Researchers at Staten Island College, City University of New York are working on what may turn out to be the world’s smallest gyroscope. Manipulating two light waves spinning around a microscopic track – one rotating clockwise, the other counterclockwise – may yield a device measuring a fraction of the width of a human hair. Applications for such a gyro would be limited only by one’s imagination.
For example, NASA claims it costs about $10,000 for every pound lifted into orbit. So it stands to reason that designing smaller and lighter components is a constant struggle for engineers and project managers. The new gyroscope looks like a more than viable candidate for air and space travel.
Li Ge, a physicist at the Graduate Center, states, “We have found a new detection scheme that may lead to the world's smallest gyroscope. Though these so-called optical gyroscopes are not new, our approach is remarkable both in its super-small size and potential sensitivity.” Ge’s colleagues include physicist Hui Cao and her student Raktim Sarma, both at Yale University in New Haven, Connecticut. Together they recently published their results in The Optical Society’s (OSA) new high-impact journal Optica.
Unlike mechanical gyroscopes, optical gyroscopes like the one under research here have no moving parts. Mechanical gyroscopes employ Newtonian laws of motion for stability and orientation, but the same physics principles do not apply to light, which requires measuring motion by looking for subtle and specific optical signals instead.
One such signal is known as the Sagnac effect. This phenomenon creates a measurable interference pattern when light waves split and then recombine upon leaving a spinning system. Commercial optical gyroscopes built on this principle vary in size ranging from about that of a baseball to a basketball. If made much smaller, measurements would require a greater level of sensitivity that is not available at this time.
Two approaches for making optical gyroscopes are based on the Sagnac effect: one uses an optical cavity to confine light and the other uses an optical fiber to guide light. However, to miniaturize the package, optical cavities seem to be the preferable option, where the Sagnac effect manifests as a subtle color change. The problem is that the sensitivity of this type of optical gyroscopes degrades as the cavity gets smaller. According to Ge, “This issue was the roadblock that has hindered scientists from developing tiny optical gyroscopes. There have been several attempts to get around this limitation, but they could not get around the real problem, the Sagnac effect itself.”
The researchers overcame this hurdle by using a unique principle based on far-field emission. Instead of measuring the color change of the light waves, they measured the pattern the light produced as it exited the cavity.
To initiate the optical gyroscope, light waves are first pumped into the optical cavity. This naturally produces light waves traveling in both clockwise and counterclockwise directions.
Carefully designing the shape of the optical cavity, the researchers were able to control where both waves would exit. The researchers needed to balance the light trapping properties of the cavity with the need for some light to escape to create a far-field emission pattern. They observe pattern by placing a pair of camera-like detectors facing the cavity at different angles that move along with the cavity. This allows continuous monitoring of the pattern for distortions that would reveal the speed of rotation.
Although this reveals just one plane of motion, multiple sensors at different orientations would be able to provide a three-dimensional view of how the object is moving. According to the researchers, more work is necessary to enable many light paths to exist simultaneously in the cavity. Their far-field emission patterns may change in different ways, which causes a reduction of the sensitivity to rotation. To view the paper: L. Ge, R. Sarma, and H. Cao, “Rotation-induced evolution of far-field emission patterns of deformed microdisk cavities,” Optica, 2, 4, 323-328 (2015), visit: http://dx.doi.org/10.1364/OPTICA.2.000323