Single-mode photonic crystal fiber with an indefinitely large core

Summary form only given. The photonic crystal fiber (PCF) is made of undoped silica with a hexagonal array of air holes running down its length. One hole is missing. The resulting central solid region is the core, while the remaining two-dimensional photonic crystal, with the array of holes, is the cladding. The core´s diameter is roughly twice the separation /spl Lambda/ (or pitch) of the holes. The fiber is drawn from a stack of capillaries (and one central solid rod) in a conventional drawing tower. Light is guided by total internal reflection, because the cladding has an effective refractive index lower than that of silica. PCFs can be single mode throughout a broad wavelength range, from 337 nm to beyond 1550 nm, because of the unusual properties of the cladding´s effective index. Unlike conventional fibers, in the limit of short wavelength the V-value of a PCF tends to a finite value V/sub /spl infin// which depends on the diameter d of the holes relative to /spl Lambda/. If the holes are small enough for V/sub /spl infin// to be less than the V-value at second-mode cutoff, the fiber is single mode at all wavelengths. Our experiments suggest that the cutoff V-value is near 2.5, close to the conventional value.