Tech Briefs Magazine - April 2021 - MD-22

Laser-Light-Based Accelerometer
The tiny device accurately measures acceleration in smaller navigation systems and other devices.
National Institute of Standards and Technology, Gaithersburg, Maryland

n automobile airbag deploys thanks
to an accelerometer - a sensor that
detects sudden changes in velocity.
Accelerometers keep rockets and airplanes on the correct flight path, provide navigation for self-driving cars, and
rotate images so that they stay right-side
up on cellphones and tablets, among
other essential tasks.
A team of researchers has developed
an accelerometer a millimeter thick that
uses laser light instead of mechanical
strain to produce a signal. Although a
few other accelerometers also rely on
light, the design of the new instrument
makes the measuring process more
straightforward, providing higher accuracy. It also operates over a greater range
of frequencies and has been more rigorously tested than similar devices.

The optomechanical accelerometer is
much more precise than the best commercial accelerometers and does not
need to undergo the time-consuming
process of periodic calibrations. In fact,
because the instrument uses laser light
of a known frequency to measure acceleration, it may ultimately serve as a
portable reference standard to calibrate
other accelerometers now on the market, making them more accurate. The
accelerometer also has the potential to
improve inertial navigation in such critical systems as military aircraft, satellites,
and submarines, especially when a GPS
signal is not available.
The device consists of two silicon
chips with infrared laser light entering
at the bottom chip and exiting at the
top. The top chip contains a proof mass

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Illustration of the optomechanical accelerometer,
which uses light to measure acceleration. (Credit: F.
Zhou/NIST)

suspended by silicon beams, which
enables the mass to move up and down
freely in response to acceleration. A
mirrored coating on the proof mass and
a hemispherical mirror attached to the
bottom chip form an optical cavity. The
wavelength of the infrared light is chosen so that it nearly matches the resonant wavelength of the cavity, enabling
the light to build in intensity as it
bounces back and forth between the
two mirrored surfaces many times
before exiting. When the device experiences an acceleration, the proof mass
moves, changing the length of the cavity
and shifting the resonant wavelength.
This alters the intensity of the reflected
light. An optical readout converts the
change in intensity into a measurement
of acceleration.
The optomechanical accelerometer
detects accelerations as tiny as 32 billionths of a g, where g is the acceleration due to Earth's gravity. That's a
higher sensitivity than other accelerometers now on the market with similar
size and bandwidth. With further improvements, the device could be used
as a portable, high-accuracy reference
device to calibrate other accelerometers without having to bring them into a
laboratory.
For more information, contact Ben P. Stein
at benjamin.stein@nist.gov; 301-975-2763.
Motion Design, April 2021

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Tech Briefs Magazine - April 2021

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