Photo of two men in lab

University of Illinois mechanical sciences engineering professor Gaurav Bahl, left, and graduate student Seunghwi Kim confirmed that backscattered light waves can be suppressed to reduce data loss in optical communications systems. [Image: Julia Stackler]

Rayleigh backscattering can cause problems in all types of subwavelength waveguides, from communications fibers to nanostructures on an optical chip. Researchers at a U.S. university have devised an optomechanical technique for counteracting this lossy mess inside dielectric resonators (Optica, doi: 10.1364/OPTICA.6.001016).

Team members hope this method will lead to better optical data transmission despite the inevitable flaws in the transmission medium.

Breaking symmetry

The study by mechanical engineering professor Gaurav Bahl and colleagues at the University of Illinois at Urbana-Champaign gets around the backscattering by breaking the fundamental principle of time-reversal symmetry in the medium. Numerous groups have explored and exploited this approach in recent years for different types of photonic structures, but generally in particular circumstances involving magneto-optics.

To apply this approach to more common optical dielectric materials, the Illinois team turned to Brillouin scattering, a nonlinear dance of photons and phonons. To demonstrate the principle, the researchers built a tiny, high-Q whispering-gallery resonator from a silica sphere connected to optical waveguides. In normal, optical-only operation, the light signals running around the resonator keep hitting a defect inside the resonator, which increases the backscattering to the point where the signal’s directionality is lost.

The researchers then introduced a second near-infrared light beam at a frequency that triggered the interaction between light and sound. The resulting broken symmetry and Brillouin scattering stopped the undesirable Rayleigh backscattering. Introducing the direction-sensitive Brillouin scattering “is like creating a one-way mirror,” Bahl said in a press release. “By blocking the backward propagation of a light wave, it has nowhere to go when it encounters a scatterer, and no other option than to continue moving forward.”

Toward better optical cable

Although this study was carried out with a specific resonator geometry, the Illinois engineers are confident that the symmetry-breaking principle can be extended to other types of silica-based waveguides, including fiber networks. The team hopes that the technique will lead to better optical cable, as well as ways to bypass damaged sections of existing optical cables.

One of the study’s co-authors is with the Joint Quantum Institute, part of the U.S. National Institute of Standards and Technology and the University of Maryland, USA.