The ability to image biological structures and tissues deep within the human body is met with varying levels of success through such methods as optical coherence tomography (OCT), multiphoton microscopy, and endoscopy. Challenged by the tradeoff of imaging resolution vs. depth, a researcher at the University of Dundee (Dundee, Scotland), with support from the Scottish Universities Physics Alliance (Glasgow, Scotland), has built on earlier work to improve understanding of holographic light transport processes through optical fibers, enabling in vivo imaging through flexible multimode fibers—a feat not previously possible.
Because today's flexible endoscopes with 1 mm diameter or larger fiber bundles are too invasive for sensitive tissues, hair-thin multimode optical fibers were selected to transmit image information along the fiber via several concurring optical modes propagating at different phase velocities. Although the output optical fields are randomly appearing superpositions of these modes with highly complex phase relations, digital holography can characterize these scrambled images empirically, revealing the transmission matrix of the optical system and the details of the image. The most common approach is based on phase-shifting of the input modes such that they interfere constructively at a selected point behind the fiber output facet, thus creating a diffraction-limited focal point. A matrix of such focal points is then used to raster-scan the object to form its image. Currently available components can be used to achieve sub-micron resolution with typically tens of kilopixels at a few frames per second, but the performance is likely to grow steeply in the near future. Reference: M. Plöschner et al., Nat. Photonics, 9, 8, 529-535 (2015).
