Now I am studying a bit of optical fibers. And of course generally optical fiber works because of total reflection property of the light, which happens when the light from higher medium hits the lower medium material. However I recently found out that there also is a thing 'Hollow optical fiber' which the core is hollowed out, filled with an air. I think the propagation should not happen cuz total reflection won't work here, but it works perfectly fine. I've hit some papers and got some keywords like 'band gap waveguide', 'photonic crystal' etc... but still can not understand it. Can you give me a brief explanation, or just let me know where should I start with first?