Laser-based Method Derives Data from Disordered Media

Researchers from TU Wien and Utrecht University have developed a laser-dependent method for gathering data through disordered and complicated media, using a mathematical procedure.

In contexts such as biomedicine, structures that are embedded in an irregular, complicated environment can be difficult to measure with light; present irregularities can deflect, scatter, and refract, rendering gathered information inaccurate.

Disordered structures such as a turbid pane of glass alter the path of light, making it difficult to obtain accurate information about objects behind them, though a mathematical procedure developed by TU Wien and Utrecht University allows for highly accurate measurements. Courtesy of TU Wien.

“You always want to achieve the best possible measurement accuracy — that’s a central element of all natural sciences,” said Stefan Rotter from TU Wien. “Let’s think, for example, of the huge LIGO facility, which is being used to detect gravitational waves. There, you send laser beams onto a mirror, and changes in the distance between the laser and the mirror are measured with extreme precision.”

Due to the ultrahigh-vacuum environment, the light travels undisturbed, enabling the high level of accuracy of its measurements.

“Let’s imagine a panel of glass that is not perfectly transparent, but rough and unpolished like a bathroom window,” said Allard Mosk of Utrecht University. “Light can pass through, but not in a straight line. The lightwaves are altered and scattered, so we can’t accurately see an object on the other side of the window with the naked eye.”

The researchers determined that by identifying the specific changes that are being made to the beam of light, they can then in essence turn the situation around by creating a complicated wave pattern that is then transformed into the desired shape by the disordered media through which it passes.

“To achieve this, you don’t even need to know exactly what the disturbances are,” said Dorian Bouchet, first author of the work’s research paper. “It’s enough to first send a set of trial waves through the system to study how they are changed by the system.”

The researchers jointly developed a mathematical procedure that can be used to calculate the optimal wave from the test data.

“You can show that for various measurements there are certain waves that deliver a maximum of information as, for example, on the spatial coordinates at which a certain object is located,” Bouchet said.

Scientists at Utrecht University tested the technique using a turbid pane of glass, similar to those used in bathroom windows. In characterizing the scattering behavior of the medium, the researchers were able to calculate the optimal wave to analyze an object behind the glass and, ultimately, to achieve information accurate to the nanometer range.

To test the limits of the technique, the researchers significantly reduced the number of photons. They found that they were still able to achieve accurate results.

“We see that the precision of our method is only limited by the so-called quantum noise,” Mosk said. “This noise results from the fact that light consists of photons — nothing can be done about that. But within the limits of what quantum physics allows us to do for a coherent laser beam, we can actually calculate the optimal waves to measure different things — not only the position, but also the movement or the direction of rotation of objects.”

The research was published in Nature Physics (www.doi.org/10.1038/s41567-020-01137-4).