Lensless Endoscope Produces 3D Images of Ultrasmall Objects

Researchers at Dresden University of Technology (TU Dresden) have developed a tiny, 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 μm across.

To build a self-calibrating endoscope, the researchers added a 150-μm-thick glass plate to the tip of a coherent fiber bundle (a type of optical fiber that is commonly found in endoscopy applications). The fiber bundle is about 350 μm wide and consists of 10,000 cores.

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

When the central fiber core is illuminated, it emits a beam that is reflected back into the fiber bundle. The beam serves as a reference point for measuring the transfer of light. This measurement provides the endoscope with the data it uses to calibrate itself as needed. A spatial light modulator is used to manipulate the direction of the light and enable remote focusing. A camera captures light that is back-reflected from the fiber bundle. The back-reflected light is superposed with a reference wave to measure the light phase.

The position of the beam that is reflected back into the fiber bundle guides the focus of the device, with a minimal focus diameter of about 1 μm. The researchers used an adaptive lens and a 2D galvometer mirror to shift the focus and enable scanning at different depths.

The team tested the new endoscope by using it to image a 3D specimen under a 140-μm-thick cover slip. When scanning the image plane in 13 steps over 400 μm with an image rate of 4 cycles per second, the endoscope 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 said that future work could address this limitation. In addition, using a galvometer scanner with a higher frame rate could allow faster image acquisition.

The minimally invasive device can provide high-contrast imaging and robust stimulation. Its self-calibration capabilities will allow it to tolerate bending or twisting of the fiber. The endoscope could be used in optogenetics to stimulate cellular activity and during medical procedures to monitor cells and tissue.

“The novel 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,” professor Juergen W. Czarske said.

The research will be presented at the Frontiers in Optics + Laser Science (FIO + LS) conference, Sept. 15-19, in Washington, D.C.