Autonomous vehicles relying on light-based image sensors often struggle to see through blinding conditions such as fog. Sub-terahertz wavelengths, which are between microwave and infrared radiation on the electromagnetic spectrum, can be detected through fog and dust clouds with ease, whereas the infrared-based LiDAR imaging systems used in autonomous vehicles struggle. To detect objects, a sub-terahertz imaging system sends an initial signal through a transmitter; a receiver then measures the absorption and reflection of the rebounding sub-terahertz wavelengths. That sends a signal to a processor that recreates an image of the object.
Implementing sub-terahertz sensors into driverless cars is challenging. Sensitive, accurate object-recognition requires a strong output baseband signal from receiver to processor. Traditional systems, made of discrete components that produce such signals, are large and expensive. Smaller, on-chip sensor arrays exist, but they produce weak signals.
A two-dimensional, sub-terahertz-radiation receiving system array on a chip was developed that could help steer driverless cars when traditional methods fail. The array is orders of magnitude more sensitive, meaning it can better capture and interpret sub-terahertz wavelengths in the presence of a lot of signal noise.
To achieve this, researchers developed a scheme of independent signal-mixing pixels — called heterodyne detectors — that are usually very difficult to densely integrate into chips. They drastically shrank the size of the heterodyne detectors so that many of them can fit into a chip. The trick was to create a compact, multipurpose component that can simultaneously down-mix input signals, synchronize the pixel array, and produce strong output baseband signals.
A prototype was built that has a 32-pixel array integrated on a 1.2-square-millimeter device. The pixels are approximately 4,300 times more sensitive than the pixels in today's best on-chip sub-terahertz array sensors. The chip could potentially provide “electric eyes” in driverless cars and autonomous robots.
The researchers incorporated into their chip a component called a phase-locked loop that locks the sub-terahertz frequency of all 32 local oscillation signals to a stable, low-frequency reference. Because the pixels are coupled, their local oscillation signals all share identical, high-stability phase and frequency. This ensures that meaningful information can be extracted from the output baseband signals. This entire architecture minimizes signal loss and maximizes control.