Photonic Crystal Fibers
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In 1842, the Swiss physicist Daniel Colladon demonstrated that light could be guided along a curved path within a tube of transparent material. Colladon's “light pipe” was simply a narrow stream of water issuing through a hole in the side of a tank. The underlying guidance mechanism in this case was, of course, total internal reflection (TIR)—the phenomenon by which standard single-mode fiber (SMF) guides light. A typical SMF designed for communications wavelengths has a Ge-doped silica core (synthesized by modified chemical vapor deposition), a core-cladding refractive index difference of a few percent, and a core diameter of ∼ 10 µm, and possesses astonishing optical clarity (0.2 dB/km at 1550 nm). Although SMF has been outstandingly successful in optical telecommunications, there is still pressure to further improve its performance. For example, optical nonlinearities in the solid glass core are proving complex to understand and awkward to control, and are contributing to bit error rate deterioration over long spans in wavelength division multiplexed systems. In other fields, there is a long-standing need for fibers that can carry higher power; act as versatile sensors or hosts for rare earth ions; and have multiple cores, higher nonlinearities, lower nonlinearities, higher birefringence, and widely engineerable dispersion.