Tilted Fiber Bragg Grating Sensing Demodulation Technology And Basal Study On Network

Fiber gratings, as novel photonic devices, make many complicated all-fiber communication network and sensor network executable and significantly broaden the scope of the application of optical fiber technology. Tilted fiber Bragg gratings (TFBGs), a kind of fiber gratings, have many unique advantages in addition to characteristics of ordinary fiber Bragg grating (FBG).In this dissertation, we mainly study on the sensing and demodulation characteristics of TFBG in theory and experiment. Based on the sensing characteristics of TFBG different from those of FBG, we carry out a more accurate estimate on the measurands using data fusion theory. In combination with sensor technology, embedded computing technology, modern communication and network technology, we apply FBG to the sensor network and design a cellular sensor network.With the subjects supported by the National High Technology Research and Development Program of China (Grant No. 2006AA01Z217), the National Natural Science Fund projects (Grant No. 60736039) and the Key Laboratory Fund of Optoelectronic Information Technical Science, Ministry of Education of China, the main contents of this dissertation are as following:1. Analysis methods of fiber gratings are presented and FBG and TFBG are analyzed respectively in theory using mode coupling method and volume current method. This is the base of future work and will provide theory foundation for experiment.2. Sensing characteristics and applications of TFBG are mainly studied. We have make progress including:(1) The wavelength shifts of core mode and cladding modes of TFBG are nearly consistent in temperature sensing experiment .Temperature sensitivity of TFBG can be improved by using different backing material of different thermal-expansion coefficient. (2) Strain can make the resonance peaks of TFBG core mode and cladding modes shift and the influences of strain to core mode and cladding modes of TFBG are different. A kind of system of measuring temperature and strain simultaneously is designed based on strain and temperature characteristics of TFBG.(3) A kind of method to measure bending radius using different modes of TFBG is presented for the first time by analyzing the bending of simple supported beam in theory. The bending of simple supported beam make the resonance peak wavelength of each mode of TFBG shift and resonance peak amplitude unchangeableness. So the wavelength of TFBG can be used to define the bending radius of simple supported beam. The response of different modes of TFBG are different. Temperature and bending can be measured simultaneously with a single TFBG based on this characteristic and temperature characteristic of TFBG.(4) In pure bending experiment of TFBG, it can be concluded that the resonance wavelengths of TFBG core mode and cladding modes are not sensitive to bending, resonance peak transmission power of cladding modes and curvature present linear relationship with different bending radius and transmission power of core mode and cladding modes of TFBG are not sensitive to temperature. Bending and temperature can be measured simultaneously using these characteristics to respectively measure core mode resonance wavelength and transmission power with a single TFBG.3. Demodulation and Multiplexing characteristics of fiber gratings are mainly studied. We have make progress including:(1) In some project application, it is more important to design a demodulation method which has the characteristics of simple structure and low cost than to design a demodulation method with a high resolution. We design a kind of demodulation system which is composed of double edges filter and has the advantages of low cost, quick response, easy operation and so on.(2) A FBG strain demodulation system based on TFBG, which is insensitive to temperature, is designed. When ambient temperature is changed, dynamic demodulation of FBG strain using a single TFBG without additional temperature compensation, which can eliminate the influence of temperature noise to strain signal, can be achieved. (3) There are many edges presenting linearity in transmission spectrum of high birefringence fiber loop mirror which can be used as edge filter groups. A multiplexing and demodulation system based on high birefringence fiber loop mirror, which has advantages of simply structure, wide linear edge and easy modulation.4. A strain measuring method based on measurands estimation data fusion is presented using many modes of TFBG in strain sensing experiment. Different modes of TFBG are used as different strain sensors in this method for the first time. This kind of method, which can get more accurate result than the method of calculating the arithmetic mean of limited sensors and can be used in sensors measuring system with normal distribution measuring result, has the advantages of easy calculation, easy program and high reliability.5. Common concept of sensor network is simply presented and a kind of celluar sensor network based on fiber grating is put forward. The celluar sensor network not only improves the multiplex ability but also change the simple sensor to be an intelligent sensor module while traditional fiber grating sensor network only considers its multiplex ability in design. This kind of sensor can measure several parameters simultaneously, improve the use factor of channel, communicate with other devices and also can ask to the center control computer. Queuing theory is used to analyze th system performance and calculate the average waiting time which can improve the reliability of the system.