Caltech scientists invent new silicon chips for fast 3D imaging at a low cost

Imagine you want an identical copy of an object, and then imagine that you take your phone out of your pocket, take a photo with the integrated 3D imager on your phone, and send it to a 3D printer. You get it in a few minutes. A precise replica of the latter with an accurate error of the original object in the micrometer range. Thanks to a new, high-resolution miniature 3D imager invented by the California Institute of Technology, this vision may soon become a reality.

If you want to create a precise replica with a 3D printer, you first need to use a 3D camera to scan the object at high resolution and measure its length, width and height. Such 3D imaging technology has been around for decades, but the most sensitive systems are still too large and too expensive to be suitable for consumer product applications.

In contrast, a new, inexpensive, compact, and highly accurate Nanophotonic Coherent Imager (NCI) can change this. Using silicon wafers that are not expensive and do not exceed 1 square millimeter, NCI offers the highest depth measurement accuracy that any nanophotonic 3D imaging device can achieve. The laboratory, led by Ali Hajimiri, a professor of electrical engineering at the California Institute of Technology's School of Engineering and Applied Sciences, was published in the February issue of Optics Express.

In a normal camera, each pixel represents the density of light received at a particular point in the image, regardless of whether it is near or far from the camera, which means that the pixel does not provide information about the relative distance of the object from the camera. In contrast, each pixel in the image created by the California Institute of Technology's NCI provides distance and density information. “Every pixel on the chip is a separate interferometer – this device uses an interference of light waves for accurate measurements – the interferometer detects its phase and frequency in addition to its strength.” Hajimire said.

This latest chip utilizes an existing detection and ranging technology called LIDAR, which uses a scanning laser beam to illuminate a target object. Based on the wavelength of the laser used, scientists can analyze the light reflected from the object, while LIDAR can collect information about the size of the object and the distance from the laser to create an image of the surrounding environment. “By coherent imagers with small LIDAR arrays, we can simultaneously image different parts of the object or scene without any mechanical movement within the imager,” explains Hajimiri.

NCI can provide such high-resolution images and information thanks to an optical concept called coherence. If the two waves are coherent, then the waves have the same frequency and the peaks and troughs of the waves will align with each other. Light reflected from the object will be received by the detector on the chip, the grating coupler, forming a "pixel" because the light detected by each coupler represents one pixel on the 3D image. The NIC chip detects the phase, frequency, and density of light reflected from different points of the object and is used to determine the exact distance of the target point.

Since coherent light has a uniform frequency and wavelength, it is often used as a reference for measuring the difference between reflected light. In this way, NCI uses coherent light as a precise measure of the size of the object and the distance of each point on the object from the camera. This light is then converted into an electrical signal containing the density and distance information of each pixel - all of which is essential for creating 3D images.

The combination of coherent light not only achieves the highest depth measurement accuracy in silicon photonics in 3D imaging, but also greatly reduces the size of the device. “By coupling, limiting and processing the reflected light in a small tube of silicon, we were able to reduce each LIDAR element to a few hundred microns – small enough that we could put it down in an active area of ​​300 microns*300 microns. 16 such coherent imagers,” Hajimiri said.

The first proof of the NCI concept has only 16 coherent pixels, which means that there are only 16 pixels in the 3D image it produces at any distance. However, the researchers also invented a new way to image larger objects by first imaging the 4*4 pixel area, then moving the object in units of 4 pixels to image the next area, and so on. Using this method, the researchers used the device to scan at a distance of 0.5 meters and created a 3D image of a "dale" on the front side with a resolution of micron.

In the future, Haji Miri said that the current 16-pixel array can easily scale to hundreds of thousands of pixels. One day, by creating these miniature LIDARs arrays, the imager can be used in a wider range of fields, from accurate 3D scanning and printing to helping unmanned vehicles avoid collisions, and then improving the motion sensitivity of hyperfine human-machine interfaces – Even the slightest movement of the patient's eyes or changes in the patient's heart rate per minute can be detected.

"The small size and high quality of this latest chip-based imager will result in significant cost reductions. By integrating these systems into personal devices, such as smartphones, it will result in thousands of new applications for such systems." Hajimire said. Other co-authors of the study included former postdoctoral fellow at the California Institute of Technology, Frooz Aflatouni, an assistant professor at the University of Pennsylvania, and graduate students Behrooz Abiri and Anger. Angad Rekhi. The study was funded by the California Institute of Technology's Innovation Program.

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