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Nanophotonic coherent imager developed for superfine 3D resolution

Researchers from the California Institute of Technology (Caltech) have developed a nanophotonic coherent imager (NCI) for producing accurate three dimensional images of objects so they can be replicated with a 3D printer. The device, published in Optics Express, contains tiny lidars to produce 3D images with the highest level of depth-measurement accuracy ever achieved in silicon photonics.

Any time an exact copy of an object is producing using a 3D printer, the first step is to acquire a high-resolution scan of the object with a 3D camera that measures its height, width, and depth. Although 3D imaging has been around for decades, the most sensitive systems are typically too large and expensive to be used in consumer applications.

However, the new imager developed at Caltech uses an inexpensive silicon chip less than a millimetre square in size.

Each pixel in an image captured by the device provides both distance and intensity information. ‘Each pixel on the chip is an independent interferometer…which detects the phase and frequency of the signal in addition to the intensity,’ explained Ali Hajimiri, the Thomas G Myers Professor of Electrical Engineering at Caltech.

The instrument contains tiny lidars, which means different parts of an object or a scene can be imaged simultaneously, without the need for mechanical movements within the imager. Lidar, a detection and ranging technology, works by illuminating an object with a laser beam, which allows information to be gathered about the object's size and its distance from the laser to create an image of its surroundings.

In the NCI, the object is illuminated with coherent light, and because coherent light has a consistent frequency and wavelength, it is used as a reference with which to measure the differences in the reflected light.

The reflected light is then analysed by on-chip detectors, called grating couplers, which serve as ‘pixels’, as the light detected from each coupler represents one pixel on the 3D image. On the NCI chip, the phase, frequency, and intensity of the reflected light from different points on the object is detected and used to determine the exact distance of the target point.

The light is then converted into an electrical signal that contains intensity and distance information for each pixel − all of the information needed to produce a 3D image.

The incorporation of coherent light allows for 3D imaging with the highest level of depth-measurement accuracy ever achieved in silicon photonics, but it also allows for very small devices. ‘By coupling, confining, and processing the reflected light in small pipes on a silicon chip, we were able to scale each lidar element down to just a couple of hundred microns in size − small enough that we can form an array of 16 of these coherent detectors on an active area of 300 by 300µm,’ Hajimiri said.

The first proof-of-concept of the NCI contains just 16 coherent pixels, meaning that the 3D images can only be 16 pixels at any given instance. However, the researchers also developed a method for imaging larger objects by first imaging a four-pixel-by-four-pixel section, then moving the object in four-pixel increments to image the next section. With this method, the team used the device to scan and create a 3D image of the US penny with micron-level resolution and from half a metre away.

According to Hajimiri, the current array of 16 pixels could also be easily scaled up to hundreds of thousands, which could allow the imager to be applied to a broad range of applications, from precise 3D scanning and printing to helping driverless cars avoid collisions, to improving motion sensitivity in superfine human machine interfaces, where the slightest movements of a patient's eyes and the most minute changes in a patient's heartbeat can be detected on the fly.

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Further information

Paper in Optics Express: Nanophotonic Coherent Imager

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