ABSTRACT -- Fiber Bragg grating (FBG) sensors are one of the most exciting developments in the field of fiber optic sensors in recent years. FBG sensors have all advantages of fiber-optic sensors including immunity to electromagnetic interference, electrical safety, small size, and light weight. In work, we report two new types of gas and chemical sensors based on fiber Bragg gratings. The first type is an optical hydrogen sensor with an FBG coated with palladium thin film. The sensing mechanism in this device is based on mechanical stress that is induced in the palladium coating when it absorbs hydrogen. The stress in the palladium coating stretches and shifts the Bragg wavelength of the FBG. Hydrogen and thermal sensitivities of the sensors were calculated using a simple elastic model. The second type is a refractive index sensor based on an FBG and evanescent wave interaction. The sensing element of the device is the FBG whose cladding layer has been removed almost entirely. The interaction between the evanescent field and the sensed solution changes the propagation constant of the FBG. Changes in refractive index of the sensed solution are determined by measuring the device's Bragg wavelength shifts. The sensor was modeled as a three-region cylindrical waveguide, and the matrix method was used to study the sensor response to the refractive index of the sensed medium. In addition to its unique multiplexing capability, this sensor can be used in most corrosive environments.

KEYWORDS -- Fiber Bragg Gratings, Fiber-Optic Sensors, Chemical Sensors, Hydrogen Sensors