Study On General Demodulation System For FBG And Fiber F-P Sensor Based On DSP

With the development of infrastructures in China, health monitoring of those structures has become a hot point. As fiber optical sensors have more advantages over traditional ones, they have been studied widely. There are two typical ones, fiber Bragg grating (FBG) sensor and fiber Fabry-Perot (F-P) sensor. FBG sensor is often needed due to its wavelength-decoded characteristic and multiplexing ability, while fiber F-P sensor is utilized due to the advantages of high measurement accuracy, low cost and insensitivity of temperature fluctuation. Different interrogation units have been developed according to different sensing principle of those sensors at present. In many practical complicated sensing systems, those two types of fiber sensors may be both wanted, therefore a set of general demodulation system for those two sensors is demanded. The purpose of this thesis is to study a general demodulation method for FBG and fiber F-P sensors, and design relevant demodulation system. The main points of the thesis are as follows: Firstly, the technology characteristics of FBG and fiber F-P sensors are analyzed, and the principles of classical demodulation methods for them are investigated and compared. Then a conclusion is drawn that a general demodulation system can be realized based on tunable fiber F-P filter, and the setup of general demodulation system is put forwarded. Secondly, as the spectrum measurement precision of tunable fiber F-P filter was affected by its performance defect, a standard wavelength reference is required in the general demodulation system. The wavelength reference theory is then studied, and a single wavelength reference based on thermostatic FBG is presented. Further research shows that there is shortage of single wavelength reference, and a new wavelength reference scheme to increase spectrum measurement accuracy is put forwarded. With this scheme, the single wavelength reference can be easily converted to dual wavelength references. Therefore, an actual wavelength reference device is designed and manufactured. Other main optical components, such as tunable fiber F-P filter, the board band source, the photo detector and so on, are selected to meet the requirements of the designed system. Thirdly, analog circuits are designed to condition sensor's output signal and the methods of generating periodic driving signal for tunable fiber F-P filter are also analyzed. In order to sample two channels synchronously, the method of DA+DSP signal generating is adopted. The hardware based on DSP (TMS320VC5416) system is developed, and peripheral circuits of the DSP system are accomplished including FLASH, AD, DA, etc. In addition, communication interfaces, such as serial interface with PC and HPI interface with host processor, are also developed. Fourthly, the software for upper PC and DSP system are written. The main functions of the upper PC software (in C++ Builder language) are to send commands to DSP system, receive data form DSP system, and display, store results etc. As for DSP system, the program is used for sampling and processing data with high speed. Finally, temperature test of SLD driver is carried out, and the results show that SLD driver works well. In order to compare the performances of wavelength reference schemes, single wavelength and dual wavelength references, FBG temperature sensing experiment is carried out. It shows that measurement accuracy of dual wavelength reference is higher than that of the single one. Measuring experiments are also done on fiber F-P sensor with general demodulation system. All results demonstrated the feasibility of the general demodulation system, and it indicates a potential possibility of using the system in practical engineering applications.