“Shed Light” at Right Angle: ITMO Specialists Improve Optical Fiber for Data Transfer

The University specialists have improved the quality of data transfer when using a modernized optical fiber. With the help of a new light coupling technology, they succeeded in eliminating "blind spots" that emerged at large incidence angles. The improved optical fiber can be used to improve endoscopic and laparoscopic images, in quantum technologies and fiber-optic sensors. The concept proposed by the scientists got on the front cover of the October issue of ACS Photonics.

Optical fibers are widely used in the field of telecommunications, allowing the optical signal to transfer almost a million times more data than an electric signal does via a copper cable. This technology is used in everyday medicine, for example, for the in vivo (within living organisms or cells) studies. For instance, endoscopy methods employing optical fibers help make quick and precise diagnoses. However, the efficiency of light coupling by optical fibers rapidly decreases at large incidence angles. This happens so fast that at the incidence angle of 15 degrees, the fiber becomes almost useless. This fundamental task was solved by ITMO scientists Oleh Yermakov and Andrey Bogdanov in collaboration with the Leibniz Institute of Photonic Technology (Germany) and Australian National University.

The researchers used a dielectric nanostructure made of silicon nitride with a high refractive index, implemented at the end face of the optical fiber. The nanostructure represents a ring-like diffraction grating that makes it possible to capture light of any polarization and incident from any direction at a large incidence angle. At the same time, silicon nitride doesn't absorb light compared to other counterparts used. These factors help increase the efficiency of light coupling by about ten thousand times in comparison with optical fibers with a metal nanostructure or without one.

"I think we succeeded due to the harmonic combination of specialists in the fields of optical fibers and optical nanostructures, and teamwork between theoreticians and experimenters," explains Oleh Yermakov, a staff at ITMO's Department of Physics and Engineering. "We advanced from a superficial understanding of the problem to a detailed concept, analytical description, and a precise numerical model. This helped us come up with the optimal structure design which was then fabricated and measured by our German colleagues."

In the nearest future, the researchers plan to speed up, simplify and cheapen the process of nanostructures fabrication by using nanoimprint lithography, assuming the polymer resist stamping and the consequent etching. For now, scientists are working on an automatic algorithm in order to define a nanostructure design for any particular task.

"The implemented device is useful for any application that demands to remotely collect light under a large angle, as for instance in in-vivo spectroscopy or quantum technology," comments Markus Schmidt, Professor for Fiber Photonics at Leibniz-IPHT and the head of the project.

"For many years a success of nanophotonics developed for an efficient control of light-matter interaction was not connected with the progress in fiber technologies aiming to operate on much larger scales. This work provides a bridge between two fields, and this fact was acknowledged by highlighting the paper on a front cover of ACS Photonics with the impact-factor 7.3," says Yuri Kivshar, an Honorary Professor of ITMO University and a fellow of the Australian Academy of Science.
The research was conducted with support from the Foundation for the Advancement of Theoretical Physics and Mathematics "BASIS", the German-Russian Interdisciplinary Science Center (G-RISC), and the Russian Science Foundation.

More information:
Oleh Yermakov et al. Nanostructure-Empowered Efficient Coupling of Light into Optical Fibers at Extraordinarily Large Angles, ACS Photonics (2020). DOI: 10.1021/acsphotonics.0c01078

Provided by ITMO University