Microstructured Silica as an Optical-Fiber Material
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Microstructured Silica as an Optical-Fiber Material
J.C. Knight, T.A. Birks, B.J. Mangan, and P.St.J. Russell Introduction Conventional optical fibers are fabricated by creating a preform from two different glasses and drawing the preform down at an elevated temperature to form a fiber. A waveguide core is created in the preform by embedding a glass with a higher refractive index within a lower-index “cladding” material. Over the last few years, researchers at several laboratories have demonstrated very different forms of optical-fiber waveguides by using a drawing process to produce two-dimensionally microstructured materials in the form of fine “photoniccrystal fibers” (PCFs). One such waveguide is represented schematically in Figure 1. It consists of a silica fiber with a regular pattern of tiny airholes that run down the entire length. The optical properties of the microstructured silica cladding material enable the formation of guided waves in the pure silica core. Fabrication processes to create twodimensional (2D) microstructures using fiber-drawing were described by Kaiser1 and later by Tonucci2 and Inoue and colleagues.3 We have used a procedure based on stacking and drawing hollow silica tubes to form periodic samples with an index contrast and pitch (periodic spacing) suitable for interesting and useful optical effects. By considering the microstructured material as a single optical medium with its own unique properties (a “photonic crystal”), we have been able to incorporate these materials into our existing understanding of fiber-waveguide performance. This has provided us with greatly improved insight into the performance and limitations of the new structures and is pointing
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the way to new optical-fiber structures that can outperform conventional fibers in several critical respects.
Redrawn and Microstructured Silica Silica is an incredibly versatile material and has several properties that make it ideal for drawing optical fibers. One is its very low intrinsic optical loss. Another
Figure 1. Schematic illustration of a “photonic-crystal fiber” (PCF). A fine silica fiber has a two-dimensional array of airholes running down its length. This photonic-crystal material can then be used to trap light in a core region. In the case shown here, the core is defined by a missing airhole in the center of the array.
is the slow change in viscosity with temperature, which makes the fiber-drawing process relatively insensitive to changes in temperature. A third is the very high stability of the glass matrix and its slow devitrification. These same properties make silica the first choice for fabricating PCFs. However, the intrinsic properties of silica also limit the range of microstructure parameters that can be produced; for example, the strength of a strand of silica limits the viscosity at which it can be drawn, while its relatively low refractive index limits the width of bandgaps that can be attained.
Photonic-Crystal Fiber Waveguide Design Considering regularly microstructured silica as a single ph
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