# Reviews on Corrugated Diaphrags in Miniature Fiber-Optic Pressure Sensors
## 1 Introduction  
Pressure sensors are widely used in many fields of life. The applications of pressure sensors can be divided into static pressure sensors (such as atmospheric pressure or hydrostatic pressure) and dynamic pressure (such as acoustic pressure) sensors. Compared with the traditional electronic pressure sensors, the fiber optic pressure sensors enjoy the advantages of high sensitivity, wide frequency response, large dynamic range and strong anti-electromagnetic interference, along with small size, light weight and miniaturization. These make them suitable for use in harsh environments, such as limited space, high temperature, high pressure, strong corrosion and radiation. Therefore, the fiber optic pressure sensors have been highly valued and extensively used in many key areas, such as national defense security [1], medical health [2], oil and gas exploration [3] and underwater monitoring [4].  
According to the difference between sensing elements, fiber-optic pressure sensors can be classified as intrinsic or extrinsic pressure sensors. For the intrinsic fiber-optic pressure sensor, the length and refractive index of optical fiber, when subjected to external pressure, will change and result in the variation of the internal transmission light state (polarization, phase, amplitude, etc.). In contrast, for the extrinsic fiber-optic pressure sensor the optical fiber is only used as a light guide element and the sensing of the pressure will be realized by an external pressure sensitive diaphragm. Due to the flexibility of the diaphragm's structural design, the diaphragm-based fiber-optic pressure sensor has attracted widespread attention. Pressure sensitive structures based on the diaphragm have been extensively studied by [PERSON] (M-Z) [5; 6; 7], [PERSON] [8; 9], [PERSON] [10; 11; 12] and Fabry-Perot (F-P) [13; 14; 15] interference structures. Among them, the extrinsic Fabry-Perot interferometer (EFPI) structure, usually formed by two interference beams sharing a common optical path, is insensitive to the disturbances in the transmission fiber, and thus is widely used in pressure sensing.  
As the core component of the EFPI pressure sensor, the diaphragm's material and structure directly determine the performance of the sensor. Up to now, the probe manufacturing for diaphragm-based EFPI sensors is usually realized using the microelectromechanical system (MEMS). According to the structures, the diaphragms could be mainly divided into three types: flat diaphragms (FDs), Mesa-diaphragms and Corrugated diaphragms (CDs) [16]. The FD, the traditional structure, is simple to design and fabricate. However, the nonlinear deformation under large applied pressure limited the FD's application. Meanwhile, the mechanical sensitivity of the FD will decrease rapidly, when there is a large initial stress in the FD. Generally, there are two methods to improve the performance of the diaphragm-based sensors. The first is to add a boss structure to the diaphragm. The boss structure can improve the optical performance of the sensor by declining the nonlinear deformation in the diaphragm's central region, but the fabrication of the boss structure is pretty complex, and the introduce of boss structure will enhance the sensor's undesirable sensitivity to vibration. The second is to add a corrugated structure to the diaphragm. The corrugated structure can not only reduce the impact of initial stress on the diaphragm (if there is any) and increase the linear range effectively, but also can easily be fabricated using MEMS technology.  
Recently, the application of CDs in miniature fiber-optic pressure sensors has been developing rapidly [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. In this paper, the mechanical properties of CD are analyzed in detail. We are trying to summarize the application of CD in miniature fiber-optic pressure sensors, to point out the existing problems and to explore the potential application of CDs in miniature fiber-optic pressure sensors.