# 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
The metal CDs were made of silver, their diameter, thickness, corrugation width and number of corrugations were 2.5 mm, 210 nm, 50 \(\upmu\)m and 7 respectively. Tests in air showed that the sensor based on metal corrugation diaphragm with the corrugation depth of 2.3 \(\upmu\)m had a relatively uniform frequency response between 80 and 1000 Hz, the phase sensitivity was \(-\)123.42 dB re 1 rad/\(\upmu\)Pa, and the corresponding mechanical sensitivity was 83.24 nm/Pa. The MDP at 1 kHz was 57.45 \(\upmu\)Pa/Hz\({}^{1/2}\) and the dynamic range was 67.41 dB. In 2018, [PERSON] et al. [26] designed and fabricated a fiber-optic microphone with EFPI structure based on corrugated silver diaphragm. The structure is shown in Figure 12. The rectangular corrugated structure was introduced into the diaphragm by \"gray-scale exposure\", and the corrugation depth could be easily controlled by adjusting the exposure duration. The radius, thickness, corrugation depth, corrugation width and corrugation period of the diaphragm were 1.25 mm, 200 nm, 3 \(\upmu\)m, 0.05 mm and 0.1 mm, and a number of corrugations of 7. The experimental results showed that the mechanical sensitivity of the CD was 52 nm/Pa, which was twice that of the FD under the same parameters. The FOM had a uniform frequency response in the range of 63-1 kHz, the phase sensitivity was \(-\)127.5 dB re 1 rad/\(\upmu\)Pa, and the MDP and dynamic range were 86.97 \(\upmu\)Pa/Hz\({}^{1/2}\) and 64.88 dB at 1 kHz, respectively. The proposed FOM was simple in manufacturing technology and low in cost, which was expected to be applied in the field of weak acoustic sensing. Similarly, in 2018, [PERSON] et al. [27] developed a miniature fiber-optic microphone based on a silicon nitride CD to improve the MDP and frequency response range of the microphone. The CD was fabricated using the bulk-silicon micromachining technology of MEMS. The diaphragm had a size of 1.9 mm \(\times\) 1.9 mm, a corrugation width of about 25 \(\upmu\)m, and a corrugation number of 5. The FOM with a thickness of 400 nm was tested to have a flat frequency response and a linear sound pressure response in the audible sound frequency range. The acoustic pressure sensitivity was 2.23 V/Pa and the maximum measurable pressure was 165 mPa at the acoustic pressure signal of 1 kHz. Under the acoustic pressure signal of 37.76 mPa and 1 kHz, the signal-to-noise ratio was 79.08 dB, and the corresponding MDP reached 2.97 \(\upmu\)Pa/Hz\({}^{1/2}\). Similarly, the performance parameters of CD acoustic pressure sensor are compared with those of commercially electronic acoustic pressure sensor (B&K 4192), as shown in Table 2.  
At present, CD-based acoustic pressure sensors have shown great superiority in sensitivity, and the frequency response range has also gradually improved. But the shortcomings, such as high self-noise, low consistency and poor stability, still need to be resolved. There is a trade-off between the bandwidth and mechanical sensitivity of this sensor, and the diaphragm may be easily damaged under strong acoustic pressure, so a more suitable diaphragm structure needs to be designed. With further research and development, CD-based acoustic pressure sensors will have great potential in practical applications.