Theoretical Analysis And Experimental Investigation On Phase-shifted Fiber Grating

Fiber Bragg grating(FBG) sensing systems have a number of advantages, such as high sensitivity, easily multiplexing capability, miniature size, and so on, When the fiber grating is used to detect weak dynamic signals, the wavelength shift of FBG needs to be changed to phase shift utilizing a fiber interferometer, and makes the 3 d B linewidth of the reflection spectrum of FBG become the main factor which influences the performance of the whole sensing system. To narrow the linewidth of the spectrum need to increase the length of the FBG, or decrease the exposure of ultraviolet, which will increase the dimension of the sensor head as well as the insert loss.By inducing a phase shift in the exposing area of the fiber grating, a rather narrow window is introduced to the reflection spectrum as well as the transmission spectrum, which can be extracted out to be a novel FBG with narrow linewidth through optical filtering. When it is influenced by the external disturbance, there will be a wavelength shift, which means that it could be used to design interferometric fiber grating sensing system. This method can provide a probability to decrease the noise floor and improve the stability of the sensing system, while the dimension of the sensor and the insert loss is kept to be a certain level.This thesis mainly did research on the interferometric phase-shifted fiber grating sensing system. The transmission spectrum of the phase-shifted FBG is analyzed theoretically by treated as a Fabry-Perot cavity, and the influences of the manufacture parameters to the location, depth and width of the window are simulated. Phase-shifted FBG with different characteristic parameters are produced through the shielding method. Several interferometric phase-shifted fiber grating sensing systems are integrated designed and tested.The main and innovatory results are as follow.1. The reflection spectrum with 3 dB linewidth as narrow as 0.02 nm are got through the perfect matching of two phase-shifted FBGs, which makes the length of the optical path difference of the demodulation interferometer can be increased greatly, and get a noise floor as low as-88dB/ @1kHz.2. The phase-shifted FBGs are coated by the parallel packaging method, and the noise floor of the sensing system is tested when the signals are loaded. The experiments show that the signals will not mismatch the two phase-shifted FBGs, neither increase the noise floor of the system.