High-Throughput Measurement Speeds Production of Large Optics

Mike Zecchino, !%4D Technology Corp.%!

Dynamic laser interferometry is an established technique for measuring optical-grade surfaces in the presence of vibration and air turbulence. Unlike traditional laser interferometry, dynamic interferometry does not require expensive vibration isolation systems, extensive mechanical coupling between the instrument and test optic, or air flow abatement. The method provides a cost-effective means of assessing the quality of large optics, often 1 m in diameter or larger, even when the optic must be placed many meters from the interferometer to enable measurement.

While dynamic interferometry is useful for verifying an optic’s final shape, it also can provide accurate shape data to guide polishing. This data, gathered early in – and even throughout – the process, allows more aggressive polishing that greatly shortens manufacturing time.

There are challenges in measuring large optics during polishing, but dynamic interferometry has been applied to overcome them.

Measuring optics during polishing

Measuring a large optic in a shop-floor environment presents multiple difficulties. Figure 1 shows a typical setup for measuring a large aspheric optic. The size and curvature of the optic dictate that it must be located many meters from the measurement system.

Figure 1. This diagram shows a setup for measuring a large aspheric optic. Courtesy of 4D Technology Corp.

This measurement poses difficult challenges for a temporal phase-shifting laser interferometer. Because a temporal phase-shifting system acquires data over a sequence of frames, its acquisition time is long, on the order of hundreds of milliseconds. Over the duration of the measurement, vibration and air flow would hinder or, more likely, prevent the acquisition of data. Vibration and turbulence would therefore need to be tightly controlled, which would mean coupling the instrument and optic through a single vibration-isolated pad or stand, as well as carefully controlling air flow and temperature differentials in the facility. All of these measures would be expensive and would require significant alteration of a facility.

The polishing process presents additional metrology challenges. Maximizing polishing throughput requires that the measurement process, including setup, alignment, data acquisition and analysis, be as fast as possible.

Also, the surface quality of an optic in its rough shape will exceed the measurement capability of most systems. Early in the polishing process, a large optic may have local surface deviations exceeding 40 μm. Such high local slopes are difficult for measurement systems to resolve. Qualitative methods provide some degree of process feedback, but the sooner a manufacturer can acquire quantitative data, the faster and more accurate the polishing process can be. This early quantitative feedback becomes even more critical for guiding automated polishing equipment.

Polishing feedback

In an instantaneous interferometric measurement, all data is acquired simultaneously in a single frame, rather than sequentially across several frames. Acquisition time can be only tens of microseconds, making the systems virtually insensitive to vibration. This insensitivity allows measurement when the test optic is located far from the instrument, even on a separate concrete pad. Furthermore, the effects of air turbulence can be easily removed from data using averaging techniques. Where a traditional interferometric measurement will be compromised by even minor air perturbations, an instantaneous measuring system actually performs better with a degree of fast-moving turbulence.

Several methods have been developed and commercialized for instantaneous phase data acquisition:

1. In the software-based spatial carrier method, high-frequency tilt fringes are applied by tilting the reference surface relative to the test beam. The tilt fringes are then filtered out by the software algorithms that determine the phase and thus the surface heights.

2. In an off-axis Fizeau interferometer, the test and reference beams pass through separate internal apertures, which select the correct polarization states from the test and reference optics. The apertures also filter the tilt fringes and high spatial frequencies.

3. In a stitching interferometer, multiple data sets covering small portions of an optic are stitched together to provide a full measurement of the total surface.

4. In dynamic interferometry, polarization elements separate the test beam into four or more phases. All phase data is recorded simultaneously and analyzed to determine surface heights.

Each of these instantaneous methods may be able to provide final quality control for large optics. However, most methods cannot provide feedback for the early stages of polishing. In a spatial carrier system, the filtering required to remove the tilt fringes limits the spatial resolution and thus the range of slopes that can be resolved. High spatial frequencies are also filtered out by the apertures used in Method 2, again resulting in a reduced range of measurable slopes. A stitching interferometer retains high spatial frequencies, but great care must be taken to preserve low-frequency data during the stitching process. Only dynamic interferometry has proved capable of measuring the full range of spatial frequencies present during polishing across a large aperture in an environment typical of a polishing process.

Measuring a large aspheric optic

Optical Surface Technologies of Albuquerque, N.M., designs, manufactures and coats custom optical components for specialized applications. In a current project, the company is polishing six 1.4-m-diameter parabolic mirrors. Stringent requirements dictate that the final surface quality will have to be 29 nm root mean square (rms) roughness, so both large-scale shape and small-scale structure are important to monitor during polishing.

Figure 2. Pictured is a measurement setup for a 1.4-m parabolic optic. The interferometer is in the foreground, along with the computer-generated hologram and optic. Courtesy of Optical Surface Technologies.

The test setup requires the optic to be located approximately 3 m from the interferometer, which means the optic must be located on a separate concrete slab. The measurements must be completed in an environment with significant background noise and vibration since the facility is located near the intersection of two major expressways.

The manual polishing process consists of two phases: first polishing the blank to a sphere and then polishing in the final shape. Acquiring reliable measurement data early in the process is critical for producing the mirrors on a tight schedule, noted Rod Schmell, optical fabrication manager at Optical Surface Technologies. “If we can see surface deviations when the surface quality is at 33 waves rms, then we can polish more aggressively and converge on the final shape faster,” he said.

For these measurements, the company first employed an off-axis Fizeau interferometer, but because of the extreme local slopes, reliable data could not be obtained. A dynamic interferometer was then used and was found to measure accurately (Figure 3) even at the rough blank stage. “We had data we could believe, and we tracked it throughout the polishing run,” Schmell said. “The data gave us confidence to work the part more quickly.”

Figure 3. Dynamic interferometric measurement of a large aspheric surface during the early stages of polishing is seen in this image. Courtesy of 4D Technology Corp.

Also important was the dynamic interferometer’s ease of use for in-process measurement. Setting up the off-axis Fizeau system required alignment of two spots per optical element; determining which spot was correct required significant time and effort. The Twyman-Green-type dynamic interferometer did not have this limitation. This advantage, along with minimal system drift and other benefits, enabled the polishing team to reduce downtime for a measurement to 15 min.

Dynamic interferometry provides quantitative feedback for manual and automatic polishing operations, from rough polishing to final shaping. Overcoming the difficult measurement environments found in most polishing operations, dynamic interferometry enables aggressive polishing with less downtime, so manufacturers can deliver high-quality finished optics in less time.

Meet the author

Mike Zecchino is applications engineer at 4D Technology Corp. in Tucson, Ariz.; e-mail: mike.zecchino@4dtechnology.com.