Freeform metrology using subaperture stitching interferometry

Transcription

1 Freeform metrology using subaperture stitching interferometry APOMA November 10-11, 2016 Presented By: Christopher Hall QED Optics Sr. Engineer, QED Technologies Copyright QED Technologies 2016

2 Interferometry Without Nulls - SSI Key benefits of sub-aperture metrology: Magnify Locally null Higher resolution resolves more fringes QED Algorithms Enable automation Compensate for systematic errors Motion Reference wave Re-trace SSI-A 2 Copyright QED Technologies 2016

3 Subaperture Stitching Interferometry for Freeform Two new developments QIS: QED Interferometer optimized for Stitching Algorithms extended to freeform geometries QIS Enables capture of higher fringe densities New Algorithms From rotationally symmetric only freeform shapes New motion equations & other algorithm extensions Maintains all the benefits of stitching parts with rotational symmetry (reference wave calibration, distortion calibration, etc.) Still under development 3 Copyright QED Technologies 2016

4 Subaperture Stitching Interferometry Interferometry can typically provide higher lateral resolution and precision than CMM metrology or profilometry ASI platform excels at measuring mid-spatial frequency errors Extended some aspects of our stitching algorithms to support nonrotationally symmetric geometries Freeform geometry input Freeform motion equations Still more work to be done Using a 4 f/7.2, a 5 mm subaperture patch could be measured ~16 µm freeform departure over 5 mm Φ 166 sub-aps collect to cover 28 mm Φ area; ~600 µm freeform departure 4 Copyright QED Technologies 2016

5 Subaperture Stitching Interferometry Results Low order error agrees very well Greatly increased resolution over CMM 1200 pix (SSI) vs 60 pix (CMM) 23 µm/pix vs 460 µm/pix Mid-spatial frequencies are characterized much more clearly by SSI Vertical trough is clearly seen in SSI measurement, but barely visible in CMM measurement Horizontal ripples have higher definition For high precision figure correction, both CMM and SSI measurements are valuable SSI Color Scale +/- 40 µm POW & alignment errors removed Color Scale +/ µm 4 mm high-pass filter CMM Vertical trough seen by SSI but not by CMM 28 mm 26 mm 5 Copyright QED Technologies 2016

6 Freeform telescope mirror Dominant terms in Q-polynomial freeform representation: Q 0 2 = 66 µm (astigmatism-like) Q 1 1 = 50 µm (coma-like) Q 0 0 = 21 µm (spherical-like) Q 0 3 = 20 µm (trefoil-like) 106 μm PV departure from best-fit sphere 6 Copyright QED Technologies 2016

7 Metrology of telescope mirror CGH vs SSI For CGH: 4 interferometer on horizontal table 4 f/1.5 TS ~600 x 600 pixels For Stitching: ASI(Q) Interferometer 6 f/3.5 TS 47 Subapertures ~2K x 2K pixels Lattice Design All data reported over 100 mm clear aperture 7 Copyright QED Technologies 2016

8 ASI Repeatability Three repeat measurements RMS: 78 nm RMS: 77 nm RMS: 76 nm Pixel-by-pixel mean and standard deviation ASI(Q) gives very repeatable measurements RMS: 77 nm RMS: 2 nm 8 Copyright QED Technologies 2016

9 Comparison with CGH Metrology Difficulty of measurement execution Lateral resolution Mid-spatial frequency characterization Distortion 9 Copyright QED Technologies 2016

10 Comparison with CGH metrology CGH Null ASI(Q) Freeform stitching PV = 518 nm RMS = 77 nm PV = 527 nm RMS = 77 nm Results agree very closely >10x more pixels (250,000 vs 3,000,000) 10 Copyright QED Technologies 2016 ASI(Q) gives very good low-order accuracy

11 ASI measurement is easier to execute Software-assisted alignment (initial setup takes minutes instead of hours) Automatic subaperture positioning Easy-to-use data analysis tools Actual fringes Expected fringes 11 Copyright QED Technologies 2016

12 ASI measurement is easier to analyze Automatic removal of alignment error On a freeform, can t remove Zernikes like you can for a sphere or asphere (no 1-1 relationship between alignment errors & Zernikes) X shift: Z1, Z5, and Z6 (X tilt, Y primary astigmatism, and X primary coma), Y shift: Z2, Z4, and Z7 (Y tilt, X primary astigmatism, and Y primary coma), and Rotation: Z5, Z6, and Z9 (Y primary astigmatism, X primary coma, and X trefoil). Instead, figure error due to rigid-body alignment errors are removed Rigid-body alignment error removal Zernike removal Difference RMS: 77 nm RMS: 35 nm RMS: 72 nm If Zernike removal is performed instead of a rigid-body fit, some figure error will be mistakenly attributed to alignment error. 12 Copyright QED Technologies 2016

13 ASI gives higher resolution metrology PSD Data of central subaperture Red curve = CGH data Green curve = ASI(Q) data CGH data appears to have higher level of MSF, but ASI(Q) data shows result is actually quite smooth at this level 13 Copyright QED Technologies 2016

14 ASI gives better MSF fidelity Hotspots from higherorder diffraction visible in CGH data Again, CGH data has MSF information that is not actually true surface shape information 14 Copyright QED Technologies 2016

15 ASI gives better MSF fidelity Ripple from ghost fringes ASI(Q) Freeform Stitching has accurate low-order information, and has more high-frequency information and accuracy at the same time 15 Copyright QED Technologies 2016

16 ASI gives distortion-free measurement ASI distortion free +Y -Y In ASI measurements, rings are at the same radial position at +Y and -Y CGH Up to 1.2mm lateral error In CGH measurements, distortion causes rings to appear at different distances from the center, making a deterministic correction of these features impossible 16 Copyright QED Technologies 2016

17 Correction with MRF Initial After MRF 7x rms improvement demonstrates high convergence as a result of reproducible metrology 17 Copyright QED Technologies 2016

18 Summary ASI(Q) Freeform Stitching is bringing high accuracy and high resolution to full 3-D freeform metrology Critical for deterministic figure and MSF correction using processes like MRF Freeform measurements are very repeatable and show very good agreement with CGH cross-tests Several benefits of ASI measurement over CGH Easier measurement execution Improved lateral resolution Better fidelity of mid-spatial frequency (MSF) features Distortion-free measurement 18 Copyright QED Technologies 2016