Fundamentals of Plastic Optical Fibers

4Materials

Since the first POF (plastic optical fiber) was invented in the mid-1960s, considerable efforts have been made to develop materials in order to enhance POF's performance. The fundamental requirements for choosing or designing materials for POF fabrication are that the polymer should be (i) completely transparent, (ii) resistant to high temperatures, (iii) able to be drawn into a fiber, and (iv) mechanically flexible. Although the temperature requirement depends on where and how the POFs are utilized, in most cases it is preferred that their T_g should be above 80 °C. The above-mentioned four requirements are applicable irrespective of whether polymers are used as the core or for cladding. However, the core polymer of graded-index (GI) POFs should also possess the following requirements: (v) low refractive index, and (vi) low material dispersion. Though the GI profile can be fabricated by several methods, as will be discussed in Chapter 5, the most established method today is to diffuse a dopant from the core into the cladding. In such a polymer–dopant system, the biggest concern is that the dopant significantly decreases T_g because of the plasticization effect. Thus, it is preferable that the refractive index of the polymer is as low as possible so that a lower dopant concentration can be used to form a GI profile. The material dispersion is not important for step-index (SI) POFs because their bandwidth is almost dominated by modal dispersion. However, for GI POFs, ...