Multiplexible gas and chemical sensors based on fiber Bragg and diffraction gratings.

摘要

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 addition, the grating-based sensors have a built-in self-reference and an excellent multiplexing capability. Most of the FBG sensors reported in the literature are designed for distributed strain and temperature measurements. The challenge, however, is to design and implement FBGs as gas and chemical sensors. In this thesis, we have developed 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. Using FBGs with different wavelengths, many such hydrogen sensors can be multiplexed on a single fiber. Hydrogen and thermal sensitivities of the sensors were calculated using a simple elastic model. Additionally, to quantify the amount of stress in the palladium film as a function of hydrogen concentration, a novel and very sensitive method was devised and used to detect deflections in a palladium-coated cantilever using an evanescent microwave probe. This stress was about three times larger than that found in the bulk palladium for the same range of hydrogen concentrations.;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. The sensor was fabricated using a two-step active-monitoring etching technique, In addition to its unique multiplexing capability, this sensor can be used in most corrosive environments.;Furthermore, dispersion characteristics of multimode fiber optics coupled with a metal diffraction grating are investigated. Two coupling configurations were used. In the first configuration, the metal grating is physically contacted with the exposed fiber core. In the second configuration, the fiber is situated directly above the grating at a certain angle. In both cases, the fiber optics itself acts as a narrow slit permitting only a certain range of the wavelengths that are reflected back by the grating to be intercepted by the fiber optics. The spectra of the intercepted light are asymmetric with a long tail on the shorter-wavelength regions. These spectra are explained by using the well-known diffraction equation and fiber optics characteristics. Applications of these gratings-coupled devices include narrowband filters and chemical or gas sensors.