4.1 Conclusion
In this thesis, we have designed and fabricated multiple phase-shifted long-period fiber gratings (LPGs). We modulated the refractive index of the fiber core periodically with the point-by-point UV sequential writing method. Considering the values of the induced refractive index change and the beam size, we chose the coupling cladding mode LP08 and the grating period of 260 nm as the main working regime. We fabricated a variety of uniform LPGs with different exposure times to determine the induced refractive index changes.
Together with the coupled-mode theory, we got the calibrated results for the fiber core refractive index change versus wavelength and the modulated refractive index change versus the exposure time. After the calibration, we used the single phase-shifted, double phase-shifted and multi-phase-shifted LPGs to check our calibrated results. Then we used the verified parameters and the Evolutionary Programming (EP) algorithm to design the optimal coupling coefficient distribution function for flat-band LPGs. By controlling the exposure time and the phase shift between each uniform LPGs, in principle the flat-band LPGs can be constructed by the sequential point-by-point writing method.
When comparing our preliminary experimental result of the flat-band LPG with the simulated one, they are quite different at the first sight. We found that this is mainly caused by
the difference between the fabricated and simulated coupling coefficient distribution functions.
From our analysis, we also identified that the main error should be caused by the difference in the fringe visibility of the index change between the fabricated coupling coefficient distribution function and the simulated one.
4.2 Future Work
The final goal of our study is to fabricate flat-band LPGs in order to use them as filters or mode converters in optical communication. To achieve this goal, there are still some problems remained to be solved. Because of the difference between the simulated and the experimental results as explained before, we still have to fabricate a new flat-band LPG to justify our design and analysis. After solving these problems, we can then use our verified design and fabrication capability to fabricate more complicated LPG devices for different applications.
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