Development and Fabrication of High-Precision Dichroic Filters, 18-9332

Principal Investigator
James Arps

Inclusive Dates: 07/01/02 - 11/01/02

Background - High-performance multilayer optical thin films are critical and enabling elements used in an expanding range of leading-edge technologies. Specialized coated optics are commonly found in systems such as spacecraft instrumentation, military night vision gear, lasers, and telecommunications equipment. During the last decade, a dramatic increase in demand in this later area has resulted in purchases and mergers of several leading optical coating companies, leaving more traditional aerospace and defense customers in an severely underserved situation.

Key industry contacts were recently established in this new area and resulted in our first significant request to produce a series of special dichroic filters used in a heads-up display application. A quick-look research support was requested to move from an "in-principle" capability to a demonstrated competence in optical coatings to respond aggressively to this specific near-term requirement and several related opportunities that are developing concurrently.

Approach - Three major tasks were associated with this effort. The first activity focused on the preparation and configuration of the chamber for optical coating work. This step included installation of new fixtures and equipment, as well as modification of existing components. Also included was the procurement of test optics, coating materials, and supplies related to cleaning and preparation of the optics. The next step undertaken was to calibrate each candidate coating material, ZrO₂, TiO₂, and SiO₂, for their optical and physical characteristics under varying evaporation conditions. Included in this task was the development of the necessary optical testing and characterization methods to optimize the deposition process. The final task was to fabricate a technically challenging type of dichroic filter required by the potential customer. This step required several iterations between the results obtained during the thin film optimization phase and an optical coating program to generate a design that could then be applied photometrically in the vacuum chamber to accomplish the desired results.

Accomplishments - Substrate temperature and coating uniformity were established and adjusted. Temperature was determined at different settings and soak times utilizing a surface thermometer with the values recorded for future use. Coating uniformity was assessed by depositing a series of 9-layer stacks of alternating, optical quarter-wave thicknesses. Zirconium dioxide (ZrO₂) was chosen as the high refractive index material, with silicon dioxide (SiO₂) used as the low refractive index material. Several test runs were made, with adjustments to the chamber geometry made after each iteration. The final result was a uniformity of ±0.5% across a 16-inch diameter. This is suitable for almost all optical coatings, including the dichroics that were the focus of this project. Figure 1 shows a selection of samples fabricated during this effort.

All three of the primary tasks of this project were completed. While there are still improvements to be made, SwRI now has a system capable of producing state-of-the-art optical coatings. There is a design and implementation protocol available now that did not exist before. This combination of technology and equipment has enabled SwRI to initiate new complex optical coating projects.

Figure 1. Sample Filters Produced Under the Project.

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