Investigation of laser systems for 3D endomicroscopy using coherent fiber bundles

• Applicant: Professor Dr.-Ing. Jürgen W. Czarske
• Subject Area: Biomedical Systems Technology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454289351

Multimodal 3D imaging with submillimeter-thin and flexible endoscopes enables important advances in biomedical research and clinical practice. Available fiber bundle endoscopes are based on the evaluation of light intensity and distal imaging optics. As a result, they are limited in their minimum diameter, their 2D imaging and their spatial bandwidth product. During the first funding period, a novel minimally invasive 3D endoscopy technology was fundamentally investigated that also evaluates the light phase and dispenses with distal imaging optics. For this purpose, a self-calibration technique was validated that allows the optical transfer function (OTF) of coherent fiber bundles to be measured holographically without distal access. The OTF could be corrected using programmable optics. It was shown that the method enables unpixelated imaging with an order of magnitude better spatial bandwidth product and 3D imaging with distal diameters smaller 500 µm. At the same time, it was shown that for fibers with advantageous transmission properties, a correction of the transfer function can also be realized for static phase objects, which potentially makes it possible to realize robust and easy-to-use fiber optic imaging systems. However, the technology is currently still limited by the small spectral width of the phase correction and by diffraction due to the large core-to-core distance and the low photon efficiency of the fiber bundle. The aim of the requested second funding period is therefore to investigate how multicore fiber (coherent fiber bundles) with improved transmission properties can be used for high-resolution endomicroscopy, especially of broadband radiation such as autofluorescence. To this end, the photon efficiency of fiber bundles is to be increased by thermal expansion of the fiber cores in order to increase the signal-to-noise ratio. By tapering the distal fiber end to 200 µm, the fibers are to be less invasive and losses due to optical diffraction are to be reduced. Numerically and experimentally, optimized microstructures will be investigated that can be laser-fabricated on the fiber facet to increase the numerical aperture of the fiber cores and the spectral width of the phase correction. The result will be a completely new optical component for high-resolution endomicroscopy, which will be validated for confocal autofluorescence imaging with a spatial resolution of better than 1 µm. Paradigm shifts for biomedicine with single-shot high-resolution fluorescence 3D imaging are promising.

• DFG Programme: Research Grants