Optomechanical Accelerometer Enhances Accuracy

A laser light-powered accelerometer — a sensor that detects sudden changes in velocity — operates over a greater range of frequencies than comparable, nonoptical instruments, and at a higher rate of accuracy. Researchers from NIST developed the optomechanical accelerometer device, which is 1 mm thick and bypasses reliance on mechanical strain by using light as its activator. The device, the researchers said, also does not require periodic calibrations.

Though other light-powered accelerometers exist, the NIST-design delivers higher accuracy than similar and comparable instruments.

All accelerometers record changes in velocity by tracking proof mass, or the position of a freely moving mass, relative to an unmoving reference point inside the accelerometer itself. The distance between the proof mass and reference point changes only if the accelerometer changes speed or direction.

In a vehicle moving at a constant velocity, for example, the distance between a seated passenger and the dashboard stays the same. However, if the car brakes suddenly and throws the passenger forward, that distance changes.

In motion, the proof mass generates a detectable signal that an accelerometer can perceive. The NIST researchers used infrared laser light to gauge the difference in distance between two highly reflective surfaces bookending an empty space. They suspended the proof mass using flexible beams one-fifth the width of a human hair. If the free mass moved freely, it supported one of the mirrored surfaces. The other reflecting surface consisting of an immovable, microfabricated concave mirror functioned as the fixed reference point.

The mechanism of the surfaces and free space formed a cavity in which the infrared light resonated, or bounced back and forth, at a precise wavelength that was determined by the distance between the mirrors. This built intensity; if the proof mass moved in response to acceleration, changing the separation between the mirrors, the resonant wavelength also changed.

The researchers used a stable, single-frequency laser that they locked to the cavity to track any changes in the resonant wavelength with high sensitivity. That, paired with an optical frequency comb, allowed them to measure the cavity light with a high degree of accuracy.

The NIST 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 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. 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. Courtesy of F. Zhou/NIST.

When the proof mass moved during a point of acceleration, which changed the length of the cavity, the reflected light’s intensity changed as the wavelengths associated with the teeth of the frequency comb moved in and out of resonance with the cavity.

The design ensured that the proof mass and supporting beams functioned as a harmonic oscillator, or simple spring, that vibrated a single frequency in the accelerometer’s operating range. Converting the displacement of the proof mass into an acceleration, the researchers said, is an operation that has plagued existing optomechanical accelerometers.

In testing, the accelerometer allowed the engineering team to achieve minimal measurement uncertainty acceleration frequencies spanning 1 to 20 kHz. They did not need to calibrate the instrument. Current iterations of the device were capable of sensing displacements of the proof mass that are less than 100 thousandths the diameter of a hydrogen atom. It detected accelerations as small as 32 billionths of a g (where “g” is the acceleration due to Earth’s gravity).

Further improvements, the team said, would give the optomechanical accelerometer use as a portable, high-accuracy reference device for the calibration of other accelerometers outside of laboratory settings.

NIST researchers Jason Gorman, Thomas LeBrun, David Long, and colleagues published the research in Optica (www.doi.org/10.1364/OPTICA.413117).