Re: How does Bragg reflections occurs through propgation in
Well, the defect core, which has a lower refractive index, operates like a cavity for the wavelengths into the photonic bandgap regime. In normal conditions (without the photonic crystal structure in the fiber), light can not propagate through the core because it has a lower refractive index than the cladding.
Photonic crystals are periodic structures that do not allow certain wavelenghts to propagate through them, working like reflectors. Bragg reflectors are almost the same thing. So, if light gets into the core, it is trapped into it because it reflects to the photonic crystal "walls"(bandgap effect), and so it propagates through core even if core has a lower refractive index than cladding.
Hope I explained well.