I wonder if folk truly understand their humble tape measures

Distance change between two micromirrors

The new accelerometer developed by Jason Gorman, Thomas LeBrun, David Long and colleagues at the US National Institute of Standards and Technology (NIST) uses infrared light to measure the change in distance between two micromirrors in a configuration known as a Fabry–Perot cavity. In their set-up, the proof mass is a single crystal of silicon with a mass of between 10 and 20 mg, and it is suspended from the first mirror using a set of 1.5 μm-thick flexible silicon nitride (Si3N4) beams. Being suspended in this way allows the proof mass to move freely, with nearly ideal translational motion. The second mirror, which is concave, acts as the accelerometer’s fixed reference point and thus cannot move.

When the team directs infrared laser light into the optical cavity, most frequencies of light are entirely reflected. However, light of a certain frequency can resonate – or bounce back and forth – between the two mirrors in the cavity, increasing its intensity. When the team makes the device accelerate, the proof mass displaces relative to the concave mirror, and this displacement produces a change in the intensity of light reflected from the cavity.

The NIST researchers track this change using a stable single-frequency laser “locked” to the cavity’s resonant frequency. By continually matching the laser’s frequency to the resonant frequency of the cavity, they can determine how much the device has accelerated. The result is a device that can sense displacements of the proof mass that are smaller than a femtometre (10–15 m) and detect accelerations as low as 3.2 × 10-8 g, where g is the acceleration due to Earth’s gravity. This is better than any accelerometer on the market today of comparable size and bandwidth, the team says.