Stimulated Brillouin scattering (SBS) is a typical third-order optical nonlinear effect describing the coupling between coherent light and phonon, which is the strongest nonlinear effect in the materials. Since the first realization in quartz and sapphire crystals in 1964, more work has been performed to study the SBS in various media, such as optical fibers, optical microcavities, gases and fluids. For the properties of narrow-linewidth and opposite-direction of the light generated by SBS, SBS lasers have many potential applications in the fields of laser, optical sensing, and optoelectronic devices related to fast and slow lights.
Optical gyroscope is an all solid-state angular velocity sensor based on the Sagnac effect with the advantages of high precision and high reliability. It has three main categories: ring laser gyroscope (RLG), fiber optical gyroscope (FOG) and micro optical gyroscope (MOG). RLG is sensitive to angular velocity by resonant laser beat frequency detection, which has the advantage of high stability of scale factor, but it has harsh requirements on the fabrication process. Interferometric fiber optical gyroscope (IFOG) owns the advantages of relatively simple structure and easy process, but the scale factor is not as stable as that of RLG. To improve the precision, hundreds or even thousands of kilometers of fiber are required to be accurately wound onto the ring-pillar. After decades of study, RLG and IFOG have been widely used in many optical sensor fields and navigation systems. In recent years, with the rapid development of autonomous driving, unmanned aerial vehicle and satellite platform, the demand for miniaturization and high precision of gyroscope is put forward, while the traditional gyroscope is difficult to meet these new requirements.
The sensing element of a resonant optical gyroscope is fiber ring or waveguide resonator. It only needs tens of meters of fiber or an on-chip microcavity with the diameter of millimeters to realize high-precision gyroscope, which is helpful for the miniaturization of the whole gyroscope devices theoretically. Based on the criterion that whether or not there is a laser source generated inside the fiber cavity, resonant optical gyroscope can be classified into active and passive ones, respectively. Subject to the disadvantages in linewidth, stability, various optical noise and complexity of the system, the passive fiber gyroscope is still under research in the laboratory, not completely used in real devices. On the other hand, with the continuous progress of nanophotonics and microfabrication technology, the active optical gyroscope based on SBS laser generation in the microcavity, combining both the advantages of laser gyroscope and traditional fiber optical gyroscope, provides a new way for the study of the miniaturized and integrated optical gyroscope. Theoretically, it could realize the excellent gyroscope with the advantages of easy integration, low threshold, high gain, large dynamic range and high sensitivity. However, the study on the micro-resonator Brillouin gyroscope has not reached such a high level yet. Therefore, it is meaningful to advance the study on the micro-resonator (such as microcavity, waveguide) Brillouin gyroscope for the development of integrated optical gyroscope and extend their applications in the real civil devices, weapons, and navigation systems.
The basic parameters to characterize the stimulated Brillouin scattering effect mainly include the full width at half maximum (FWHM) of Brillouin gain spectrum, peak Brillouin gain and threshold. For solids and fluids, they are expressed by different equations, as shown in Eqs. (1)-(5). The Brillouin gain coefficient can be increased by doping and designing device structure. The corresponding Brillouin parameters in different types of fiber, microcavity, gas and liquid are summarized in Table 1 and, at the same time, their typical applications are introduced (Figs. 2-5). The research progress of miniaturized and integrated optical gyroscope based on SBS in the fiber and microcavity is thoroughly introduced (Fig. 6), including the study of exceptional point to achieve optical gyroscope with ultra-high sensitivity (Fig. 7).
Stimulated Brillouin scattering laser plays an important role in the field of miniaturization and integration of active resonating optical gyroscope. In this paper, the SBS effect and its corresponding applications in different media are detailedly summarized. Especially, the SBS effect is an important tool for the integration and miniaturization of optical gyroscope both in the fiber and micro-resonator systems. Hence, the studies on the SBS fiber optical gyroscope and micro-resonator gyroscope are reviewed to provide the recent progress of the integrated optical gyroscope. More interestingly, exceptional point excited in the optical microcavity has been found to own the possibility of significantly enhancing the sensitivity of the gyroscope, paving a new avenue for the study of a highly sensitive integrated optical gyroscope.
