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This post was last edited by Hot Ximixiu on 2024-7-22 08:28
Intelligence and automation are a major trend in the development of future machines. Machines need to autonomously perceive and interact with their surroundings, which is inseparable from various three-dimensional sensors .
LiDAR uses light waves to perform three-dimensional measurements and can achieve higher resolution. It has unique advantages over other three-dimensional sensors and has therefore been widely used in driverless cars, robots, drones, consumer electronics and other fields.
There are various technical solutions for lidar, but most of the current medium and long-range lidars use a scanning beam solution, that is, emitting one or several beams of collimated lasers, measuring the distance to one or several points on the object each time, and then changing the direction of the emitted light in turn to scan the entire field of view.
Traditional LiDARs generally use mechanical rotation to scan beams, which makes most LiDAR products still have shortcomings such as large size and high cost. Miniaturization and integration are the development direction of LiDAR. If solid-state LiDAR can be realized on a chip, it will greatly reduce the power consumption and cost of LiDAR and improve reliability.
The integration of beam scanning devices is one of the major technical difficulties. In recent years, academia and industry have proposed two integrated beam scanning solutions, namely optical phased array and focal plane switch array .
The focal plane switch array has the advantages of simple control, strong scalability, and easy realization of two-dimensional array. Limited by the power consumption and size of the interferometric silicon optical switch, the largest focal plane switch array reported in the literature has only 512 pixels , which is far from the pixel number requirement of practical lidar.
Recently, Professor Wu Mingqiang 's research group at the University of California, Berkeley, used a unique micro-electromechanical system optical switch to realize a large-scale focal plane switch array of 16,384 pixels, and demonstrated the use of the chip as a laser radar implemented as a beam scanning device.
It is worth mentioning that Intel announced the establishment of the Integrated Photonics Research Center in December 2021, which brings together world-renowned researchers in photonics and circuits, with the goal of paving the way for computing interconnection in the next decade. Professor Wu is one of the seven researchers involved in the research center.
The team integrated 128x128 MEMS optical switches and antenna arrays on a 1 cm silicon photonic chip , which can quickly scan and switch the lidar beam between 16,384 different directions within a range of 70°x70° .
The research was published in Nature under the title A large-scale microelectromechanical systems-based silicon photonics LiDAR . Dr. Xiaosheng Zhang and Dr. Kyungmoo Kwon from the University of California, Berkeley are the co-first authors of the paper.
It is worth mentioning that Dr. Zhang Xiaosheng was the winner of the Tsinghua Undergraduate Special Award in 2016 (the highest honor for undergraduates at Tsinghua University).
Focal Plane Switch Arrays and Their Advantages
The principle of the focal plane switch array designed by the team is shown in Figure 1. They integrated a two-dimensional grating antenna array on a silicon photonic chip, in which each antenna is connected to the input port through an optical switch and an optical waveguide. By turning on the corresponding optical switch, the light input to the chip can be controlled to be transmitted to a specified grating antenna and emitted from the antenna. Using optical principles similar to those of a camera, this silicon photonic chip is placed on the focal plane of a convex lens. The light emitted from antennas at different positions will be converted into collimated beams transmitted in different directions after passing through the lens, thereby realizing beam scanning.
Figure 1 Schematic diagram of focal plane switch array principle
Image credit: Zhang Xiaosheng, University of California, Berkeley
The optical switches in the focal plane switch array only require digital switch control, without complex analog control signals, and the two-dimensional array can reduce the number of control signals through row and column addressing, so it has good scalability and is easy to realize large-scale arrays. For the same array chip, using lenses with different focal lengths and apertures (similar to changing the lens of a camera) can flexibly change the range, resolution and other parameters of the beam scanning to adapt to different application scenarios.
MEMS Optical Switch
The working principle of the MEMS optical switch used in this array is shown in Figure 2. The input port of each grating antenna (green in the figure) is controlled by a MEMS actuator and can be switched between two positions, upper and lower. When the switch is off, the input port of the grating antenna is in the upper position, away from the waveguide on the silicon wafer (yellow in the figure), and does not affect the propagation of light in the waveguide. When the switch is on, the input port of the grating antenna is in the lower position, close to the waveguide on the silicon wafer, and the light propagating in the waveguide will be coupled into the grating antenna. This MEMS optical switch has the advantages of small area, low loss, and fast switching speed, and is particularly suitable for large-scale arrays.
Figure 2 Working principle of MEMS optical switch
Image credit: Zhang Xiaosheng, University of California, Berkeley
The 128x128 focal plane switch array designed and processed by the team is shown in Figure 3. The micro-nano processing technology used is compatible with the existing CMOS process and can be mass-produced.
Figure 3 Microscope photo of the 128x128 focal plane switch array (a), and magnified photos of the grating antenna and MEMS optical switch (b, c)
Image credit: Zhang Xiaosheng, University of California, Berkeley
Large-Scale Solid-State LiDAR
The authors combined a 128x128 focal plane switch array with a frequency modulated continuous wave ranging system to demonstrate a lidar system. In the experiment, 3D imaging with a 10-meter ranging range was verified, and the measured 3D point cloud is shown in Figure 4. The beam scanning range and measurement resolution obtained in the experiment are consistent with the designed values.
Figure 4 3D point cloud measured by LiDAR and corresponding physical photos
Image credit: Zhang Xiaosheng, University of California, Berkeley
Future Outlook
To further improve the number of pixels, resolution and scanning range of the focal plane switch array to better meet the needs of the LiDAR system, it is necessary to integrate more optical switches and antennas on the chip and reduce the unit size. The research team pointed out that by further optimizing the design of the MEMS optical switch, it is expected to reduce the unit size to 10 microns, and then integrate a 1000x1000 pixel solid-state LiDAR on a 1 cm chip and achieve a scanning resolution better than 0.1°.
The realization of large-scale solid-state lidar in the future may greatly expand the application scenarios and scope of application of lidar three-dimensional sensors, allowing people to obtain three-dimensional information more conveniently, and smart machines can perceive the surrounding environment more accurately, providing greater convenience for production and life.
Written by Zhang Xiaosheng (PhD, University of California, Berkeley)
Note: This article was contributed by the author of the paper
Paper Information
Zhang, X., Kwon, K., Henriksson, J. et al. A large-scale microelectromechanical-systems-based silicon photonics LiDAR. Nature 603, 253–258 (2022).
https://doi.org/10.1038/s41586-022-04415-8
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