0.37 dB/km, and for hollow core is 1.2 dB/km Construction Generally, such fibers are constructed by the same methods as other optical fibers: first, one constructs a "preform" on the scale of centimeters in size, and then heats the preform and draws it down to a much smaller diameter (often nearly as small as a human hair), shrinking the preform cross section but (usually) maintaining the same features. In this way, kilometers of fiber can be produced from a single preform. Air holes are most commonly created by gathering hollow rods into a bundle, and heating the bundle to fuse it into a single rod with ordered holes before drawing, although drilling/milling was used to produce the first aperiodic designs. This formed the subsequent basis for producing the first soft glass and polymer structured fibers. Most photonic crystal fibers have been fabricated in silica glass, but other glasses have also been used to obtain particular optical properties (such as high optical non-linearity). There is also a growing interest in making them from polymer, where a wide variety of structures have been explored, including graded index structures, ring structured fibers and hollow core fibers. These polymer fibers have been termed "MPOF", short for microstructured polymer optical fibers. A combination of a polymer and a chalcogenide glass was used by Temelkuran et al. in 2002 for 10.6 μm wavelengths (where silica is not transparent). Modes of operation Photonic crystal fibers can be divided into two modes of operation, according to their mechanism for confinement: index guiding and photonic bandgap. Index guiding photonic crystal fibers are characterized by a core with a higher average refractive index than that of the cladding. The simplest way to accomplish this is to maintain a solid core, surrounded by a cladding region of the same material but interspersed with air holes, as the refractive index of the air will necessarily lower the average refractive index of the cladding. These photonic crystal fibers operate on the same index-guiding principle as conventional optical fiber — 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. Alternatively, they can also be made with much lower effective index contrast. Alternatively, one can create a photonic bandgap photonic crystal fiber, in which the light is confined by a photonic bandgap created by