Trapping Light Without Back Reflections

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 backwards, which is disturbing their function. Researchers have now demonstrated a method for suppressing these unwanted back reflections. Their findings can help improve a multitude of micro­resonator-based applications from measure­ment technology such as sensors used for example in drones, to optical infor­mation processing in fibre networks and computers. The method is developed by a team spanning the Max Planck Institute for the Science of Light (Germany), Imperial College London, and the National Physical Labora­tory (UK).

An optical microresonator and a sharp tungsten tip. (Source: A. Svela)

Researchers 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 amount of back reflection, or back­scattering, of light due to material and surface imper­fections. The back reflected light negatively impacts of the usefulness of the tiny glass structures. To reduce the unwanted back­scattering, the British and German scientists were inspired by noise cancelling headphones, but rather using optical than acoustic inter­ference. “In these headphones, out-of-phase sound is played to cancel out undesirable background noise,” says Andreas Svela from the Quantum Measurement Lab at Imperial College London. “In our case, we are intro­ducing out-of-phase light to cancel out the back reflected light,” Svela continues.

To generate the out-of-phase light, the researchers position a sharp metal tip close to the micro­resonator surface. Just like the intrinsic imper­fections, the tip also causes light to scatter backwards, but there is an important difference: The phase of the reflected light can be chosen by controlling the position of the tip. With this control, the added backscattered light’s phase can be set so it anni­hilates the intrinsic back reflected light – the researchers produce darkness from light. “It is an unintui­tive result, by intro­ducing an additional scatterer, we can reduce the total back­scattering,” says Pascal Del’Haye at the Max Planck Institute for the Science of Light. The results show a record suppression of more than 30 db compared to the intrinsic back reflec­tions. In other words, the unwanted light is less than a thousandth of what it was prior to applying the method.

“These findings are exciting as the technique can be applied to a wide range of existing and future micro­resonator techno­logies,” comments Michael Vanner from the Quantum Measure­ment Lab at Imperial College London. For example, 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 allows us to transport more infor­mation even faster. (Source: ICL)

Reference: A. Ø. Svela et al.: Coherent suppression of backscattering in optical microresonators, Light: Sci. & App9, 204 (2020); DOI: 10.1038/s41377-020-00440-2

Links: Microphotonics, National Physical Laboratory, Teddington, UK • Microphotonics, Max Planck Institute for the Science of Light, Erlangen, GermanyQuantum Measurement Lab, Imperial College London, London, UK

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