Breakthrough could help Nobel Prize-winning technology measure distances and timing with pinpoint accuracy limited only by the quantum nature of light

An improvement to a Nobel Prize-winning technology called a frequency comb allows it to measure arrival times of light pulses with greater sensitivity than before – potentially improving distance measurements as well as applications such as timing of accuracy and atmospheric detection.

The innovation, created by scientists at the National Institute of Standards and Technology (NIST), represents a new way to use frequency comb technology, which scientists have dubbed a “time-programmable frequency comb.” Until now, frequency comb lasers had to create light pulses with metronomic regularity to achieve their effects, but the NIST team showed that manipulating pulse timing can help frequency combs perform precise measurements under a wider set of conditions than was possible. .

“We basically broke that rule of frequency combs that requires them to use fixed pulse spacing for precision operation,” said Laura Sinclair, a physicist at NIST’s Boulder campus and one of the authors of the article. “By changing the way we control the frequency combs, we got rid of the compromises we had to make, so we can now get high precision results even when our system has little light to work with.” »

The work of the team is described in the log Nature.

Often described as a ruler for light, a frequency comb is a type of laser whose light consists of many well-defined frequencies that can be accurately measured. Looking at the laser spectrum on a screen, each frequency would stand out like a tooth of a comb, giving the technology its name. After earning NIST’s Jan Hall part of the 2005 Nobel Prize in Physics, frequency combs have found use in a number of applications ranging from precision timekeeping to searching for Earth-like planets to detecting greenhouse gas.

Despite their many current uses, frequency combs have limitations. The team’s paper attempts to address some of the limitations that arise when using frequency combs to make accurate measurements outside of the lab in more challenging situations, where signals can be very weak.

Shortly after their invention, frequency combs enabled very precise distance measurements. In part, this accuracy stems from the wide range of light frequencies used by the combs. Radar, which uses radio waves to determine distance, is accurate from a few centimeters to several meters depending on the pulse width of the signal. A frequency comb’s optical pulses are much shorter than radio, potentially allowing precise measurements down to nanometers (nm) or billionths of a meter, even when the detector is several miles from the target. The use of frequency combing techniques could potentially enable precise formation flight of satellites for coordinated Earth or space sensing, improve GPS, and support other ultra-fast navigation and timing applications. precise.

Distance measurement using frequency combs requires two combs whose timing of the laser pulses is closely coordinated. The pulses of a comb laser bounce off a distant object, just as radar uses radio waves, and the second comb, slightly offset in the repetition period, measures their return timing with great precision.

The limitation that accompanies this high accuracy concerns the amount of light that the detector must receive. By design, the detector can only register photons from the ranging laser that arrive at the same time as the pulses from the second comb laser. Until now, due to the slight shift in the repeat period, there was a relatively long “dead time” period between these pulse overlaps, and any photons that arrived between the overlaps were wasted information, useless to measurement effort. This made some targets hard to see.

Physicists have a term for their aspirations in this case: they want to make measurements at the “quantum limit,” meaning they can account for every available photon that contains useful information. More detected photons mean greater ability to spot rapid changes in distance to a target, a goal in other frequency comb applications. But despite all of its accomplishments to date, frequency comb technology has operated far from that quantum limit.

“Frequency combs are commonly used to measure physical quantities such as distance and time with extreme precision, but most measurement techniques waste the vast majority of light, 99.99% or more,” said Sinclair. “Rather, we’ve shown that by using this different control method, you can get rid of this waste. This can mean an increase in measurement speed, accuracy, or it allows a much smaller system to be used. »

The team’s innovation involves the ability to control the timing of the second comb’s pulses. Advances in digital technology allow the second comb to “lock” to the return signals, eliminating the dead time created by the previous sampling approach. This happens despite the controller having to find a “needle in a haystack” – the pulses are relatively short, lasting only 0.01% as long as the dead time separates them. After an initial acquisition, if the target moves, the digital controller can adjust the time output so that the second comb pulses speed up or slow down. This allows the pulses to realign, so that the pulses from the second comb always overlap with those returning from the target. This adjusted time output is exactly twice the distance to target, and it is returned with the extreme precision characteristic of frequency combs.

The result of this time-programmable frequency comb, as the team calls it, is a detection method that makes the best use of available photons and eliminates dead time.

“We found that we could quickly measure the distance to a target, even if we only had a weak signal coming back,” Sinclair said. “Since each returning photon is detected, we can measure the distance near the standard quantum limit in precision. »

Compared to standard dual-comb telemetry, the team found a 37-decibel reduction in the received power required, in other words, requiring only about 0.02% of the photons previously needed.

The innovation could even enable future nanometer-level measurements of distant satellites, and the team is studying how its time-programmable frequency comb could benefit other frequency comb sensing applications.

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