The fibers of a new “optical brush” are connected to an array of photosensors at one end and left to wave free at the other. (Image courtesy of MIT/Barmak Heshmat.)
A new type of imaging device consisting of a loose bundle of optical fibres without lenses or a protective housing is being developed by a team of researchers at MIT .
The bundle of optical fibers is connected to an array of photosensors at one end with the other ends left loose, like the bristles of a brush. Because the fibers of this “optical brush” are flexible and spread out, this method of imaging shows promise for a number of unique applications.
For example, the fibers could potentially pass individually through micrometer-scale gaps in a porous membrane to image the interior or the other side.
Other applications could include feeding the flexible bundles through pipes or immersing them in fluid, using them to image underground resources such as oil fields and aquifers, or to navigate plumbing, industrial pipe or irregularly shaped tunnel systems.
The researchers also see medical applications for devices such as endoscopes, due to the optical fibers’ flexibility and the potential for a narrow bundle diameter since the device does not require any additional electronics.
Imaging with the “Optical Brush”
The position of the fibers’ free ends does not need to correspond to the position of the photodetectors in the array. Instead, the researchers used a measurement technique called “time of flight,” which measures the differing times at which short pulses of light reach the photodetectors.
This allows the device to determine the relative locations of the fibers. In the current prototype version of the device, the researchers are using external lasers to provide the calibrating bursts of light.
“Time of flight, which is a technique that is broadly used in our group, has never been used to do such things,” said Barmak Heshmat, a postdoc at MIT’s Media Lab and lead on the project. “Previous work has used time of flight to extract depth information. But in this work, I was proposing to use time of flight to enable a new interface for imaging.”
In their experiments, the research team used a bundle of 1,100 fibers with one end left waving free, and positioned it opposite a screen featuring some projected symbols. The other end of the bundle was attached to a beam splitter, which was then connected to both an ordinary camera as well as a high speed camera capable of distinguishing the optical light pulses’ time of arrival.
The researchers positioned two ultrafast lasers perpendicular to the tips of the fibers at the bundle’s loose end and to each other. These lasers fired short bursts of light, with the high-speed camera recording the light pulses’ time of arrival along each fiber.
The bursts of light came from two different directions, which allowed the researchers to use software to measure the differences in the arrival time of each light burst. The result is a two-dimensional map of the positions of the fibers’ tips. The software then uses that information to unscramble the jumbled image captured by the conventional camera.
Image resolution for the system is limited by the number of fibers. The 1,100-fiber prototype produces an image measuring approximately 33 by 33 pixels. Since there is a certain amount of ambiguity in the image reconstruction process, the images produced in the researchers’ experiments remained fairly blurry.
However, the prototype sensor also used off-the-shelf optical fibers that were 300 micrometers in diameter. Fibers that are only a few micrometers in diameter can be commercially manufactured, meaning the resolution could be increased significantly without increasing the bundle size.
In a commercial application, of course, the system wouldn’t have the luxury of two perpendicular lasers positioned at the fibers’ tips. Instead, bursts of light would be sent along individual fibers and the system would gauge the time they take to reflect back. Many more pulses would be required to form an accurate picture of the fibers’ positions, but these pulses are so short that calibration would still only require a fraction of a second.
Increased Image Resolution and Medical Applications
The use of interferometric methods could improve the quality of the image for medical applications requiring the diameter of the bundle, and therefore the number of fibers, to remain low.
In these methods, an outgoing light signal is split in two and half of it, the reference beam, is kept locally. The other half, called the sample beam, bounces off objects in the scene and returns. The two signals are then recombined. The ways in which they interfere with each other yield very detailed information about the sample beam’s trajectory.
The researchers have not used this technique in their experiments, but they did perform a theoretical analysis showing that it should enable more accurate scene reconstructions.
“It is definitely interesting and very innovative to combine the knowledge we now have of time-of-flight measurements and computational imaging,” said Mona Jarrahi, an associate professor of electrical engineering at the University of California at Los Angeles. “And as the authors mention, they’re targeting the right problem, in the sense that a lot of applications for imaging have constraints in terms of environmental conditions or space.”
Relying on laser light piped down the fibers themselves “is harder than what they have shown in this experiment,” Jarrahi cautioned. “But the physical information is there. With the right arrangement, one can get it.”
“The primary advantage of this technology is that the end of the optical brush can change its form dynamically and flexibly,” adds Keisuke Goda, a professor of chemistry at the University of Tokyo. “I believe it can be useful for endoscopy of the small intestine, which is highly complex in structure.”
For more information, the team’s research is published in Nature Scientific Reports and is available here , or you can visit the Optical Brush team’s project page .