Optical Frequency Combs measure exact frequencies

By Bhalchandra Chorghade, Editor, CNCTimes.com

Optical frequency combs are specialized lasers that act like a ruler for light. They measure exact frequencies of light — from the invisible infrared and ultraviolet to visible red, yellow, green and blue light — quickly and accurately.

These Nobel Prize-winning devices fill an important technological gap. Optical frequency combs allow scientists to measure and control light waves as if they were radio waves. With optical frequency combs, technologies that employ radio and microwave frequencies — such as clocks, computers and communications — are now seamlessly connected to optical waves that oscillate at 10,000 times higher frequencies.

Optical frequency combs began as part of NIST scientists’ vision for better optical atomic clocks in the late 1990s. Today, NIST scientists are at the forefront of advancing these tools, and they have found uses beyond just timekeeping.

What a Comb Can DoWhat a Comb Can Do

By making a simple measurement, the optical frequency comb has changed science. So what can a comb do? A lot—and the possibilities are constantly expanding.

How are optical frequency combs used?

Timekeeping

Optical frequency combs have been revolutionary for atomic clocks and timekeeping. Optical atomic clocks mark the passage of time by counting the natural oscillation of atoms in the same way a grandfather clock counts the swings of a pendulum. These atoms oscillate about 500,000 billion times a second — a much higher frequency than standard microwave-based atomic clocks. The current electronic systems that are used to measure frequency for microwave-based atomic clocks simply can’t count the optical “ticks.”

Because the teeth of an optical frequency comb are evenly spaced and precise, the comb acts like the gears of a clock, taking the faster optical frequencies and dividing them down to the lower-frequency microwave signals used by electronics and current atomic clocks. This lets scientists link optical atomic clocks’ higher-frequency “ticks” to microwave-based clocks’ lower-frequency “ticks” and electronics used by present day computers and communications systems. 

Optical frequency combs can act like gears in a clock, allowing scientists to easily translate high frequency optical frequencies to lower frequency microwaves and vice versa. They can even help scientists translate between different optical frequencies.

With these “gears” carrying accurate signals between electronics, microwave-based tools and optical atomic clocks, scientists can use these powerful new clocks for faster, more accurate timekeeping systems. Optical atomic clocks may eventually redefine the second.

In order for these new clocks to be used for national and global timekeeping, scientists need to be able to compare signals from clocks across distances. Optical frequency combs can help achieve that too. NIST and JILA, a joint research institute of NIST and CU Boulder, used lidar to send time signals through the air, comparing two different kinds of atomic clocks.

Improved timekeeping systems are crucial in many technological applications, from stock trading to navigation. Global Positioning System (GPS) satellites and receivers send radio signals back and forth and use the timing of those signals to pinpoint a user’s location. GPS uses military time, and those clocks check their timing periodically with civilian clocks, like NIST’s optical atomic clocks and others like them around the world. Scientists hope to have optical atomic clocks on navigation satellites in the future, making the system even more precise and allowing GPS to pinpoint locations within centimeters.

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

https://www.nist.gov/topics/physics/optical-frequency-combs#:~:text=Optical%20frequency%20combs%20are%20specialized,fill%20an%20important%20technological%20gap.

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