A technique developed at the University of Rochester “flips” the optical wavefront of an image for both polarizations simultaneously, so that it can be transmitted through a multimode fiber without distortion. The research overcomes a barrier to the use of multimode fibers to boost the informational capacity of the internet caused by a phenomenon called modal crosstalk. When a well-defined image propagates from the right-hand side to the left-hand side through a 1-km-long multimode fiber, its spatial profile and polarization will be strongly distorted. By flipping the wavefront of the distorted image for both polarizations simultaneously, a technique referred to as vectorial time reversal, an undistorted beam is formed after it passes from left to right through the optical fiber. Courtesy of Yiyu Zhou, University of Rochester. “Obviously a multiple-lane highway is faster than a single lane,” said lead author Yiyu Zhou, a Ph.D. candidate in the lab of Rochester professor Robert Boyd. “But if a courier is forced to change from lane A to lane B, the package will be delivered to the wrong destination. When this happens in a multimode fiber — when one spatial mode is coupled to another during the propagation through the fiber — it’s what we call modal crosstalk. And we want to suppress that.” Researchers at the University of South Florida and at the University of Southern California collaborated on the project. The team’s method digitally pre-shapes the wavefront and polarization of a forward-propagating signal beam to be the phase conjugate of an auxiliary, backward propagating probe beam, in an experimental realization of vectorial time reversal. “When an optical beam with perfect wavefronts passes through the multimode fiber, it comes out badly distorted,” explained Boyd, who also serves as the Canada Excellence Research Chair in Quantum Nonlinear Optics at the University of Ottawa. “If we use a mirror to send the wavefront back, it will become even more distorted. But if we instead reflect it off a mirror, and also flip the wavefront from front to back, the distortion becomes undone as the waves go back through that distorting medium. In particular, we need to perform this procedure for both polarizations simultaneously when the distorting medium is a long multimode fiber.” The researchers were able to demonstrate the technology in a 1-km multimode fiber in which they were able to support 210 channels. “Our technique can be used to realize mode-division multiplexing over long, standard multimode fibers to significantly enhance the channel capacity of optical communication links,” Zhou said. “It can potentially be used to increase the internet speed by one or two orders of magnitude.” The work was published in Nature Communications (www.doi.org/10.1038/s41467-021-22071-w).