# Measurement of Structural Loads Using a Novel MEMS Extrinsic Fabry-Perot Strain Sensor
## 1 Introduction  
Fiber optic sensors are desirable in engineering measurement due to their advantages such as small size, anti-electromagnetic interference, high resolution, and reusability [1; 2]. As a basic physical parameter, the strain can be successfully measured by sensors including the fiber Bragg grating [3; 4], interferometric fiber-optic sensor [5; 6; 7] and distributed fiber sensor [8; 9]. Among them, the Fabry-Perot (FP) sensors are an attractive choice, owing to their compact structure, and especially their advantages in terms of sensitivity and high-temperature resistance [10; 11; 12]. Previous studies have proposed FP force sensors; the theory of the axial contact force sensor reported by [13] corroborated that the wavelength of interference dip will have a shift as the applied axial force increases. In [14], [PERSON] et al. implemented a new fiber-optical force sensor as a modular sensor used in surgery. The sensitivity of the FP sensor depends on the dimensions and shape of the microcavity, including the spheroidal cavity [15], hollow tube [16], and microbubble [17]. [PERSON] et al. proposed an intrinsic cost-effective FP sensor by splicing two optical fibers previously destroyed by the catastrophic fuse effect [18]. The sensor was embedded into a polymer casing with a maximum sensitivity of \(59.39\pm 1.7\) pm/kPa. [PERSON] et al. proposed an integrated optical fiber Fabry-Perot interferometer (FPI)sensor, which was composed of an in-fiber microcavity and a tiny segment of single-mode fiber (SMF). The sensor performed for a temperature and pressure double parameter measurement with a sensitivity of 10.8 pm/\({}^{\circ}\)C and 4.1587 nm/MPa, respectively [19]. Microstructured optical fibers (MOFs) have been used as more effective sensors in recent years: [PERSON] et al. achieved strain measurement based on a photonic crystal fiber (PCF) with an embedded coupler, which was fabricated by use of a femtosecond (fs)-laser-assisted selective infiltration method, resulting in a high strain sensitivity of \(-\)22 pm/\(u\epsilon\)[20]. However, its temperature measurement range is too narrow, which severely limits its practical applications. On this basis, the bubble structure has developed from a simple air bubble to the \"fiber in capillary\". [PERSON] et al. [21] reported a strain force sensor based on a fiber inline Fabry-Perot microcavity plugged by cantilever taper \(L_{1}\), and the hollow tube length \(L_{2}\); the strain force sensitivity can be improved by reducing the sidewall thickness. However, it has an ultra-thin sidewall that sacrifices the mechanical strength. [PERSON] et al. [22] proposed a new ultra-thin crescent-shaped FP cavity by multimode optical fiber with higher sensitivity than conventional elliptical or D-shaped sensors. However, all these methods involve various complicated steps, expensive photonic crystal fibers (PCFs) [23], or other special optical fibers. Much research has been carried out to overcome these shortcomings, and various kinds of new technologies are presented to fabricate FPIs. [PERSON] et al. [24] designed a gas-cavity fiber Fabry-Perot interference strain sensor by using arc discharge technology and invading liquid to make microbubbles. [PERSON] et al. [25] used a femtosecond laser processing and phase masking method to write a Type II fiber grating around the miniature fiber Fabry-Perot interferometer to measure the strain and high temperature, simultaneously. Moreover, the combination of FP fiber sensors with microelectromechanical systems (MEMS) technology can provide a new solution for the miniaturization of an optical fiber sensing systems [26]. The fabrication and preliminary testing of a new MEMS pressure sensor is described in [27], while the entire MEMS structure is fabricated directly on the optical fiber. [PERSON] et al. proposed an FP sensor based on MEMS technology, which consists of a 45\({}^{\circ}\) polished optical fiber and silicon thin film to form an air chamber that can simultaneously measure pressure and temperature [28].