Ultra-precise optomechanical accelerometer needs no calibration

March 29, 2021 // By Rich Pell
Ultra-precise optomechanical accelerometer needs no calibration
Researchers at the National Institute of Standards and Technology (NIST) say they have developed an accelerometer measuring only a millimeter thick that uses laser light instead of mechanical strain to produce a signal.

Known as an optomechanical accelerometer, the device is designed to address the increasing demand to accurately measure acceleration in smaller navigation systems and other devices. Consisting of two silicon chips, with infrared laser light entering at the bottom chip and exiting at the top (see image), the design of the instrument makes the measuring process more straightforward, providing higher accuracy, say the researchers.

The device also operates over a greater range of frequencies and has been more rigorously tested than similar light-based devices. Not only is device much more precise than the best commercial accelerometers, it does not need to undergo the time-consuming process of periodic calibrations.

In fact, say the researchers, 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 device 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.

Accelerometers 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 motion of the proof mass creates a detectable signal. The new NIST accelerometer 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. 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. An optical frequency comb - a device that can be used as a ruler to measure the wavelength of light - is employed 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, say the researchers. However, the 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, say the researchers, 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. The researchers say thay 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 - a higher sensitivity than all accelerometers now on the market with similar size and bandwidth.

With further improvements, say the researchers, 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. For more, see " Broadband thermomechanically limited sensing with an optomechanical accelerometer."

NIST

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