A New Optical Sensor for Acceleration

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.

Illustration of an optomechanical accelerometer, which uses light to measure acceleration. The device consists of two silicon chips, with infrared laser light entering at the bottom chip and exiting at the top. (Source: NIST)

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. Not only is the NIST device, 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. Accelero­meters, including the new 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 motion of the proof mass creates a detectable signal. The accelero­meter developed 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 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. The researchers have also employed an optical frequency comb to measure the cavity length with high accuracy. The markings of the ruler 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. Now, 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 opto­mechanical acce­lerometer could be used as a portable, high-accuracy reference device to calibrate other acce­lerometers without having to bring them into a labora­tory. (Source: NIST)

Reference: F. Zhou et al.: Broadband thermomechanically limited sensing with an optomechanical accelerometer, Optica 8, 350 (2021); DOI: 10.1364/OPTICA.413117

Link: Photonics and Optomechanics Group, National Institute of Standards and Technology, Gaithersburg, USA

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