Embed Size (px)
Citation preview
REPLICATION OF A DIFFRACTIVE OPTICAL ELEMENT
Nelson E. Claytor*, Oscar M. Lechuga, and Jay Udayasankaran
Fresnel Technologies, Inc., 101 W. Morningside Drive, Fort Worth, TX 76110
Diffractive optical elements (DOEs) find application in a wide variety of products. In particular, computer–generated holograms consist of micro–scale features over a rather large area. The micro-scale features must be accurately reproduced in order that the DOE displays high efficiency and thus does not waste light from the source. Replication of these features at reasonable cost, while main-taining the required accuracy, is difficult.
A DOE that serves as a fanout grating—that is, makes many spots from a single incoming laser beam—was replicated in acrylic (PMMA or polymethylmethacrylate) by two different methods, com-pression molding and injection molding. The pattern produced by the DOE is shown in Figure 1: six-teen spots in a circle, with the zero-order (undiffracted) beam in the center. The picture also shows many higher-order spots, which rob energy from the desired spots and are not desirable. These higher-order spots exist because the DOE is of the binary type, with 2 levels, rather than having a continuous surface relief. The higher-order spots are not visible in room light, though they are cer-tainly very visible in the dark.
Figure 1. Diffraction pattern (633 nm light) of the DOE studied here.
Typical methods for originating DOEs involve some form of lithography and etching. The master for the DOE studied here was made by reactive ion etching in quartz, so the master we received was a small piece of quartz with the desired surface relief structure etched into it. A small piece of the re-lief structure is shown in Figure 2, which shows that the features to be replicated are of the order of 1-3 µm wide and 0.5 µm deep.
Figure 2. Trace of a portion of the quartz master for the DOE studied here.
Quartz is not a suitable material for either compression molding or injection molding, and we also wished to make many copies of the same element in order to mold many DOEs at once and thereby reach an economical production cost. Thus we first metallized, then electroformed the quartz master in order to obtain a master nickel electroform. The intensity of one of the sixteen spots relative to the center spot was chosen as a good measure of the quality of replication, with compression molded rep-licas of the master nickel electroform taken to be the standard against which other molded DOEs were measured.
The master nickel electroform was then replicated in nickel again, to form a multiple-cavity com-pression mold and a single-cavity injection mold. Moldings from eight of the cavities (the four corners and four from the center) of the compression mold were compared to compression moldings from the master.
The master nickel electroform was also replicated and incorporated into a single-cavity injection mold. Moldings from the injection mold were compared to compression moldings from the master.
We also wished to determine more directly the quality of replication by the compression molding and injection molding methods. We therefore made scanning electron micrographs of the two types of replica, and also measured the microstructure of each type of replica using white light scanning in-terferometry. Data on the quality of replication from the two methods will be presented.
*corresponding author: [email protected], +1 817 926 7474
