# Reviews on Corrugated Diaphrags in Miniature Fiber-Optic Pressure Sensors
## 5 Conclusions  
In this paper, the application status of CDs in miniature fiber-optic pressure sensors has been reviewed. It was proved by both theory and experiment that the introduction of a corrugated structure can effectively reduce the influence of diaphragm's initial stress and improve the mechanical sensitivity and linear range of the diaphragm, and thus improve the overall sensitivity, measurement pressure range and stability of the sensor. CD-based fiber optic pressure sensors have been developing towards being high performance, anti-harsh environment, micro-sized, low in cost and practical in application. However, up to now, most of the sensors have still been in the experimental stage, and their performance in some aspects needs to be improved compared with commercially electronic pressure sensors. The main obstacles to the practical application of CD-based miniature fiber-optic pressure sensors are as follows:  
* The fabrication of CDs: At present, almost all CDs are fabrication by MEMS technology, including photolithography, thin film technology, etching technology, and so on. Some of the processing parameters are difficult to control accurately, which makes the center area of the CD uneven, resulting in a reduction in the accuracy and consistency of the CD, and seriously affecting the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Dynamic Range** & **Frequency Response** & **Sensitivity** & **MDP** & **Temperature** \\ \hline B\&K 4192 & 19–162 dB & 3.15 Hz–20 kHz & 12.5 mV/Pa & — & –30–300 \({}^{\circ}\)C \\ [PERSON] [24] & — & — & — & 7.3374 mV/Pa & 115.04 μ/Hz\({}^{1/2}\) & — \\ [PERSON] [17]1  & 9.17–76.58 dB & 80–1000 Hz & 83.24 nm/Pa & 57.45 μ/Hz\({}^{1/2}\) & — \\ [PERSON] [17]2  & 63.6–130.94 dB & 31.5–800 Hz & 3.31 nm/Pa & 1514.22 μ/Hz\({}^{1/2}\) & — \\ [PERSON] [26] & 12.77–77.65 dB & 63–1000 Hz & 52 nm/Pa & 86.97 μ/Hz\({}^{1/2}\) & — \\ [PERSON] [27] & \(\sim\)165 mPa & 20 Hz–20 kHz & 2.23 V/Pa & 2.97 μ/Hz\({}^{1/2}\) & — \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance of CD-based acoustic sensors and commercially electronic acoustic sensor. Figure 12: Schematic of the proposed OFM based on CD. (**a**) Schematic diagram of the proposed OFM; (**b**) Schematic, and (**c**) section diagram of the CD. (Reproduced from [26]). performance of the sensor. It could be said that the manufacturing technology is currently the main factor that limits the practical application of the CD-based miniature fiber-optic pressure sensor. * The bonding method of the sensor: At present, most components of the CD-based miniature fiber-optic pressure sensor are fixed by curing glue. Due to the large thermal expansion coefficient of the curing gel, the sensor's performance is greatly affected by temperature, and the length of the F-P cavity will be changed during the use, which makes the temperature-pressure crossover sensitivity of the sensor large, limiting the high temperature performance of the sensor. * The packaging of the sensor: At present, MEMS devices mainly use chip-level packaging, that is, dicing before releasing, which can avoid damage to the diaphragm in dicing process, but this method cannot be mass-produced, and the size of the product is large with high cost. * For the existing problems, the following improvements can be made:
* Develop more suitable and easy-to-control MEMS technology to reduce the difficulty of controlling technological parameters and improve the accuracy and consistency of the CD. * Develop new diaphragm materials, design new structures of diaphragm and sensor to improve the performance of diaphragm-based pressure sensors to meet the needs of different application environments. * Consider the packaging of the sensor according to the actual situation, select the appropriate bonding method, and use wafer-level packaging which releases the diaphragm first and then performs dicing, so as to improve the production efficiency.