Improve the recording resolution of solid state lidar sensor

When Google unveiled its first self-driving car in 2010, the roof-spinning cylinder did stand out. This is the vehicle's light Detection and ranging (LiDAR) system, which works in a similar way to light-based radar. Lidar works with cameras and radar to map the environment and help the car avoid obstacles and drive safely.

Since then, cheap, chip-based cameras and radar systems have become mainstream for collision avoidance and highway autonomous driving. However, lidar navigation systems are still bulky, mechanical pieces of equipment that cost thousands of dollars.

That may be about to change thanks to a new high-resolution lidar chip developed by Ming Wu, professor of electrical engineering and computer science at the University of California, Berkeley, and co-director of the Berkeley Sensor and Actuator Center. The new design was published Wednesday, March 9, in the journal Nature.

Engineers at the University of California, Berkeley, have used microelectromechanical systems (MEMS) switches to significantly improve the resolution of chip-based lidar sensors. In this schematic of a lidar chip, a laser is emitted from an optical antenna connected to a tiny switch. The reflected light is captured by the same antenna. A 3D image is obtained by turning on switches in the array one by one. Credit: Zhang Xiaosheng, University of California, Berkeley.

Wu's lidar is based on the focal plane Switching Array (FPSA), a semiconductor-based antenna matrix that collects light like a sensor in a digital camera. Its 16,384 pixel resolution may not sound impressive compared to the millions of pixels on a smartphone camera, but it pales in comparison to the 512 pixels or less currently available on FPsas, Wu said.

Equally important, the design uses the same complementary metal oxide semiconductor (CMOS) technology used to produce computer processors, expandable to megapixel sizes. This could lead to a new generation of powerful, low-cost 3-D sensors for self-driving cars, drones, robots and even smartphones.

Lidar barrier

Lidar works by capturing the reflected light of lasers. By measuring the time it takes for light to return or changes in the frequency of the beam, lidar can map the environment and record how fast objects are moving around it.

Mechanical lidar systems have powerful lasers that can see objects hundreds of yards away even in the dark. They also generate 3D maps with enough resolution to allow the vehicle's AI to distinguish between vehicles, bicycles, pedestrians and other hazards.

However, putting these functions on a chip has puzzled researchers for more than a decade. The most powerful obstacle is the laser.

"We wanted to illuminate a very large area," Wu said. "But if we tried to do that, the light would be too weak to get far enough. So, as a design trade-off for maintaining light intensity, we reduced the area of laser illumination."

This is where the FPSA comes in. It consists of a matrix of tiny light transmitters or antennas and switches that turn them on and off quickly. In this way, it can transmit all available laser power simultaneously through a single antenna.

Scanning electron micrograph of a lidar chip, showing a raster antenna. Photo credit: Kyungmok Kwon, University of California, Berkeley.

MEMS switch

However, switching can cause problems. Almost all silicon-based lidar systems use thermo-optical switches, which rely on large changes in temperature to produce small changes in refractive index and bend and redirect the laser from one waveguide to another.

Thermo-optical switches, however, are large and power-hungry. Put too much stuff on the chips, and they generate too much heat to work properly. This is why existing FPsas are limited to 512 pixels or less.

Wu's solution replaces it with a microelectromechanical system (MEMS) switch that physically moves the waveguide from one location to another.

"The building is very similar to the highway exchange," he said. "Suppose you're a beam of light going from east to west. We can mechanically lower a slope so that you suddenly turn 90 degrees and go from north to south."

MEMS switches are known techniques for routing light in communication networks. This is the first time they have been used for lidar. Compared to thermo-optical switches, they are smaller, consume less power, switch faster, and have very low optical loss.

That's how Wu was able to fit 16,384 pixels into a chip measuring 1 centimeter by 1 centimeter. When the switch turns on the pixel, it fires a laser beam and captures the reflected light. Each pixel corresponds to 0.6 degrees of the array's 70-degree field of view. Using a fast-cycling array, Wu's FPSA builds a 3D picture of the world around it. Installing several of these in a circular configuration will produce a 360-degree view around the vehicle.

Smartphone size

Before his system is ready for commercialization, we need to improve the resolution and range of the FPSA. While optical antennas are difficult to make smaller, switches are still the largest components, and researchers think they can be made even smaller.

If so, traditional CMOS production techniques are expected to make cheap chip-level lidar part of our future.

"Look at how we use the camera," Wu said. "They are embedded in vehicles, robots, vacuum cleaners, surveillance devices, biometrics and doors. Once we shrink lidar down to the size of a smartphone camera, there are many more potential applications."