Volume 12 Issue 2 - January 1, 2010 PDF
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Process equipped with a sloped UV lamp for the fabrication of gradient-refractive- index lenses
Jui-Hsiang Liu*, Yi-Hong Chiu
Department of Chemical Engineering, College of Engineering, National Cheng Kung University
 
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In this investigation, a method for the preparation of radient-refractive-index (GRIN) lenses by UV energy-controlled polymerization has been developed. A glass reaction tube equipped with a sloped UV lamp was designed. Methyl methacrylate and diphenyl sulfide were used as the reactive monomer and nonreactive dopant, respectively. Ciba IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl- ketone) was used as the initiator. The effects of initiator concentration, the addition of acrylic polymers, and the preparation conditions on the optical characteristics of the GRIN lenses produced by this method were also investigated. Refractive index distributions and image transmission properties were estimated for all GRIN lenses prepared.
Fig. 1. Schematic representation of the equipment for UV-controlled polymerization of plastic rods. The UV lamp is equipped with a small slope angle. The highest energy exists at the bottom and decreases gradually from the bottom to the top, as indicated by the dotted lines. This is consistent with gel zone formed in the reaction tube between A and B.

Figure 2 shows the dependence of the initiator concentration on the △n distribution of the GRIN lens, where np and Rp denote the refractive index at the periphery and the radius of the plastic rod, respectively. The results of the refractive index distribution suggest that the concentration of high-refractiveindex dopant decreases from the center axis to the periphery of the gel rod, resulting in a GRIN distribution. Higher initiator concentrations were found to increase the polymerization rate. By contrast, nonreactive DS was fixed more easily in the polymer matrix, leading to a decrease in refractive index at the central axis.
Fig. 2. Dependence of the initiator concentration on the △n distribution of the GRIN rods. Initiator concentrations: circles, 1.0 wt.%; diamonds, 1.5 wt.%; triangles, 2.0 wt.%. Increased initiator concentration decreases the refractive index difference.

Figure 3 shows a color (online) image transmitted through a freshly fabricated GRIN lens with a 15 mm diameter and 80 mm length. An inverted virtual image was obtained through the plastic rod fabricated in this investigation. The distance between the image and the face of the GRIN lens was 60 mm in this trial. The quality of these results suggests that GRIN lenses can be successfully fabricated via UVcontrolled polymerization to produce GRIN lenses with excellent optical properties.
Fig. 2. Dependence of the initiator concentration on the △n distribution of the GRIN rods. Initiator concentrations: circles, 1.0 wt.%; diamonds, 1.5 wt.%; triangles, 2.0 wt.%. Increased initiator concentration decreases the refractive index difference.

In conclusion, we have demonstrated that a UVcontrolled polymerization can be used to fabricate GRIN plastic rods. This method is a technique that is easy to perform and requires relatively inexpensive equipment and materials.
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