Optical accelerometer could transform microelectronics
PASADENA, Calif. – An ultrasensitive miniature optical accelerometer uses laser light rather than electrical circuits to gauge movement, a change that could transform the microelectronics industry.
The optical cavity of the accelerometer, developed at California Institute of Technology, is only about 20 x 1 µm and a few tenths of a micron thick. Despite its size, the device is an extremely sensitive probe of motion. Its low mass enables operating at a large range of frequencies, making it sensitive to motions that occur in tens of microseconds – thousands of times faster than the motions that today’s most sensitive sensors can detect. It could pave the way for a new class of microsensors that could aid in oil and gas exploration or be used in personal navigation devices.
A scanning electron microscope image of an array of optomechanical accelerometer devices formed in the surface of a silicon microchip. The areas highlighted in green represent proof masses, which are suspended by nanoscale tethers across the open etched areas of the chip. Even smaller, and barely visible, are optical cavities (highlighted in pink), which sensitively read out the motion of the proof masses.
“The new engineered structures we made show that optical sensors of very high performance are possible, and one can miniaturize them and integrate them so that they could one day be commercialized,” said Oskar Painter, an applied physics professor and co-director of Caltech’s Kavli Nanoscience Institute.
Because they can be small and inexpensive, microchip accelerometers are common, used to deploy vehicle airbags and in navigation systems, and in conjunction with other sensors in cellphones and cameras.
They use sensitive displacement detectors to measure the motion of a flexibly mounted mass, called a proof mass. Usually, that detector is an electrical circuit. However, because laser light is one of the most sensitive ways to measure position, scientists are making such a device with an optical readout. Lasers can detect very small movements because they are typically limited by the quantum properties of light itself; they have small intensity fluctuations because they can have very little intrinsic noise.
Miniature versions of such large-scale interferometers have been attempted previously, but with limited success. A hangup in miniaturizing the technology has been that the larger the proof mass, the larger the resulting motion when the sensor accelerates. In this case, it would be easier to detect accelerations with larger sensors. It also can be challenging to integrate all the optical accelerometer’s components – the lasers, detectors and interferometer – into a micropackage.
“What our work really shows is that we can take a silicon microchip and scale this concept of a large-scale optical interferometer all the way down to the nanoscale,” Painter said. “The key is this little optical cavity we engineered to read out the motion.”
The optical cavity consists of two silicon nanobeams, situated similar to a zipper, with one side attached to a proof mass. 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 (about the diameter of a proton) being probed on the timescale of a second.
Because the cavity and proof mass are so small, the light bouncing back and forth in the system pushes the proof mass. When the proof mass moves away, the light helps push it farther, 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,” Painter said. “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. Much engineering work still needs to be done to make this happen, Painter said.
“It is very exciting to envision the ways this research might transform the microelectronics industry and our daily lives,” said Ares Rosakis, chairman of Caltech’s Division of Engineering and Applied Science.
A report on the device and its capabilities was published in Nature Photonics