Fan-beam steering device using a photonic crystal slow-light waveguide with surface diffraction grating

Compact unidirectional waveguide grating emitter with enhanced wavelength sensitivity based on the hybrid plasmonic mode

Waveguide grating antennas are widely adopted in beam-steering devices, typically enabling the beam steering in longitudinal direction within a two-dimensional scanning optical array by changing the input wavelength. However, traditional waveguide grating antennas suffer from limited tuning range due to low dispersion of the gratings. In this paper, a compact silicon grating waveguide antenna array is proposed with enhanced wavelength sensitivity by introducing a periodically modulated hybrid plasmonic mode. The hybrid plasmonic mode is supported by the hybrid plasmonic waveguides (HPWs) composed of silicon waveguides and periodic subwavelength silver strips. In order to convert the guided waves to the radiated waves, a series of silicon emitting segments are deposited above the HPWs. Additionally, the horizontally arranged array of HPWs also acts as a reflector of the downward radiation, resulting in an effective unidirectional emission. Through the optimization of physical parameters, the proposed antenna array achieves a wavelength-length tuning efficiency up to 0.3°/nm within the wavelength range of 1500∼1600 nm, exhibiting a significant improvement compared with traditional ones. Moreover, an average upward emissivity exceeding 80% with a maximum value of 89% within the 100 nm bandwidth is demonstrated through the numerical simulations. The proposed compact antenna array provides an alternative solution in realizing large-scale integrated high-tuning-efficiency optical beam-steering devices.

Deep learning improves performance of topological bending waveguides

This study introduced design informatics using deep learning in a topological photonics system and applied it to a topological waveguide with a sharp bending structure to further reduce propagation loss. The sharp bend in the topological waveguide composed of two photonic crystals wherein dielectrics having C6v symmetry were arranged in triangle lattices of hexagons, and the designing of parameters individually for 6 × 6 unit cells near the bending region using deep learning resulted in an output improvement of 60% compared to the initial structure. The proposed structural design method has high versatility and applicability for various topological photonic structures.

Demonstration of a highly efficient topological vertical coupler

A defect structure is proposed for enhancing the coupling efficiency of vertically incident circularly polarized light in a topological waveguide. In the topological edge-state waveguide based on triangle lattices of hexagons consisting of six nanoholes respecting C6v symmetry in a silicon optical circuit, the vertical coupling rate is improved by removing the nanoholes from one hexagonal cell near the line. The coupling efficiency was evaluated with and without the defect structure. The introduced defect structure operates suitably for focused beams of left- and right-handed circularly polarized light, enhancing the optical communication wavelength bandwidth by up to 10 dB.

Topological Photonic Lattice for Uniform Beam Splitting, Robust Routing, and Sensitive Far-Field Steering

Far-field optical beam steering is a fast-growing technology for communications, spatial ranging, and detections. Nonmechanical optical phased arrays based on straight waveguides have been studied recently, where the beam emission angle to the propagation axis can be scanned by conveniently tuning the wavelength. However, the dispersion of the waveguide limits the wavelength sensitivity of beam steering and the deliberately created emitters inevitably introduce in-line backscattering on-chip. To overcome these limitations, here, we report a robust and back-reflection-free topological photonic integrated circuit, where different functionalities, such as beam splitting, routing, and far-field steering, are defined by strategic arrangements of lattices with different topological modulations simply controlled by a single lattice deformation parameter. Benefiting from the robust topological scheme, an extra band flattening is applied to achieve far-field steering with high wavelength sensitivity.

Two-dimensional scanning of silicon-based focal plane array with field-of-view splicing technology

All-solid-state beam scanning chip is ideal for next-generation LiDAR due to its reliability and small size. Here we propose a focal plane array chip for two-dimensional scanning using field-of-view splicing technology on silicon photonics platform. The chip has two rotationally symmetric structures, each including a 1 × 64 antenna array accompanied by a 1 × 64 micro-ring optical switch array. We demonstrate a two-dimensional scanning equivalent to an 8-line LiDAR with a field-of-view of 82° × 32°, a beam divergence angle of 0.07° × 0.07°, and a background suppression ratio of over 20 dB. Our chip works in such a simple way that only one optical switch needs to be turned on each time the beam is emitted. And the chip is scalable that a larger range of two-dimensional scanning can be achieved when using more antennas for field-of-view splicing and cascading more optical switches.

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.

Phase-combining unit for aliasing suppression in an optical phased array

Integrated optical phased array (OPA) devices have been widely studied as a solution for solid-state light detection and ranging technology in the autonomous driving application. In this work, a phase-combining unit (PCU) is proposed and studied. With a given number (N) of phase shifters, instead of the general N (phase shifters) to N (emitters) control, the PCU can enable an N to 2N-1 control, which efficiently suppresses the aliasing effect. The theoretical analysis, numerical simulation, and experimental proof-of-concept have been completed in this work. The results show that a maximum suppression of 92.54% can be achieved for the grating lobes in simulation, and an average 53.76% is tested for one grating lobe in the experiment. In conclusion, the PCU can be used as a universal aliasing suppression unit on many types of integrated OPA devices.

Highly efficient vertical coupling to a topological waveguide with defect structure

In this study, we propose a defect structure that enhances the vertical coupling efficiency of circularly polarized light incident on topological waveguides consisting of triangle nanoholes with C_6v symmetry arranged in honeycomb lattice. The defect structure was formed by removing triangle nanoholes from a certain hexagonal unit cell around the topological waveguide. As a result of comparing the coupling efficiency with and without the defect structure through three-dimensional finite-difference time-domain analysis, significant improvement in the vertical coupling efficiency was observed over the entire telecom C band (4460%@1530 nm). In addition, it was also found that the wavelength showing maximum coupling efficiency can be controlled over the entire C band by changing the arrangement of the dielectric around the defect structure.

Solid-state FMCW LiDAR with two-dimensional spectral scanning using a virtually imaged phased array

The beam-steering device is a critical component in LiDAR systems for 3D imaging. Solid-state beam-steering devices attract the most attention for their advantages of robustness, fast beam-steering speed, and stability. However, solid-state beam-steering devices, such as optical phased arrays (OPAs), are challenging to realize 2D scanning ability. Here we employed a virtually imaged phased array (VIPA) in the LiDAR system to realize all solid-state two-dimensional (2D) beam-steering based on dispersion only. A frequency swept laser source is used for performing optical frequency-modulated continuous-wave (FMCW) ranging and 2D beam steering simultaneously. The 2D disperser is compact and can be easily implemented owing to its simple structure. The mechanism of continuous scanning and ranging is beneficial for obtaining high lateral resolution, and a lateral resolution of 0.06° is achieved. 3D maps of the object at a distance of 2 m are obtained with cm-level ranging precision. The frame rate of the proposed LiDAR system only depends on the wavelength-tuning speed of the swept laser source, with the potential to realize ultrafast solid-state LiDAR systems.

A Survey on LiDAR Scanning Mechanisms

In recent years, light detection and ranging (LiDAR) technology has gained huge popularity in various applications such as navigation, robotics, remote sensing, and advanced driving assistance systems (ADAS). This popularity is mainly due to the improvements in LiDAR performance in terms of range detection, accuracy, power consumption, as well as physical features such as dimension and weight. Although a number of literatures on LiDAR technology have been published earlier, not many has been reported on the state-of-the-art LiDAR scanning mechanisms. The aim of this article is to review the scanning mechanisms employed in LiDAR technology from past research works to the current commercial products. The review highlights four commonly used mechanisms in LiDAR systems: Opto-mechanical, electromechanical, micro-electromechanical systems (MEMS), and solid-state scanning. The study reveals that electro-mechanical scanning is the most prominent technology in use today. The commercially available 1D time of flight (TOF) LiDAR instrument is currently the most attractive option for conversion from 1D to 3D LiDAR system, provided that low scanning rate is not an issue. As for applications with low size, weight, and power (SWaP) requirements, MEMS scanning is found to be the better alternative. MEMS scanning is by far the more matured technology compared to solid-state scanning and is currently given great emphasis to increase its robustness for fulfilling the requirements of ADAS applications. Finally, solid-state LiDAR systems are expected to fill in the gap in ADAS applications despite the low technology readiness in comparison to MEMS scanners. However, since solid-state scanning is believed to have superior robustness, field of view (FOV), and scanning rate potential, great efforts are given by both academics and industries to further develop this technology.

Polarization-division and spatial-division shared-aperture nanopatch antenna arrays for wide-angle optical beam scanning

Chip-based optical beam scanners hold promise for future compact high-speed light detection and ranging (LIDAR) systems. Many of the demonstrated chip-based optical beam scanners are designed based on diffraction-based waveguide gratings as on-chip antennas. The waveguide grating antenna, however, only provides a typical field-of-view (FOV) of roughly 10° by tuning the input light wavelength. In this paper, polarization-division and spatial-division multiplexed nanoantenna arrays are proposed to expand the FOV of on-chip antennas. The proposed device, based on silicon-on-insulator (SOI) platform, consists of three nanoantenna groups which are densely packed and fed by a common silicon nanostrip. It is demonstrated that the combination of the optical mode-multiplexing technique and the antenna engineering allows independent controls over the interactions between multiple nanoantenna groups and the waveguide. By proper engineering of the antenna dimensions, the proposed device achieves a FOV of over 40° within a 100 nm wavelength tuning range, almost tripling that of the conventional waveguide grating antenna.

Compound period grating coupler for double beam generation and steering

Grating couplers are one of the most basic integrated photonic structures. They have raised tremendous research interest due to their outstanding performance in compact nonmechanical beam steering. Here we propose a new compound period grating coupler formed by combining two grating structures with different periodicities. The new compound period grating coupler structure can couple the waveguide mode into two radiation modes with different angles. Therefore, the beam steering range is doubled due to the extra beam. We numerically demonstrate this idea, and a 26.20° steering range is observed within a wavelength tuning range of 1500 to 1600 nm. The compound period grating structure with a distributed Bragg reflector as the substrate is also numerically demonstrated, and its energy leakage to the substrate is highly suppressed. In addition, the investigation of fabrication tolerance shows that the new structure can be fabricated with the current CMOS technology.

Wavelength-division multiplexing Si photonic crystal beam steering device for high-throughput parallel sensing

We proposed and demonstrated a wavelength-division multiplexing (WDM) optical beam-steering device consisting of a thermally controlled doubly periodic Si two-dimensional bulk photonic crystal waveguide and coupled microring multiplexers. Beam forming and steering while maintaining a sharp profile is much easier in this device than with optical phased arrays which need the fine phase control. By dividing the range of beam-steering angles into different wavelength channels, it is possible to cover a wide range of angles, even when each angle is small. In this study, we fabricated a device with four wavelength channels, each of which showed beam steering of 4°-5° as a result of heating, resulting in a total of 16°. Two-dimensional steering is also achieved by loading a collimator lens and selecting one waveguide from those arrayed. We evaluated 112 resolution points with four wavelengths and 448 points in total by switching four waveguides. If this WDM concept is introduced into light detection and ranging and the number of wavelengths is increased, it will be possible to increase the sensing throughput, which is usually constrained by the round-trip time of light, by simultaneous parallel operation.

Experimental simulation of ranging action using Si photonic crystal modulator and optical antenna

Time of flight light detection and ranging (LiDAR) has been tested and used as a key device for auto-driving of vehicles. Frequency-modulated continuous-wave (FMCW) LiDAR potentially achieves a high sensitivity. In this study, we fabricated and tested two components of FMCW LiDAR based on Si photonics. The ranging action was also experimentally simulated. A Si photonic crystal slow light Mach-Zehnder modulator was driven by linearly frequency-chirped signals to generate quasi-frequency-modulated signal light. Then, the light was inserted into a fiber delay line of 20-320 m. Its output was irradiated to a photonic crystal slow beam steering device that acted as an optical antenna via a free-space transmission. The detected light was mixed with the reference light branched after the modulation in balanced photodiodes. A sufficiently sharp beat spectrum was observed, whose frequency well agreed with that expected for the delay line. The experimental simulation of the FMCW LiDAR, thus, was achieved.

Thermally controlled Si photonic crystal slow light waveguide beam steering device

The doubly periodic Si photonic crystal waveguide radiates the guided slow light into free space as an optical beam. The waveguide also functions as a beam steering device, in which the steering angle is changed substantially by a slight variation in the wavelength generated due to the large angular dispersion of the slow light. A similar function is obtained when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we tested two kinds of integrated heater structures and observed the beam steering using the thermo-optic effect. For a p-i-p doped waveguide, the heating current was made to flow directly across the waveguide and a beam steering range of 21° was obtained with a relatively low heating power and high-speed response of the order of 100 kHz, maintaining a narrow beam divergence of 0.1-0.3° and a 120 resolution points. We also performed a preliminary life test of the device but did not observe any severe degradation in the temperature variation of 80-430 K for the duration up to 20‒40 h. For a TiN heater device, we obtained the comparable beam steering characteristics, but the required heating power increased, and the response speed decreased drastically.

Two-dimensional beam-steering device using a doubly periodic Si photonic-crystal waveguide

We demonstrate a nonmechanical, on-chip optical beam-steering device using a photonic-crystal waveguide with a doubly periodic structure that repeats the increase and decrease of the hole diameter. We fabricated the device using a complementary metal-oxide-semiconductor process. We obtained a beam-deflection angle of 24° in the longitudinal direction, while maintaining a divergence angle of 0.3°. Four such waveguides were integrated, and one was selected by a Mach-Zehnder optical switch. We obtained lateral beam steering by placing a cylindrical lens above these waveguides. By combining the lateral and longitudinal beam steering, we were able to scan the collimated beam in two dimensions, with 80 × 4 resolution points.