A light sensor developed by the University of Texas at Austin (UT Austin) and the University of Virginia is able to amplify weak signals from far away with greater accuracy than current technology, giving autonomous vehicles a fuller picture of what’s happening on the road.
The detector is able to amplify weak signals by drastically reducing noise associated with the detection process. Excessive noise causes systems to miss signals, which in the case of autonomous vehicle applications, can put passengers at risk.
“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.
Electrons multiply as they roll down the 'staircase' as part of the avalanche photodiode. Courtesy of University of Texas at Austin.
To meet these needs, research has focused on the development of avalanche photodiodes (APDs). What sets this detector apart from other efforts, however, is its staircase-like configuration in which electrons roll down physical steps, multiplying along the way to create a stronger electrical current for light detection.
“In detectors with internal gain, one source of noise is randomness associated with the gain mechanism, which results in temporal variation of the gain,” UT Austin professor of electrical and computer engineering Seth Bank told Photonics Media. “Compared to a conventional APD, where impact ionization occurs essentially randomly throughout the multiplication region, the staircase steps drive impact ionization at specific deterministic locations.”
The spatial determinicity, he added, therefore implies temporal determinicity, which reduces noise. In contrast with photomultiplier tubes (PMT) where gain also occurs at spatially deterministic locations (at each dynode), the noise advantage of the staircase APD stems from the fact that each staircase can be designed to initiate only one impact ionization event for each electron that crosses the step, Bank said.
“The electron is like a marble rolling down a flight of stairs,” Bank said. “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: one, two, four, eight, and so on.”
That predictability is not found in photomultiplier tubes. The secondary electron yield there varies between one to four electrons, causing the gain to fluctuate from one incident electron to the next.
“Mathematically, impact ionization from a staircase step is described by a Bernoulli distribution, while the gain from a PMT dynode is described by a Poisson distribution,” Bank said. “Bernoulli statistics describe physical situations like flipping a coin; here we can design the staircase step as a loaded coin that is guaranteed to come out to heads some 90% of the time, which produces fundamentally weaker variations (hence lower amplification noise) than a Poisson distribution.”
The concept for this technology is not exactly new — it’s been around for decades. However, technological barriers held back its realization. In the 1980s, Federico Capasso conceived of the avalanche photodiode technology the researchers have been studying. However, the processes needed to create the device were not far enough along.
“At the time, AlGaAs/GaAs and InGaAs/InAlAs were really the only options; they do not possess all of the fundamental materials properties necessary for true staircase operation,” Bank told Photonics Media. “In particular, alloys of AlInAsSb offer very large conduction band offsets and very small bandgap materials that enable us to inject electrons with sufficiently high kinetic energy into small bandgap material (at the bottom of the step) to trigger impact ionization.”
The device is particularly well suited for lidar receivers that require high-resolution sensors that detect optical signals reflected from distant objects.
The researchers plan to focus next on adding more steps to the device to increase its sensitivity. Additionally they intend to marry the multistep staircase device with an avalanche photodiode they created last year that is sensitive to NIR light. That pairing holds potential for applications such as fiber optic communications and thermal imaging.
“This should give us the best of both worlds: response to a wider range of colors and greater sensitivity to weak signals because of the lower noise amplification that comes naturally from the staircase architecture,” Bank said.
The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00814-x).