Researchers at NIST have developed a method to enable frequency combs to measure light pulse arrival times with greater sensitivity than was previously possible — potentially improving measurements of distance, along with applications such as precision timing and atmospheric sensing. The NIST team attempted to address some of the limitations that arise when using frequency combs to make precise measurements outside the laboratory in more challenging situations, where signals can be very weak. Until now, frequency comb lasers needed to create light pulses with metronomic regularity to achieve their effects. The NIST team showed that manipulating the timing of the pulses can help frequency combs make accurate measurements under a broader set of conditions than has been possible. The researchers termed the technology a “time programmable frequency comb.” In addition to improving existing frequency comb applications such as measurement and precision timekeeping, the innovation could enable future nanometer-level measurements of distant satellites. The developing team is exploring how its time-programmable frequency comb could benefit other frequency comb sensing applications. In part, the high accuracy of distance measurements that frequency combs enable stems from the broad array of frequencies of light that the combs use. The optical pulses from a frequency comb are far shorter than radio, for example, potentially allowing measurements accurate to nanometers even when the detector is many kilometers from the target. Use of frequency comb techniques could eventually enable precise formation flying of satellites for coordinated sensing of Earth or space, improving GPS, and supporting other ultraprecise navigation and timing applications. Distance measurement using frequency combs requires two combs whose lasers’ pulse timing is tightly coordinated. The pulses from one comb laser are bounced off a faraway object, just as radar uses radio waves, and the second comb, slightly offset in repetition period, measures their return timing with great accuracy. The limitation that comes with this great accuracy relates to the amount of light that the detector needs to receive. By nature of its design, the detector can only register photons from the ranging laser that arrive at the same time as pulses from the second comb’s laser. Up to now, due to the slight offset in repetition period, there was a relatively lengthy period of “dead time” between these pulse overlaps. Any photons that arrived between the overlaps were lost information, useless to the measurement effort. This made some targets hard to see. A frequency comb — the pulse time and phase of which are digitally controlled with ±2-attosecond accuracy — enables quantum-limited sensitivity in sensing applications, as the programmable comb can be configured to coherently track weak returning pulse trains at the shot-noise limit. The 'time programmable frequency comb' can be used to perform light pulse arrival time measurements with greater sensitivity than previously possible. The technology supports precision timing and atmospheric sensing, among other applications. Courtesy of NIST. Physicists have aspired to make measurements at the “quantum limit” — to take account of every available photon that carries useful information, since more photons detected means greater ability to spot fast changes in distance to a target, which is a goal in other frequency comb applications. According to Laura Sinclair, a physicist at NIST’s Boulder, Colorado, campus and one of the paper’s authors, most measurement techniques waste a great majority of the light used to operate the technique — 99.99% or more. “We have instead shown that by using this different control method, you can get rid of that waste. This can mean an increase in measurement speed, in precision, or it allows using a much smaller system,” Sinclair said. The team’s innovation involves the ability to control the timing of the second comb’s pulses. Advances in digital technology permit the second comb to “lock on” to the returning signals, eliminating the dead time created by the previous sampling approach. This occurs despite the fact that the controller must find a “needle in a haystack” — the pulses are comparatively brief, lasting only 0.01% as long the dead time between them. After an initial acquisition, if the target moves, the digital controller can adjust the time output such that the second comb’s pulses speed up or slow down. This allows the pulses to realign, so that the second comb’s pulses always overlap with those returning from the target. This adjusted time output is exactly twice the distance to the target, and it is returned with the pinpoint precision characteristic of frequency combs. The upshot of this time-programmable frequency comb is a detection method that makes the best use of the available photons — and eliminates dead time. “We found we can measure the range to a target fast, even if we only have a weak signal coming back,” Sinclair said. “Since every returning photon is detected, we can measure the distance near the standard quantum limit in precision.” Compared to standard dual-comb ranging, the team saw a 37-dB reduction in required received power — in other words, only requiring around 0.02% of the photons needed previously. “We’ve essentially broken this rule of frequency combs that demands they use a fixed pulse spacing for precision operation,” Sinclair said. “By changing how we control frequency combs, we have gotten rid of the trade-offs we had to make, so now we can get high-precision results even if our system only has a little light to work with.” The research was published in Nature (www.doi.org/10.1038/s41586-022-05225-8).