Investigation On Fabrication And Sensing Applications Of Fiber Bragg Gratings

Optical fiber grating sensing technology has become one of the promising technologies in sensing area in recent years. Fiber grating sensors, based on the measurement of wavelength, an absolute physical parameter, possess many advantages such as high sensitivity, compact size, immunity to electromagnetic interference, capability for sensor networking and so on. This thesis studies the optical fiber grating applications in many areas, such as civil structures, oil and power industry, aerospace systems, seismology, ocean survey, medical instrument, and chemical and biological industry. This thesis studies the principle, mechanism, fabrication and sensing applications of fiber Bragg gratings (FBGs), analyzes several conventional fiber grating fabrication methods and expatiates on the fabrication system put up by ourselves. FBG-based devices for tilt measurement, strain sensing applications and acceleration sensing applications have been analyzed and studied in detail. The problem of cross sensitivity of temperature and strain in measurements has been discussed.FBGs were fabricated by using phase-mask method with a frequency-doubled argon laser. Long-period gratings (LPGs) were fabricated by using point-by-point inscription technique with a CO2 laser. We studied the characteristics of LPGs and FBGs based on the transfer matrix method and coupled mode theory. Temperature and strain (including the axial strain and transversal load) sensing characters of the FBG were experimentally studied.Two single FBG-based simple sensor head designs were proposed and experimentally demonstrated for simultaneous measurement of strain and temperature. The first is formed by encapsulating half of a FBG into a steel cannula and fixing them with epoxy glue. Bragg wavelengths of the two FBG halves responded differently to the applied strain and temperature due to their different equivalent Young's moduli and coefficients of thermal expansion, so that strain and temperature can be measured simultaneously. The second design is realized by chemical etching of half of the FBG to make it obviously thinner than another half. The initial single Bragg reflection peak of the FBG splits into two, whose wavelengths are sensitive to both force and temperature, but the separation of their wavelengths is determined only by the applied force. As a result, simultaneous measurement of force and temperature are realized by measuring wavelengths of the two peaks. We designed a new suspension type fiber Bragg grating (FBG) tilt sensor consisting of a weight hanged to a circular plate through four equal-long optical fibers on which FBGs were inscripted in-between. Bonding points of the fibers on the brim of the circular plate were separated equally so that each pair of FBGs with the opposite position can sense the inclination in the plane they belong. It can be used to detect the magnitude as well as the direction of a two-dimensional inclination. The temperature effect can be eliminated completely without need of additional temperature compensation schemes.A novel accelerometer based on a strain-chirped FBG was proposed and experimentally studied. The FBG was glued in a slanted direction onto a right-angled triangle cantilever beam's lateral side, and a mass was bonded on its free end. Vertical acceleration applied to the cantilever beam leads to a uniform bending along the beam length. As a result, the FBG is chirped and its reflection bandwidth changes linearly with the applied acceleration. The accelerometer is temperature-independent because temperature only changes the central Bragg wavelength of the FBG, but not the reflective bandwidth.An intensity-modulated optical fiber Bragg grating (FBG) acceleration sensor incorporating a tapered fiber is proposed and experimentally demonstrated. The sensing mechanism is based on the measurement of the FBG-reflected optical power, which is changed with the applied vertical acceleration because acceleration-induced fiber bending in the tapered area introduces variable insertion loss. Vibration measurement has also been carried out. The power detection method reduces the cost and complexity of the sensor system.