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A lens-free endoscope captures 3D images of objects smaller than a cell

Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than one cell Credit J Czarske, TU Dresden, Germany

Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than one cell. Without a lens or any optical, electrical or mechanical components, the tip of the endoscope measures only 200 microns, about the width of several human hairs twisted together.

As a minimally invasive imaging tool inside living tissues, the extremely thin endoscope can allow for various examinations and medical applications. The research will be presented at the Frontiers in Optics + Laser Science (FIO + LS) conference, held September 15-19 in Washington, D.C., USA.

According to Juergen W. Czarske, director and C4 professor at TU Dresden, Germany and lead author of the paper: "The endoscope without objective fibers is approximately the size of a needle, allowing it to have minimally invasive access and high contrast imaging as well as stimulation with robust calibration against bending or twisting of the fiber. "The endoscope is probably particularly useful for optogenetics – research approaches that use light to stimulate cellular activity. It may also be useful for monitoring cells and tissues during medical procedures as well as for technical examinations.

Self-calibration system

Conventional endoscopes use cameras and lights to capture images inside the body. In recent years, researchers have developed alternative ways of capturing optical fiber images, eliminating the need for bulky cameras and other bulky components, allowing significantly thinner endoscopes. However, despite their promise, these technologies suffer from limitations such as the inability to withstand temperature fluctuations or the bending and twisting of the fiber.

A major obstacle to realizing these technologies is that they require complex calibration processes, in many cases, while the fiber collects images. To deal with this, the researchers added a thin glass plate with a thickness of only 150 microns to the tip of a coherent fiber bundle, a type of optical fiber commonly used in endoscopic applications. The coherent fiber bundle used in the experiment was about 350 microns wide and consisted of 10,000 cores.

When the central core of the fiber is illuminated, it emits a beam that is reflected back into the fiber bundle and serves as a virtual guiding star to measure how light is known as the optical transmission function. The optical transfer feature provides important data that the system uses to calibrate on the go.

Keep the look in focus

A key component of the new setup is a spatial light modulator, which is used to manipulate the light direction and to focus remotely. The spatial light modulator compensates for the optical beam and fiber imaging function. The reflected light from the fiber bundle is captured on the camera and superimposed with 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 approximately one micron. Researchers use an adaptive lens and a 2D galvanometer mirror to shift focus and allow scanning at different depths.

Demonstration of 3D images

The team is testing their device, using it to display a 3D specimen under a 140 micron thickness. By scanning the image plane in 13 steps over 400 microns at an image rate of 4 cycles per second, the device successfully renders particles in the top and bottom of the 3D pattern. However, his focus worsened as the angle of the mirror of the galvometer increased. Researchers suggest that future work can cope with this limitation. In addition, using a higher-frame-rate galvanometer scanner can allow for faster image acquisition.

"The new approach allows for both real-time calibration and minimal-invasive imaging, important for 3D on-site imaging, lab-on-chip, mechanical cell manipulation, deep tissue in vivo optogenetics and technical key checks," said Czarske.

Paper: "Fast 3D image with holographic endoscopy without lens using coherent fiber bundles,"By Juergen W. Czarske, Elias Scharf and Robert Kuschmierz, will be presented on Monday, September 16, 2019, from 11:15 a.m. EDT at the Washington 6 Room at the Marriott Wardman Park Hotel in Washington, D.C.

Frontiers in Optics + Laser Science Award

Juergen W. Czarske will receive the 2019 Joseph Fraunhofer Award / FiO + LS Robert M. Burley Award for Excellence in Optical Engineering. He has been honored with a major contribution to the field of digital interferometric and holographic sensors. Czarske is also an Fellow of OSA, EOS, SPIE, an elected Fellow of the Saxon Academy of Sciences and the Scientific Society for Laser Technology. He received the Berthold Leibinger Innovation Award, the Reinhart Kozelk Project from the German Research Foundation, the AHMT Measurement Technique Award and many other honors.

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