The use of multimode optical fibers to increase the information capacity of the Internet is severely hampered by the distortions that occur in image transmission due to a phenomenon called modal crosstalk.
However, researchers at the University of Rochester’s Institute of Optics have developed a new technique, described in Communications of nature, to “flip” the optical wavefront of an image for both polarizations simultaneously, so that it can be transmitted through a multimode fiber without distortion. Researchers from the University of South Florida and the University of Southern California collaborated on the project.
Lead author Yiyu Zhou, a doctoral candidate in the Rochester lab of optics professor Robert Boyd, draws an analogy to a multi-lane highway when describing the challenge the researchers faced.
“Obviously, a multi-lane highway is faster than a single lane,” Zhou says. “But if a courier is forced to switch from lane A to lane B, the package will be delivered to the wrong destination. When this happens in a multimode fiber – when one space mode is coupled to another during propagation to through the fiber – – that’s what we call modal crosstalk. And we want to remove that. “
The solution the researchers devised is to digitally preform the wavefront and polarization of a forward-propagating signal beam to be the phase conjugate of a backward-propagating auxiliary probe beam. – in an experimental realization of vector inversion of time.
“When an optical beam with perfect wavefronts passes through multimode fiber, it comes out strongly distorted,” explains Boyd, also holder of the Canada Excellence Research Chair in Nonlinear Quantum Optics at the University of Paris. ‘Ottawa.
“If we use a mirror to reflect the wavefront, it will get even more distorted. But if we reflect it on a mirror instead, and also reverse the wavefront back and forth, the distortion breaks down. as the waves come back. In particular, we need to do this procedure for both polarizations simultaneously when the distortion medium is a long multimode fiber. “
Researchers demonstrate that this technology can improve channel capacity in a 1 km long multimode fiber
“Our technique can be used to perform mode division multiplexing over long standard multimode fibers to dramatically improve the channel capacity of optical communication links,” Zhou explains. “It can potentially be used to increase Internet speed by an order of magnitude or two.”
The technique could also potentially be used to improve endoscopic imaging of the brain and other biological tissues, Zhou says.
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