Caltech engineers create microscale optical accelerometer

Pasadena, CA--An optical accelerometer developed by engineers at the California Institute of Technology (Caltech) is potentially smaller, more sensitive, and more versatile than the electrical accelerometers commonly found in cell phones. In addition to use in consumer devices, the accelerometer could find use in aircraft, oil and gas exploration, and biomedical applications. The sensor is sensitive to motions that occur in tens of microseconds.

The accelerometer's optical cavity is only about 20 microns long, a micron wide, and a few tenths of a micron thick. It consists of two nanosized silicon mechanical beams, situated like the two sides of a zipper, with one side attached to a proof mass (which serves as the inertial reference). When laser light enters the system, the nanobeams act like light pipes, bringing the light into an area where it bounces back and forth between holes in the nanobeams. When the tethered proof mass moves, it changes the gap between the two nanobeams, resulting in a change in the intensity of the laser light being reflected out of the system. The reflected laser signal is in fact tremendously sensitive to the motion of the proof mass, with displacements as small as a few femtometers (roughly the diameter of a proton) being probed on the timescale of a second.

It turns out that because the cavity and proof mass are so small, the light bouncing back and forth in the system pushes the proof mass in a dampening way: when the proof mass moves away, the light helps push it further, and when the proof mass moves closer, the light pulls it in. As a result, the laser light softens and damps the proof mass's motion.

"Most sensors are completely limited by thermal noise, or mechanical vibrations—they jiggle around at room temperature, and applied accelerations get lost in that noise," says Oskar Painter, the team leader. "In our device, the light applies a force that tends to reduce the thermal motion, cooling the system." This cooling—down to a temperature of 3 K in the current devices—increases the range of accelerations that the device can measure, making it capable of measuring both extremely small and extremely large accelerations. At the same time, the sensor can measure very large accelerations.

The team envisions its optical accelerometers becoming integrated with lasers and detectors in silicon microchips. Painter says that a lot of engineering work still needs to be done to make this happen.

The new device and its capabilities are described in an advance online publication of the journal Nature Photonics.