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Optical Alignment with
Computer Generated Holograms
James H. Burge, Rene Zehnder, Chunyu Zhao
College of Optical SciencesSteward Observatory
University of Arizona
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
Accuracy and flexibility
• CGHs transform wavefronts with very high accuracyErrors are typically < /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
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
Interferometer
Spherical wavefront
aspherical wavefront
CGH
Alignment of CGH
• Reflect wavefront back into the interferometer
• Use this to align the CGH to the wavefront
Interferometer
Spherical wavefront
Reflection CGH
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
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
CGH alignment for testing off axis parabola
CGH alignment of a 24-in off axis parabola(600-in ROC, 60 inches off axis)
CGH null lens incorporates alignment marksEasily align axis to 0.020” by eye
/20 rms
Phase map
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
Use of CGH for optical alignment
Aligning the test for a 1.7-m off axis parabola
1.7m diameter OAP
50 cm spherical mirroraligned within 7m
CGHaligned within 7m
Projecting alignment marks through other optics
Aligning test for a 1.7-m off axis parabola
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
Clocking mark
Positioning mark
CGH
Tilted spherical mirror
Relay Lens
Inte
rfero
met
er
Creating desired alignment features
Aligning the OAP
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
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 betweenballs can be measured
with metering rods
Lateral position of the balldefined by light
Axial position defined bymetering rod
Attaching the mirror to three balls defines its positionThe fourth ball gives redundant information
CGH
Alignment of tooling balls to light created by CGH
Beam with ball at focus well aligned
Misaligned ball cases return beam to shift
Very sensitive to lateral motion of the ball but not for axial motion
Use tooling balls because they provide good mechanical interface
Ball alignment tool
1. Align a tool to the projected beam
2. Use the tool to laterally align the ball
Sensitivity comes from the geometry
CCD
Ball at mirrorBall at mirrorCCD cameraCCD camera
Beam splitterBeam splitterApertureAperture
Direction of Direction of the reference the reference
beambeam
Ball Alignment Tool
~2 µm resolution
Metering rods in action
Use of CGHs for optical alignment
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
Single pattern, creating four 1st order references
Pattern that projects spots to 4 different distances
a)
Peak intensities along the z axis
500 1000 1500 2000 25000
10
20
30
40
50
60
70
80
90
100
Propagation distance [mm]
Rel
ativ
e pe
ak in
tens
ity [
%]
Relative peak intensity at different propagation distances
b)
c) Peak at z=720mm
d) Peak at z=1080mm
e) Peak at z=1440mm
f) Peak at z=1790 mm
a) Binary Phase-only amplitude multiplexed CGH b) Relative peak intensities at different propagation distances from the CGH. Relative to the maximum peak intensity c),d)e)f) spot shapes at desired distances. All these are simulated results.
Single CGH with multiple references
CGH creating multiple wavefronts Position sensing detector
Conclusion
• CGHs are probably the most accurate and flexible things in optics
• Whatever your problem is, you can probably solve it with a CGH.