# Optical accelerometers for Detecting Low-Frequency Micro-Vibrations
## 3 Optical Path Modulation
As shown in Figure 7b, [PERSON] et al. designed a highly sensitive 2D optical accelerometer based on a modified DVD pickup head with \(x\)- and \(y\)-axis sensitivities of 22.9 V/g and 21.3 V/g, respectively [27]. Figure 4: Accelerometer with a fully symmetric leaf spring: (**a**) structure schematic and (**b**) photograph. Reprinted with permission from Ref. [22]. Copyright 2019 IEEE. Figure 5: Principle of LTG: (**a**) optical path and (**b**) output signal. Figure 6: Accelerometer based on the LTG [7]: (**a**) principle, (**b**) structure schematic, and (**c**) photograph. Although the use of a DVD pickup head is convenient; this part is extremely compact, such that any component change is impossible. Following the optical configuration of a DVD pickup head, a compact 2D angle sensor based on the laser autocollimator (LAC) principle was developed to expand the measurement range [28]. [PERSON] et al. designed a 2D optical accelerometer based on this modified 2D angle sensor [29] (Figure 8). The accelerometer uses a hollow cylindrical arc-cut 2D flexure hinge, which references its stability, and is based on an optical sensing method, which imbues it with high sensitivity and resolution. Its \(x\)- and \(y\)-axis sensitivities are 25.2 V/g and 25.6 V/g, respectively, and its test resolution is down to 60 \(\upmu\)g. Compared with that in [PERSON] et al., the performance and serviceability of this optical accelerometer are improved, enhancing its application value. In addition to 2D angle sensors based on the LAC principle, 2D angle measurement based on the laser interference (LI) principle was proposed by [PERSON] et al. This method utilizes the spectral separation properties of angular cone prisms and gratings with directional reflection characteristics to achieve a coherent beam with an error of less than \(\pm\)0.01\({}^{\circ}\)  
Figure 8: 2D accelerometer based on a modified 2D angle sensor. Reprinted with permission from Ref. [26]. Copyright 2021 Elsevier. Figure 7: Accelerometer based on the LAC: (**a**) 2D angle sensor and (**b**) 2D accelerometer based on a DVD pickup head. Figure (**a**) is reprinted with permission from Ref. [26]. Copyright 2019 Springer Nature. for 2D angle measurement. On the basis of this principle, [PERSON] et al. designed a 2D optical accelerometer that can reach a measurement range of 177 g with a measurement error of less than 0.1 g [30]. This study achieved a large range but sacrificed measurement accuracy. In summary, optical accelerometers based on optical path modulation are characterized by a simple structure and convenient signal processing; high sensitivity can be achieved by optimizing the mechanical [22] or optical sensing units [23]. However, the measurement accuracy of this type of optical accelerometer is directly related to the structure of the optical path and the performance of the photodetector. To increase the sensitivity of the accelerometer, the optical range of the optical lever should be widened, and a high-sensitivity photodetector should be used, increasing the size and cost of the accelerometer. The parameters of the structures and the optical accelerometer based on optical path modulation (i.e., resonant frequency, working frequency band, and sensitivity) are provided in Table 1.