# Optical accelerometers for Detecting Low-Frequency Micro-Vibrations
## 7 Conclusions and Prospect  
The accurate detection of low frequency (frequency below 10 Hz or even below 1 Hz) micro-vibration is of great significance for the development of high precision inertial navigation, structural health monitoring of large buildings, precision measurement/processing, wind turbine fault detection, earthquake detection, and other fields. The low-frequency micro-vibrations produce small acceleration amplitudes down to milligram or even micro-gram magnitude, requiring accelerometers with excellent low-frequency response characteristics. Accelerometers have made significant advances in recent decades. By taking advantage of optical modulation technology, accelerometers are currently capable of achieving impressive levels of performance.  
This study analyzed the practical application requirements of optical accelerometers and then classified accelerometers into four categories: optical path, optical intensity, optical phase, and optical wavelength modulations. With the improvement in light source technology, photodetectors, optical signal modulation, and demodulation in recent years, the performance of accelerometers has been significantly improved, as evident in Tables 1-4. Each class of accelerometers exhibits advantages and disadvantages and are suitable for different requirements. For example, accelerometers based on optical path modulation and light intensity modulation typically have relatively simple optical path structures and signal processing circuits, as well as their mechanical structures being easy to design and machine, which easily improves their low-frequency response characteristics. However, their measurement accuracy depends on the performance of the light source and detector. In addition, the light source and detector are arranged internally in both types of optical  
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accelerometers, making them complex, large, and heavy. The use of both types of optical accelerometers is suitable for applications wherein the size and mass requirements of the accelerometer are insensitive, such as precision active vibration isolation systems, structural health monitoring, and earthquake monitoring. Accelerometers based on optical phase and wavelength modulations have also been extensively investigated in recent years. The optical paths of these types of optical accelerometers are mostly arranged in optical fibers, while the light source and detector are placed outside. In addition, the mechanical structures of these two types of accelerometers are also mostly designed and processed using microelectromechanical techniques, with an overall compact structure and small size. However, these two types of accelerometers also have the problems of high cost and complicated output signal modulation and demodulation technology, and the convenience of using them is different from the first two types. Therefore, these types of accelerometers can measure micron-level acceleration with high accuracy in mass- and size-sensitive application scenarios, such as inertial navigation and wind turbine fault monitoring. A summary of the performance of the four types of optical accelerometers is provided in Table 5.  
Research on high-precision optical accelerometers will continue to flourish in the coming decades, focusing on the collaborative design of optical and mechanical components, the application of precision micromachining techniques, and the study of new optical measurement technologies. The development of high-precision optical accelerometers should be promoted toward high sensitivity, stability, integration, and anti-interference capability. Moreover, the application range of optical accelerometers should be broadened to make them convenient and easy-to-use precision instruments instead of being limited to laboratory application scenarios.  
Not applicable.  
The authors declare no conflict of interest.