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The Working Principle and Application of Optical Frequency Comb

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An optical frequency comb is a special kind of laser, like a ruler of light. They can quickly and accurately measure the precise frequency of light, from invisible infrared and ultraviolet light to visible red, yellow, green and blue light.

These Nobel Prize-winning devices fill an important technological gap. Optical frequency combs enable scientists to measure and control light waves in the same way they measure radio waves. With optical frequency combs, technologies such as clocks, computers, and communications that use radio and microwave frequencies can now seamlessly connect with light waves that oscillate 10000 times more frequently.

In the late 1990 s, NIST scientists began working on better optical atomic clocks, and optical frequency combs were a part of it. Today, NIST scientists are at the forefront of advancing these tools, and they're finding that they can be used for more than just timing.

With simple measurements, optical frequency combs have transformed science. So what can a comb do? There are many, and the possibilities are expanding.

How the frequency comb works

Frequency combs measure unknown optical frequencies by measuring the repetition rate of a train of consecutive light pulses-this belongs to the larger, easy-to-measure range of radio frequencies.

Light contains many colors, and they travel in the form of waves. From invisible infrared and ultraviolet light to red, blue or yellow visible light, each color of light has a corresponding frequency, or the number of peaks passing through a fixed point per second.

Radio waves and microwaves also travel at the speed of light, but their crests are farther apart, so modern electronics can easily count and track them.

The optical frequency comb emits a series of very short, closely spaced pulses of light, which contain a million different colors, from opaque infrared light to visible light to the ultraviolet spectrum.

Thanks to a technique called mode-locking, all frequencies in each pulse are phase-activated and synchronized with each other. The result is like the teeth of a comb, separating each frequency into a distinct spike-hence the device's name. The teeth of these combs are so finely spaced and perfectly uniform that they are like scales on a ruler that allow extremely accurate measurements of the light emitted by stars, atoms, other lasers, etc.

Application of optical frequency comb

Timing

Optical frequency combs are revolutionary in atomic clocks and timekeeping. Optical atomic clocks mark the passage of time by counting the natural oscillations of atoms, just as a grandfather clock counts the swing of a pendulum. These atoms oscillate about 50000 billion times per second, much higher than the frequency of standard microwave atomic clocks. Current electronic systems for measuring the frequency of microwave atomic clocks simply cannot calculate the optical "ticking" sound.

Because the tooth spacing of the optical frequency comb is uniform and accurate, the comb teeth act like the gears of a clock, which can receive faster optical frequencies and decompose the faster optical frequencies into low-frequency microwave signals used by electronic clocks and current atomic clocks. In this way, scientists can link the high-frequency "ticks" of optical atomic clocks with the low-frequency "ticks" of microwave clocks and the electronic devices used in today's computers and communication systems.

With these "gears" that transmit precise signals between electronic devices, microwave tools and optical atomic clocks, scientists can use these powerful new clocks to build faster and more accurate timing systems. Optical atomic clocks could eventually redefine the second.

The optical frequency comb acts like a gear in a clock, allowing scientists to easily convert high-frequency optical frequencies into low-frequency microwaves and vice versa. They can even help scientists switch between different optical frequencies.

In order to use these new clocks for national and global timekeeping, scientists need to be able to compare clock signals from different distances. An optical frequency comb also helps to achieve this goal. NIST and JILA (a joint institute of NIST and CU Boulder) use lidar to send time signals over the air to compare two different atomic clocks.

Improving timing systems is essential in many technology applications, from stock trading to navigation. The satellites and receivers of the Global Positioning System (GPS) send radio signals back and forth, and use the time of these signals to determine the location of the user. GPS uses military time, and these clocks regularly check the time with civilian clocks (such as the optical atomic clock of the National Institute of Standards and Technology and other similar clocks around the world).

Optical atomic clocks are also useful in studying quantum physics. By dividing time into incredibly small pieces, scientists can use these clocks to measure previously undetectable changes, such as gravitational redshifts on short-distance scales, the effect of gravity on the passage of time.

Astronomy and Cosmology

Advanced optical atomic clocks also allow scientists to study nature's constants beyond our own Earth. For example, with the help of optical frequency combs, NIST scientists are using these improved clocks to find elusive dark matter.

Optical frequency combs are also helping scientists search for exoplanets around distant stars. By tracking the exact color of the light emitted by these stars, they can look for wobbles in the stellar motion, which would indicate that there is an Earth-like planet orbiting the star.

Accurate distance measurement

The optical frequency comb can work from a distance. In 2013, NIST received a patent for lidar, a light detection and ranging system that uses an optical frequency comb to measure the distance of an object by analyzing the light reflected by the object.

The system has been used in several research applications. NIST's Fire Research Laboratory uses frequency combs to "see" through flames and identify melting objects. Frequency comb based lidar is also used to draw 3D maps. Ultimately, lidar using optical frequency combs could allow satellites and other space instruments to fly in close formation, acting as a single instrument.

Atmospheric Science and Greenhouse Gases

Atoms and molecules can be identified by the frequency at which they absorb light. Since the optical frequency comb can generate millions of frequencies in short pulses, it can be used to quickly and efficiently study the number, structure and dynamics of various molecules and atoms.

This has many potential applications and has been used to study pollution problems. Scientists at JILA used optical frequency combs to study the short-lived molecules that link burning fossil fuels to air pollution. Frequency combs can also probe the structure and dynamics of large complex molecules.

Scientists are also working on using optical frequency combs to detect trace amounts of various molecules in gases. In 2019, scientists and engineers from NIST, the University of Colorado Boulder, and the Technologies of LongPath developed a dual-comb portable spectroscopy system for detecting trace methane emissions from oil and gas fields.

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