## 1 Introduction  
The composite laminates have extraordinary mechanical properties such as high strength, light weight, strong corrosion resistance and low temperature distortion [1]. Thus, the application of composite laminates in lieu of traditional alloys has significantly increased in aircraft manufacturing [2]. Specifically, carbon fiber reinforced polymer (CFRP) laminates have been widely used in the manufacture of the wing skin. The CFRP laminates are made of a number of carbon-fiber fabrics impregnated with polymer. The fabrics are stacked together in specified orientations and formed at a high temperature and a high pressure. The transverse damage resistance of CFRP laminates is weak due to the lack of through-thickness reinforcement [3]. The damage mechanism of the CFRP wing skin is much more complex compared with the traditional alloy skin [4; 5]. Strain testing is important for composite structural health monitoring [6]. Once wing skin strain under different loads is measured accurately, aircraft designers and engineers can obtain its ultimate limit states, which is important for reducing the risk of composite structural failures. Therefore, the accurate strain measurement for CFRP laminates is of great importance [7; 8; 9; 10]. Fiber Bragg grating (FBG) sensors, a type of optical fiber sensor, have been recognized as ideal choices for aviation structure strain measurement [11]. FBG sensors have many advantages such as their small size, high sensitivity, multiplexing capability and immunity to electromagnetic interference [12; 13; 14]. FBG is made by inscribing grating in an optical fiber core. The refractive index of the grating experiences a periodic modulation, which makes the fiber act as a band-pass filter. When lights of different wavelength are launched into the fiber, the grating reflects the light of one particular wavelength, named the Bragg wavelength. The reflection wavelength of the FBG sensor is determined by the Bragg condition [15]. When the diffraction order is equal to 1, the Bragg condition can be described as follows:  
\[\lambda_{\text{B}}=2n_{\text{eff}}\Lambda \tag{1}\]  
where \(n_{\text{eff}}\) is the effective refractive index of the fiber core and \(\Lambda\) is the grating period. The grating period represents the geometrical distance between two consecutive repeated alterations of refractive index in a Bragg grating [16]. The Bragg wavelength can be detected by an FBG interrogator [17]. A scanning diode laser combined with a photoelectric detector is a common design for a standard commercial interrogator. A tunable narrow band light is generated by a diode laser. The wavelength of the light is adjusted by a scanning voltage, which changes periodically. The light is launched into the optical fiber. The light with Bragg wavelength can be reflected by FBG sensors, and then detected by the photoelectric detector. At the moment, the scanning voltage is recorded, and the Bragg wavelength can be obtained according to relationship between the scanning voltage and the wavelength of the light. FBG sensors are sensitive simultaneously to both temperature and strain changes [18]. When environmental temperature varies, the reflection wavelength of an FBG sensor will also change. The relationship between the reflection wavelength shift, represented by \(\Delta\lambda_{\text{T}}\), and the temperature variation, represented by \(\Delta T\), can be described [19] according to  
\[\Delta\lambda_{\text{T}}=\lambda_{\text{B}}(\alpha+\zeta)\Delta T \tag{2}\]  
where \(\alpha\) and \(\zeta\) represent the thermal-expansion and thermo-optical coefficients, respectively. When axial strain is applied to the optical fiber, the wavelength shift of the FBG sensor, represented by \(\Delta\lambda_{\varepsilon}\), can be calculated [20; 21] according to  
\[\Delta\lambda_{\varepsilon}=\lambda_{\text{B}}\{1-\frac{n_{\text{eff}}^{2}[p_{ 11}-v(p_{11}+p_{12})]}{2}\}\varepsilon_{\text{ms}} \tag{3}\]  
where \(\varepsilon_{\text{ms}}\) represents the axial strain and \(\
u\) is the Poisson's ratio.