OPA units contributes to reduced peak intensity fluctuation of the main lobe across the large FOV. An expedient polishing enabling the angled facet was diligently accomplished through the implementation of oblique polishing techniques applied to the 90° angle of the chip. For each dispersive OPA unit, we engineered an array of delay lines with progressively adjustable delay lengths, enabling a passive wavelength-tunable beam scanning. Experimental validation of the proposed OPA revealed efficient beam scanning, achieved by wavelength tuning from 1530 to 1600 nm and seamless switching between multiline OPAs, yielding an FOV of 152° with a main lobe intensity fluctuation of 2.8 dB. The measured efficiency of dispersive scanning was estimated at 0.97°/nm, as intended.

Non-uniform distributed silicon optical phased array for high directionality and a wide steering range

A non-uniform distributed silicon optical phased array (OPA) is proposed and numerically demonstrated to realize high directionality and a wide range for beam steering. The OPA is composed of grating antennas with dual-layer corrugations along silicon strip waveguides, which can achieve a high directionality of 0.96 and a small divergence angle of 0.084°. To reduce the crosstalk between adjacent antennas and realize a wide steering range, the genetic algorithm is improved and utilized to arrange the locations of grating antennas. As a proof of concept, a 32-channel non-uniform distributed OPA is designed and thoroughly optimized. The simulation results successfully demonstrate a two-dimensional wide steering range of 70∘×18.7∘ with a side-mode suppression ratio (SMSR) over 10 dB.

Integrated liquid-crystal-based variable-tap devices for visible-light amplitude modulation

In this Letter, we propose and experimentally demonstrate the first, to our knowledge, integrated liquid-crystal-based (LC-based) variable-tap devices for visible-light amplitude modulation. These devices leverage the birefringence of LC medium to actively tune the coupling coefficient between two waveguides. First, we develop the device structure, theory of operation, and design procedure. Next, we summarize the fabrication and LC packaging procedure for these devices. Finally, we experimentally demonstrate amplitude modulation with 15.4-dB tap-port extinction within ±3.1 V for a 14-µm-long device at a 637-nm operating wavelength. These small-form-factor variable-tap devices provide a compact and low-power solution to integrated visible-light amplitude modulation and will enable future high-density integrated visible-light systems.

Absorption and scattering limits of silicon nitride integrated photonics in the visible spectrum

Visible-light photonic integrated circuits (PICs) promise scalability for technologies such as quantum information, biosensing, and scanning displays, yet extending large-scale silicon photonics to shorter wavelengths has been challenging due to the higher losses. Silicon nitride (SiN) has stood out as the leading platform for visible photonics, but the propagation losses strongly depend on the film's deposition and fabrication processes. Current loss measurement techniques cannot accurately distinguish between absorption and surface scattering, making it difficult to identify the dominant loss source and reach the platform's fundamental limit. Here we demonstrate an ultra-low loss, high-confinement SiN platform that approaches the limits of absorption and scattering across the visible spectrum. Leveraging the sensitivity of microresonators to loss, we probe and discriminate each loss contribution with unparalleled sensitivity, and derive their fundamental limits and scaling laws as a function of wavelength, film properties and waveguide parameters. Through the design of the waveguide cross-section, we show how to approach the absorption limit of the platform, and demonstrate the lowest propagation losses in high-confinement SiN to date across the visible spectrum. We envision that our techniques for loss characterization and minimization will contribute to the development of large-scale, dense PICs that redefine the loss limits of integrated platforms across the electromagnetic spectrum.

Liquid-crystal-based visible-light integrated optical phased arrays and application to underwater communications

In this Letter, we present the first, to the best of our knowledge, liquid-crystal-based integrated optical phased arrays (OPAs) that enable visible-light beam forming and steering. A cascaded OPA architecture is developed and experimentally shown to emit a beam in the far field at a 632.8-nm wavelength with a power full width at half maximum of 0.4°×1.6° and 7.2° beam-steering range within ±3.4 V. Furthermore, we show the first visible-light integrated-OPA-based free-space-optical-communications transmitter and use it to demonstrate the first integrated-OPA-based underwater-wireless-optical-communications link. We experimentally demonstrate a 1-Gbps on-off-keying link through water and an electronically-switchable point-to-multipoint link with channel selectivity greater than 19 dB through a water-filled tank.

Ultra-Compact and Broadband Nano-Integration Optical Phased Array

The on-chip nano-integration of large-scale optical phased arrays (OPAs) is a development trend. However, the current scale of integrated OPAs is not large because of the limitations imposed by the lateral dimensions of beam-splitting structures. Here, we propose an ultra-compact and broadband OPA beam-splitting scheme with a nano-inverse design. We employed a staged design to obtain a T-branch with a wavelength bandwidth of 500 nm (1300-1800 nm) and an insertion loss of -0.2 dB. Owing to the high scalability and width-preserving characteristics, the cascaded T-branch configuration can significantly reduce the lateral dimensions of an OPA, offering a potential solution for the on-chip integration of a large-scale OPA. Based on three-dimensional finite-difference time-domain (3D FDTD) simulations, we demonstrated a 1 × 16 OPA beam-splitter structure composed entirely of inverse-designed elements with a lateral dimension of only 27.3 μm. Additionally, based on the constructed grating couplers, we simulated the range of the diffraction angle θ for the OPA, which varied by 0.6°-41.6° within the wavelength range of 1370-1600 nm.

High-performance optical phased array for LiDARs demonstrated by monolithic integration of polymer and SiN waveguides

Optical phased array (OPA) beam scanners for light detection and ranging (LiDAR) are proposed by integrating polymer waveguides with superior thermo-optic effect and silicon nitride (SiN) waveguides exhibiting strong modal confinement along with high optical power capacity. A low connection loss of only 0.15 dB between the polymer and SiN waveguides was achieved in this work, enabling a low-loss OPA device. The polymer-SiN monolithic OPA demonstrates not only high optical throughput but also efficient beamforming and stable beam scanning. This novel integrative approach highlights the potential of leveraging heterogeneous photonic materials to develop advanced photonic integrated circuits with superior performance.

Gallium arsenide optical phased array photonic integrated circuit

A 16-channel optical phased array is fabricated on a gallium arsenide photonic integrated circuit platform with a low-complexity process. Tested with a 1064 nm external laser, the array demonstrates 0.92° beamwidth, 15.3° grating-lobe-free steering range, and 12 dB sidelobe level. Based on a reverse biased p-i-n structure, component phase modulators are 3 mm long with DC power consumption of less than 5 µW and greater than 770 MHz electro-optical bandwidth. Separately fabricated 4-mm-long phase modulators based on the same structure demonstrate single-sided Vπ·L modulation efficiency ranging from 0.5 V·cm to 1.22 V·cm when tested at wavelengths from 980 nm to 1360 nm.

Integrated lithium niobate optical phased array for two-dimensional beam steering

Optical phased arrays (OPAs) with high speed, low power consumption, and low insertion loss are appealing for various applications, including light detection and ranging, free-space communication, image projection, and imaging. These OPAs can be achieved by fully harnessing the advantages of integrated lithium niobate (LN) photonics, which include high electro-optical modulation speed, low driving voltage, and low optical loss. Here we present an integrated LN OPA that operates in the near-infrared regime. Our experimental results demonstrate 24 × 8° two-dimensional beam steering, a far-field beam spot with a full width at half maximum of 2 × 0.6°, and a sidelobe suppression level of 10 dB. Furthermore, the phase modulator of our OPA exhibits a half-wave voltage of 6 V. The low power consumption exhibited by our OPA makes it highly attractive for a wide range of applications. Beyond conventional applications, our OPA's high speed opens up the possibility of novel applications such as high-density point cloud generation and tomographic holography.

Microcantilever-integrated photonic circuits for broadband laser beam scanning

Laser beam scanning is central to many applications, including displays, microscopy, three-dimensional mapping, and quantum information. Reducing the scanners to microchip form factors has spurred the development of very-large-scale photonic integrated circuits of optical phased arrays and focal plane switched arrays. An outstanding challenge remains to simultaneously achieve a compact footprint, broad wavelength operation, and low power consumption. Here, we introduce a laser beam scanner that meets these requirements. Using microcantilevers embedded with silicon nitride nanophotonic circuitry, we demonstrate broadband, one- and two-dimensional steering of light with wavelengths from 410 nm to 700 nm. The microcantilevers have ultracompact ~0.1 mm² areas, consume ~31 to 46 mW of power, are simple to control, and emit a single light beam. The microcantilevers are monolithically integrated in an active photonic platform on 200-mm silicon wafers. The microcantilever-integrated photonic circuits miniaturize and simplify light projectors to enable versatile, power-efficient, and broadband laser scanner microchips.

Bidirectional wide-angle waveguide grating antennas with flat-top far-field patterns for optical phased arrays

To build advanced all solid-state LiDAR, optical phased arrays (OPAs) with a large field of view are highly desirable. As a critical building block, a wide-angle waveguide grating antenna is proposed here. Instead of aiming at the elimination of downward radiation of waveguide grating antennas (WGAs) to improve efficiencies, we in turn utilize the downward radiation and double the range of beam steering. In addition to widened field of views, the steered beams in two directions come from a common set of power splitters, phase shifters and antennas, which greatly reduces chip complexity and power consumption, especially for large-scale OPAs. Beam interference and power fluctuation in the far field due to downward emission can be decreased by specially designed SiO₂/Si₃N₄ antireflection coating. The WGA exhibits balanced emissions in both the upward and downward directions, in which the field of view in each direction is more than 90°. The normalized intensity remains almost the same with a small variation of 10% from -39° to 39° for the upward emission and from -42° to 42° for the downward emission. This WGA is featured by a flat-top radiation pattern in far field, high emission efficiency and good tolerance to device fabrication errors. It holds good potential to achieve wide-angle optical phased arrays.

Monolithic integration of polymer waveguide phase modulators with silicon nitride waveguides using adiabatic transition tapers

Polymer waveguide phase modulators (PMs) demonstrate high thermal confinement with outstanding thermo-optic properties and can provide stable low-power phase modulation in optical phased arrays (OPA). On the other hand, silicon nitride (SiN) waveguides produce stronger optical confinement with smaller waveguide core sizes than polymer waveguides and can handle high optical power without nonlinear effects. In this work, a high-performance PM was achieved by monolithic integration of a polymer waveguide and tapered SiN input and output waveguides. The integration of heterogeneous waveguide materials on a single substrate will enable the fabrication of efficient OPAs for advanced imaging, display, sensing, and communications applications.

Ultra-wideband integrated photonic devices on silicon platform: from visible to mid-IR

Silicon photonics has gained great success mainly due to the promise of realizing compact devices in high volume through the low-cost foundry model. It is burgeoning from laboratory research into commercial production endeavors such as datacom and telecom. However, it is unsuitable for some emerging applications which require coverage across the visible or mid infrared (mid-IR) wavelength bands. It is desirable to introduce other wideband materials through heterogeneous integration, while keeping the integration compatible with wafer-scale fabrication processes on silicon substrates. We discuss the properties of silicon-family materials including silicon, silicon nitride, and silica, and other non-group IV materials such as metal oxide, tantalum pentoxide, lithium niobate, aluminum nitride, gallium nitride, barium titanate, piezoelectric lead zirconate titanate, and 2D materials. Typical examples of devices using these materials on silicon platform are provided. We then introduce a general fabrication method and low-loss process treatment for photonic devices on the silicon platform. From an applications viewpoint, we focus on three new areas requiring integration: sensing, optical comb generation, and quantum information processing. Finally, we conclude with perspectives on how new materials and integration methods can address previously unattainable wavelength bands while maintaining the advantages of silicon, thus showing great potential for future widespread applications.

Comparative analysis of speckle-based single-pixel imaging using uniform and non-redundant optical phased arrays

An optical phased array (OPA) is a compact high-speed wavefront modulation device that is promising for next-generation optical sensing systems. In particular, speckle-based single-pixel imaging (SSPI) using OPA is an attractive scheme since precise tuning of optical phases is unnecessary. In this work, we present a comprehensive analysis of SSPI using an OPA with N phase shifters by comparing two classes of OPAs: uniformly spaced OPA (UOPA) and non-redundant OPA (NROPA). Through singular value decomposition analysis of the illumination patterns generated from the OPA, we clarify the theoretical limit of the imaging resolution for each case. As a result, the number of resolvable points can be as large as N ²-N+1 for the case of NROPA. This is in clear contrast to the case of UOPA, where the number of resolvable points can only be as large as 2N-1. Finally, imaging results of a test target are compared to study the impact of the array layout in OPA-based SSPI. Our work provides theoretical understanding of OPA-based SSPI and reveals the effectiveness of SSPI using NROPA.

Design of Monolithic 2D Optical Phased Arrays Heterogeneously Integrated with On-Chip Laser Arrays Based on SOI Photonic Platform

In this work, heterogeneous integration of both two-dimensional (2D) optical phased arrays (OPAs) and on-chip laser arrays based on a silicon photonic platform is proposed. The tunable multi-quantum-well (MQW) laser arrays, active switching/shifting arrays, and grating antenna arrays are used in the OPA module to realize 2D spatial beam scanning. The 2D OPA chip is composed of four main parts: (1) tunable MQW laser array emitting light signals in the range of 1480-1600 nm wavelengths; (2) electro-optic (EO) switch array for selecting the desired signal light from the on-chip laser array; (3) EO phase-shifter array for holding a fixed phase difference for the uniform amplitude of specific optical signal; and (4) Bragg waveguide grating antenna array for controlling beamforming. By optimizing the overall performances of the 2D OPA chip, a large steering range of 88.4° × 18° is realized by tuning both the phase and the wavelength for each antenna. In contrast to the traditional thermo-optic LIDAR chip with an external light source, the overall footprint of the 2D OPA chip can be limited to 8 mm × 3 mm, and the modulation rate can be 2.5 ps. The ultra-compact 2D OPA assembling with on-chip tunable laser arrays using hybrid integration could result in the application of a high-density, high-speed, and high-precision lidar system in the future.

Dispersive silicon-nitride optical phased array incorporating arrayed waveguide delay lines for passive line beam scanning

As optical phased arrays (OPAs), used as solid-state beam scanning elements, swiftly stride towards higher efficiency and faster scanning speed, the line beam scanner is emerging as a viable substitute for its counterpart relying on point-beam-incorporated raster scanning. However, line-beam scanners require active phase shifters for beam scanning; thus, they consume more power and have complex device designs. This study proposes and demonstrates a dispersive silicon-nitride OPA that allows for passive wavelength-tuned steering of a line beam with an elongated vertical beamwidth. To steer the line beam passively covering the two-dimensional field of view, we deployed an array of delay lines with progressive delay lengths across adjacent channels. Furthermore, adiabatic tapers that allow precise effective array aperture adjustment are used as emitter elements to flexibly realize different vertical beamwidths. Combinations of different delay-length differences and taper tip-widths resulted in beam coverage (lateral × vertical) ranging from 6.3° × 19° to 23.8° × 40° by tuning the wavelength from 1530 to 1600 nm. The main lobe emission throughput was as small as - 2.8 dB. To the best of our knowledge, the embodied OPA is the first demonstration of a passive line beam scanner facilitating an adjustable beam coverage with quick operation and enhanced efficiency.

Monolithically integrated, broadband, high-efficiency silicon nitride-on-silicon waveguide photodetectors in a visible-light integrated photonics platform

Visible and near-infrared spectrum photonic integrated circuits are quickly becoming a key technology to address the scaling challenges in quantum information and biosensing. Thus far, integrated photonic platforms in this spectral range have lacked integrated photodetectors. Here, we report silicon nitride-on-silicon waveguide photodetectors that are monolithically integrated in a visible light photonic platform on silicon. Owing to a leaky-wave silicon nitride-on-silicon design, the devices achieved a high external quantum efficiency of >60% across a record wavelength span from λ ~ 400 nm to ~640 nm, an opto-electronic bandwidth up to 9 GHz, and an avalanche gain-bandwidth product up to 173 ± 30 GHz. As an example, a photodetector was integrated with a wavelength-tunable microring in a single chip for on-chip power monitoring.

Wafer-level calibration of large-scale integrated optical phased arrays

We present the wafer-level characterization of a 256-channel optical phased array operating at 1550 nm, allowing the sequential testing of different OPA circuits without any packaging steps. Using this, we establish that due to random fabrication variations, nominally identical circuits must be individually calibrated. With this constraint in mind, we present methods that significantly reduce the time needed to calibrate each OPA circuit. In particular, we show that for an OPA of this scale, a genetic optimization algorithm is already >3x faster than a simple hill climbing algorithm. Furthermore, we describe how the phase modulators within the OPA may be individually characterized 'in-situ' and how this information can be used to configure the OPA to emit at any arbitrary angle following a single, initial calibration step.

High modulation efficiency and large bandwidth thin-film lithium niobate modulator for visible light

We experimentally demonstrate an integrated visible light modulator at 532 nm on the thin-film lithium niobate platform. The waveguides on such platform feature a propagation loss of 2.2 dB/mm while a grating for fiber interface has a coupling loss of 5 dB. Our fabricated modulator demonstrates a low voltage-length product of 1.1 V·cm and a large electro-optic bandwidth with a roll-off of -1.59 dB at 25 GHz for a length of 3.3 mm. This device offers a compact and large bandwidth solution to the challenge of integrated visible wavelength modulation in lithium niobate and paves the way for future small-form-factor integrated systems at visible wavelengths.

Electrically Driven Reprogrammable Vanadium Dioxide Metasurface Using Binary Control for Broadband Beam Steering

Resonant optical phased arrays are a promising way to reach fully reconfigurable metasurfaces in the optical and near-infrared (NIR) regimes with low energy consumption, low footprint, and high reliability. Continuously tunable resonant structures suffer from inherent drawbacks such as low phase range, amplitude-phase correlation, or extreme sensitivity that makes precise control at the individual element level very challenging. We computationally investigate 1-bit (binary) control as a mechanism to bypass these issues. We consider a metasurface for beam steering using a nanoresonator antenna and explore the theoretical capabilities of such phased arrays. A thermally realistic structure based on vanadium dioxide sandwiched in a metal-insulator-metal structure is proposed and optimized using inverse design to enhance its performance at 1550 nm. Continuous beam steering over 90° range is successfully achieved using binary control, with excellent agreement with predictions based on the theoretical first-principles description of phased arrays. Furthermore, a broadband response from 1500 to 1700 nm is achieved. The robustness to the design manufacturing imperfections is also demonstrated. This simplified approach can be implemented to optimize tunable nanophotonic phased array metasurfaces based on other materials or phased shifting mechanisms for various functionalities.

Reconfigurable scan lens based on an actively controlled optical phased array

Integrated photonics provides a path for miniaturization of an optical system to a compact chip scale and offers reconfigurability by the integration of active components. Here we report a chip-scale reconfigurable scan lens based on an optical phased array, consisting of 30 actively controlled elements on the InP integrated photonic platform. By configuring the phase shifters, we show scanning of a nearly diffraction-limited focused spot with a full width at half maximum spot size down to 2.7 µm at the wavelength of 1550 nm. We demonstrate the key functions needed for a laser-scanning microscope, including light focusing, collection, and steering. We also perform confocal measurements to detect reflection at selective depths.

Two-dimensional silicon optical phased array with large field of view

Optical phased array (OPA) is a promising beam steering component for light detection and ranging (LiDAR) systems. For most LiDAR applications, two-dimensional (2D, lateral and longitudinal) beam steering with large field of view is required. To achieve large lateral and longitudinal field of view, waveguide with nonuniform spacing and broadband tunable laser source is commonly utilized, resulting in complex structure and high cost. Here, a 2D OPA with large field of view is proposed and demonstrated on the silicon-on-insulator platform. Assisted by an improved optical antenna and polarization switch, lateral and longitudinal steering range could be both significantly improved. The experimental results show the steering ranges are 99.24° × 15.62° and 96.48° × 16.08° for transverse electric mode and transverse magnetic mode, respectively. The proposed scheme provides a promising approach to realize the integrated OPA with large field of view.

Free-Space Applications of Silicon Photonics: A Review

Silicon photonics has recently expanded its applications to delivering free-space emissions for detecting or manipulating external objects. The most notable example is the silicon optical phased array, which can steer a free-space beam to achieve a chip-scale solid-state LiDAR. Other examples include free-space optical communication, quantum photonics, imaging systems, and optogenetic probes. In contrast to the conventional optical system consisting of bulk optics, silicon photonics miniaturizes an optical system into a photonic chip with many functional waveguiding components. By leveraging the mature and monolithic CMOS process, silicon photonics enables high-volume production, scalability, reconfigurability, and parallelism. In this paper, we review the recent advances in beam steering technologies based on silicon photonics, including optical phased arrays, focal plane arrays, and dispersive grating diffraction. Various beam-shaping technologies for generating collimated, focused, Bessel, and vortex beams are also discussed. We conclude with an outlook of the promises and challenges for the free-space applications of silicon photonics.

High-performance optical beam steering with nanophotonics

The ability to control and steer optical beams is critical for emerging technologies. Among these are light detection and ranging (LiDAR), laser display, free space communication, and single pixel imaging. Improvements in these areas promise enhanced 3D data collection capabilities, orders of magnitude increase in wireless data rate, less expensive cameras, and ever more immersive virtual/augmented reality (VR/AR) consumer electronics. Bulk mechanical or liquid crystal devices are conventionally utilized platforms that achieve optical beam steering, but they are bulky and limited in speed and reliability. Instead, chip-scale photonic platforms offer faster and more elegant mechanisms to manipulate light, capable of minimizing device size, weight, and power. Additionally, a critical device metric is its far field resolution, which influences fine feature detection in imaging applications, laser display quality, and signal power and fidelity of free space communication links. Strong light matter interaction achieved with nanophotonic approaches generally makes devices smaller and more efficient, yet ultimately these effects must be scaled to suitable aperture sizes to maintain good resolution. Recent years have seen rapid development in these performance characteristics, spurred by research on active metasurfaces, slow light waveguides, and waveguide phased arrays, with different architectures encountering unique tradeoffs between device complexity, resolution, and speed, in attempting to achieve groundbreaking values for all three. We review these diverse emerging nanophotonic approaches that aspire to achieve high-performance optical beam steering.

Superiorly low half-wave voltage electro-optic polymer modulator for visible photonics

Chip-scale optical devices operated at wavelengths shorter than communication wavelengths, such as LiDAR for autonomous driving, bio-sensing, and quantum computation, have been developed in the field of photonics. In data processing involving optical devices, modulators are indispensable for the conversion of electronic signals into optical signals. However, existing modulators have a high half-wave voltage-length product (VπL) which is not sufficient at wavelengths below 1000 nm. Herein, we developed a significantly efficient optical modulator which has low V_πL of 0.52 V·cm at λ = 640 nm using an electro-optic (EO) polymer, with a high glass transition temperature (Tg = 164 °C) and low optical absorption loss (2.6 dB/cm) at λ = 640 nm. This modulator is not only more efficient than any EO-polymer modulator reported thus far, but can also enable ultra-high-speed data communication and light manipulation for optical platforms operating in the ranges of visible and below 1000 nm infrared.

Fast-running beamforming algorithm for optical phased array beam scanners comprised of polymeric waveguide devices

The phase error imposed in optical phased arrays (OPAs) for beam scanning LiDAR is unavoidable due to minute dimensional fluctuations that occur during the waveguide manufacturing process. To compensate for the phase error, in this study, a fast-running beamforming algorithm is developed based on the rotating element vector method. The proposed algorithm is highly suitable for OPA devices comprised of polymer waveguides, where thermal crosstalk between phase modulators is suppressed effectively, allowing for each phase modulator to be controlled independently. The beamforming speed is determined by the number of phase adjustments. Hence, by using the least square approximation for a 32-channel polymer waveguide OPA device the number of phase adjustments needed to complete beamforming was reduced and the beamforming time was shortened to 16 seconds.

Parallel emitted silicon nitride nanophotonic phased arrays for two-dimensional beam steering

In this Letter, a two-dimensional (2D) beam steering on silicon nitride (SiNx) nanophotonic phased arrays from visible to near-infrared wavelengths is reported for the first time, to the best of our knowledge. In order to implement beam steering along the transverse direction for one-dimensional waveguide surface grating arrays, wavelengths from 650 to 980 nm provided by the supercontinuum laser are used to excite the phased array. Then the beams are parallel radiated with steering angles in a sequence of 26.84° to -16.54^∘ along the transverse direction, and a continuous line in the far field consisting of parallel emitted spots is produced with a total view angle of 43.38°. Moreover, this continuous far-field line is steered along the longitudinal direction with massive wavelengths simultaneously tuned by phase shifts from -π/2 to over +π/2. This method with massive parallel wavelengths emitted paves a new way for 2D steering on SiNx nanophotonic phased arrays.

Low-loss broadband bi-layer edge couplers for visible light

Low-loss broadband fiber-to-chip coupling is currently challenging for visible-light photonic-integrated circuits (PICs) that need both high confinement waveguides for high-density integration and a minimum feature size above foundry lithographical limit. Here, we demonstrate bi-layer silicon nitride (SiN) edge couplers that have ≤ 4 dB/facet coupling loss with the Nufern S405-XP fiber over a broad optical wavelength range from 445 to 640 nm. The design uses a thin layer of SiN to expand the mode at the facet and adiabatically transfers the input light into a high-confinement single-mode waveguide (150-nm thick) for routing, while keeping the minimum nominal lithographic feature size at 150 nm. The achieved fiber-to-chip coupling loss is about 3 to 5 dB lower than that of single-layer designs with the same waveguide confinement and minimum feature size limitation.

Investigation and demonstration of a high-power handling and large-range steering optical phased array chip

The optical power handling of an OPA scanning beam determines its targeted detection distance. So far, a limited number of investigations have been conducted on the restriction of the beam power. To the best of our knowledge, we for the first time in this paper explore the ability of the silicon photonics based OPA circuit for the high power application. A 64-channel SiN-Si based one-dimensional (1D) OPA chip has been designed to handle high beam power to achieve large scanning range. The chip was fabricated on the standard silicon photonics platform. The main lobe power of our chip can reach 720 mW and its peak side-lobe level (PSLL) is -10.33 dB. We obtain a wide scanning range of 110° in the horizontal direction at 1550 nm wavelength, with a compressed longitudinal divergence angle of each scanning beam of 0.02°.

Increasing wavelength-controlled steering range of an optical phased array by using two subarrays

We demonstrate an optical phased array that consists of two subarrays based on the silicon on insulator (SOI) platform, each subarray including 16 independent channels. The demonstrated field of view of the optical phased array is 36.6^∘×32.6^∘ with a spot size of 1.68^∘×0.0673^∘. A steering range of 32.6° is achieved by combining two subarrays with different periods and tuning the wavelength from 1500 nm to 1600 nm. In another dimension, the steering is realized by introducing phase differences between channels.

Compact solid-state optical phased array beam scanners based on polymeric photonic integrated circuits

Optical phased array (OPA) devices are being actively investigated to develop compact solid-state beam scanners, which are essential in fields such as LiDAR, free-space optical links, biophotonics, etc. Based on the unique nature of perfluorinated polymers, we propose a polymer waveguide OPA with the advantages of low driving power and high optical throughput. Unlike silicon photonic OPAs, the polymer OPAs enable sustainable phase distribution control during beam scanning, which reduces the burden of beamforming. Moreover, by incorporating a tunable wavelength laser comprising a polymer waveguide Bragg reflector, two-dimensional beam scanning is demonstrated, which facilitates the development of laser-integrated polymeric OPA beam scanners.

Subwavelength structure enabled ultra-long waveguide grating antenna

Because of the high index contrast, current silicon photonics based optical phased arrays cannot achieve small beam divergence and large field-of-view simultaneously without increasing fabrication complexity. To resolve the dilemma, we propose an ultra-long waveguide grating antenna formed by placing subwavelength segments within the evanescent field of a conventional strip waveguide. Bound state in the continuum effect is leveraged to suppress the sidewall emission. As a proof of concept, we theoretically demonstrated a millimeter-long through-etched waveguide grating antenna with a divergence angle of 0.081° and a feature size compatible with current silicon photonics foundries.

Integrated Optical Phased Arrays for Beam Forming and Steering

Integrated optical phased arrays can be used for beam shaping and steering with a small footprint, lightweight, high mechanical stability, low price, and high-yield, benefiting from the mature CMOS-compatible fabrication. This paper reviews the development of integrated optical phased arrays in recent years. The principles, building blocks, and configurations of integrated optical phased arrays for beam forming and steering are presented. Various material platforms can be used to build integrated optical phased arrays, e.g., silicon photonics platforms, III/V platforms, and III–V/silicon hybrid platforms. Integrated optical phased arrays can be implemented in the visible, near-infrared, and mid-infrared spectral ranges. The main performance parameters, such as field of view, beamwidth, sidelobe suppression, modulation speed, power consumption, scalability, and so on, are discussed in detail. Some of the typical applications of integrated optical phased arrays, such as free-space communication, light detection and ranging, imaging, and biological sensing, are shown, with future perspectives provided at the end.

Integrated multi-beam optical phased array based on a 4 × 4 Butler matrix

We design and demonstrate the first, to the best of our knowledge, silicon-based multi-beam optical phased array (MOPA), incorporating a 4×4 Butler matrix beamforming network. The one-dimensional end-fire array consists of 16 emitters at a uniform pitch together with their corresponding phase shifters and is shared among the beams to realize large-scale aliasing-free beam-steering at reduced complexity. Experimental results show that the device is capable of individual beam aliasing-free operation with a field of view up to 46°. The steering envelope shows a plateau where the peak intensities fluctuate within 0.5 dB. The beamforming and beam-steering performance are also evaluated for simultaneous multi-beam operation. Our work validates the feasibility of beamforming-network-based MOPAs, which are promising for applications including light detection and ranging and free-space optical communication.

High-speed programmable lithium niobate thin film spatial light modulator

High-speed spatial modulation of light is the key technology in various applications, such as optical communications, imaging through scattering media, video projection, pulse shaping, and beam steering, in which spatial light modulators (SLMs) are the underpinning devices. Conventional SLMs, such as liquid crystal (LC), digital micromirror device (DMD), and micro-electro-mechanical system (MEMS) ones, operate at a typical speed on the order of several kilohertz as limited by the slow response of the pixels. Achieving high-speed spatial modulation is still challenging and highly desired. Here, we demonstrate a one-dimensional (1D) high-speed programmable spatial light modulator based on the electro-optic effect in lithium niobate thin film, which achieves a low driving voltage of 10 V and an overall high-speed modulation speed of 5 MHz. Furthermore, we transfer an image by using parallel data transmission based on the proposed lithium niobate SLM as a proof-of-principle demonstration. Our device exhibits improved performance over traditional SLMs and opens new avenues for future high-speed and real-time applications, such as light detection and ranging (LiDAR), pulse shaping, and beam steering.

Optimization of aperiodic 3D optical phased arrays based on multilayer Si3N4/SiO2 platforms

Silicon-based optical phased arrays (OPAs) have been widely explored, while the design of the structure with high sidelobe level reduction, remains a big challenge. This work investigated the optimization of the optical path-modulated 3D OPAs with Si₃N₄ as the core layer and SiO₂ as the cladding layer. We used the particle swarm optimization algorithm to optimize high-performance random distributed OPAs. Our study provides an effective pathway to optimize the random distributed OPAs within a controllable time frame among a vast number of parameters.

2D beam steerer based on metalens on silicon photonics

Beam steering with solid-state devices represents the cutting-edge technology for next-generation LiDARs and free-space communication transceivers. Here we demonstrate a platform based on a metalens on a 2D array of switchable silicon microring emitters. This platform enables scalable, efficient, and compact devices that steer in two dimensions using a single wavelength. We show a field of view of 12.4° × 26.8° using an electrical power of less than 83 mW, offering a solution for practical miniature beam steerers.

Broadband silicon nitride nanophotonic phased arrays for wide-angle beam steering

In this Letter, the broadband operation in wavelengths from 520 nm to 980 nm is demonstrated on silicon nitride nanophotonic phased arrays. The widest beam steering angle of 65° on a silicon nitride phased array is achieved. The optical radiation efficiency of the main grating lobe in a broad wavelength range is measured and analyzed theoretically. The optical spots radiated from the phased array chip are studied at different wavelengths of lasers. The nanophotonic phased array is excited by a supercontinuum laser source for a wide range of beam steering for the first time to the best of our knowledge. It paves the way to tune the wavelength from visible to near infrared range for silicon nitride nanophotonic phased arrays.

On the performance of optical phased array technology for beam steering: effect of pixel limitations

Optical phased arrays are of strong interest for beam steering in telecom and LIDAR applications. A phased array ideally requires that the field produced by each element in the array (a pixel) is fully controllable in phase and amplitude (ideally constant). This is needed to realize a phase gradient along a direction in the array, and thus beam steering in that direction. In practice, grating lobes appear if the pixel size is not sub-wavelength, which is an issue for many optical technologies. Furthermore, the phase performance of an optical pixel may not span the required 2π phase range or may not produce a constant amplitude over its phase range. These limitations result in imperfections in the phase gradient, which in turn introduce undesirable secondary lobes. We discuss the effects of non-ideal pixels on beam formation, in a general and technology-agnostic manner. By examining the strength of secondary lobes with respect to the main lobe, we quantify beam steering quality and make recommendations on the pixel performance required for beam steering within prescribed specifications. By applying appropriate compensation strategies, we show that it is possible to realize high-quality beam steering even when the pixel performance is non-ideal, with intensity of the secondary lobes two orders of magnitude smaller than the main lobe.