Research On Tapered Fiber Grating Strain Sensor And Its Demodulation Technology

The study of the strain and temperature sensing technology is the most important part of the fields of smart material and structure and health monitoring of large civil infrastructure. Because of their special characters, optical fiber grating sensors have been considered the critical sensing components in these two fields. With the development of the optical communication technology in past ten years, optical fiber Bragg grating (FBG) sensors have progressed very rapidly, many research works, such as the fiber grating fabricating technology, signal demodulating technology, have been made a great progress, and the application fields of the optical fiber grating sensor have grown wider and wider.However, there are many problems which still limit the practice application of optical fiber grating sensing technology, such as the problem which is called cross sensitivity of strain and temperature of FBG, the wavelength demodulating equipments for FBG sensors are usually complex and expensive, and so on. How to solve these problems has been the key to ensure the sensing properties and make FBG sensors advancing in practical applications. It must show great academic value and practical importance.Through collecting and studying of many documents about FBG sensing technology, this thesis makes studies of the transmitting theory, the sensing principle, and the interrogation technology of FBG sensors. Then, a novel optical fiber grating strain sensor which can be used to solve the problem of cross sensitivity is proposed, the sensing property of this kind of strain sensor and its signal demodulation system are studied by experiment. The main innovative contributions of this thesis are as follows:On the basis of the studies of tapered fiber grating sensing principle and its characters, a strain sensor which is based on the measurement of reflected bandwidth of tapered fiber grating is proposed. This strain sensor still retains the wavelength modulation characteristic of the optical fiber grating, and it possesses characteristics as simple structure, easy to achieve. The results of the experiment have shown that, the reflected bandwidth of this strain sensor has a strict linear relationship with the stress and strain, but it is insensitive to the temperature in a wide changing range. It has also been demonstrated that this kind of strain sensor can be used in distributed sensing applications. So it must have great value to strain sensing in fields of smart structure and health monitoring of large civil infrastructure.On the basis of analysis of the characteristics of the reflected bandwidth, several methods to demodulate the reflected bandwidth of the tapered fiber grating are proposed. Because the bandwidth of the reflected light will be modulated only by strain, it is insensitive to the change of temperature. By demodulating both reflected bandwidth and the center reflected wavelength, this tapered fiber grating sensor could be used for simultaneous measurement of the strain and temperature. Compared with the method which is based on the principle of dual-wavelength shifted by two optical fiber gratings, this sensing method has so many advantages, such as simpler system structure, lower cost, easy to achieve, and so on.During the course of interrogation of the sensing system, a signal pre-procession method called threshold limiting method is used to promote the demodulation accuracy. That is, only the data which are higher than a certain value will be transmitted into the demodulation system to be processed, and the others which are lower will be eliminated, this certain value is called threshold value. By adopting this method in data processing, the experimental result shows that the influence of the side pulses in reflected light on measurement result is eliminated, and the processing amount of sensing data is greatly decreased. So, the demodulating speed and the measurement precision of this sensing system would be approved simultaneously. All of these advantages are very important to the tapered fiber grating sensors when they are used in distributed sensing applications.The algorithm to solve the synthesis problem of the optical fiber grating has also been studied, and two methods to reconstruct the physical parameters of FBG from the target reflected spectrum are proposed. These two methods are based on the same principle to transform the synthesis problem of FBG into a global optimization problem, and then solve this optimization problem by simulated annealing algorithm and modified genetic algorithm respectively. The presented numerical examples show that these two methods all can give results that are comparable or better than other design methods.