Date: Aug 18, 2014 Source: Company Data (
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Advanced surface metrology for meter-class optics
M. Valente1, B. Lewis1 , N. Melena1, M. Smith1, J. H. Burge1,2, C. Zhao2
1Arizona Optical Systems, 925 W Grant Rd., Tucson, AZ 85705
2Arizona Optical Metrology, 8943 N. Veridian Dr., Tucson, AZ 85743
ABSTRACT
Refinements in computer controlled optical surfacing allow efficient grinding and polishing of meter-class optics to accuracy limited only by the surface metrology. W e present a categorization of metrology methods and their implementation for meter-class optical components. Interferometry with computer generated holograms provides nanometer accuracy for full surface measurements of a wide range of convex and concave aspheric surfaces. F or measuring off-axis and freeform aspheric surfaces, the holograms include features that provide references for alignment. V ery high spatial resolution is achieved with subaperture interferometric measurements which can be stitched together to provide a full-aperture map. S canning systems complement the capabilities of interferometry by achieving larger dynamic range and providing independent corroboration. O ptical coordinate measurement machines (CMMs) provide non-contact measurements of surfaces in their ground state to guide figuring, as well as highly accurate measurements of finished optics. S canning systems for measuring flat mirrors provide excellent resolution and absolute accuracy. T he performance and practical issues for this full array of measurement techniques are presented to show the relative strengths of each method.
Keywords: Metrology, interferometry, large optics, aspherics
Contact Information: mvalente@arizonaopticalsystems.com
INTRODUCTION
Like most precision surfaces, the manufacture of large optics relies on the ability to accurately measure the shape at all stages of the processing. The surface measurements are used as process controls and for direct feedback into the shaping operations. Since there are a variety of shapes and several different manufacturing stages, numerous types of surface measurement are employed, even by a single organization. We classify the surface measurement methods as:
• Snapshot systems covering the full aperture
• Snapshot systems covering a subaperture of the part, coupled with scanning systems
• Scanning systems with single point sensor
• Scanning systems with multiple sensors
Initial shaping operations for optics usually use stiff machines and diamond cutting tools. The requirement for accuracy is loose enough (> 5 µm or so) that the surface can be measured using the same stylus profiler or touch-probe coordinate measurement machine (CMM) that was developed for measuring general mechanical parts. The situation becomes more interesting when the subsequent manufacturing steps require sub-micron accuracy and when non-contact methods must be used.
After initial shaping, the optical surfaces are lapped with finer and finer abrasives such that the materials and processes control surface finish (and subsurface damage), and feedback from the measurement systems allows the material removal to be biased such that the surface form errors are improved. This basic sequence is shown in Figure 1.
Figure 1. Precision optical surfaces are manufactured by following a set of lapping and metrology steps that drive the shape to its required form and finish. At AOS, we use a high speed shear mode grinding operation followed by computer controlled polishing. T he measurement methods that guide processing are frequently dictated more by efficiency than by performance.
The diversity of optical shapes and the need to go through multiple processing steps leads to a rich variety of surface measurement techniques. Typically, measurements are made with scanning systems to guide the coarse operations of grinding, followed by the use of snapshot interferometry to guide the polishing and demonstration of final performance. We find it useful in many cases to depart from this convention to more economically achieve the measurement goals. The use of shear mode grinding provides rapid removal, and leaves a specular surface that can be measured directly with optical tests. The use of single- and multi-probe scanning systems achieves sufficient accuracy to provide feedback for all the polishing of most optics. Since these measurement systems are built on the same platform as the computer controlled polisher, we do not need to move the optic from the machine between measurement and polishing cycles, allowing rapid production.
This paper discusses some particular optical tests, providing an assessment of the relative strengths and limitations for each one. Scanning measurement methods that support the initial grinding stages of fabrication are presented in Section 2. Section 3 presents some scanning methods that use multiple simultaneous sensors to achieve high accuracy. Section 4 discusses several snapshot interferometric testing systems and presents some discussion on subaperture stitching.
SCANNING MEASUREMENT OF GROUND SURFACES
Initial lapping typically leaves a n on-specular surface that cannot be measured using reflection of visible light. Interferometry using infrared light is possible, but is too expensive for most applications. W e utilize two different scanning methods to guide the grinding: surface profiling using a laser tracker and full surface measurement with the Swingarm Optical CMM (SOC) that uses a diffuse reflectance sensor.
Laser trackers have found increasing applications for manufacture and alignment of large optics. 1 , 2 The principle of surface measurement is simple -- the laser tracker monitors position of a corner cube reference that is scanned across the surface. The corner cube is mounted into a ball such that the apex of the mirrors coincides with the center of curvature of the ball. When the appropriate geometry is used, and careful attention is paid to the scanning and environmental issues, submicron measurement accuracy can be achieved.
Table 1. Issues for surface measurements using a laser tracker
Benefits Limitations
Useful for measuring nearly any surface form in the generated or ground state
Good accuracy with careful setup, ~ 1µm using DMI
Single point measurement means extended data collection time
For 1µm accuracy, must be near optic CoC -- can be
difficult
Limited spatial resolution
Ground surfaces can also be measured right on the grinding machine using a Swingarm Optical CMM that uses a triangulation type sensor that utilizes diffuse reflection.3 This system relies on accuracy of the scanning geometry and careful alignment to achieve accuracy of ~100 nm rms for most surfaces. The measurements are performed as a set of scans across the part which are combined to provide a full surface map. Figure 2 shows the measurement geometry, an example machine, and a layout of a pattern of 64 scans.
Figure 2. The Swingarm Optical CMM (SOC) scans a noncontact probe across the surface. Multiple scans are combined to give a full surface measurement.
Table 2. Issues for surface measurements using the Swingarm Optical CMM with triangulation probe
Benefits Limitations
Useful for measuring nearly any surface form in Requires careful alignment, calibration
the ground or polished state
Excellent accuracy with careful setup, 0.1 µm Does not measure power (need secondary radius
of curvature measurement )
High spatial resolution for in-scan direction Limited spatial resolution in azimuthal direction
(between scans)
In situ measurement on CCP machine, very Possibly long measurement time
efficient
SNAPSHOT INTERFEROMETRIC SURFACE MEASUREMENTS
Most precise optical surfaces are measured using interferometers that can achieve a full measurement in a s ingle snapshot. Commercial interferometers are available that measure with nanometer precision, capturing data in milliseconds to decrease sensitivity to vibration. These are incorporated into different systems that use auxiliary optics or computer generated holograms (CGHs) for measuring meter-class optics. In some cases, the auxiliary optics become too large or expensive to support the measurement of a p articular optic, but a s ubaperture measurement can be performed. The data from a set of subapertures is then stitched together to give a full surface map.
Classic interferometric surface measurements
Interferometers are available to measure small flats and convex spheres (≤ 6" diameter) and concave spheres without auxiliary optics. The limitations for these measurements come from calibration and environmental effects. Large flats are measured using the Ritchey-Common geometry with a spherical mirror. Large convex aspheres and paraboloids can be measured using the Hindle geometry and autocollimation respectively that utilize the natural conjugate pairs for these optics. Figure 3. shows some interferometers configured for such tests.
R-C test of 1m flat
Figure 3. Interferometry provides snapshot measurements of simple optical surfaces.
Table 3. Issues for surface measurements using classical interferometry with auxiliary optics
Benefits Limitations
Useful for measuring simple spheres, flats (size limited on flats and convex spheres), and conjugate tests for aspheres
High resolution and accuracy provided by commercially available interferometers Limited flexibility for different surface forms
Custom configuration of auxiliary optics can be costly and time consuming
Short data acquisition time Measurements over long air paths are sensitive to
atmospheric effects
Interferometric surface measurements utilizing computer generated holograms
Aspheric surfaces are measured using interferometry combined with computer generated holograms.4 The simplest embodiment is shown in Figure 4 where the CGH acts as a n ull corrector, essentially transforming the spherical wavefront from the interferometer to an aspheric shape that fits the desired aspheric mirror. A different technique is required for large convex aspherics, shown in Figure 5. The convex aspheric is measured by viewing through the substrate and interfering a reflection from the aspheric surface with a r eflection from a master concave sphere. A computer generated hologram compensates the difference. T he CGH allows accurate testing of general, non-
symmetrical aspheric shapes.5 Both of these aspheric tests have been proven at AOS to achieve accuracy of 3 nm rms for steep, 1-m class off-axis mirrors.
OAP test set with CGH
Figure 4. Interferometry with CGH null correctors allow accurate measurement of concave aspheric optics with relying on symmetry.
Mandrel
Figure 5. Fizeau interferometry with CGH correction provides accurate measurement of off-axis convex aspherics, including a 1.5-m convex off-axis asphere measured in the AOS test tower shown.
Table 4. Issues for surface measurements using classical interferometry with auxiliary optics
Benefits Limitations
Flexible for measuring general aspheric surfaces Custom CGH required for each aspheric surface
High spatial resolution and surface measurement accuracy
CGHs provide accurate references that are used for aligning the optical test
Measurements over long air paths are sensitive to atmospheric effects
Short data acquisition time
Subaperture interferometric surface measurements
In some cases, it is not practical to use auxiliary optics or CGHs to achieve snapshot measurements of a full aperture for large optics. However, it may be cost effective to measure a portion of the surface at a time, then to combine or stitch the data to create a full surface map. Recent developments allow the use of multiple subaperture measurements to not only cover the full surface under test, but to calibrate the measurement system.6 For example, the annular aspheric mirror shown in Figure 6 can be measured using subapertures that are each corrected only over the smaller measurement region. Systematic errors in the measurement system will be constant for all measurements, while figure irregularity in the annular part under test will vary according to the region being measured. This diversity allows a s eparation of features in the test from those in the mirror. A simulation for this case is shown in Figure 6, where the reference is calibrated by the same data that was used to determine the shape of the mirror.
Figure 6. Stitching interferometry provides advantages for measuring highly aspheric mirrors, and also achieves high accuracy using self-calibration. Simulated results are shown here for simultaneous calibration of the systematic errors in the measurement system as well as the irregularity in the annular surface under test.
Table 5. Issues for surface measurements using subaperture stitching interferometry
Benefits Limitations
Cost effective solution for particular classes of optic (large flat or convex asphere)
High spatial resolution and surface measurement accuracy
Self-calibration enables high accuracy Long data acquisition times, many measurements may be required
Can require precision motion of large optical systems
SCANNING MULTI-PROBE MEASUREMENTS
A new class of measurement systems has been developed that incorporates the best of the scanning systems (in situ measurement that does not require custom optics for each part) with some of the best features from interferometry (nanometer accuracy and high spatial resolution). These systems use high performance sensors and rely on the diversity from multiple simultaneous measurements to achieve high measurement accuracy in the presence of environmental and alignment perturbations.7 Some of these systems are shown below in Figure 7.
Multi-head sensor applied to the Swingarm Optical CMM Multi-DMI system configured for measurement of a 1.5-m mirror
Figure 7. Scanning multiprobe measurements achieve sensitivity limited by the probes and accuracy limited by the calibrations made possible with simultaneous measurements.
A specialized scanning system for measuring flat mirrors uses an autocollimator and pentaprisms to make slope measurements across the diameter of the mirror.8 The system uses one fixed sensor and a second scanning beam. The motions of the mirror under test with respect to the measurement system are compensated using the fixed sensor. Multiple scans are combined to create a full surface map. This method provides an accurate calibration of flat in the surface, which is elusive for most other tests. Figure 8 below shows the test concept and implementation at AOS for 1.8¬m flat mirrors.
Figure 8. The scanning pentaprism system measures surface slope variations with 0.1 µrad precision over 1.8 meters.
Table 6. Issues for surface measurements using multiprobe scanning systems
Benefits Limitations
Systems can be reconfigured to measure mirrors from 20 cm to several meters
Measurements can be performed in situ, on the polishing table
Self-calibrating to achieve excellent surface measurement accuracy
High spatial resolution in scan direction Can require long data acquisition times
Can be sensitive to alignment and environmental issues
Unconventional! Many customers require
corroboration with interferometry
DISCUSSION
A variety of surface measurement techniques were presented. We provide a co mparison of the relative merits and limitations. Figure 9 shows the tradeoff between spatial resolution and accuracy for the measurements, normalized for a 1.5-m mirror. Also, the various measurement methods are listed in the tables below.
Figure 9. The various measurements presented here allow a tradeoff between accuracy and spatial resolution.
Table 7. Comparison of measurement methods for meter class aspheric mirrors
Measurement technique
Full aperture interferometry
Applications
Conjugate tests for simple shapes
Pros
High accuracy, low cost
Cons
May require space, auxiliary optics
Full aperture interferometry
Full aperture interferometry
Use CGH for concave aspheres
Use CGH Fizeau for convex aspheres
High accuracy
High accuracy
Requires access to center of curvature
Requires concave reference sphere
Scanning sub-aperture interferometry
Swingarm Optical CMM multi-interferometric probes
Large aspheric, convex , or flat surfaces
Measure surfaces during polishing, large aspheric, convex or concave
Lower cost than full aperture test
Excellent accuracy, measures on the polishing machine
Data acquisition is more difficult
Sensitive to calibration, does not measure power
Scanning multiprobe DMI system
Measurement of any surface
Self-calibrating, High accuracy
Sensitive to calibration, does not measure power
Table 8. Comparison of measurement methods for meter class flats
Measurement technique
Scanning sub-aperture interferometry
Applications
Large aspheric, convex , or flat surfaces
Pros
Lower cost than full aperture test
Cons
Data acquisition is more difficult
Swingarm Optical CMM multi-interferometric probes
Measure surfaces during polishing, large aspheric, convex or concave
Excellent accuracy, measures on the polishing machine
Sensitive to calibration, does not measure power
Scanning multiprobe DMI system
Scanning pentaprism system
Measurement of any surface
Measures flat, especially power
Self-calibrating, High accuracy
Very high accuracy
Sensitive to calibration, does not measure power
Does not measure high order aberrations
REFERENCES
1. T. L. Zobrist, J. H. Burge, H. M. Martin, "Laser tracker surface measurements of the 8.4m GMT primary mirror segment," Proc. SPIE 7426, (2009).
2. J. H. Burge, P. Su, T. Zobrist, C. Zhao, "Use of a commercial laser tracker for optical alignment" Proc. SPIE 6676, (2007).
3. Y. Wang, P. Su, R. E. Parks, C. J. Oh, J. H. Burge "Swing arm optical coordinate-measuring machine: high precision measuring ground aspheric surfaces using a laser triangulation probe," Opt. Eng. 51(7), 073603 (2012).
4. J. H. Burge, "Applications of computer-generated holograms for interferometric measurement of large aspheric optics," Proc. SPIE 2576, 258-269 (1995).
5. J. H. Burge, M. Dubin, C. Zhao, "Measurement of aspheric mirror segments using Fizeau interferometry with CGH correction," Proc. SPIE 7739, (2010).
6. J. H. Burge and C. Zhao, "Applications of subaperture stitching interferometry for very large mirrors," Proc. SPIE 8450 (2012).
7. P. Su, Y. Wang, C.J. Oh, R. E. Parks, J. H. Burge, "Swing arm optical CMM: self calibration with dual probe shear test," Proc. SPIE 8126 (2011).
8 J. Yellowhair and J. H. Burge, "Analysis of a scanning pentaprism system for measurements of large flat mirrors," Appl. Opt. 46, 8466-8474 (2007).
