One critical challenge for engineers is to measure distances with ever greater accuracy. In recent years, this has become possible with nanometer resolution over distances of a few meters. That means to an accuracy equivalent to about the width of a human DNA strand. Although impressive, engineers would dearly love to have that kind of accuracy over much longer distances.

Now Yan-Wei Chen at the University of Science and Technology of China in Hefei, and colleagues, have found a way to do just that. They say they can measure distances greater than 100 kilometers with nanometer accuracy and that their breakthrough will have immediate applications in a wide range of frontier science and engineering projects.

For many years, the most accurate distance measurements have employed light-based technologies such as laser radar or lidar. This measures the time photons are in flight to determine the distance they travel. It is limited ultimately by the resolution of timing devices but nevertheless achieves accuracies of tens of micrometers over many kilometers.

Interference Patterns

That might sound impressive, but it can be significantly improved by combining it with interferometry. This technique reflects a laser beam off a distant mirror creating an interference pattern that depends on the distance and any changes in it.

Although better, the accuracy of this technique is limited by the periodicity of electromagnetic waves, which introduces inevitable ambiguities.

So physicists reduce this ambiguity by combining measurements from a variety of wavelengths spanning the electromagnetic spectrum. The tool they use to do this is called an optical frequency comb, which generates a spectrum of evenly spaced optical frequencies, resembling the teeth of a comb.

While the teeth of the comb have specific optical frequencies, the gaps between the teeth are determined by lower-frequency radio signals. In this way, optical frequency combs span the electromagnetic spectrum, making them ideal for improving the accuracy of distance measurements. Indeed, this is the technology that has made it possible to measure distances with nanometer-scale precision over a few meters.

But longer distance measurements have not been possible because of transmission losses and atmospheric noise caused by variations in air pressure, temperature, and humidity. These can distort light signals and reduce measurement accuracy.

That’s where Yan-Wei and colleagues have stepped in with a technique that overcomes these issues. Conventional optical frequency combs measure the round-trip distance of a reflected beam. By contrast, Yan-Wei and co’s innovation uses two combs, one at each end of the distance in question. The beams then interfere at each end of the experiment, creating patterns from which the distance can be inferred.

Opposite Beams

Although the two combs are identical, the interference patterns they create at each end are not, since the atmosphere effects each slightly different because the beams travel in opposite directions. “By comparing the results of two independent optical frequency comb interference systems, we can effectively capture real-time atmospheric drift and mitigate its impact,” they say.

The result is a system with breath-taking accuracy. “By using these techniques, we have demonstrated high-precision absolute distance measurements over a path of 113 km with a precision of 82 nm,” say the team. “To the best of our knowledge, this study represents the first instance of such precise absolute distance measurement over a path exceeding 100 km.”

That immediately suggests a variety of applications. For example, precise distance measurements are critical for satellite formation flying, which requires precise separation between probes to enable coordinated observations. By maintaining accurate baselines between satellites, researchers can create large synthetic apertures for high-resolution imaging and astrophysical observations such as very-long-baseline interferometry for radio astronomy.

Various projects around the world are designing exactly these kinds of systems, which is why the breakthrough has immediate application. In principle it can dramatically increase the distance accuracy wherever it is applied. Impressive work!

Ref: 113 km Absolute Ranging With Nanometer Precision : arxiv.org/abs/2412.05542