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The researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than one cell. Without a lens or optical, electrical or mechanical components, the tip of the endoscope measures only 200 microns in diameter, about the width of some human hair twisted together.
As a minimally invasive tool for imaging features inside living tissues, the extremely thin endoscope could enable a variety of research and medical applications. The research will be presented at the Frontiers in Optics + Laser Science Conference (FIO + LS), September 15-19 in Washington, DC, USA.
According to Juergen W. Czarske, director and professor C4 at TU Dresden, Germany, and lead author of the paper: "The fiber-optic endoscope without lens is about the size of a needle, which allows it to have a minimally invasive access and a high contrast. as well as stimulation with robust calibration against bending or twisting of the fiber. The endoscope will probably be particularly useful for optogenetics – research approaches using light to stimulate cellular activity. This could also prove useful for monitoring cells and tissues during medical procedures as well as for technical inspections.
A self-calibrating system
Conventional endoscopes use cameras and lights to capture images in the body. In recent years, researchers have developed other methods to capture images using fiber optics, eliminating the need for bulky cameras and other large components, thus allowing to much thinner endoscopes. Despite their promises, however, these technologies suffer from limitations such as their inability to tolerate temperature fluctuations or bending and twisting of the fiber.
A major obstacle to putting these technologies into practice is that they require complicated calibration processes, in many cases, while the fiber is collecting images. To remedy this problem, the researchers added a thin, 150 micron thick glass plate at the tip of a coherent fiber bundle, a type of optical fiber commonly used in applications such as microwaves. endoscopy. The coherent fiber bundle used in the experiment had a width of about 350 microns and consisted of 10,000 nuclei.
When the central fiber core is illuminated, it emits a beam that is reflected back into the fiber bundle and serves as a virtual guide star to measure the mode of light transmission, called the optical transfer function. The optical transfer function provides crucial data that the system uses to calibrate on the fly.
Keep the view clear
A key component of the new configuration is a spatial light modulator, used to manipulate the direction of light and enable remote focus. The spatial light modulator compensates for the optical transfer function and the images on the fiber bundle. The light reflected by the fiber bundle is captured by the camera and superimposed on a reference wave to measure the phase of the light.
The position of the virtual guide star determines the focus of the instrument, with a minimum focus diameter of about one micron. The researchers used an adaptive lens and a 2D galvometer mirror to move the focus and allow scanning at different depths.
Demonstration of 3D imaging
The team tested the device using it for imaging a 3D specimen under a 140-micron thick slide. By digitizing the 13-step image plane over 400 microns with an image rate of 4 cycles per second, the device was able to image particles at the top and bottom of the sample 3D. However, its focus deteriorated as the angle of the galvometer mirror increased. The researchers suggest that future work could address this limitation. In addition, the use of a galvometer scanner with a higher frame rate could allow faster acquisition of images.
"The new approach allows for both calibration and real-time imaging with minimal invasion, which is important for in situ 3D imaging, mechanical cell manipulation by the lab-on-a-chip technique, and optogenetics. In vivo deep tissues and technical inspections of keyholes, says Czarske.
Paper: "Rapid 3D imaging with objective-free holographic endoscopy using coherent fiber bundles,"From Juergen W. Czarske, Elias Scharf and Robert Kuschmierz, will be presented on Monday, September 16, 2019 at 11:15 am EDT in Room Washington 6 of the Marriott Wardman Park Hotel in Washington, DC.
Award in Frontiers in Optics + Laser Science
Juergen W. Czarske will receive the Robert Fraffen / Joseph Fraunhofer 2019 Prize at FiO + LS for his achievements in optical engineering. He is honored for his fundamental contributions to the field of interferometric and holographic digital sensors. Czarske is also a member of OSA, EOS and SPIE, an elected member of the Saxon Academy of Sciences and the Scientific Society for Laser Technology. He has received the Berthold Leibinger Innovation Award, the Reinhart Koselleck Project from the German Research Foundation, the AHMT Measurement Technique Award and many other honors.
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