Operation Modes of Photonic Crystal Fiber

   Photonic crystal fibers can be divided into two modes of operation, according to their mechanism for confinement. Those with a solid core, or a core with a higher average index than the micro structured cladding, can operate on the same index-guiding principle as conventional optic fiber cable— however, they can have a much higher effective- refractive index contrast between core and cladding, and therefore can have much stronger confinement for applications in nonlinear optical devices, polarization-maintaining fibers, (or they can also be made with much lower effective index contrast). Alternatively, one can create a "photonic band gap" fiber, in which the light is confined by a photonic band gap created by the macrostructure cladding – such a band gap, properly designed, can confine light in a lower-index core and even a hollow (air) core. Band gap fibers with hollow cores can potentially circumvent limits imposed by available materials, for example to create fibers that guide light in wavelengths for which transparent materials are not available (because the light is primarily in the air, not in the solid materials). Another potential advantage of a hollow core is that one can dynamically introduce materials into the core, such as a gas that is to be analyzed for the presence of some substance. PCF can also be modified by coating the holes with sol-gels of similar or different index material to enhance its transmittance of light.