To control backscattering and reduce energy loss during optical data transmission, researchers from the University of Illinois at Urbana-Champaign exploited the interaction between light and sound waves. They were able to achieve robust light propagation in spite of scatterers and defects by dynamically suppressing backscattering using time-reversal symmetry breaking. The team sent lightwaves into a microresonator made from silica. As the light inside the resonator traveled along a circular path, it encountered defects in the silica repeatedly, which amplified the backscattering effect. The researchers used a second laser beam to engage the light-sound interaction in the backward direction only, blocking the possibility of light scattering backward. Instead of losing energy to backscattering, the light continued to move forward, in spite of defects in the resonator. The near-complete suppression of backscattering was experimentally confirmed by measuring the reduction in intrinsic optical loss. The researchers also observed a reduction in the back-reflections caused by disorder. The time-reversal symmetry in the silica resonator was broken through a Brillouin scattering interaction that is available in all materials, the researchers said. Mechanical sciences engineering professor Gaurav Bahl, (l), and graduate student Seunghwi Kim confirmed that backscattered lightwaves can be suppressed to reduce data loss in optical communications systems. Courtesy of Julia Stackler. Lightwaves travel through most materials at the same speed irrespective of direction, be it forward or backward. “But, by using some direction-sensitive optomechanical interactions, we can break that symmetry and effectively shut down backscattering,” professor Gaurav Bahl said. “It is like creating a one-way mirror. By blocking the backward propagation of a lightwave, it has nowhere to go when it encounters a scatterer, and no other option than to continue moving forward.” Some of the light will still be lost to side scattering, which scientists have no control over, the researchers said. “The advance is therefore very subtle at this stage and only useful over a narrow bandwidth,” Bahl said. “However, simply verifying that we can suppress backscattering in a material as common as silica glass suggests that we could produce better fiber optical cable or even continue to use old, damaged cable already in service at the bottom of the world’s oceans, instead of having to replace it.” The researchers plan to test their findings in fiber optic cable to demonstrate that this phenomenon is possible at the bandwidths required in optical fiber communications. “The principle that we explored has been seen before,” Bahl said. “The real story here is that we have confirmed that backscattering can be suppressed in something as simple as glass, using an opto-mechanical interaction that is available in every optical material. We hope that other researchers examine this phenomenon in their optical systems as well, to further advance the technology.” The research was published in Optica, a publication of OSA, The Optical Society (https://doi.org/10.1364/OPTICA.6.001016).
