| Popis: |
A periodic multi-layer thin film stack, which behaves as one-dimensional photonic crystal, exhibits "superprism effect", or high spatial dispersion, which can be used for wavelength multiplexing and demultiplexing. For application purposes, thin-film stacks of multi-cavity Gires-Tournois structure are more suitable to design linear spatial shifts than periodic multi-layer structures. Using the modified design procedure for all-pass filters, a mathematical model of the spatial dispersion with normalized frequency is set up to simulate the designed spatial dispersion at given wavelength band. As a simulated result, the all-pass polynomial coefficients, and reflectivity coefficients of reflectors are obtained. According to the calculation based on resonate theory, the spatial dispersion of the designed structure is in accordance with the required dispersion. Then, this structure is implemented in thin film stacks, with layers of different periodical numbers as different reflectors and materials of high or low refractive index as cavity materials. Adjusting the cavity lengths also performs optimization. In our design, a linear spatial shift of 25μm caused by dispersion is designed at 850nm and 1550nm, and implemented in thin film stacks. Comparisons are made between designs of different center wavelength, different cavity lengths, and different number of cavities. |
