image: Image quality comparison before and after the thermal modification of the multicore fiber endoscope. (a) Scheme of the multicore fiber modification (top) and microscope image of the fiber before (bottom left) and after (bottom right) modification showing the enlargement of the individual fiber cores; Endoscopic imaging of (b) a point source, (c) test chart and (d) 10 micron beads, performed with an unmodified and modified fiber showing the increase in light collection and image contrast after fiber modification.
Credit: by Kinga Zolnacz, Ronja Stephan, Jakob Dremel, Katharina Hausmann, Matthias Ließmann, Michael Steinke, Juergen Czarske, Robert Kuschmierz
Endoscopes are tools allowing for transmitting an image from hard-to-reach areas of the body and are widely used during medical diagnostics and surgery. Microendoscopes based on optical fibers offer the smallest size, comparable to the thickness of a single hair, while maintaining a high resolution of the transmitted image. Therefore, they are a promising solution for minimally invasive medical procedures in areas where space is limited, such as brain or cochlea. Unfortunately, optical fibers currently used in endoscopy still suffer from a few limitations, for example low light collection, which currently restrict their use in real world applications. Increasing light collection efficiency is a challenge that has to be overcome to open the gate for endoscopes to biomedical applications, such as imaging of tissue fluorescence used for cancer diagnostics.
In a new paper published in Light: Advanced Manufacturing, a team of scientists from Poland’s Wroclaw University of Science and Technology and Germany’s Technical University of Dresden and Leibniz University Hannover have developed a method for increasing the light collection efficiency in multicore optical fiber. By controlled local heating of the fiber in a focused high-power laser beam, they increased the diameter of the cores guiding the image without changing the size of the probe.
“Our key result is that by a modification of a multicore fiber, we can collect 2.3 times more light than with its unmodified version. This leads to more than five times higher signal-to-noise ratio and, therefore, significant gain in image contrast. Moreover, we expand the cores inside of the fiber but the outer diameter of the fiber remains as small as in the original fiber, therefore, the fiber still meets the criteria of a minimally invasive tool.”
The core expansion is driven by a diffusion of the cores dopants to the cladding area. The diffusion is induced by heating the fiber locally to a high temperature but below the glass melting point. The whole heating procedure takes less than two minutes and is applied over the length of just a few hundred microns from the fiber’s end. In this way the general transmission properties of the fiber do not change but the larger cores at the end of the fiber collect more light. In this way the scientists were able to increase the light collection efficiency without increasing the inter-core crosstalk along the fiber, which in endoscopic imaging results in image degradation over time, or when the fiber is touched.
“For demonstration we used a self-fabricated specialty fiber with pseudo-randomly organized cores to also show the reduction of ghost images induced by core aperiodicity. But the same modification can be performed on any commercially available glass fiber. Moreover, the same approach can be used also on longitudinally twisted optical fibers which allow for not only bending independent image quality, but also image position. Such a solution is essential for example in cochlear implant placement surgery, where knowledge of exact position is required” they explain.
Journal
Light: Advanced Manufacturing
Article Title
Multicore fiber with thermally expanded cores for increased collection efficiency in endoscopic imaging