A new spin on optical gyroscope technology could allow the devices, which are crucial to satellite and rocket guidance, to increase in sensitivity while shrinking to about 10 μm in size. Researchers at the City University of New York (CUNY) and Yale University have demonstrated that far-field emission patterns of light interact strongly with rotating microdisk optical cavities. This finding presents an alternative to current optical gyroscopes, which are limited by their dependence on the Sagnac effect. This phenomenon creates a measurable interference pattern when light waves split and then recombine upon leaving a spinning system. Schematics showing the far-field emission pattern of a microdisk cavity changing from symmetric to strongly asymmetric. Two cameras on the right monitor the change. Courtesy of Dr. Li Ge/Graduate Center and Staten Island College, CUNY. At slow rotation speeds, the researchers said, the Sagnac effect is negligible. Far-field emission patterns, on the other hand, “already exhibit a significant rotation-induced asymmetry, which increases linearly with the rotation speed.” Gyroscopes based on this principle could be made small enough to be integrated onto circuit boards. This could drastically reduce the equipment cost in space missions, opening the possibility for a new generation of micro-payloads. To start the new optical gyroscope, light waves are first pumped into an optical cavity and begin travelling in in both clockwise and counterclockwise directions. By carefully designing the shape of the optical cavity, the researchers can control where both waves exit. Normally optical cavities are designed to trap light as long as possible. Here, 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. A pair of detectors move with the cavity, allowing continuous monitoring of the pattern for distortions that reveal the speed of rotation. Though this only reveals one plane of motion, multiple such sensors with different orientations would be able to give a fully 3-D picture of how the object is moving, the researchers said. Sagnac limitations Current optical gyroscopes are baseball- to basketball-sized and operate on different principles related to the Sagnac effect. One kind uses an optical cavity to confine light, while the other uses an optical fiber to guide light. The second approach has, to date, been most practical because its sensitivity can be easily enhanced by using longer sections of optical fiber, some up to 5 km long. These lengths of fiber are wrapped around an object about 5 cm in diameter. Though the system is sensitive to rotation, there are practical limits to how long the fiber can be and how tightly it can be wrapped before the fiber itself is damaged. In optical cavities, the Sagnac effect manifests as a subtle color change. The problem, however, has been that the sensitivity of this type of optical gyroscopes degrades as the cavity gets smaller. “This issue was the roadblock that has hindered scientists from developing tiny optical gyroscopes,” said Dr. Li Ge, a professor at CUNY’s Graduate Center and Staten Island College. “There have been several attempts to get around this limitation, but they could not get around the real problem, the Sagnac effect itself.” Further studies are needed to consider the possibility that many modes of light exist simultaneously in the cavity. Their far-field emission patterns may change in different ways, which would cause a reduction of the device’s sensitivity to rotation. The researchers are now working on methods to control this effect. The research was published in Optica (doi: 10.1364/optica.2.000323). For more information, visit www.gc.cuny.edu.