Resolving the LiDAR Scanning Limitations of Autonomous Vehicles

July 20, 2020 by Gary Elinoff

Researchers employ mechanical means for the control of laser light to improve LIDAR performance.

Autonomous vehicles must not only detect fast-moving objects, but they must also be able to identify them. Splitting a single-frequency laser beam into multiple wavelengths will multiply resolution capabilities to achieve this necessary goal.

Lidar stands for light detection and ranging. Simply put, a lidar device measures the time it takes for a light beam to travel to a target and get reflected, thereby ascertaining its distance. The industry is now exploring “frequency modulated continuous wave” (FMCW) lidar. This method involves splitting a single mono-frequency laser beam into multiple laser light wavelengths, called  “frequency combs”, to achieve better resolution. 

The present means of achieving this result have been bulky and cumbersome. Still, researchers at Purdue University and the Laboratory of Photonics and Quantum Measurements at École polytechnique fédérale de Lausanne (EPFL) in Switzerland have developed a methodology depends on acoustics to create and control this photonic choir of multiple laser frequencies on a silicon chip. 


What Are Frequency Combs?

Laser frequency combs present hundreds of laser emissions, each occupying a defined range of frequencies. Then, for the next range of frequencies, there is no laser emission. Then there is the next laser emission, and it, too, holds to a defined range of frequencies.

A graphical plot of frequency vs. laser radiation intensity would show one peak that corresponds to each laser’s frequency channel. There are empty voids between these channels with no measured energy that correspond to frequency spans that contain no laser emissions. 

The pattern resembles nothing so much as the outline of a hair comb, ergo the name. A frequency comb is illustrated below.


Acoustics control a frequency comb created from a single laser light.

Acoustics control a frequency comb created from a laser light, allowing for more capable lidar. Image credited to Purdue University

Microelectromechanical Systems 

The researchers published the results of their work in Nature Communications. The technology involved using microelectromechanical systems (MEMS) transducers made of aluminum nitride to modulate the frequency comb with frequencies ranging from the megahertz to gigahertz ranges.

A MEMS is a tiny electromechanical machine that can be as small as a micrometer that can be fabricated along with an IC. In this instance, the combination is with a silicon nitride photonics wafer.

This technique for light modulation integrates mechanics with optics, and also the fabrication processes for both. The MEMS transducers are actually fabricated on top of the silicon nitride photonics. This makes for easier manufacturing and thereby for greater commercial viability. 

“This achievement, bridging integrated photonics, MEMS engineering, and nonlinear optics, represents a new milestone for the emerging chip-based microcomb technology,” said Junqiu Liu, the first author the Nature paper. Liu also leads the fabrication of silicon nitride photonics chips at EPFL’s Center of MicroNanoTechnology. 

As per Tobias Kippenberg, a physics professor at EPFL, “As yet unforeseen applications will follow up across multiple communities.” He says, “It’s been shown time, and again that hybrid systems can obtain advantages and functionality beyond those attained with individual constituents.” 


Around the Industry

Kippenberg’s words were prescient, as laser frequency combs are emerging as an essential area of research for various purposes.

The National Institute of Standards and Technology (NIST) and the University of California at Santa Barbara (UCSB) are exploring laser frequency combs to cram more signals simultaneously on the same fiber optical cable.

A team of researchers from UCSB, Caltech, and the Swiss Federal Institute of Technology Lausanne (EPFL) are exploring this technology, with an eye to producing optical clocks that are smaller, lighter, and less expensive.

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