Study Of Fluorescence Optical Fiber Temperature Measuring System Based On Hilbert-Huang Transform

The technology of fluorescent optical fiber temperature measurement has attracted more and more attention and shows powerful vitality in recent years. The application of the technology is very broad, ranging from simple temperature measurement to high temperature,strong electromagnetic interference and other extreme environments, and it can realize the real-time temperature measuring to great precision. This is principally because optical fiber temperature sensor has good insulation, anti-electromagnetic interference and high reliability,etc.First of all, the research status at home and abroad are analyzed and the basic principle and characteristics of fluorescence optical fiber temperature measurement technology are introduced. Starting from the production mechanism of fluorescence optical fiber temperature sensor, this paper discussed the phenomenon of photo-luminescence, fluorescent light-emitting mechanism and spectral characteristics. For the sake of research, three different types of fluorescent materials are introduced, which are the solid laser crystal and rare earth ions doped materials. Then we have studied the temperature property of fluorescent materials,including the attenuation characteristic of the fluorescence intensity, the principle of measuring temperature based on fluorescent lifetime, temperature independence of fluorescent lifetime, and determined the method of of fluorescent lifetime was used to measure the temperature.Secondly, The overall design of the fluorescence optical fiber temperature measurement system is studied, respectively on the part of the system in detail. For the working principle of excitation light source, working life and driving mode considered, LED was selected as the excitation light source because of high luminous efficiency, easy modulation and cheaper. In the process of fluorescence signal processing, according to the processing sequence, the various stages of the device and circuit design are analyzed in detail, including the photoelectric detector, signal amplification and filter circuit design, and the types of noise contained in the measured signal, and some simple and effective methods were proposed to restrain or eliminate the noise. Then pulse and phase modulation method, two fluorescent lifetime test methods, and the two point method, the integral area ratio method and weighted logarithm, three different kinds of fluorescence data fitting methods, are elaborated. Then the way of fluorescence signal processing based on Hilbert-Huang transform(HHT) was presented.In this paper, an improved HHT is present. This method absolutely eliminates the endpoint effect which happened in the process of empirical mode decomposition while using the HHT algorithm. In detail, the endpoints of original signal are expanded by the method of mirror extension, then the signal is turned into a ring without any endpoint. After that, onepart of the IMFs which produced during the process of EMD are the needful. The position they come from just was the signal’s place in the ring. So the phenomenon of wing in the IMFs is avoided, and it is because that there is no extreme value in the endpoint of IMFs.Eventually, the second step of HHT is Hilbert transform, and we will get the Hilbert spectrum.Finally, experimental investigation shows that, the fluorescent signal was decomposed into a collection of IMF by empirical mode decomposition, then Hilbert spectral analysis was performed for each of the IMF. According to the difference of signal and noise characteristics,HHT can generate adaptive modal functions and remove the noise from signal effectively. At the same time, using this method is simple, fast and better resolution. Furthermore, in different temperature range and interval, the difference between measured data and the actual temperature is no more than 0.1 ℃, which proved that the fluorescence optical fiber temperature measuring system structure design is reasonable, the scheme is feasible,meanwhile possesses the advantages of good stability and high precision.