# 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
(a: fiber flange; b: ceramic ferrule; c: glass ring; d: CD) (Reproduced from [19]). Figure 9: The structure diagram of optical fiber F-P cavity pressure sensor. (Reproduced from [21]). be ultra-miniature. In 2003, [PERSON], [PERSON] et al. [58; 59] presented an F-P micro cavity structure with a novel, single deep corrugated diaphragm (SDCD). The F-P micro cavity was fabricated on a single-chip using both surface and bulk micromachining techniques which facilitated the mass production and F-P micro cavity was significantly miniaturized, they could apply the advantage of small size and dense arrays, which played an important role in medical applications. The proposed F-P micro cavity sensing component, as shown in Figure 10, was composed of three parts: the SDCD structure that served as a moving mirror composed of a polysilicon/silicon nitride/polysilicon composite diaphragm; a bottom diaphragm that served as the stationary mirror; and the gap between the moving and the stationary mirrors of the F-P micro cavity. The theoretical analyses, as well as the measurements, showed that the SDCD-based structure could enhance the flatness of the diaphragm and significantly reduce the signal-averaging effect for the proposed F-P micro cavity pressure sensor. The next year, based on the previous work, the research team [51] proved that the proposed SDCD structure could substantially reduce the cross-sensitivity to temperature of the F-P pressure sensor. In 2006, [PERSON] et al. [22] designed a dual-fiber F-P pressure sensor using this SDCD F-P micro cavity structure for blood pressure measurement. Structural parameters of the F-P micro cavity and the structure of the sensor are shown in Figure 11a,b respectively. The bottom area of the SDCD was 150 \(\upmu\)m \(\times\) 150 \(\upmu\)m and the corrugation depth was 40 \(\upmu\)m. The aim of adopting a double fiber structure was to minimize such unwanted perturbation in the output signal. Experimental results showed that the proposed pressure sensor achieved less than 2% nonlinearity in the blood pressure range of 0-150 mmHg (0-20 kPa). In 2018, [PERSON] et al. [23] proposed a CD-based miniature EFPI pressure sensor model for pulse pressure detection. The diaphragm's material was silicon nitride, and the total diameter of the sensor was only 650 \(\upmu\)m. The simulation results showed that the sensor designed gained a static pressure range of 0-300 mmHg (0-40 kPa) and the sensitivity was \(2.13\times 10^{-3}\) mm/mmHg (16 \(\upmu\)m/MPa). At present, the CD-based pressure sensors applied in the medical field are still in the experimental stage, and the pressure measurement range and structural size basically meet the requirements, but the sensor also has many defects, such as low sensitivity, poor linearity, and poor stability. Figure 10: (**a**) Three-dimensional schematic with cross section of the proposed SDCD FP microcavity pressure-sensing element (Reproduced from [58]); (**b**) Schematic view of the proposed F-P microcavity pressure sensor (Reproduced from [51]). Performance comparisons were made between the CD-based static pressure sensor and conventionally commercialized electric pressure sensor, as shown in Table 1. In this paper, three electronic pressure transmitters applied to different fields produced by JUMO of Germany were selected, and their types are 404366, 402050 and 404327 respectively. (Note: Some of the performance parameters in the table are not given or qualitatively described, as the corresponding parameters are not explicitly given in the literature or data sheet.). One can find that CD-based fiber optic pressure sensors have been used in all the fields that commercial electronic pressure sensors have been used. However, the performance of CD-based static pressure sensors still has many shortcomings, such as low sensitivity, small measurement pressure range, and poor stability. Especially in harsh environments (high temperature, high pressure, strong corrosion, limited space, etc. ), the performance of the sensor is still not satisfactory.