Polymerization interfacial-gel

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. 

Isothermal Frontal Polymerization (IFP), also called Interfacial Gel Polymerization, is a slow process in which polymerization occurs at a constant temperature and a localized reaction zone propagates because of the gel effect (64,65). Using IFP (27), one can control the gradient of an added material like a dye, to generate materials useful, for example, in optical applications (66,67). Lewis et al. provide experimental and theoretical results in chapter 14. 

A transparent polymer rod was prepared under these conditions. The method does not require extra treatments such as preparation of a tube or diin rod made of PMMA before polymerization or reaction conditions requiring high pressure. It might be an alternative to interfacial gel polymerization for manufiicturing radial functionally gradient materials. 

Spade, C.A. and Volpert, V.A. (1999) Mathematical modeling of interfacial gel polymerization. Math. Comput. Model.,... 

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. 

G1 preforms can also be fabricated by an interfacial-gel polymerization technique. The principle is basically the same as that of the photo-copolymerization method discussed above, except for the mechanism that forms the initial gel phase. In this method, the core solution (the monomer) is placed in a polymer tube rather than in a glass tube. The gel phase in the photo-copolymerization method is referred to as a prepolymer with a conversion of less than 100%, whereas in this method the gel phase comprises the polymer layer on the inner wall of the tube swollen by the core monomer. The reaction is carried out under UV irradiation or heating. 

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. 

Figure 5.11 Interfacial-gel polymerization technique using dopants.

Figure 6.8 shows the DMA of a GI POF fabricated by interfacial-gel polymerization and another with double cladding fabricated by coextrusion (see Chapter 5). The attenuation of the GI POF fabricated by coextrusion increases rapidly as the launching position shifts from the core center to the core-cladding boundary, whereas a slight increment in the attenuation is observed in the GI POF fabricated by interfacial-gel polymerization. This is because the structural imperfections at the core-cladding interface that are formed during the coextrusion... 

Figure A.7 Formation process of Gl distribution by the interfacial-gel polymerization technique.

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. 

The production method up until around 2005 was the preform method (a manufacturing method where a G1 preform is created, which is then made into Gl-POF through heat-drawing) with a focus on the interfacial-gel polymerization method. However, from around 2005, we began to develop the continuous extrusion method in earnest, and by 2008 succeeded in 40-Gb transmission. This was the world s fastest transmission speed, surpassing the Gl-type silica optical fiber. 

For all the work carried out on polymeric GRIN materials via interfacial gel polymerization in the 1980s and 1990s, little work was performed on the actual hont propagation process. Golubev et al proposed a mechanism in 1992. Gromov and Frisch proposed a mathematical model that was flawed. Ivanov et al. did work with IFF in 1997 and 2002. °... 

As far as GI-POFs are concerned, Ohtsuka and co-workers reported several GI-POFs with low losses. By using the interfacial-gel-polymerization technique, the GI-POF attenuation loss was reduced to nearly the same value as that of SI-POF. Single-mode POF was also reported by several researchers around 1992. ... 

As the main part of the GI-POFs is composed of PMMA, the loss spectrum is nearly the same as that of SI-POF with PMMA core. The attenuation loss of GI-POF with the gel-copolymerization technique at 652 nm is 134 dB/kra Koike s group, Keio University, has used an interfacial-gel-polymerization technique where bro-mobenzene or other chemicals are used as unreactive components instead of vinyl phenyl acetate or vinyl benzoate in the interfacial-gel-copolymerization method. An attenuation loss of 90 dB/km at 572 nm was obtained. MMA-dg was also used as a monomer instead of MMA, and the deuterated polymer core GI-POF was successfully fabricated. Fluorinated acrylate monomer was also used to fabricate moisture-resistant GI-POF. Attenuation losses of 113 and 155 dB/km at 780 nm wavelength were obtained for deuterated and fluorinated POFs, respectively. These POFs are Oj pected to serve as the signal transmission medium with high information capacities in local area network systems. However, this GI-POF has not been commercially available so far because of the fabrication difficulty of the technique in a mass production level with reasonable attenuation loss and fabrication cost. 

Figure 15 Differential mode attenuation of Gl POFs based on the PMMA-DPS systems prepared by the rod-in-tube method and the interfacial-gel polymerization technique. Adapted with permission from Noda, T. Koike, Y. Opt Exp. 2010, 18 3128, 2010 OSA.

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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