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Is MEMS plus photonics the next big thing in sensors?

Microelectromechanical systems or MEMS have truly revolutionized sensing transducers since their introduction several decades ago for airbag triggers replacing the ball-in-tube design. We now see MEMS-based devices used for a wide range of applications including microphones, acoustic sources, motion and pressure sensors, antenna tuners, and more.

Perhaps no single area of sensing has benefited more from MEMS technologies than accelerometers, which have gone from the relatively low accuracy required for airbags to devices with performance better by many orders of magnitude. When combined with fascinating MEMS gyroscopes, these accelerometers are key to the inertial measurement units (IMUs) on drones, UAVs, autonomous vehicles, missiles, and more. The MEMS-driven accelerometer revolution—that’s a word I am normally hesitant to use, but not in this case—has given us devices with astounding combinations of extreme accuracy, tiny size, ultralow power, and very low cost.

There’s another technology which is making major advances, also by leveraging MEMS-like fabrication and processing: optoelectronics using silicon and optical-friendly substrates to hold, carve, or create lasers, sensors, gratings, interferometers, and spectrometers. Due to processing constraints, many of these photonic devices are primarily optical with lesser electronic content but that, too, is changing. Researchers are now succeeding in combining MEMS technology with photonics to build devices which were previously not possible, and they are using the pairing to achieve outstanding performance along multiple parameters.

For example, a team based at the National Institute of Standards and Technology (NIST) has built a MEMS-based accelerometer. At its core are two mirrored chips facing each other which create an optically resonant cavity. The freely moving proof mass supports one of the mirror surfaces while the other mirror is the fixed reference surface.

They then inject infrared light into this cavity at the cavity’s resonant wavelength, using a fixed-frequency laser which is wavelength-locked to the cavity’s resonance. If the proof mass moves due to acceleration, the wavelength at which the cavity resonates changes as well. Finally, they employ an optical comb—another amazing photonic structure—as a tunable filter to assess changes in the cavity’s resonance.

Although it seems like a lot of technology devoted to solving a problem which MEMS devices and other accelerometers seem to have already addressed, there are significant benefits. First, no calibration is needed to achieve full performance. Second, it’s the performance which is astounding: the researchers say this device can sense displacements of the proof mass which are less than one-hundred-thousandth the diameter of a hydrogen atom—after all, they are using optical wavelengths here—and thus acceleration as tiny as 32 billionth g over a bandwidth of 1 kHz to 20 kHz.

The NIST team has published two papers on the project: “Broadband thermomechanically limited sensing with an optomechanical accelerometer” covering the accelerometer design, fabrication, and extensive test and performance results, and “Electro-optic frequency combs for rapid interrogation in cavity optomechanics” on the optical comb. There’s also an introductory two-minute video “Measuring Acceleration with Light.”

Have you had any experience using photonics devices, beyond basic emitters—LEDs and laser diodes, for example—and photodetectors? What about photonics and MEMS devices? Do you see this merging of MEMS and photonics as a possible “next big thing”?

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