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Optical Alignment with Computer Generated Holograms
James H. Burge, Rene Zehnder, Chunyu Zhao College of Optical Sciences Steward Observatory University of Arizona
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Computer Generated Holograms
Use diffraction to create a desired wavefront Modern fabrication provides >100 mm patterns with <0.1 µm pixels. That’s > 1012 pixels! Incredible dynamic range
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Accuracy and flexibility
CGHs transform wavefronts with very high accuracy Errors are typically < l/100 Any wavefront shape can be created No special solution for spheres Multiple wavefronts can be created from the same CGH The registration between the different wavefronts is also very accurate
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CGH for interferometric measurement of aspheric surfaces
Interferometers use light to measure to ~1 nm surface errors, for spherical or flat surfaces CGH can change spherical wavefronts to aspheric, allowing the use of interferometers for measuring aspheric surfaces Aspheric surface to be measured aspherical wavefront Spherical wavefront Interferometer CGH
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Alignment of CGH Reflect wavefront back into the interferometer
Use this to align the CGH to the wavefront Spherical wavefront Interferometer Reflection CGH
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CGH for aligning the aspheric mirror
Use numerous holograms on a single substrate to provide both wavefront and alignment information. For alignment, the CGH can project bright crosshair patterns
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CGH for testing off axis parabola
A single substrate provides: - reference for interferometer - null lens for aspheric surface - creates 5 reference marks, 4 around edge, 1 on optical axis
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CGH alignment for testing off axis parabola
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CGH alignment of a 24-in off axis parabola (600-in ROC, 60 inches off axis)
Phase map l/20 rms CGH null lens incorporates alignment marks Easily align axis to 0.020” by eye
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Projection of fiducial marks
The positions of the crosshairs can be controlled to micron accuracy The patterns are well defined and can be found using a CCD Measured pattern at 15 meters from CGH. Central lobe is about 100 µm FWHM
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Use of CGH for optical alignment
Aligning the test for a 1.7-m off axis parabola 50 cm spherical mirror aligned within 7m CGH aligned within 7m 1.7m diameter OAP
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Projecting alignment marks through other optics
Aligning test for a 1.7-m off axis parabola Tilted spherical mirror We need to place the OAP to the right place Projecting a mark onto the OAP gives lateral position Need a second mark to get the clocking right CGH Interferometer Relay Lens Clocking mark Positioning mark
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Creating desired alignment features
Aligning the OAP
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Use of CGHs for optical alignment
Aligning the Sphere to within 7m The position of the sphere is known if 3 points on its surface are known
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Use of CGHs for optical alignment
Aligning the Sphere to within 7m Placing a ball concentric to zero order gives a very good reference Distance between balls can be measured with metering rods Lateral position of the ball defined by light Axial position defined by metering rod CGH Attaching the mirror to three balls defines its position The fourth ball gives redundant information
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Alignment of tooling balls to light created by CGH
Use tooling balls because they provide good mechanical interface Beam with ball at focus well aligned Very sensitive to lateral motion of the ball but not for axial motion Misaligned ball cases return beam to shift
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Ball alignment tool 1. Align a tool to the projected beam
2. Use the tool to laterally align the ball CCD Sensitivity comes from the geometry
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Direction of the reference beam
Ball Alignment Tool CCD camera Ball at mirror Aperture Beam splitter Direction of the reference beam ~2 µm resolution
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Use of CGHs for optical alignment
Metering rods in action
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Multiple patterns We use multiple patterns of the same substrate Divide the regions on the CGH. Each has a single pattern Derive a single pattern the gives simultaneous wavefronts
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Single pattern, creating four 1st order references
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Single CGH with multiple references
Position sensing detector CGH creating multiple wavefronts
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Conclusion CGHs are probably the most accurate and flexible things in optics Whatever your problem is, you can probably solve it with a CGH.
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