1 Effect of Epoxy Bonding on Strain Sensitivity and Spectral Behavior of Reflected Bragg Wavelength Abstract—We present the effect of epoxy bonding on strain sensitivity of fiber grating sensors. We show experimentally that, on average, sensors mounted onto aluminum plates by epoxy gluing both ends of the Bragg grating (epoxy free grating zone) display 16% more sensitivity to applied strain compared to those with epoxy covering the length of the grating zone (conventional method). We also show that in the conventional method, the epoxy layer contracts upon curing, forming an additional buffer that extends the breaking point of the sensor. Finally, adopting the conventional approach will also result in both spectral broadening and reduction in peak power of the reflected Bragg wavelength caused by the non-linearly distributed axial strain. Index Terms—Fiber Bragg grating (FBG), fiber optics, strain measurements, fiber sensor. I. INTRODUCTION ibre Bragg gratings (FBG) were first utilised for sensing purposes in the late 80s [1-3]. Since then, the technology has undergone rapid developments. These sensors find applications in aerospace, marine, oil and gas, composites and smart structures, to name a few [2]. In addition, the ability offered by photonic based solutions for continuous monitoring of structural health in a non-destructive manner has become widely recognised and accepted [3]. The basic sensing principle commonly used in an FBG based sensor system is by monitoring the shift in wavelength of the returned (reflected) Bragg signal with the changes in the measured strain, temperature or pressure. With such a device, injecting spectrally broad laser will result in a reflected narrowband spectral component at the Bragg wavelength. Any changes in fibre properties, such as strain, temperature, or polarization which varies the modal index or grating pitch, will change the Bragg wavelength [4]. For strain measurement, there are two common techniques for surface-bonding of an FBG strain sensor. The conventional practice is to distribute cement/epoxy uniformly along the fibre grating which was demonstrated in [3,5]. These methods provide a protective coating and housing for fibre sensors that find use in harsh environments [3]. FBG sensors fabricated by this method show poor calibration accuracy and repeatability compared to commonly used resistance strain sensors as the former is not a point sensor [5]. Recently, Hwang et. al. [6] proposed an improvement to surface bonding for a mounted fibre grating strain sensor and verified the sensor’s performance via simulation work. Their work suggested that applying cement/epoxy at both ends of a grating zone, instead of covering the length of the grating, provides uniform distribution of average strain over the sensing medium. This paper provides experimental proof to these observations. We show the difference in strain characteristics for FBG sensors mounted using the two aforementioned bonding methods. For this work, FBG sensors mounted with epoxy adhesive only at both ends of the FBG (without covering the grating zone) will be referred to as Method I and the more conventional method of applying epoxy along the length of the sensing medium, which is called Method II. We observe that Method I displays higher sensitivity to applied strain, while preserving the shape and peak power of the narrowband reflected Bragg spectra. However, as expected, the sensors break at lower applied strain values to that of Method II. We believe that this is due to the added robustness provided by the epoxy surrounding the sensing medium in the case of Method II. II. METHODOLOGY A. FBG Strain Sensor Mounting and Sample Preparation. It is necessary to develop a suitable encapsulation or mounting technique for the FBG sensor [3]. To this end, an aluminium plate measuring 140 mm long x30 mm wide x1.5 mm thick was designed and fabricated. A number of FBG sensors were prepared and mounted along the long axis of the plate. M.R.A. Hassan, N. Tamchek, M.A. Ismail, T.F. Izam, A. F. Abas, R.M. Johar, S.S Chong and F.R. Mahamd Adikan F