A new technique for suppressing back reflections of light

Trapping light without back reflections.

As light propagates along a waveguide, a fraction of the field can be reflected by Rayleigh scatterers. In high-quality-factor whispering-gallery-mode microresonators, this intrinsic backscattering is primarily caused by either surface or bulk material imperfections.

Microresonators are small glass structures in which light can circulate and build up in intensity. Due to material imperfections, some amount of light is reflected backward, which is disturbing their function.

To suppress these unwanted back reflections, scientists at the Imperial College London have demonstrated a technique that can help improve many microresonator-based applications from measurement technology such as sensors used, for example, in drones, to optical information processing in fiber networks and computers.

The study was conducted in collaboration with the Max Planck Institute for the Science of Light (Germany) and the National Physical Laboratory (UK).

Scientists and engineers are discovering many uses and applications for optical microresonators, a type of device often referred to as a light trap. One limitation of these devices is that they have some back reflection, or backscattering, of light due to material and surface imperfections. The back-reflected light negatively impacts the usefulness of the tiny glass structures. To reduce the unwanted backscattering, the British and German scientists were inspired by noise-canceling headphones, but rather using optical than acoustic interference.

Lead author Andreas Svela from the Quantum Measurement Lab at Imperial College London said, “In these headphones, the out-of-phase sound is played to cancel out undesirable background noise. In our case, we are introducing out-of-phase light to cancel out the back-reflected light.”

By positioning a sharp metal tip close to the microresonator surface, scientists generated the out-of-phase light. Likewise, the intrinsic imperfections, the tip also causes light to scatter backward. Still, there is a significant difference: The reflected light phase can be chosen by controlling the tip’s position. With this control, the added backscattered light’s phase can be set, annihilating the intrinsic back-reflected light—the researchers produce darkness from light.

An optical microresonator and a sharp tungsten tip
Top: An optical microresonator and a sharp tungsten tip. The tip’s position can control the amount of back reflections in the microresonator. The authors show >30 dB suppression below the intrinsic backscattering. Bottom: The unwanted (intrinsic backscattered) light to the left is cancelled out by the out-of-phase light (“anti-light” similar to “anti-noise” in noise-canceling headphones) introduced by the metal tip. Credit: Andreas Svela

Co-author and principal investigator Pascal Del’Haye at the Max Planck Institute for the Science of Light said, “It is an unintuitive result. By introducing an additional scatterer, we can reduce the total backscattering.”

The study shows that a record suppression of more than 30 decibels compared to the intrinsic back reflections. In other words, the unwanted light is less than a thousandth of what it was before applying the method.

Principal investigator Michael Vanner from the Quantum Measurement Lab at Imperial College London said, “These findings are exciting as the technique can be applied to a wide range of existing and future microresonator technologies.”

Scientists noted, “The method can be used to improve gyroscopes, sensors that for instance, help drones navigate; or to improve portable optical spectroscopy systems, opening for scenarios like built-in sensors in smartphones for detection of dangerous gasses or helping check the quality of groceries. Furthermore, optical components and networks with better signal quality allow us to transport more information even faster.”

Journal Reference:
  1. Andreas Ø. Svela et al. Coherent suppression of backscattering in optical microresonators, Light: Science & Applications (2020). DOI: 10.1038/s41377-020-00440-2

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