Simultaneous Measurement Technique Of Strain And Temperature Based On Fluorescence Fiber Gratings

As a kind of important photonics components, fiber gratings are wildly used in optical communication and optical sensing area. Variant kinds of purposes and applied environments need variety of kinds of fiber gratings, so that a series of fabrication methods and a number of demodulation methods of fiber gratings are developed. Fiber grating sensors are important parts of smart structures or materials, because it can be used to measure many physical or chemical measurands such as strain, temperature, press, movements and concentration. On the other hands, the multi-sensing of the fiber grating sensors suffered from the cross-talk of the different parameters, especially, the cross-talk of temperature and strain. The strain measurement is the most important applied area of fiber grating sensors, and the temperature is an important factor could not be ignored. In order to realize simultaneous measurement of strain and temperature, a series of technologies are developed.Fluorescence fiber sensors are broadly used to measure temperature, concentration, and pH value etc. The fluorescence sensing systems based on different principles, namely, fluorescence intensity, fluorescence intensity ratio, and fluorescence lifetime. The fluorescence lifetime is an effective parameter for sensing purpose, because it is independent on the intensity of the pumping source and does not need expensive narrow-band filter.Fluorescence fiber gratings, as a new type of photonics components, are attempted to realize the simultaneous measurement of strain and temperature. The fluorescence fiber gratings have the advantages of fiber gratings and fiber fluorescence sensors. Some theoretical and practical studies are conducted below. (1) Based on the theoretical analyses of those common fiber gratings, the light-transmitting theory of the fluorescence fiber gratings is discussed. Especially, a new analytic method, the fluorescence superposition method, is introduced to describe the character of a kind of the fluorescence fiber gratings, the coalescent fluorescence fiber grating. Some simulation results are obtained.(2) After summarizing the fabrication methods of different kinds of fiber gratings, the fabrication methods of the fluorescence fiber gratings are proposed as fabricating FBG in hydrogen-loading fluorescence fibers or connecting FBG and fluorescence fibers. Some samples are produced to measure strain and temperature at the same time.(3) The fluorescence lifetime is an important sensing parameter of the fluorescence fiber sensors. A new estimating method based the fast Fourier transform (FFT method) is introduced. This method has a series of advantages, such as high processing speed, high accuracy and immunity from the floor signal. To overcome the decline of the fluorescence decay signal, a correcting method is introduced. To assist the correct of the signal, a high-speed and stable estimating method, weighed log-fit method is proposed to replace the Levenburg-Marquardt method, the standard nonlinear estimating method.(4) An experimental system has been established. Some samples with different structures and contents are produced to get the relationship of strain and temperature vs. wavelength and fluorescence lifetime. Linear and non-linear fitting methods are introduced with the analyses of fitting deviations. The testing results show that the fluorescence fiber gratings can be used to measure the strain and temperature simultaneously.