An 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 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.