Double-index fibers

The double-crucible assembly consists of two silica crucibles inserted one into the other with an open nozzle at the lower end. The inner crucible is charged with the core glass, while the outer crucible 

Extrusion also allows the fabrication of core/clad preforms. Savage et al. successfully reported the coextrusion of a Ge-As-Se preform and its subsequent fiber drawing [152], The coextrusion technique produces preforms with optimum core/clad interface quality. Fibers with optical losses of 1.7 dB/m at 6.0 pm were fabricated. Recently Abouraddy et al. have extended the technology by coextruding composite macroscopic preforms made of two ChG materials and a thermoplastic polymer cladding [153], yielding mechanically robust fibers. 

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Le Coq D., Boussard-Pledel C., Fonteneau G., Pain T., Bureau B., and Adam J. L., Chalcogenide double index fibers Fabrication, design, and application as a chemical sensor. Mater. Res. Bull, 38, 1745-1754 (2003). 

Step-index fibers are manufactured using the rod/tube or the double crucible process, in which the core and cladding glasses are melted separately from ultrapure powders and transferred into two crucibles with concentric orifices at the bottom and drawn into a fiber. To obtain a semi graded refractive index profile, a diffusion process can be applied in a curing furnace. Step index fibers with a transmission loss of < 10 dB/km at a wavelength of 0.8. pm can be obtained. 

In this section we will review the major techniques used for the fabrication of single-index, double-index, and PhC fibers. Ultimately the materials properties and targeted applications dictate the fabrication route. 

Fig. 16-2 Contours of constant refractive index for (a) the infinite linear profile of Eq. (16-24) and (b) the double parabolic fiber profile of Eq. (16-30), where d is the separation of the fiber centers.

Neutron diffraction studies of cellulose I (cotton crystallites) showed that the a andc dimensions (b is the fiber axis) of the conventional unit-cell should be doubled. The dimensions deduced are a = 1.678 nm, b (fiber axis) = 1.03 nm, c = 1.588 nm, and /3 = 82°. These are the same as the values proposed by Honjo and Watanabe,33 except that the b dimension is less than the value of 1.058 nm proposed by them. It was found that, in the region of 101, 101, and 002 reflections, there are a number of additional reflections that cannot be indexed by using the Meyer-Misch unit cell, but they can be indexed as 102, 102, 211, 211, 203, 203, 121, and 121 by using the larger cell. 

Fig. 29. Double refraction of collagen fibers produced by fibroblast cultures of chick embryo explants, after Pfeiffer (172). Retardation, r, in m/t is plotted, as ordinate, against the refractive index, n, of the immersion medium. Curve A is for fibers fixed in Helly s fluid (containing formalin), and curve B shows the result of fixation in tannic acid.

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