Microwave imager chip promises hand-held near-field imaging
In the Optica journal, their paper “Single-chip nanophotonic near-field imager” describes a 2- x 2-mm microwave imager chip containing more than 1,000 photonic components such as waveguides, directional couplers, photodiodes and ring modulators. The 121-element imager integrated on a silicon chip is capable of simultaneous processing of ultra-wideband microwave signals and achieves 4.8° spatial resolution for near-field imaging, while being orders of magnitude smaller than alternative benchtop implementations.
The imager uses four antennas to receive microwave signals reflected from an object. These microwave signals are then encoded into an optical signal and are optically processed, emulating a microwave lens, to form an image. One of the essential components is the optical delay element network used for signal processing, which consists of more than 280 delay cells.
“This system is significantly smaller and more efficient than its electronic equivalent because the delay cells are more than 10 times smaller and more than 10 times more efficient,” said Farshid Ashtiani, a graduate student in Aflatouni’s group and coauthor on the paper. “They can also operate with significantly shorter microwave pulses, which produces higher imaging resolution.”
“Today’s practical microwave imagers are bench-top systems that are bulky and expensive,” said research team leader Firooz Aflatouni from the University of Pennsylvania, USA. “Our new near-field imager uses optical, rather than electronic, devices to process the microwave signal. This enabled us to make a chip-based imager similar to the optical camera chips in many smartphones.”
Hand-held near-field microwave imagers would be useful for many applications including high-resolution brain imaging and monitoring heart motion and breathing. Miniaturization of microwave imagers would also benefit applications such as tracking objects in radar systems and low-power, high-speed communication links.
As a demonstration, the microwave imager was used to detect objects with metallic surfaces, including metallic squares measuring 24 centimeters on each side and the UPenn logo. After short microwave pulses illuminated each object placed in front of the imager, the four antennas received the reflected signals, which were used to form the image of each target object.
Now that they’ve demonstrated a chip-based microwave imager, the researchers plan to increase the number of pixels by upping the number of on-chip delay lines, using more advanced fabrication technologies and stitching together smaller images. They also want to use shorter microwave pulses to achieve higher resolution.
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