Light detector adds awareness to autonomous vehicles

Autonomous vehicles could be given extra awareness thanks to a unique light detecting device that can more accurately amplify weak signals reflected from faraway objects.

Autonomous vehicle (Image by falco from Pixabay)

Developed by engineers at the University of Texas at Austin and the University of Virginia, the first-of-its-kind device promises to give autonomous vehicles a fuller picture of what is happening on the road.

The new device is claimed to be more sensitive than other light detectors because it eliminates inconsistency, or noise, associated with the detection process.

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“Autonomous vehicles send out laser signals that bounce off objects to tell you how far away you are. Not much light comes back, so if your detector is putting out more noise than the signal coming in you get nothing,” said Joe Campbell, professor of electrical and computer engineering at the University of Virginia School of Engineering.

Researchers globally are working on so-called avalanche photodiodes to meet these needs, but it is this device’s staircase-like alignment that makes it unique. It includes physical steps in energy that electrons roll down, multiplying along the way and creating a stronger electrical current for light detection as they go.

avalanche photodiode
Electrons multiply as they roll down the “staircase” as part of the avalanche photodiode (Image: The University of Texas at Austin)

In 2015, the researchers created a single-step staircase device, but in this new discovery, detailed in Nature Photonics, they have shown a staircase avalanche photodiode with multiple steps for the first time.

“The electron is like a marble rolling down a flight of stairs,” said Seth Bank, professor in the Cockrell School’s Department of Electrical and Computer Engineering who led the research with Campbell.

“Each time the marble rolls off a step, it drops and crashes into the next one. In our case, the electron does the same thing, but each collision releases enough energy to actually free another electron. We may start with one electron, but falling off each step doubles the number of electrons: 1, 2, 4, 8, and so on.”

The new pixel-sized device is claimed to be ideal for Light Detection and Ranging (lidar) receivers, which require high-resolution sensors that detect optical signals reflected from distant objects.

Adding steps increases sensitivity and consistency of the device; the consistent multiplication of electrons with each step makes the electrical signals from the detector more dependable, even in low light conditions.

“The less random the multiplication is, the weaker the signals you can pick out from the background,” said Bank. “For example, that could allow you to look out to greater distances with a laser radar system for autonomous vehicles.”

This type of sensing capability has existed for decades, but technological barriers held back its advancement. Photomultiplier tubes long represented the “holy grail” of this form of sensing, Bank said, but that technology has been around for over 50 years and uses outdated lighting components and vacuum tubes. In the 1980s, inventor Federico Capasso first conceived of the avalanche photodiode technology the researchers have been studying. But the tools and techniques to make it a reality just weren’t far enough along.

The science behind this breakthrough comes in a new way of growing materials, Bank said. Instead of growing materials with randomly distributed atoms, they created layered alloys composed of binary compounds stacked on top of each other.

“What this allows is to change the electron’s energy landscape in a very simple way to create the structure that Capasso envisioned in the early 80s, but unfortunately there just wasn’t the ability to synthesise crystals that had all the requisite properties,” Bank said.

Another important piece of this device is that it can operate at room temperature. Today, the most sensitive light detectors need to be kept at temperatures hundreds of degrees below zero, making them too expensive and impractical for applications such as lidar.

The researchers have funding through US Army Research Office and The Defense Advanced Research Projects Agency to continue refining their process to add more steps to the devices. They are working also with a semiconductor company to commercialise the technology.