This paper aims to review the state of the art of Light Detection and Ranging (LiDAR) sensors for automotive applications, and particularly for automated vehicles, focusing on recent advances in the field of integrated LiDAR, and one of its key components: the Optical Phased Array (OPA). LiDAR is still a sensor that divides the automotive community, with several automotive companies investing in it, and some companies stating that LiDAR is a ‘useless appendix’. However, currently there is not a single sensor technology able to robustly and completely support automated navigation. Therefore, LiDAR, with its capability to map in 3 dimensions (3D) the vehicle surroundings, is a strong candidate to support Automated Vehicles (AVs). This manuscript highlights current AV sensor challenges, and it analyses the strengths and weaknesses of the perception sensor currently deployed. Then, the manuscript discusses the main LiDAR technologies emerging in automotive, and focuses on integrated LiDAR, challenges associated with light beam steering on a chip, the use of Optical Phased Arrays, finally discussing current factors hindering the affirmation of silicon photonics OPAs and their future research directions.

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A Review and Perspective on Optical Phased Array for Automotive LiDAR

Three-dimensional (3D) imaging sensors allow machines to perceive, map and interact with the surrounding world1. The size of light detection and ranging (LiDAR) devices is often limited by mechanical scanners. Focal plane array-based 3D sensors are promising candidates for solid-state LiDARs because they allow electronic scanning without mechanical moving parts. However, their resolutions have been limited to 512 pixels or smaller2. In this paper, we report on a 16,384-pixel LiDAR with a wide field of view (FoV, 70° × 70°), a fine addressing resolution (0.6° × 0.6°), a narrow beam divergence (0.050° × 0.049°) and a random-access beam addressing with sub-MHz operation speed. The 128 × 128-element focal plane switch array (FPSA) of grating antennas and microelectromechanical systems (MEMS)-actuated optical switches are monolithically integrated on a 10 × 11-mm2 silicon photonic chip, where a 128 × 96 subarray is wire bonded and tested in experiments. 3D imaging with a distance resolution of 1.7 cm is achieved with frequency-modulated continuous-wave (FMCW) ranging in monostatic configuration. The FPSA can be mass-produced in complementary metal-oxide-semiconductor (CMOS) foundries, which will allow ubiquitous 3D sensors for use in autonomous cars, drones, robots and smartphones. 

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A large-scale microelectromechanical-systems-based silicon photonics LiDAR

Optical phased arrays (OPAs) implemented in integrated photonic circuits could enable a variety of 3D sensing, imaging, illumination, and ranging applications, and their convergence in new LIDAR technology. However, current integrated OPA approaches do not scale - in control complexity, power consumption, and optical efficiency - to the large aperture sizes needed to support medium to long range LIDAR. We present the serpentine optical phased array (SOPA), a new OPA concept that addresses these fundamental challenges and enables architectures that scale up to large apertures. The SOPA is based on a serially interconnected array of low-loss grating waveguides and supports fully passive, two-dimensional (2D) wavelength-controlled beam steering. A fundamentally space-efficient design that folds the feed network into the aperture also enables scalable tiling of SOPAs into large apertures with a high fill-factor. We experimentally demonstrate the first SOPA, using a 1450 - 1650 nm wavelength sweep to produce 16,500 addressable spots in a 27x610 array. We also demonstrate, for the first time, far-field interference of beams from two separate OPAs on a single silicon photonic chip, as an initial step towards long-range computational imaging LIDAR based on novel active aperture synthesis schemes.

Serpentine optical phased arrays for scalable integrated photonic LIDAR beam steering

The experimental realization of a Kerr frequency comb represented the convergence of research in materials, physics, and engineering. This symbiotic relationship continues to underpin efforts in comb innovation today. While the initial focus developing cavity-based frequency combs relied on existing microresonator architectures and classic optical materials, in recent years, this trend has been disrupted. This paper reviews the latest achievements in frequency comb generation using resonant cavities, placing them within the broader historical context of the field. After presenting well-established material systems and device designs, the emerging materials and device architectures are examined. Specifically, the unconventional material systems as well as atypical device designs that have enabled tailored dispersion profiles and improved comb performance are compared to the current state of art. The remaining challenges and outlook for the field of cavity-based frequency combs are evaluated.

Emerging material systems for integrated optical Kerr frequency combs

In the article the authors discuss light detection and ranging (LiDAR) for automotive applications and the potential roles Si photonics can play in practice. The authors review published research work on Si photonics optical phased array (OPA) and other relevant devices in the past decade with in-depth technical analysis with respect to practical system design considerations. The commercialization status of certain LiDAR technologies is briefly introduced.

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Si Photonics for Practical LiDAR Solutions

This paper reports on large field-of-regard, high-efficiency, and large aperture active optical phased arrays (OPAs) for optical beam steering in LIDAR systems. The fabricated 5 mm-long silicon photonic OPA with a 1.3 μm waveguide pitch achieved adjacent waveguide crosstalk below −12dB. A relatively large and uniform emission aperture has been achieved with a low-contrast silicon nitride assisted grating (~20 dB/cm) whose emission profile can be further optimized using an apodized design. The fabricated silicon-photonic OPA demonstrated > 40° lateral beam steering with no sidelobes in a ± 33° field-of-regard and 3.3° longitudinal beam steering via wavelength tuning by 20 nm centered at 1550 nm. We have fully integrated the silicon photonic OPA device with electronic controls and successfully demonstrated 2-dimensional coherent optical beam steering of pre-planned far-field patterns. Future improvements include placement of a distributed Bragg reflector (DBR) underneath the grating emitter in order to achieve nearly a factor of two improvement in emission efficiency.

Sub-wavelength-pitch silicon-photonic optical phased array for large field-of-regard coherent optical beam steering

This paper discusses recent advances on the newly developed high-density three-dimensional photonic integrated circuits (3-D PICs). In particular, we introduce our efforts in design, fabrication, and characterization toward a silicon photonic wafer-scale light detection and ranging (LIDAR) system populated by 3-D PIC unit cells. The 3-D PIC unit cell includes vertical U-shaped photonic couplers created by a combination of anisotropic etching, vertical α-silicon via formation, and wafer bonding. Using the U-shaped photonic couplers, the 3-D PIC unit cell forms a 120-channel folded single tile optical phased array (OPA) with 2- μ m pitch. Ultracompact vertical U-shaped coupler arrays with 1- μ m pitch are also fabricated and tested. The 3-D PIC unit cells will be tiled on a wafer-scale interposer, transmitting and receiving signals through equal-power splitters with pathlength-matched waveguides, and 3-D arbitrarily shaped waveguides or evanescent couplers. Designs of such interposers are shown and low-loss and misalignment tolerant chip-to-interposer coupling using evanescent coupler is demonstrated. Loss values of arbitrarily shaped waveguides fabricated by ultrafast laser inscription on the deposited-SiO2 cladding and their alignment tolerance to conventional silicon nitride edge couplers are reported. A path toward realizing 3-D integrated LIDAR is discussed.

High-Density Wafer-Scale 3-D Silicon-Photonic Integrated Circuits

While compact and low-loss optical coupling to ultrahigh-quality-factor (Q) crystalline resonators is important for a wide range of applications, the major challenge for achieving this coupling stems from the relatively low refractive index of the crystalline resonator host material compared to those of the standard waveguide coupling materials. We report the first demonstration of a single-mode waveguide structure (prism-waveguide coupler) integrated on a low-loss compact silicon nitride platform resulting in low-loss and efficient coupling to magnesium fluoride crystalline resonators by achieving the phase-matched and the mode-matched evanescent wave coupling. The coupling is characterized with 1 dB loss at 1550 nm wavelength. We further present a photonic integrated chip containing a pair of waveguides successfully coupling light into and out of the resonator, demonstrating a planar-waveguide-coupled crystalline resonator with a loaded Q of 1.9×109. We assemble this waveguide-coupled resonator and a distributed-feedback-laser chip into a butterfly package to realize a miniature Kerr optical frequency comb source using self-injection locking of the distributed feedback laser to the waveguide-coupled crystalline resonator.

Low-loss prism-waveguide optical coupling for ultrahigh-Q low-index monolithic resonators

We demonstrate a chip-scale germanium-silicon optical phased array (OPA) fabricated on a CMOS-compatible platform capable of 2D beam steering in the mid-infrared wavelength range. The OPA included a specially designed grating emitter waveguide array with uniform emission intensity along the mm -length waveguide propagation to realize very sharp instantaneous field-of-view (IFOV) and wide beam-steering total-field-of-view (TFOV). The experimental results indicated lateral beam-steering TFOV up to 12.7° by phase-tuning the waveguide array and longitudinal TFOV up to 12° by wavelength tuning. The 3-dB beam divergence is 3.08° × 0.18°. The demonstrated OPA architecture can employ wafer-scale fabrication and integration while supporting sensing and imaging applications in the mid-infrared spectral range.

Solid-State MWIR Beam Steering Using Optical Phased Array on Germanium-Silicon Photonic Platform

We demonstrate a 3D fan-in/fan-out device for multi-core fiber integration fabricated by ultrafast laser inscription using an automatic 3D waveguide routing algorithm. The low propagation loss of 0.3 dB/cm and 0.16 dB/cm have been achieved for two types of glasses.

Yi-Chun Ling

Papers

Sub-wavelength-pitch silicon-photonic optical phased array for large field-of-regard coherent optical beam steering

High-Density Wafer-Scale 3-D Silicon-Photonic Integrated Circuits

Low-loss prism-waveguide optical coupling for ultrahigh-Q low-index monolithic resonators

Solid-State MWIR Beam Steering Using Optical Phased Array on Germanium-Silicon Photonic Platform

Low-Loss Three-Dimensional Fan-in/Fan-out Devices for Multi-Core Fiber Integration