## 1 Introduction  
Over the past years, fiber Bragg gratings have been used as sensors for structural health monitoring and smart structure applications. Compared to conventional strain sensors like resistance strain gauges or extensometers, FBG sensors have several advantages. Small size, light weight, immunity to electromagnetic fields and lightning, capability to be multiplexed in a single optical fiber, material integratability, and the ability to withstand high dynamic loadings, make FBG strain sensors especially suitable for structure monitoring when performance accuracy and reliability are of principle interest.  
FBG are optical wavelength filters written into an optical fiber by an UV laser. Using the phasemask technique, the index of refraction of the core of the fiber has been locally and periodically modified. From the light spectrum travelling in the fiber, a small spectral band will be reflected at the grating, while the rest of the light will be transmitted. The Bragg condition defines the centre wavelength \(\lambda_{\text{B}}\) of the small reflected band [1],  
\[\lambda_{\text{B}}=2n_{\text{eff}}\Lambda \tag{1}\]  
with \(n_{\text{eff}}\) effective index of refraction and \(\Lambda\) grating period. FBG are both sensitive to strain and temperature. The shift in Bragg wavelength due to strain and temperature is given by  
\[\Delta\lambda_{\text{B}}=2\left(\Lambda\frac{\partial n_{\text{eff}}}{ \partial l}+n_{\text{eff}}\frac{\partial\Lambda}{\partial l}\right)\Delta l+2 \left(\Lambda\frac{\partial n_{\text{eff}}}{\partial T}+n_{\text{eff}}\frac{ \partial\Lambda}{\partial T}\right)\Delta T \tag{2}\]  
with \(\Delta l\) change in grating length, and \(\Delta T\) change in temperature. A major advantage of FBG sensors is, additionally, that the sensor signal is encoded in the frequency domain and therefore is not affected by signal power fluctuations.  
FBG sensors in bare optical fibers are very brittle and, therefore, difficult to handle, especially for untrained users. Subsequently, producers offer FBGs integrated or applied to a carrier material in order to simplify the handling and installation of the sensors. These optical strain gauges are also named \"patches.\" Patches are available from different manufacturers and, depending on the patch design, their characteristics are different. In previous investigations [2], we compared the performance of several commercially available patches to their specifications in the datasheets. Although an international guideline for the use and testing of FBG-based optical strain sensors [3], exists and manufacturers are strongly recommended to use it, our experimental investigations showed deviations in the patch performance compared to the specified characteristics by the manufacturer. Additionally, we observed some of the surface-applied patches delaminated from the specimen during alternate bending tests at a flexural strain of 4 mm/m at a frequency of 0.5 Hz, as shown in Figure 1.  
Figure 1: Alternate bending test with FBG-based patches. The optical signal of patch 1 decays at a flexural strain of 4 mm/m. The patch started to delaminate from the specimen.  
For this reason, it was necessary to develop a FBG patch with reliable and long-term stable mechanical and spectral properties. However, a carrier material glass-fiber-reinforced plastic (gfrp) was chosen so that the manufacturing technique could be easily adapted for the integration of optical fibers into composite structures.