Silicon photonics is an enabling technology that provides integrated photonic devices and systems with low-cost mass manufacturing capability. It has attracted increasing attention in both academia and industry in recent years, not only for its applications in communications, but also in sensing. One important issue of silicon photonics that comes with its high integration density is an interface between its high-performance integrated waveguide devices and optical fibers or free-space optics. Surface grating coupler is a preferred candidate that provides flexibility for circuit design and reduces effort for both fabrication and alignment. In the past decades, considerable research efforts have been made on in-plane grating couplers to address their insufficiency in coupling efficiency, wavelength sensitivity and polarization sensitivity compared with out-of-plane edge-coupling. Apart from improved performances, new functionalities are also on the horizon for grating couplers. In this paper, we review the current research progresses made on grating couplers, starting from their fundamental theories and concepts. Then, we conclude various methods to improve their performance, including coupling efficiency, polarization and wavelength sensitivity. Finally, we discuss some emerging research topics on grating couplers, as well as practical issues such as testing, packaging and promising applications.

https://www.mdpi.com/2072-666X/11/7/666/pdf

Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues.

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

Compact beam steering in the visible spectral range is required for a wide range of emerging applications, such as augmented and virtual reality displays, optical traps for quantum information processing, biological sensing, and stimulation. Optical phased arrays (OPAs) can shape and steer light to enable these applications with no moving parts on a compact chip. However, OPA demonstrations have been mainly limited to the near-infrared spectral range due to the fabrication and material challenges imposed by the shorter wavelengths. Here, we demonstrate the first chip-scale phased array operating at blue wavelengths (488 nm) using a high-confinement silicon nitride platform. We use a sparse aperiodic emitter layout to mitigate fabrication constraints at this short wavelength and achieve wide-angle beam steering over a 50° field of view with a full width at half-maximum beam size of 0.17°. Large-scale integration of this platform paves the way for fully reconfigurable chip-scale three-dimensional volumetric light projection across the entire visible range.

Chip-scale blue light phased array

We present a $1\times 16$ silicon optical phased array using electro-optic phase shifters to attain high-speed operation with low power consumption. The phase shifters are constructed using a $p$ - $i$ - $n$ junction structure with the $p$ - and $n$ -doped regions formed on both sides of the $i$ -region. The $i$ -region width of the phase shifter is optimized considering the power consumption for phase-tuning and the propagation loss in the phase shifter. The fabricated $p$ - $i$ - $n$ phase shifter exhibits a fast operating speed of 20 MHz and a low phase-tuning power of 1.7 mW/ $\pi $ . From a 1-D optical phased array integrated with 2- $\mu \text{m}$ -pitch grating radiators, a wide beam-steering range of 45° is attained along the transversal direction at a $1.55~\mu \text{m}$ wavelength. Average power consumption for the beam-forming operation of the $1\times 16$ OPA is measured to be 39.6 mW and the average transition time to steer the beam is 24 ns.

Silicon-Based Optical Phased Array Using Electro-Optic $p$ - $i$ - $n$ Phase Shifters

A SiN-Si dual-layer optical phased array (OPA) chip is designed and fabricated. It combines the low loss of SiN with the excellent modulation performance of Si, which improves the performance of Si single-layer OPA. A novel optical antenna and an improved phase modulation method are also proposed, and a two-dimensional scanning range of 96°×14° is achieved, which makes the OPA chip more practical.

/pdf/design-and-fabrication-of-a-sin-si-dual-layer-optical-phased-55dgn84r6g.pdf

Design and fabrication of a SiN-Si dual-layer optical phased array chip

Heterogeneous silicon photonics is uniquely positioned to address the photonic sensing needs of upcoming autonomous cars and provide the necessary cost reduction for widespread deployment. This is because it allows for wafer-scale active/passive integration, including optical sources. We present our recent research and the development of interferometric optical gyroscopes and LiDAR sensors. More specifically, we show a fully integrated gyroscope front-end occupying an area of only 4.5 mm2. We also show the first dense pitch optical phased array using heterogeneous phase shifters. The 4 µm pitch heterogeneous phase shifters provide very low V2π of only 0.35-1.4 V across 200 nm, low residual amplitude modulation of only 0.1-0.15 dB for 2π phase shift, extremely low static power consumption ( 1 GHz). All of these factors make them ideal for next-generation LiDAR systems that employ optical phased arrays.

Heterogeneous silicon photonics sensing for autonomous cars.

In this paper, we demonstrate a radiation-strength-chirped metasurface centimeter-long grating antenna, whose beam quality is approaching the diffraction limit in its footprint with 90 percent efficiency.Instead of chirping the duty cycle to tune radiation strength, which calls for non-scalable fabrication resolution, we design a 2D fish-bone surface grating with apodized width of the bone.The meta surface is made by silicon nitride on top of silicon waveguide to weakly perturb the mode. Moreover, a Monte Carlo Markov Chain(MCMC) algorithm is implemented to design the radiation strength profile, making the optimized trade off between efficiency, beam quality and fabrication challenge. A metal mirror is designed to deposit on the top of the cladding for unidirectional upside-down radiation and uniform lateral gain. For the 1cm grating, our design is for a 0.01 degree beam with 37000 1D directional gain and 90% efficiency, with 200nm feature size. Finally, we fabricated the antenna and measured the beam quality with Fourier imaging system, which gives result of 0.02 degree -10dB beam divergence. To the best of our knowledge, this is the state-of-the-art on-chip antenna with CMOS- foundry-compatible technology.

Diffraction limited centimeter scale radiator: metasurface grating antenna for phased array LiDAR

In this work, we report a dense heterogeneous integration of III-V/Si phase shifters for optical phased arrays. The phase shifters have a V 2π of 0.9 V and optical loss of 0.15 dB at 1550 nm for 2π phase shift with low power consumption of less than 3 nW. We integrate those phase shifters in optical phased arrays with a pitch of 4 µm and the number of channel up to 240. 2D beamsteering is also demonstrated in a 32-channel optical phased array.

Dense III-V/Si Phase Shifters for Optical Phased Arrays

High-performance integrated optical phased arrays (OPAs) are crucial for building up energy-efficient solid-state beam steering systems on a chip. This work demonstrates a first heterogeneous III-V/Si phase-shifter based OPA that utilizes III-V multiple quantum well (MQW) structures to provide superior performance. Demonstrated phase shifters have Vpi of only 0.45 V and residual amplitude modulation (RAM) of only 0.15 dB at 1550 nm for 2pi phase shift with low power consumption of less than 3 nW, which are, to the best our knowledge, the state-of-the-art performance with a record low operating voltage, low power consumption, and low RAM in on-chip OPAs. Their bandwidth is >1 GHz across 200 nm wavelength range. Wavelength tuning is used to demonstrate two-dimensional beam steering with a field of view of 28x51 degree in the 32-channel OPA with an emitter-array pitch of 2 um. Side-mode suppression ratio of 16 dB is also achieved. The high-performance heterogeneous OPA paves the way for scalable and efficient on-chip beam steerers with large steering range and high beam quality.

Jinxi Huang

Papers

Heterogeneous silicon photonics sensing for autonomous cars.

Diffraction limited centimeter scale radiator: metasurface grating antenna for phased array LiDAR

Dense III-V/Si Phase Shifters for Optical Phased Arrays

Dense III-V/Si Phase-Shifter Based Optical Phased Array