Scientists at Columbia University, New York, have used a microchip to map the back of the eye for disease diagnosis. The interference technology used in the microchip — like bat sonar but using light instead of sound waves — has been around for a little while. However, this is the first time that technical obstacles have been overcome to fabricate a miniature device able to capture high quality images.
Ophthalmologists’ current optical coherence tomography (OCT) devices and surveyors’ light detection and ranging (LIDAR) machines are bulky and expensive. There is a push for miniaturization in order to produce cheap handheld OCT and LIDAR small enough to fit into self-driving cars.
The team demonstrated their microchip’s ability to produce high contrast OCT images 0.6 millimeters deeper than previously in human tissue. Using the technique they developed in this project, they say they can make any size system on a chip.
Central to a chip-scale interferometer is fabrication of the tunable delay line. A delay line calculates how light waves interact, and by tuning to different optical paths, which are like different focal lengths on a camera, it collates the interference pattern to produce a high contrast 3D image.
The researchers coiled a 0.4-meter Si3N4 delay line into a compact 8 mm2 area and integrated the microchip with micro-heaters to optically tune the heat sensitive Si3N4. Using the heaters enabled them to achieve delay without any moving parts, thus providing high stability, which is important for image quality of interference-based applications.
With components tightly bent in a small space, it’s hard to avoid losses when changing the physical size of the optical path. But one of the researchers, Xingchen Ji, previously optimized fabrication to prevent optical loss. He applied this method alongside a new tapered region to accurately stitch lithographic patterns together — an essential step for achieving large systems.
The team demonstrated the tunable delay line microchip on an existing commercial OCT system, showing that greater depths could be probed while maintaining high resolution images.
This technique should be applicable to all interference devices, and the researchers are already starting to scale LIDAR systems, one of the biggest photonic interferometry systems.
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