Study On The Long-distance Fiber Bragg Grating Humidity Sensor

Fiber Bragg grating has been used as optical sensors for many outstanding advantages such as anti-electromagnetic interference, low requirement on the environmental, and powerful ability to multiplex. It is widely used as physical, chemical and biological sensing devices. Using its filter characteristics to design the fiber ring laser, these two characteristics are combined together to constitute a laser sensor system which has broad application prospects in the long distance quasi-distributed sensing. The basic principles and design methods of the optical humidity sensor and its application in long-distance sensing are theoretically and experimentally discussed in detail this thesis. The main content and the obtained results are as follows:1. The characteristics of the fiber Bragg grating(FBG) have been studied. Base on the coupled mode theory, we obtain the Bragg condition of the FBG and the reflection spectrum characteristics. Meanwhile the reflection spectrum of FBG is numerically simulated, and FBG filter characteristics and wavelength tunable characteristics are discussed. In this paper, the sensing properties of the FBG strain, temperature and refractive index are experimentally demonstrated, and exploratory research on the refractive index sensing are also studied, which laid foundation for the design of FBG humidity sensor.2. The humidity sensors coated with humidity sensitive material on the side of FBG is designed based on the characteristics of the humidity sensitive material strain changes with the FBG, and the production process of the sensors, response time and humidity sensitivity characteristics of different film thickness sensors are studied in detail, which lay a foundation for the subsequent experimental design performance sensors. In the paper a conclusion that the humidity sensitive material could be measured by use of FBG, according to the measurement of the coefficient moisture expansion of the humidity sensitive material making use of FBG is obtained. 3. From the practical application point of view, high signal to noise ratio,narrow bandwidth light source must be used for long-distance, quasi-distributed sensing measurements in order to increase the number of multiplexing and transmission distance of the sensor. In the paper we design a kind of fiber laser structure based on SOA, which can provide high-noise ratio, the bandwidth of the laser output, and ideal source for long distance humidity sensor.4. The SOA-based active fiber Bragg grating humidity sensor has been studied, and a sensing system accurately measuring the changes in ambient humidity is designed and tested experimentally. According to the temperature compensation mechanism, making using of the temperature compensation sensor which is not sensitive to moisture for temperature calibration, the measured temperature coefficient of the humidity sensor is9.86×10-6/℃, and achieve less than the measurement±5%RH in the measuring range of6.6%RH-97%RH with a response time of less than8s.