Tech Briefs Magazine - October 2022 - Comsol-33

better definition of the meter, which is the
distance light travels in a vacuum in a tiny
fraction of a second. But all that relied on
being able to measure these frequencies of
light accurately and precisely.
There was a gap in measurement between
the two ends of the electromagnetic
spectrum. Scientists could measure radio
and microwave frequencies accurately,
but there were no electronics that could
count fast enough to keep up with the atom's
optical frequencies. They could use
a laser with a matching frequency to read
the atom's optical frequency. Scientists
had lasers with known, exact frequencies,
but they could only produce a single frequency
or color. Without knowing the
exact frequency of the atom, finding the
right frequency laser to read the atom
would take a lot of trial and error. NIST
scientists tried daisy-chaining several lasers
of different frequencies together to
make a rudimentary optical ruler. That
worked well enough to redefine the meter
but wasn't a long-term solution.
Enter the frequency comb, a Nobel
Prize-winning device, and the result of decades
of research from NIST and others.
The comb generates a billion pulses of
light per second, which bounce back and
forth inside an optical cavity. This creates
millions of spikes of optical frequencies
that look like rainbow-colored teeth on a
comb (hence the name). The first tooth
in that comb is set to a known frequency,
which gives scientists a starting point to
read the other frequencies. Much like a
ruler, if you know the first marker is one
millimeter and each marker is a millimeter
apart, you can easily start measuring.
Similarly, because they know exactly how
far apart these frequencies are, scientists
can translate these optical signals to microwaves
with a simple mathematical formula,
joining the two ends of the electromagnetic
spectrum. This opens a lot of
research doors.
The scientists used this new technology
to make better clocks, eventually developing
clocks that are 100 times better than
the cesium clocks used for civilian time
standards. More accurate and precise
clocks are critical for GPS navigation,
which relies on precise time signals to determine
your location. Better clocks also
have research advantages, from detecting
tiny changes in gravity to studying phenomena
of the quantum world and perhaps
finding dark matter. These clocks may
eventually change how we define a second.
All atoms and molecules emit unique
frequencies of light when they jump beTech
Briefs, October 2022
tween energy states, not just the atoms
used in clocks. If one of the frequencies
from the comb hits an atom or molecule
with the exact same frequency, scientists
can identify what kind of atom or molecule
they've hit. Using the optical frequency
comb, scientists could study the
composition of stars in exquisite detail.
Astrophysicists can tell if they've found a
new planet by measuring the changes in
frequencies of the starlight. Using frequency
combs, we can improve light-ranging
systems, which bounce light off objects
to detect them like radar or sonar.
They can see objects through flames,
helping NIST scientists study how structures
fail during a fire. The comb is also
being used to detect even the smallest
amounts of greenhouse gases in the air or
look for disease in human breath.
For more information, contact Chad
Boutin at chad.boutin@nist.gov.
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Tech Briefs Magazine - October 2022

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