An improvement on a Nobel Prize-winning technology called a frequency comb allows it to measure the arrival times of light pulses with greater sensitivity than previously possible — potentially improving distance measurements along with applications such as precision timing and atmospheric sensing.

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

“We’ve essentially broken this rule of frequency combing, which requires them to use a fixed pulse spacing for precision surgery,” said Laura Sinclair, a physicist at NIST’s Boulder campus and one of the authors of the paper. “By changing the way we control frequency combs, we’ve eliminated the tradeoffs we had to make, so now we can get very accurate results even if our system only has a little bit of light to work with.”

The team’s work is described in the journal Nature.

Often described as a ruler for light, a frequency comb is a type of laser whose light is made up of many well-defined frequencies that can be accurately measured. Looking at the laser’s spectrum on a screen, each frequency would stand out like a tooth on a comb, giving the technology its name. After NIST’s Jan Hall was awarded a share of the 2005 Nobel Prize in Physics, frequency combs have been used in a number of applications, ranging from accurate timekeeping to finding Earth-like planets to detecting greenhouse gases.

Despite their many current applications, frequency combs have limitations. The team’s paper is an attempt to address some of the limitations associated with using frequency combs to make accurate measurements outside the lab in more challenging situations, where signals can be very weak.

Since shortly after their invention, frequency combs have enabled highly accurate distance measurements. This accuracy comes in part from the wide range of light frequencies the combs use. Radar, which uses radio waves to determine distance, is accurate from centimeters to many meters, depending on the pulse width of the signal. The optical pulses from a frequency comb are much shorter than those from radio, allowing measurements down to nanometers (nm), or billionths of a meter, even when the detector is many kilometers from the target. The use of frequency combing techniques would eventually allow satellites to fly in precise formation for coordinated observation of Earth or space, improve GPS and support other ultra-precise navigation and timing applications.

Distance measurement using frequency combs requires two combs whose pulse timing of the lasers is closely coordinated. The pulses from one comb laser are reflected from 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 accuracy.

Related Post

The limitation associated with this high accuracy relates to the amount of light that the detector must receive. By design, the detector can only register photons from the distance laser that arrive at the same time as pulses from the second comb laser. Until now, because of the small shift in the repetition period, there was a relatively long period of “dead time” between these pulse overlaps, and any photons arriving between the overlaps were lost information, useless to the measurement effort. This made some targets difficult to see.

Physicists have a term for their aspirations in this case: They want to make measurements at the “quantum limit,” meaning they can take into account every available photon that contains useful information. More photons detected means greater ability to detect rapid changes in distance to a target, a target in other frequency comb applications. But for all its achievements to date, frequency combing technology has worked far from that quantum limit.

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

The team’s innovation includes the ability to control the timing of the second comb pulses. Advances in digital technology allow the second comb to “lock” on the returning 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 in between. After an initial acquisition, if the target moves, the digital controller can adjust the time output so that the pulses from the second comb accelerate or decelerate. This allows the pulses to be realigned so that the pulses from the second comb always overlap with the pulses returning from the target. This adjusted time output is exactly twice the distance to the target and is returned with the highly accurate characteristic of frequency combs.

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

“We found that we can quickly measure range to a target, even if we only get a weak signal back,” Sinclair said. “Because each returning photon is detected, we can accurately measure the distance near the standard quantum limit.”

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

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