Interfacial polymerization 316 INDEX

It is well known that the dispersion in the optical fibers is divided into three parts, modal dispersion, material dispersion, and waveguide dispersion. In the case of the SI POF, the modal dispersion is so large that the other two dispersions can be approximated to be almost zero. However, the quadratic refractive-index distribution in the GI POF can dramatically decrease the modal dispersion. We have succeeded in controlling the refractive-index profile of the GI POF to be almost a quadratic distribution by the interfacial-gel polymerization technique (2). Therefore, in order to analyze the ultimate bandwidth characteristics of the GI POF in this paper the optimum refractive index profile is investigated by taking into account not only the modal dispersion but also the material dispersion. 

Zhang, Q., Want, P., and Zhai, Y. (1997) Refractive index distribution of graded index poly(Methyl Methacrylate) preform made by Interfacial-gel polymerization. Macromolecules, 30, 7874-7879. 

In this method, nonreactive compounds are employed as the high-refractive-index component [11]. For example, MM A and bromobenzene (BB), which have higher refractive indices than PMMA, can be utilized as the monomer and the nonreactive compound, respectively. The fabrication procedure is the same as in the photo-copolymerization and interfacial-gel polymerization methods. However, the principle of formation the GI profile is different. In contrast to the previous methods that use the difference in the monomer reactivity ratios, in this method the difference in the molecular size is important. Because the molecular size of MM A is smaller than that of BB, MM A more easily diffuses into the gel phase. Thus, BB molecules are concentrated into the middle region to form the GI profile as the polymerization progresses. The mechanism is schematically described in Figure 5.11. 

The interfacial-gel polymerization technique is particularly common in acrylic GI POP studies and enables the precise control of the refractive index profile, leading to a maximal bandwidth. However, this batch process requires many complicated procedures. Furthermore, the fiber length obtained at any one time is completely dependent on the preform size. This is a serious limitation in terms of fabrication costs. 

Figire 16 Relationship between index profile coefficient gand -3 dB bandwidth for 100 m of PMMA-DPS-based GI POP at 650 nm wavelength. Solid lines are the calculated results. Closed circles are the measured bandwidths of GI POFs prepared by the interfacial-gel polymerization technique (spectral width is 3.0 nm). Adapted with permission from Koike, Y. Ishigure, T. J. Lightw. Technol. 2006,24,4541 2006 IEEE. 

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