Suppressing Backscattering of Light to Improve Optical Data Transmission

Posted Aug 22, 2019 by University of Illinois

Researchers at the University of Illinois have deduced a way to redirect misfit light waves to reduce energy loss during optical data transmission. In their study, the researchers exploited an interaction between light and sound waves to suppress the scattering of light from material defects-which could in turn lead to improved fiber optic communication. The study was published in the journal Optica.

Light waves scatter when they encounter obstacles, be it a crack in a window or a tiny flaw in a fiber optic cable. Much of that light scatters out of the system, but some of it scatters back toward the source in a phenomenon called backscattering, the researchers said. According to lead researcher and a Mechanical Science and Engineering Professor, Gaurav Bahl, there is no such thing as a perfect material.

There is always a little bit of imperfection and a little bit of randomness in the materials that are used in any engineered technology. For instance, the most perfect optical fiber used for long-range data transmission might still have some invisible flaws. These flaws can be a result of manufacturing, or they can appear over time as a result of thermal and mechanical changes to the material. Ultimately, such flaws set the limits of performance for any optical system.

A few previous studies have shown that undesirable backscattering can be suppressed in special materials that have certain magnetic properties. However, these are not viable options for today’s optical systems that use transparent, nonmagnetic materials like silicon or silica glass. In the new study, Bahl and Graduate Student Seunghwi Kim used an interaction of light with sound waves, instead of magnetic fields, to control backscattering.

Light waves travel through most materials at the same speed irrespective of direction, be it forward or backward. But, by using some direction-sensitive opto-mechanical interactions, that symmetry can be broken and backscattering can be effectively shut down. It is like creating a one-way mirror. 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.

To demonstrate this phenomenon, the team sent light waves into a tiny sphere made of silica glass, called a microresonator. Inside, the light travels along a circular path like a racetrack, encountering defects in the silica over and over again, amplifying the backscattering effect. The team then used a second laser beam to engage the light-sound interaction in the backward direction only, blocking the possibility of light scattering backward. What would have been lost energy continues moving forward, in spite of defects in the resonator.

Being able to stop the backscattering is significant, but some of the light is still lost to side scattering, which scientists have no control over. The advance is therefore very subtle at this stage and only useful over a narrow bandwidth. However, simply verifying that the team can suppress backscattering in a material as common as silica glass, suggests that they 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.

Trying the experiment in fiber optic cable will be the next step in showing that this phenomenon is possible at the bandwidths required in optical fiber communications. According to Bahl, the principle that they explored has been seen before. The real story here is that they 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. He now hopes that other researchers examine this phenomenon in their optical systems, as well, to further advance the technology.

The study was supported by the National Science Foundation, Air Force Office of Scientific Research and the Office of Naval Research.

Click here to read the published paper.