Apr 6 2015
Researchers at Caltech have developed an innovative, miniature high-resolution 3D imager capable of being integrated into a smartphone to allow capturing an exact snapshot of an object and sending it to a 3D printer for reproducing a replica which is accurate to micron level of the original object.
A 3D image produced by the new NCI chip. The image, taken from roughly half a meter (1.5 feet) away, shows the height of a US penny at various points. Image Credit: Ali Hajimiri | Caltech
To make an accurate copy of an object using a 3D printer, a high-resolution scan of the object with a 3D camera is required for measuring its height, depth, and width. This type of 3D imaging has been in existence for several years.
However highly sensitive units tend to be huge and expensive for use in consumer applications. The Caltech imager is referred to as a nanophotonic coherent imager (NCI) and is highly accurate as well as inexpensive.
The NCI is made up of an inexpensive silicon chip measuring less 1mm2 size and is capable of provides superior depth-measurement precision of similar such nanophotonic 3D imaging devices.
The entire research process of creating the NCI in the laboratory of Ali Hajimiri, the Thomas G. Myers Professor of Electrical Engineering in the Division of Engineering and Applied Science is documented in the February 2015 issue of Optics Express.
The pixel created in a conventional camera only illustrates the intensity of the light received from a definite point in the image, which could be at a distance or closer to the camera. However it does not provide any data on the object’s relative distance from the camera. On the other hand, the pixels from the image shot using the Caltech’s NCI provide not only the distance but also the intensity data.
"Each pixel on the chip is an independent interferometer—an instrument that uses the interference of light waves to make precise measurements—which detects the phase and frequency of the signal in addition to the intensity," says Hajimiri.
The novel chip uses LIDAR, an established detection and ranging technology where a target object is illuminated with scanning laser beams. The light reflected off of the object is analyzed based on the wavelength of the laser light used. The LIDAR is capable of collecting data about the size of the object and distance of the object from the laser to generate an image of its surroundings.
New Camera Chip Enables Micrometer-Resolution 3D Images
"By having an array of tiny LIDARs on our coherent imager, we can simultaneously image different parts of an object or a scene without the need for any mechanical movements within the imager," Hajimiri says.
The NCI is able to produce such high-resolution images and data due to an optical concept referred to as coherence. When two light waves are coherent, the waves possess identical frequency and the troughs and peaks of light waves are in perfect alignment with one another. The object is illuminated with this coherent light in the NCI.
Here the light reflected off of the object is collected by on-chip detectors referred to as grating couplers which act as "pixels," as the light detected from each coupler illustrates one pixel on the 3D image.
The intensity, frequency, and phase of the reflected light from different points on the object are detected on the NCI chip, and the data is used to establish the exact distance of the target point.
The coherent light possesses a consistent frequency and wavelength and these factors can be utilized to measure the dissimilarities in the reflected light. Therefore the coherent light of the NCI becomes an accurate yardstick for measuring the distance of each point on the object from the camera and the object’s size.
Next the light is converted into an electrical signal containing the distance and intensity data for all the pixels, and these factors are what are required to form a 3D image.
Adding the coherent light into 3D imaging enables the ultimate depth-measurement precision ever to be accomplished in the field of silicon photonics. Furthermore, it allows for the device to fit in a miniature size.
"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 microns by 300 microns," Hajimiri says.
The initial proof of concept of the NCI contained just 16 coherent pixels. In other words, the 3D images generated by the NCI will have only 16 pixels at any given time. However, the researchers have formulated a technique for imaging larger objects.
The first step involves imaging a four-pixel-by-four-pixel section, which is followed by moving the object in four-pixel increments to image the next section. Making use of this technique, they scanned and created a 3D image (with micron-level resolution) containing "hills and valleys" on the front face of a U.S. penny from half a meter away using the device.
Hajimiri states that going forward, current 16-pixel array can be worked on to easily scale up to hundreds of thousands. When such levels are reached, the advanced imager can be used widely in numerous applications spanning from assisting driverless cars avoid collisions to highly accurate 3D scanning and printing and enhancing motion sensitivity in superfine human machine interfaces, in which even the minor movements of a patient's eyes and the highly minute alterations in a patient's heartbeat can be detected easily.
"The small size and high quality of this new chip-based imager will result in significant cost reductions, which will enable thousands new of uses for such systems by incorporating them into personal devices such as smartphones," he says.
The study was published in a paper titled, "Nanophotonic coherent imager." In addition to Hajimiri, other Caltech coauthors include former postdoctoral scholar and current assistant professor at the University of Pennsylvania, Firooz Aflatouni, graduate student Behrooz Abiri, and Angad Rekhi.