# Dynamic Responses Measured by Optical Fiber Sensor for Structural Health Monitoring
## 1 Introduction  
Sensing technologies for real-time monitoring of engineering structures are very important, since in-service monitoring can lead to the improved safety and performance of engineering structures. Traditional sensors such as the strain gauge and the load cell are mostly based on electrical working principles to measure physical quantities such as strain, force or temperature. The main drawbacks of these sensors can be attributed to the sensitivity, cross talk, non-compatibility with harsh environments and their sizes [1]. Optical fiber sensors with small size, light weight, good sensitivity, and that are easy to be surface bonded or embedded are considered ideal sensors for structure health monitoring. The most distinguished characteristics of the optical fiber sensor is its unique capability to act as both the sensing unit and light transmission medium, allowing data acquisition equipment to be installed away from the field [2]. Fiber Bragg grating (FBG) sensors based on reflecting particular wavelengths of light with the advantages of high accuracy, corrosion resistance and convenience in forming sensor networks have received extensive attention. [PERSON] et al. [3] proposed a new type of fiber Bragg grating sensor to monitor bridge cable force. [PERSON] et al. [4] presented a highly sensitive and compact FBG sensor for simultaneous measurement of refractive index and temperature. [PERSON] et al. [5] used FBG sensors to quantitatively measure multi-axial strain fields of cross-ply laminates loaded along the three principal mechanical axes. [PERSON] et al. [6] employed longitudinally and transversely embedded FBGs to monitor the hot-press molding process of glass-fiber-reinforced polypropylene (GFPP) laminates. [PERSON] et al. [7] used two cascaded FBGs to measure both the acousticand magnetic properties simultaneously. [PERSON] et al. [8] proposed a novel integrated optical fiber sensor for temperature, pressure and flow measurements. [PERSON] and [PERSON] [9] demonstrated the capability of embedded FBG sensors in adhesive bonded composite materials for the detection of moisture contamination. [PERSON] et al. [10] investigated the effect of metal coating thickness on the temperature sensitivity of FBG sensor from cryogenic to room temperature. The effect of vibration on engineering structures caused by various phenomena can have serious consequences for their strength and safety. [PERSON] et al. [11] reported that vibration was considered one of the major concerns with respect to structural and mechanical failure, resulting in a significant reduction in the operational efficiency and life cycle of wind turbines. Adhesively bonded joints are commonly employed in automobile, aerospace and civil structures. The strength of joints and dynamic properties of the bonded structure can be severely weakened and affected by vibrating loads [12]. Blade fatigue induced by vibration is one of the most common causes of failures in gas turbines, reported to be up to 42% of total gas turbine failures [13]. Civil engineering structures such as high-rise buildings and suspension bridges are especially susceptible to the vibration due to the earthquake or high wind loads [14]. Vibrations not only result in the fatigue and failure of engineering structures, but also induce wear, tear and noise pollution [15]. To explore the vibrating behavior of structures, a reliable sensor for accurate measurement of the vibration is essential. Vibration sensors can be used for monitoring the conditions of rotating machinery, such as overheating, inadequate lubrication, excessive loading or bearing wear [16]. A variety of sensors are employed for the measurement of a vibrating structure. Among them, strain gauges can be used to measure either strain or vibrational amplitude [17], and a gyroscope was employed to determine rotation angle or angular velocity [18], while a laser vibrometer was used to detect bending and torsional displacements [19]. There is one major drawback for all these sensors.