A better way to measure acceleration
Researchers rely on a light touch

Date:
March 8, 2021
Source:
National Institute of Standards and Technology (NIST)
Summary:
Addressing the increasing demand to accurately measure acceleration
in smaller navigation systems and other devices, researchers have
developed an accelerometer a mere millimeter thick that uses laser
light instead of mechanical strain to produce a signal.



FULL STORY ========================================================================== You're going at the speed limit down a two-lane road when a car barrels
out of a driveway on your right. You slam on the brakes, and within a
fraction of a second of the impact an airbag inflates, saving you from
serious injury or even death.


==========================================================================
The 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.

Addressing the increasing demand to accurately measure acceleration
in smaller navigation systems and other devices, researchers at the
National Institute of Standards and Technology (NIST) have developed an accelerometer a mere 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 NIST 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.

Not only is the NIST device, known as an optomechanical accelerometer,
much more precise than the best commercial accelerometers, it 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. NIST researchers Jason
Gorman, Thomas LeBrun, David Long and their colleagues describe their
work in the journal Optica.



==========================================================================
This animation demonstrates the operating principles of a new
accelerometer.

This optomechanical accelerometer consists of two silicon chips. The first
chip has a proof mass suspended by a set of silicon beams, which allows
the proof mass to move vertically. The top of the mass has a mirrored
coating. The second chip has an inset hemispherical mirror. Together
the mass and hemisphere mirrors form an optical cavity. Infrared
laser light is directed into the device. Most frequencies are reflected entirely. However, light matching the resonant frequency builds up inside
the cavity, increasing in intensity, until the intensity of the light transmitted by the cavity matches the input. Light transmitted by the
cavity can be detected on the other side. When the device accelerates,
the length of the cavity changes, shifting the resonant frequency.

By continuously matching the laser to the resonant frequency
of the cavity, researchers can determine the acceleration of the
device. Animation: Sean Kelley/NIST The study is part of NIST on a Chip,
a program that brings the institute's cutting-edge measurement-science technology and expertise directly to users in commerce, medicine,
defense and academia.

Accelerometers, including the new NIST device, record changes in
velocity by tracking the position of a freely moving mass, dubbed the
"proof mass," relative to a fixed reference point inside the device. The distance between the proof mass and the reference point only changes if
the accelerometer slows down, speeds up or switches direction. The same
is true if you're a passenger in a car. If the car is either at rest or
moving at constant velocity, the distance between you and the dashboard
stays the same. But if the car suddenly brakes, you're thrown forward
and the distance between you and the dashboard decreases.

The motion of the proof mass creates a detectable signal. The
accelerometer developed by NIST researchers relies on infrared light to
measure the change in distance between two highly reflective surfaces
that bookend a small region of empty space. The proof mass, which is
suspended by flexible beams one-fifth the width of a human hair so that
it can move freely, supports one of the mirrored surfaces. The other
reflecting surface, which serves as the accelerometer's fixed reference
point, consists of an immovable microfabricated concave mirror.

Together, the two reflecting surfaces and the empty space between them
form a cavity in which infrared light of just the right wavelength can resonate, or bounce back and forth, between the mirrors, building in
intensity. That wavelength is determined by the distance between the two mirrors, much as the pitch of a plucked guitar depends on the distance
between the instrument's fret and bridge. If the proof mass moves in
response to acceleration, changing the separation between the mirrors,
the resonant wavelength also changes.

To track the changes in the cavity's resonant wavelength with high
sensitivity, a stable single-frequency laser is locked to the cavity. As described in a recent publication in Optics Letters, the researchers have
also employed an optical frequency comb -- a device that can be used as a
ruler to measure the wavelength of light -- to measure the cavity length
with high accuracy. The markings of the ruler (the teeth of the comb) can
be thought of as a series of lasers with equally spaced wavelengths. When
the proof mass moves during a period of acceleration, either shortening
or lengthening the cavity, the intensity of the reflected light changes
as the wavelengths associated with the comb's teeth move in and out of resonance with the cavity.

Accurately converting the displacement of the proof mass into an
acceleration is a critical step that has been problematic in most
existing optomechanical accelerometers. However, the team's new design
ensures that the dynamic relationship between the displacement of the
proof mass and the acceleration is simple and easy to model through
first principles of physics. In short, the proof mass and supporting
beams are designed so that they behave like a simple spring, or harmonic oscillator, that vibrates at a single frequency in the operating range
of the accelerometer.

This simple dynamic response enabled the scientists to achieve low
measurement uncertainty over a wide range of acceleration frequencies
-- 1 kilohertz to 20 kilohertz -- without ever having to calibrate the
device. This feature is unique because all commercial accelerometers
have to be calibrated, which is time-consuming and expensive. Since the publication of their study in Optica, the researchers have made several improvements that should decrease their device's uncertainty to nearly 1%.

Capable of sensing displacements of the proof mass that are less than one hundred-thousandth the diameter of a hydrogen atom, 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 all accelerometers now on the market with similar size and bandwidth.

With further improvements, the NIST optomechanical accelerometer could
be used as a portable, high-accuracy reference device to calibrate other accelerometers without having to bring them into a laboratory.

========================================================================== Story Source: Materials provided by National_Institute_of_Standards_and_Technology_(NIST).

Note: Content may be edited for style and length.


========================================================================== Journal References:
1. Feng Zhou, Yiliang Bao, Ramgopal Madugani, David A. Long, Jason J.

Gorman, Thomas W. LeBrun. Broadband thermomechanically limited
sensing with an optomechanical accelerometer. Optica, 2021; 8 (3):
350 DOI: 10.1364/OPTICA.413117
2. D. A. Long, B. J. Reschovsky, F. Zhou, Y. Bao, T. W. LeBrun, J. J.

Gorman. Electro-optic frequency combs for rapid interrogation
in cavity optomechanics. Optics Letters, 2021; 46 (3): 645 DOI:
10.1364/OL.405299 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210308152526.htm

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