03 Feb 2016
European project achieves lensless imaging of human body interiors through thin optical fibers.University of Twente's MESA+ research institute, the Max Planck Institute for the Science of Light (MPL), FOM and Carl Zeiss.
An advanced wavefront shaping method developed at Twente combined with unique optical fibers from MPL make it possible to focus lensless light at what the development team is calling “an unparalleled resolution”. FOM postdoc Dr Lyubov Amitonova and her colleagues published their findings on 29 January in Optics Letters.
Optical imaging via ultrathin fiber probes is useful for looking inside the human body in a minimally invasive manner. Unfortunately, the resolution of conventional fiber endoscopes is 1µm at best, which is not sufficient to inspect interesting and important fine features in biological cells, for example. Some endoscopes use many separate fibers bound together into a bundle. Each fiber then acts like a discrete pixel to form the final pixelated image. However such bundles tend to be quite thick, typically 1mm in diameter.
An alternative approach is to construct fiber endoscopes based on multimode fibers. These could offer imaging with a better view and be as thin as 0.1mm across. A multimode fiber uses only a single fiber core that can transmit an entire image. Howver, such images tend to become scrambled as they pass through the fiber. But certain tricks are available for unscrambling these images.
The main limiting factor for the resolution of such multimode endoscopes is that the fibers only transmit light that propagates along the fiber’s axis. Light entering the scope at a small offset from the core’s orientation can still bounce through the fiber between its walls. But if the angle of entry gets too large, the light will simply leak out of the side. Dr Amitonova and her colleagues at UT and MPL have shown that with photonic crystal fibers this limitation can be overcome.
Conventional, so-called “step-index”, fibers consist of two zones of different material (an outer cladding and an inner core) with distinct refraction indices, which enable light transmission along the fiber axis by total internal reflection. Photonic crystal fibers are constructed differently: they are made of one material only and light guiding is achieved by the presence of a specific pattern of holes in the cladding, which are filled with air.
Tailoring the cladding structure of such a fiber provides a unique tool for engineering specific fiber-optic properties. In this project, the scientists have designed and made such a fiber to focus the laser beam through the fiber down to 0.52µm, using visible red light.
A photonic crystal fiber acts as a multimode fiber in which the image typically gets scrambled due to light bouncing off the possibly irregular inside wall of the fiber. The technique of complex wavefront shaping, invented at UT, is able to undo such scrambling and make a sharp focus. This is achieved by pre-shaping the light into the precise form needed to make a sharp image behind the fiber before the light actually enters.
By this approach, the Twenty-led team has succeeded in focusing light at the output facet of different multimode fibers including several unique photonic crystal fibers. They have shown that the complex wavefront shaping technique together with a properly designed multimode photonic crystal fiber allows the creation of a tightly-focused spot at the desired position on the fiber output facet with a subwavelength beam waist.
The scientists say this paves the way towards high-resolution endoscopic imaging via fiber probes so thin that they could be inserted, for instance, into tiny blood vessels not much thicker than a human hair. The research was been made possible by funding from the Foundation for Fundamental Research on Matter (FOM), Technology Foundation STW, and the Netherlands Organization for Scientific Research (NWO).
About the Author
Matthew Peach is a contributing editor to optics.org.