The Improved Bragg Grating Chemical Sensor With Hydrogel As Sensitive Element

Fiber Bragg grating (FBG) is one of the fiber positive components, which grows up fastly and is widely used in optical communication and optical sensing applications. FBG is a simple, intrinsic sensing element and has all the advantages normally attributed to fiber sensors. The sensitivity of FBG to temperature and strain is exploited to measure a desired environment effect. Because FBG can be operated in a wavelength-coding manner, it has become an important component for optical sensing devices in measuring temperature and strains.Hydrogels are crosslinked polymers which swell to an appreciable extent in water, and are considered as biocompatible materials. The water content depends on the polymer structure, and can be made responsive to environmental factors such as inorganic salt concentration and pH value. Many research efforts have been done to study volume changes in response to different environmental factors, and develop sensors based on hydrogels for pH, soil water potentials and other chemical parameters.In this thesis, a novel FBG sensor for measuring salinity using a water swellable hydrogel as the active sensing component is proposed and developed. The sensing mechanism is based on the stress that is induced in the hydrogel coating due to its volume changing with the external environment. The stress shifts the Bragg wavelength of the FBG. By modifying the hydrogel chemistry, parameters other than salinity can be detected using the same basic approach. We etch the FBG to improve the sensitivity of FBG sensor. Because the diameter of fiber is minified, the tension tuning force can be lowered greatly, the same stress induced in hydrogels can shift the wavelength of FBG larger than that for the standard 125 u m diameter fiber without etched. Therefore, the etched fiber Bragg grating sensor based on the hydrogels becomes more sensitive and its tuning scale is also enlarged.