Research Of Multi-wavelength Filter Based On Tunable Phase Shift In A Linearly Chirped Fiber Bragg Grating

A phase-shifted fiber Bragg grating is formed when some phase shifts are inserted in a uniform fiber Bragg grating (FBG). Compared to uniform FBG, phase-shifted FBG can open one or more narrowband transmission windows inside the stop band of the reflection spectrum, thus it can be used as a transmission type filter. Furthermore, the position of transmission window can be adjusted by changing the amount of phase shift, also both the transmission and bandwidth of the window can be changed by adjusting the position of the phase shift. Thus according to the actual requirements, some transmission peaks with different bandwidth and different roll-off characteristics can be obtained by designing the number of phase shift, position and the amount of the phase shift reasonably. And phase-shifted FBG also has the advantage of low cost and low insertion loss and is not relevant to the polarization state. Based on these advantages, phase-shifted FBG has been widely used in the fields such as narrow-band filter, the fiber lasers, Wavelength division multiplexer, all optical logic devices, and fiber sensors etc.However, a chirped phase-shifted FBG is formed when some phase shifts are inserted in a chirped FBG, so it combines some important properties of chirped grating and phase-shift grating. Compared to uniform phase-shifted FBG, both the reflection bandwidth and transmission windows of a chirped phase-shifted FBG can be broadened in a wider range, it is observed that chirped phase-shifted FBG is more suitable for multichannel filtering. In this paper, based on the excellent filter characteristic of chirped phase-shifted FBG, a multi-wavelength filter that the wavelength and bandwidth can be tuned is designed by inserting some tunable phase shifts in a linearly chirped FBG The filter can be used to implement more flexible wavelength selection in actual application. The primary contents and achievements of this dissertation include are list as follows:1. Based on the orthogonality of the rules optical waveguide mode, the coupled-mode equation of phase-shifted FBG was derived in detail. For three kinds of analysis method of phase-shifted fiber grating, the transfer matrix method, multilayer membrane method and V-I transfer matrix method, carried out the detailed comparison and error analysis. The result shows that Ⅴ-Ⅰ transfer matrix method could greatly improve the computational efficiency under the premise of ensuring the accuracy of analysis. 2. V-I transfer matrix method was applied to analyze transmission spectrum of chirped phase-shifted FBG and uniform phase-shifted FBG. After comparative analysis, some regular characteristics between these two kinds of structure of grating were summarized at different phase shift parameter and different grating parameter, which could provide the basis for research of new optical devices based on phase-shifted FBG3. A temporary and stable phase shift in a linearly chirped FBG based on piezoelectric transducer (PZT) is experimentally demonstrated. V-I transfer matrix method was applied to analyze the relationship between the separation of induced phase shift positions, chirp coefficient and the wavelength separation of the double notch filters, and two linear regression equations to express these two relationships were obtained. According to the two linear regression equations and using PZTs to induce controllable phase shifts, a flexible multi wavelength filter can be obtained by designing phase shift, position and chirp coefficient reasonably.