The holographic endoscopy team has found a new way to transmit image information with virtually no distortion from multimodal fibers – even if they bend the fibers.

By T.  Čižmár // D. Boonzajer Flaes 

Endoscopes allow physicians to look inside patients’ bodies as if looking through a keyhole. The images are usually transmitted via a bundle of several hundred optical fibers. This requires endoscopes that have a relatively large diameter, which makes them difficult to use in sensitive regions of the body such as the brain. An international team led by the Leibniz IPHT scientist Prof. Tomáš Čižmár is investigating multimodal optical fiber probes that deliver high-resolution images with just a single hair-thin fiber. In order to be able to observe individual brain cells in a living organism during their “work” in the future, the probes must above all be flexible. This is a challenge because when the fibers are bent, the transmitted image is distorted in different ways.

For several years now, Čižmár has been looking for a solution to solve the problem of image transmission in multimodal fibers. “We have shown that we can use computer calculations to predict the influence of bending on light transmission in the fiber and thus restore the images. This is a complex process in which we have to know the exact deformation of the fiber,” explains Tomáš Čižmár, who has headed the fiber optics department at Leibniz IPHT since 2017.

The researchers have now found a new, simpler approach in the form of fibers with a special refractive index profile. “We came up with the idea of using gradient index fibers for image transmission. In contrast to conventional fibers, the refraction of light across the fiber cross-section does not change abruptly, but gradually in the form of a curve,” says Čižmár. The scientists first simulated the propagation of light in the fibers using theoretical models. The results suggested that the signal transmission in the fibers was insensitive to bending and movement due to the special refractive index profile. “In the experiment, we could only partially test our ideas. The gradient index fibers are actually much less susceptible to interference during bending than conventional step index fibers. We attribute the fact that the transmitted images are still slightly distorted to the not-so-perfect parabolic refractive index profile of the commercial fibers,” says Dirk Boonzajer Flaes, first author of the publication, explaining the unusual results. The scientists’ research is now focusing on producing gradient index fibers with improved optical properties and testing them for medical imaging.

The scientists published the original article entitled “Robustness of Light-Transport Processes to Bending Deformations in Graded-Index Multimode Waveguides” in the much-cited journal Physical Review Letters.

Funded by: Netherlands Organisation for Scientific Research , EU , Czech Science Foundation, EFRE, , University of Dundee , Scottish Universities Physics Alliance