To break new ground with frequency combs, an innovation plays with the beat

The innovation, created by scientists at the National Institute of Standards and Technology (NIST), represents a new way of using frequency comb technology, which the scientists have termed a "time programmable frequency comb." Up until now, frequency comb lasers needed to create light pulses 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 broader set of conditions than has been possible.

"We've essentially broken this rule of frequency combs that demands they use a fixed pulse spacing for precision operation," said Laura Sinclair, a physicist at NIST's Boulder campus and one of the paper's authors. "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 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 consists of many well-defined frequencies that can be measured accurately. Looking at the laser's spectrum on a display, each frequency would stand out like one tooth of a comb, giving the technology its name. After earning NIST's Jan Hall a portion of the 2005 Nobel Prize in Physics, frequency combs have found use in a number of applications ranging from precision timekeeping to finding Earth-like planets to greenhouse gas detection.

Despite their many current uses, frequency combs do possess limitations. The team's paper is an attempt 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.

Since shortly after their invention, frequency combs have enabled highly accurate measurements of distance. In part, this accuracy stems from the broad array of frequencies of light the combs use. Radar, which uses radio waves to determine distance, is accurate to anywhere from centimeters to many meters depending on the signal's pulse width. The optical pulses from a frequency comb are far shorter than radio, potentially allowing measurements accurate to nanometers (nm), or billionths of a meter -- 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 ultra-precise 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, and any photons that arrived between the overlaps were lost information, useless to the 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 take account of every available photon that carries useful information. More photons detected means greater ability to spot fast changes in distance to a target, a goal in other frequency comb applications. But for all 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 accuracy, but most measurement techniques waste the great majority of the light, 99.99% or more," Sinclair said. "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."

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, as the team calls it, 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-decibel reduction in required received power -- in other words, only requiring around 0.02% of the photons needed previously.

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