# Reviews on Corrugated Diaphrags in Miniature Fiber-Optic Pressure Sensors
## 4 The Application of CDs in Miniature Fiber-Optic Pressure Sensors
### In the Miniature Fiber-Optic Acoustic Pressure Sensors  
For the acoustic pressure sensor, in addition to the high sensitivity and large linear working range required by the static pressure sensor, wide and flat frequency response and small MDP are required. Vibration diaphragm with both high mechanical and optical quality is the key to determine the performance of the acoustic pressure sensor [15; 60]. The introduction of the corrugated structure can release the initial stress and improve the mechanical sensitivity. In 2015, [PERSON] [24] proposed an EFPI fiber-optic acoustic pressure sensor based on silicon dioxide annular CD to measure gas concentration. The CD was fabricated using the MEMS process and its surface \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Measurement Range** & **Temperature** & **Sensitivity** & **Accuracy** & **Application** \\ \hline JUMO 404366 & 0–60 MPa & –20–80 \({}^{\circ}\)C & — & \(\leq\) 0.2\% & \\ [PERSON] [18] & 0–1 MPa & 32–42 \({}^{\circ}\)C & 22.3 rad/bar & \(\pm\)0.15\% & general \\ [PERSON] [19] & 0–0.1 MPa & 25 \({}^{\circ}\)C & 518 \(\mu\)m/MPa & 1.05\% & \\ [PERSON] [20] & 0–0.1 MPa & 15–65 \({}^{\circ}\)C & 706.64 \(\mu\)m/MPa & good & \\ JUMO 402050 & 0–6 MPa & 3–200 \({}^{\circ}\)C & — & \(\leq\) 0.2\% & high \\ [PERSON] [21] & 0–69 MPa & 25–100 \({}^{\circ}\)C & 211 \(\mu\)m/MPa & good & temperature \\ JUMO 404327 & 0–0.1 MPa & 20–80 \({}^{\circ}\)C & — & \(\leq\) 0.2\% & \\ [PERSON] [22] & 0–20 kPa & — & — & \(<\)2\% & medical \\ [PERSON] [23] & 0–40 kPa \({}^{1}\) & — & 16 \(\mu\)m/MPa \({}^{1}\) & — & \\ \hline \hline \end{tabular} \({}^{1}\) FEM simulation.  
\end{table} 
Table 1: Performance of CD-based static pressure sensors and commercial electronic pressure sensors. Figure 11: **(a)** Structural parameters of the F-P microcavity; **(b)** Schematic view of dual optical fiber F-P pressure sensor. (Reproduced from [22]). was coated with a layer of gold film with a thickness of about 50 nm. Experimental tests showed that the probe voltage sensitivity was 7.3374 mV/Pa and the MDP was 115.04 Pa/Hz\({}^{1/2}\) at a frequency of 10 kHz. The gas concentration sensor was designed and manufactured with the main resonant frequency of the diaphragm at 4.275 kHz as the characteristic sensing frequency, and its minimum detectable concentration sensitivity was 1356.15 ppm/Hz\({}^{1/2}\). However, due to the immature release process, large residual stress, and up to 1200 nm thickness of diaphragm, the acoustic pressure response sensitivity of the diaphragm and the detectable concentration sensitivity of the fiber-optic gas concentration sensor based on this CD were not very high. In 2018, [PERSON] [25] designed and fabricated an EFPI fiber-optic acoustic pressure sensor based on an aluminum-polyimide film (Al-PI) CD using the same manufacture method. The thickness of the diaphragm was about 300 nm, and the front diameter of the acoustic pressure sensor formed was only 2 mm. However, the sensor did not perform well when tested. The fabrication of the above two kinds of diaphragms required processing both sides of the silicon substrate, and the diaphragm was released by etching off the silicon substrate, which was rather complex. The most common applications of fiber-optic acoustic pressure sensors based on CD are fiber-optic microphones (FOM) and fiber-optic hydrophones. In 2017, [PERSON] [17] adopted a simpler MEMS process to fabricate CD. Only one side of the silicon substrate was processed, and the diaphragm was fabricated on the sacrificial layer by deposition and then released by etching the sacrificial layer. This manufacturing technology had good repeatability and low production cost, which was advantageous for mass production. Based on this manufacturing technology, EFPI fiber optic sound pressure sensors based on metal CD were fabricated; the materials contain metal and polyethylene terephthalate (PET).