# Optical accelerometers for Detecting Low-Frequency Micro-Vibrations
## 4 Optical Intensity Modulation  
An optical accelerometer based on optical intensity modulation is a type of accelerometer that senses acceleration by directly modulating the radiation intensity of light. Given that it is subject to acceleration, seismic mass in the accelerometer generates an inertial displacement that directly changes output optical intensity. In conjunction with the acceleration-displacement sensitivity of the acceleration sensing structure, the magnitude of the input acceleration can be calculated from the change in the output light intensity of the accelerometer. This type of optical accelerometers exhibits the advantages of a simple structure and low cost. However, guaranteeing its precision is typically difficult due to its relatively high susceptibility to the fluctuation of the incident light source and the external environment. The simplest optical accelerometer with optical intensity modulation is the optical waveguide (OWG) accelerometer. As shown in Figure 9a, [PERSON] et al. and [PERSON] et al. fixed a transmitting fiber in the form of a cantilever beam at one end and a receiving fiber at the other end [31; 32]. When acceleration acts on this structure, the cantilever beam bends, reducing the optical flux of the receiving fiber. The acceleration value can be obtained from the change in this flux. [PERSON] et al. improved the design by not using the optical fiber directly as the pickup component, and instead, using a symmetrical elastic structure (Figure 9b), in which the mass block drives the shutter to move, and the movement of the shutter causes the light flux to change [33]. [PERSON] et al. further enhanced the design of [PERSON] et al. A fish bone-shaped self-aligning pickup mechanism based on microelectromechanical system (MEMS) processing technology was designed to simplify the initial fiber alignment problem [34; 35]. The OWG-based accelerometer exhibits the advantages of a simple structure and low cost; however, its application is limited by its high environmental sensitivity and low accuracy [36; 37]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Principle** & **Structure** & **Resonant** & **Frequency Response** & **Sensitivity** \\  & & & **Frequency (Hz)** & **Range (Hz)** & **Sensitivity** \\ \hline [PERSON] et al. [20] & FES & Cantilevered leaf spring & 130 & 0.5–24 & 12.3 V/g \\ [PERSON] et al. [21] & FES & Cantilevered wire spring & 53 & 3–8 & 24.4 V/g \\ [PERSON] et al. [22] & FES & Fully symmetrical leaf spring & 46.5 & 1–10 & 10.8 V/g \\ [PERSON] et al. [23] & FES & Fully symmetrical leaf spring & 49.3 & 0.7–11 & 192.1 V/g \\ [PERSON] et al. [7] & LTG & Fully symmetrical leaf spring & 45 & 0.4–12 & 17.4 V/g \\ [PERSON] et al. [27] & LAC & Flexure hinge & X: 92.8 & 0.5–24 & X: 22.9 V/g, \\  & & & & & \\ [PERSON] et al. [29] & LAC & Arc-cut flexure hinge & X: 44.2 & 0.8–10 & X: 25.2 V/g, \\ [PERSON] et al. [30] & LI & Hanging thread & – & – & 13.5 s\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the accelerometer based on optical path modulation. In addition to OWG, grating shutters (GSs) can also be used as optical intensity modulation elements for accelerometers. [PERSON] et al. proposed a GS-based accelerometer, the light source emits light vertically downward, with a GS-shaped sensing structure in the middle and a grating-type detector below, along with a detector for sensing ambient light, achieving common-mode rejection. The linear range of this accelerometer reaches \(\pm\)84 g, but its sensitivity is less than 0.1 V/g [38]. [PERSON] et al. fabricated an optical accelerometer with a wide band of 10 Hz-1.5 kHz on the basis of MEMS technology with an array of gratings with square holes, an array of organic light-emitting diodes as the light source, and an array of organic photodetectors as the detector [39]. Similarly, [PERSON] et al.