TUTORIAL - THE BANDGAP EFFECT
Photonic bandgap fibers are based on physical mechanisms fundamentally different from the M-TIR guiding fibers. The bandgap effect can be found in nature, where the beautiful and bright colors that are seen in butterfly wings are the result of naturally occurring periodic microstructures. The SEM picture at the bottom shows the microstructure on a butterfly wing. The structure size is in the order of a few microns.
The periodic microstructure in the butterfly wing as well as in PBG fibers results in a so called photonic bandgab, where light in certain wavelength regions cannot propagate. In the butterfly wing this light is reflected back, and is seen as the bright colors. In a PBG fiber, the core is created by introducing a defect in the PBG structure (e.g. an extra air hole), thereby creating an area where the light can propagate. As the light can only propagate at the defect region, a low index guiding core has been created. This is not possible in standard fibers, and the low index guiding of PBG fibers therefore opens a whole new set of possibilities. In this way, it is possible to guide light in air, vacuum or any gas compatible with the fiber material.
