Photonic bandgap (hollow core) fibers guide light in a hollow core that is surrounded by a microstructured cladding. Photonic bandgaps can form in materials that have a periodically structured refractive index; in Photonic Crystal Fibers (PCFs) this is achieved by using a periodic arrangement of air holes in silica. These fibers are sold based on the overall optical specifications and not the physical structure.
Like conventional fibers, solid-core PCFs guide light by Total Internal Reflection(TIR) at the boundary between a low index caldding and a high index core.In most all-solid fibers,the required index difference is created by doping either the core or the cladding glass
Polarization-Maintaining (PM) photonic crystal fibers that incorporate a non-circular core in combination with the large refractive index step between air and glass; this creates strong form birefringence. The result can be a shorter beat length that reduces the bend-induced coupling between polarization states compared with conventional PM fibers, and a much reduced thermal sensitivity of birefringence.