Gyroscopes, which are the critical components of inertial navigation devices, determine the navigation positioning and attitude control of spacecraft, cranes and vehicle load, and unmanned systems. Due to its high precision, the demand for a miniaturized integrated gyroscope has become urgent. As the market demand for inertial sensor systems grows in the future, micro-nano integrated optical gyroscope technology should be the first choice of the next generation optical gyroscope. Gyroscopes are not only suitable for spacecraft attitude control but can also be used for automatic vehicle navigation systems and safety protection. Integrated optical gyroscopes may fill in the gaps because the global navigation satellite system can be blocked and the weight of laser or optical fiber gyros is extremely large. Resonator micro-nano optical gyroscopes can be used to achieve all-optoelectronic integration, where different separation devices, such as light source, modulator, resonator, and detector, are integrated on the same chip to reduce the volume, weight, and cost of the device. Micro-nano integrated optic gyroscopes should satisfy both the demand for integration and precision.
The main problem is that the small area of the closed optical path at the micro/nanoscale decreases gyroscope sensitivity. To achieve a high-precision-resonant integrated optical gyroscope, new principles and technologies must be explored. The research progress of the integrated optical gyroscopes with optical gain, those that are dispersion-enhanced, and non-Hermitian optical gyroscopes are reviewed.
Because of the limitations of fabrication technology, the propagation loss of passive-optical waveguides cannot meet the requirement of high-precision micro-nano optical gyroscopes. To solve this problem, methods for exciting two laser beams in opposite directions or using the optical gain to compensate for the losses in the resonator are proposed. Several methods for achieving active resonators are introduced, such as semiconductor Ⅲ-Ⅴ materials, stimulated Brillouin scattering, stimulated Raman scattering, and rare earth ion doping. The development and problems of the gyroscopes with optical gain resonators are discussed. The main problems with the development of a resonant gyroscope with optical gain are as follows: (1) the mode competition in the active cavity causes nonreciprocity of the two beams, thereby making it difficult to establish the bidirectional operation of lasers; (2) for a gyroscope lock-in effect, the rough side of the wall of the processed optical waveguide causes backscattering and cross-coupling of clockwise and counterclockwise beams, resulting in no frequency difference in the output signal of the gyroscope at low rotation. (3) The optical pump technique is more complex because of the need to introduce a pump laser. However, an electric pump is more compact and suitable for developing the entire chip integration.
The dispersion relationship directly affects the propagation velocity of light, which can be divided into phase and group velocities. The Sagnac effect can be enhanced in an optical resonator by manipulating the group velocity of light. Slow and fast lights correspond to normal and abnormal dispersions, respectively. Through numerous theoretical and experimental studies, several methods have regulated the group velocity of light, which mainly fall into two categories: material and structural dispersions. Several academic papers have discussed whether the precision of a slow-light gyroscope can be enhanced under normal dispersion conditions. The Sagnac frequency shift of a resonant optical gyroscope could be enhanced under the anomalous dispersion condition. The main problem of the dispersion-enhanced gyroscope is achieving optical dispersion relation, generation mechanism of fast-light, and its influence on the Sagnac effect in active waveguide resonators. However, resonance linewidth is broadened using material or structural anomalous dispersion, which may counteract the dispersion enhancement effect of a resonant optical gyroscope.
Optical microcavities, with high-Q factor and small-mode volume, can significantly enhance the light-matter interaction. They play an important role in the research of non-Hermitian optics and have become an important platform for their research. Presently, achieving ultra-high sensitivity optical sensing based on exceptional points is a research hotspot of non-Hermitian optical systems. We are committed to achieving highly sensitive non-Hermitian optical sensing by combining the theory of non-Hermitian optics and whispering gallery mode (WGM) microcavities. Under the same perturbation, the non-Hermitic optical system at the exceptional point has a larger response to external perturbations than the traditional optical system, thus providing a new method for achieving high-performance optical sensors. The exceptional surface provides additional degrees of freedom that can shift the working point along the exceptional point (EP) surface when the system experiences undesired perturbations, thus improving the robustness of the non-Hermetic optical system. An exceptional surface, composed of chiral exceptional points, can be achieved by breaking the time-reversal symmetry of microcavity.
By combing the research progress of micro-nano integrated technology and optical gyroscope in this study, we summarize the research status, bottlenecks, and future development of the integrated resonant optical gyroscopes. Based on the possible solutions, which include new materials, structures, and physical effects, we emphasize the influence and research progress of optical gain compensation, dispersion control, and exceptional points of the non-Hermitian optical system on the sensitivity of optical gyroscopes. Finally, the related research prospect for integrated optical gyroscopes are summarized.
