The World’s Smallest Optical Gyroscope

The new optical gyroscope – shown here with grains of rice – is 500 times smaller than the current state-of-the-art device. (Source: A. Hajimiri)

Gyroscopes are devices that help vehicles, drones, and wearable and handheld electronic devices know their orien­tation in three-dimen­sional space. They are common­place in just about every bit of tech­nology we rely on every day. Originally, gyroscopes were sets of nested wheels, each spinning on a different axis. But open up a cell phone today, and you will find a micro­electro­mechanical sensor (MEMS), the modern-day equivalent, which measures changes in the forces acting on two identical masses that are oscil­lating and moving in opposite directions. These MEMS gyroscopes are limited in their sensi­tivity, so optical gyro­scopes have been developed to perform the same function but with no moving parts and a greater degree of accuracy using the Sagnac effect.

The Sagnac effect, named after French physicist Georges Sagnac, is an optical pheno­menon rooted in Einstein’s theory of general rela­tivity. To create it, a beam of light is split into two, and the twin beams travel in opposite direc­tions along a circular pathway, then meet at the same light detector. Light travels at a constant speed, so rotating the device – and with it the pathway that the light travels – causes one of the two beams to arrive at the detector before the other. With a loop on each axis of orien­tation, this phase shift, known as the Sagnac effect, can be used to calculate orien­tation.

The smallest high-perfor­mance optical gyro­scopes available today are bigger than a golf ball and are not suitable for many portable appli­cations. As optical gyroscopes are built smaller and smaller, so too is the signal that captures the Sagnac effect, which makes it more and more difficult for the gyro­scope to detect movement. Up to now, this has prevented the minia­turization of optical gyro­scopes. Caltech engineers led by Ali Hajimiri, Bren Professor of Electrical Engi­neering and Medical Engi­neering in the Division of Engi­neering and Applied Science, developed a new optical gyroscope that is 500 times smaller than the current state-of-the-art device, yet they can detect phase shifts that are 30 times smaller than those systems.

The new gyroscope from Hajimiri’s lab achieves this improved performance by using reciprocal sensi­tivity enhance­ment. In this case, “reciprocal” means that it affects both beams of the light inside the gyroscope in the same way. Since the Sagnac effect relies on detecting a difference between the two beams as they travel in opposite directions, it is considered non­reciprocal. Inside the gyro­scope, light travels through minia­turized optical waveguides. Imper­fections in the optical path that might affect the beams – for example, thermal fluc­tuations or light scat­tering – and any outside inter­ference will affect both beams similarly.

Hajimiri’s team found a way to weed out this reciprocal noise while leaving signals from the Sagnac effect intact. Reci­procal sensi­tivity enhance­ment thus improves the signal-to-noise ratio in the system and enables the inte­gration of the optical gyro onto a chip smaller than a grain of rice. (Source: Caltech)

Reference: P. P. Khial et al.: Nanophotonic optical gyroscope with reciprocal sensitivity enhancement, Nat. Phot. 12, 671 (2018); DOI: 10.1038/s41566-018-0266-5

Link: High-Speed Integrated Circuits, Dept. of Electrical Engineering, California Institute of Technology, Pasadena, USA

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