A new optical inversion strategy to unscramble light propagation through multimode optical fibers

See through the ideal MMFs. From left to right, the panels in each row display digital simulations of the field/object/scene to be imaged (leftmost panel); the image formed in perfect MMF inversion, that is to say including the spatial filtering effects due to the limited modal capacity of the fiber, but not including any mode coupling effects present in our undulator; the field at the level of the proximal fibrous facet; the field at the corrective plane; the reformed image at the output of our MPLC-based inverter. Imaging, (a) a spatially consistent speckle pattern at the distal facet of the fiber (white circle indicates core-sheath boundary of radius 40.3 μm); (b) an incoherent object at the distal facet of the fiber, evoking a sheet of confluent cells; (c) an incoherent defocused object 1 mm from the distal facet of the fiber, where the field of view, represented by a dotted white circle, has reached a radius of 100.8 μm; (d) A far-field scene of the distal facet (here the dotted white circle represents the NA fiber). Credit: Intelligent Computing (2022). DOI: 10.34133/2022/9816026

Multimode optical fibers (MMFs) are very thin strands of glass that are ubiquitous in light guiding applications. Their development has gone hand in hand with the enormous growth in the rapid transmission of information across the world.

The small footprint of MMFs also makes them attractive candidates for next-generation microendoscopes, to provide optical microscopy deep in the body. However, the practical information capacity of MMFs is limited by modal dispersion, a mechanism that blurs spatial information propagating through MMFs.

Thus, the direct transmission of images through MMFs is extremely difficult: an image projected at one end is blurred beyond recognition by the time light reaches the other end. Pioneering research over the past decade has shown how optical interference caused by MMFs can be measured and cancelled. Now, a team of researchers from the University of Exeter and the Leibniz Institute of Photonic Technology have built on that idea and come up with a new imaging strategy, called optical inversion.

The research has been published in Intelligent Computing.

“The majority of imaging techniques demonstrated so far rely on raster scanning or sequential pattern projection, which essentially means light is deciphered one spatial mode at a time.” Lead author Dr. Unė Būtaitė said:

“This currently precludes the delivery of wide-field imaging techniques via MMFs. For example, there is currently no way to achieve wide-field super-resolution imaging at the tip of an MMF, which would be a highly desirable means of gaining a deeper understanding of biological processes within the body.”

To overcome this problem, researchers propose and design a passive optical device, called an optical undulator. Dr. Būtaitė explained, “Our undulator can be understood as a tailor-made scattering medium, designed to be complementary to an MMF to cancel out its optical effects.”

Spatial information is scrambled after light from the scene travels through MMF, but the optical inverter scrambles the light exactly the opposite of the fiber, allowing the scene to be reimaged passively, and completely optically in a few nanoseconds.

Different scenarios were simulated to study the performance of the researcher’s optical inverter design. The results show that an optical undulator has the potential to achieve single wide-field imaging and super-resolution imaging via MMFs. Additionally, by incorporating optical memory effects into its design, the optical inverter can dynamically adapt to see through flexible fibers.

Dr David Phillips, lead author of the project, said: “The main advantage of our concept is that it makes possible any form of wide-field microscopy at the end of a thin strand of MMF hair, which can potentially be loaded into a needle. to visualize scenes deep inside the body. This includes powerful new imaging techniques such as location-based super-resolution imaging, as well as other emerging forms of microscopy. parallelized super resolution, structured illumination microscopy and single objective light sheet microscopy.

“Furthermore, single-shot widefield imaging at any distance beyond the distal end of a short length of MMF also becomes possible.”

In the future, the researchers predict other applications for this research. Dr Phillips said: “The optical inversion strategy we have described here can potentially be extended to decipher light that has passed through other objects, such as photonic crystal waveguides, photonic lanterns or biological tissues.”

“Finally, we anticipate that all-optical inversion of scattered light will find an array of applications beyond optical imaging: benefiting the fields of mode division multiplexing for high-capacity optical communications, as well as to quantum cryptography and classical and quantum optical computing. We are excited to see where this technology is going.”