Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than a single cell without a lens or any optical, electrical, or mechanical components. The tip of the endoscope measures just 200 microns across — about the width of a few human hairs 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.

Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than a single cell. (Credit: Czarske, TU Dresden, Germany)

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

A major hurdle to making these technologies practical is that they require complicated calibration processes — in many cases while the fiber is collecting images. To address this, the researchers added a thin glass plate, just 150 microns thick, to the tip of a coherent fiber bundle, a type of optical fiber that is commonly used in endoscopy applications. The coherent fiber bundle used in the experiment was about 350 microns wide and consisted of 10,000 cores.

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 for measuring how the light is being transmitted. This is known as the optical transfer function, which provides crucial data the system uses to calibrate itself on the fly.

A key component of the new setup is a spatial light modulator, which is used to manipulate the direction of the light and enable remote focusing. The spatial light modulator compensates the optical transfer function and images on the fiber bundle. The back-reflected light from the fiber bundle is captured on the camera and superposed with a reference wave to measure the light’s phase. The position of the virtual guide star determines the instrument’s focus, with a minimal focus diameter of approximately one micron. The researchers used an adaptive lens and a 2D galvometer mirror to shift the focus and enable scanning at different depths.

The team tested their device by using it to image a 3D specimen under a 140-micron thick cover slip. Scanning the image plane in 13 steps over 400 microns with an image rate of 4 cycles per second, the device successfully imaged particles at the top and bottom of the 3D specimen. However, its focus deteriorated as the galvometer mirror’s angle increased. The researchers suggest future work could address this limitation. In addition, using a galvometer scanner with a higher frame rate could allow faster image acquisition.

This approach enables both real-time calibration and imaging with minimal invasiveness, important for in-situ 3D imaging, lab-on-a-chip-based mechanical cell manipulation, deep tissue in vivo optogenetics, and keyhole technical inspections.

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Photonics & Imaging Technology Magazine

This article first appeared in the November, 2019 issue of Photonics & Imaging Technology Magazine.

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