# 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 Static Pressure Sensors  
The general requirements for a static pressure sensor are to have as small a size as possible and as large a linear working range on the premise of satisfying the sensitivity. For applications in harsh environments, it is generally necessary to have good heat resistance; for medical applications, it is necessary to have good biocompatibility. The introduction of the corrugated structure helps to reduce the influence temperature on the internal stress of the diaphragm, which is convenient for the design and fabrication of the pressure sensor under high temperature conditions. In 1993, [PERSON], [PERSON] et al. [18] reported the first prototype multimode fiber-based F-P high-accuracy pressure probe utilizing a CD with built-in temperature compensation. The F-P cavity of the probe consisted of the surface of a thin glass plate bonded to a CD and the end face of a multimode fiber. Within the full pressure range of 10 bar (1 MPa), a measurement range to resolution of \(3.6\times 10^{4}\)-1 and an overall measurement accuracy of \(\pm 0.15\%\) were achieved. This system represented a practical approach for industrial use. In 2015, [PERSON] et al. [19] designed and fabricated a fiber-optic F-P pressure sensor based on CD. The structure of the sensor is shown in Figure 7. The material of the annular CD was 316L stainless steel with a radius of 4600 \(\upmu\)m, a thickness of 30 \(\upmu\)m, a corrugation depth of 60 \(\upmu\)m and a corrugation width of 750 \(\upmu\)m. Experimental results showed that the sensor had good linearity over the pressure range of 0-0.1 MPa, and the sensitivity was up to 518 \(\upmu\)m /MPa, compared with the pressure sensor with the boss structure in the central area of the diaphragm which, was designed by their group [57] in 2008; the sensitivity was increased by two orders of magnitude. In 2017, [PERSON] et al. [20] designed and fabricated an annular CD-based fiber-optic pressure sensor with a novel F-P structure, which further improved the sensitivity of the sensor and reduced the influence of temperature on the measurement accuracy of the sensor. The structure of the CD was similar to the previous one, and the material was also 316L stainless steel. As shown in Figure 8, the F-P cavity was composed of sidewall of a 45\({}^{\circ}\) polished single-mode fiber which had a silver-plated end face and a CD. Experimental resultsshowed that the sensor had good linearity and repeatability over the pressure range of 0-0.1 MPa, and the sensitivity reached 705.64 um/MPa. In addition, the temperature compensation was achieved by measuring the relationship between temperature change from 15 \({}^{\circ}\)C to 65 \({}^{\circ}\)C with cavity length. In 2017, [PERSON] et al. [21] developed a CD-based EFPI fiber-optic pressure sensor, which provided a new idea for the design of pressure sensors applied in oil and gas wells. As shown in Figure 9, the F-P cavity adopted a capillary structure, and the measured medium was isolated from the fiber F-P cavity by CD, which could transmit pressure simultaneously; the influence of temperature change on the cavity length of the F-P cavity was compensated for by a fiber grating connected with fiber F-P cavity in series. The connection point material was quartz, which solved the high temperature creep problem which existed in the gelatinized package. The pressure measuring range of the sensor was 0-69 MPa, which had the advantages of high resolution, good repeatability, small hysteresis and stable long-term operation. However, the ambient temperature during measurement was only 100 \({}^{\circ}\)C, the application of this sensor in higher temperature environments needs to be detected. CD-based pressure sensors have also been used in the medical field. In addition to the good biocompatibility of the diaphragm, the size of the pressure sensor for intrava  
Figure 8: Sketch of the fiber-optic F-P pressure sensor. (Reproduced from [20]). Figure 7: Schematic of the fiber-optic Fabry-Perot sensor based on stainless steel CD.