Slow-light rotation sensors

Abstract

Precise positioning and motion tracking are enabling development of key technologies in numerous fields of research and applications such as control, navigation, stabilization, and positioning systems for robotics, medical-imaging applications, virtual reality, computer games, and many more. While rotation sensing can be based on both mechanical and optical effects, optical gyroscopes provide the highest sensitivity and accuracy. Although the details may vary depending on the specific implementation, all optical rotation sensors exploit the Sagnac effect,1 i.e., the interference pattern resulting from the different phase shifts of waves propagating in a circular path along and opposite the direction of rotation. The classical Sagnac phase shift is determined solely by the angular velocity, the optical frequency, and the area enclosed by the closed-loop optical path, and completely independent of the medium’s other properties such as the index of refraction and its dispersion properties.2 This century-old result has been the subject of ongoing debate for several decades, both because of its fundamental nature and because it inherently limits the feasibility of attaining highly sensitive miniaturized rotation sensors. Specifically, it implies that the only ways to enhance the sensitivity and accuracy of optical gyroscopes are to either increase the optical frequency or enlarge the gyro’s area. In practice, both approaches are limited. Recent studies have pointed out the advantages of exploiting the Sagnac effect, generated by electronic resonances, in slow-3 and fast-light4 media (i.e., in which the group velocity is substantially smaller or larger than the speed of light) for fabrication of ultrasensitive optical gyroscopes. However, material properties based on slow/fast-light media suffer from inherent limitations and drawbacks. In particular, such systems are bulky, lossy, require very low temperatures, and are limited to specific wavelengths according to the material’s electronic andoptical properties. Instead, we have been focusing on photonic-resonance-based structures,5, 6 taking advantage of their flexibility, compact dimensions, and excellent optical Figure 1. Schematic of a coupled-resonator optical waveguide (CROW).