Part II of this design case study focuses on issues of noise and mechanical design in the development of the interferometers for testing the large optics for the National Ignition Facility. Part I focused on optical design.
By: Erik Novak, Veeco Metrology
Understanding the PSD
Polarization and homogeneity
Specifications for the noise floor on the NIF interferometer are quite stringent. It is critical for the instrument to achieve a noise floor low enough to ensure that any measured features are real and not artifacts produced by the instrument. Lawrence Livermore has specified the surface quality of the NIF optics in terms of a power spectral density (PSD) function, so Veeco engineers characterized the system noise floor of the interferometer as a PSD also (see sidebar: Understanding the PSD Function. )
Acceptable optics must have a PSD curve which lies below 1.0* n -1.55 over the frequency range from 0.03 to 0.4 lines/mm, where n is the spatial frequency of the measured feature. According to specifications, the instrument noise floor should be less than 10% of this value. In other words, two sequential measurements subtracted from one another should produce a dataset with PSD characteristics lying below 0.1*n-1.55 .
By using a cooled, 12-bit camera and a vibrationally-insensitive phase algorithm, designers achieved this noise floor over both the 100-x-100-mm sub-aperture and the 400-x-400-mm full aperture )see Figure 6). To put it in more conventional terms, the system is able to achieve better than l/1000 (at 665-nm) repeatability over either the full 440 x 440 mm aperture or the 100 x 100 mm sub-aperture.
Polarization and homogeneity
To permit specialized measurements of certain optics for use in the NIF laser system, the interferometer was designed so that the polarization of the output beam can be rotated by 90° with a software control. Users can determine the birefringence of a part by differencing data from parts measured with horizontally-polarized light with data from parts and vertically-polarized light, (see Figure 7). Fabrication errors caused by polishing show up as streaks across the dataset.
Additionally, the homogeneity of the optics must be accurately tested. Traditionally, opticians have evaluated homogeneity using the oil-on-plate method. This process, however, can take several days, due to the use of index matching oils, which must adequately settle before accurate measurements may be obtained.
By taking four separate measurements with the interferometer and properly subtracting them, users can accurately determine the homogeneity of a test piece in a matter of hours (see Figure 8). Such time-saving devices are necessary to meet the NIF production schedule, which requires at least one large optic be completed every four hours at one of the NIF vendor sites, beginning in 1999.
Veeco engineers also faced mechanical design challenges when building the interferometers. Due to the large size of the system components, the overall length and weight of the instrument, and the strict wavefront quality requirements, the mechanical stability of the system needed extensive modeling. Engineers studied static deformation of the optics and vibrational stability of the instrument and test pieces. To increase stability, the designers placed the system on a custom air table that lowered the center of gravity. Also, the mounts for the large optics included micro-g connectors to dampen vibrations.
The optics were mounted in a thermally insensitive manner to reduce environmental effects on the system performance. Finally, special mounts with air bearings were provided to allow easy movement of both the test pieces and the return flat, whose weight would otherwise make easy and safe translation quite difficult.
The camera that captures the fringe pattern moves on a mechanical stage to allow measurement of any 100 x 100 mm sub-region. This range of motion eliminates the need for technicians to move the test optic, reducing the risk of damage to these valuable components.
Veeco is currently in the process of completing production of the final large aperture interferometers for the NIF project. The interferometers are being used to test crystals, unfinished blanks, and polished glass at various vendor sites.
As optics projects increase in scope and complexity, fast, non-contact, full surface measurements will become ever more valuable to ensure proper system performance. The large-aperture interferometers demonstrate that interferometry is able to meet the increasing demands of the optics industry.
1. D. M. Aikens, "The origin and evolution of optics specifications for the National Ignition Facility"; Proc. SPIE 2536, pp. 2, (1995).
2. J. Merle Elson, Jean M. Bennett; "Calculation of the power spectral density from surface profile data"; Applied Optics, 34, pp. 201-207, (1995).
3. D. Malacara, Optical Shop Testing, John Wiley and Sons, New York, NY (1994).
About the author…
Erik Novak is an optical scientist at Veeco Metrology Group, 2650 E. Elvira Rd., Tucson, AZ 85706. Phone: 520-741-1297; fax: 520-294-1799; e-mail: email@example.com.