Optical Alignment with Computer Generated Holograms

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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 gt100 mm patterns with
  lt0.1 µm pixels. Thats gt 1012 pixels!
  Incredible dynamic range

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Accuracy and flexibility

• CGHs transform wavefronts with very high
  accuracyErrors are typically lt 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

4
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 ?7?m
CGH aligned within ?7?m
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 ?7?m
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 ?7?m
Placing a ball concentric to zero order gives a
very good reference
Distance betweenballs can be measuredwith
metering rods
Lateral position of the balldefined by
lightAxial position defined bymetering rod
CGH
Attaching the mirror to three balls defines its
positionThe 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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Ball Alignment Tool
Ball at mirror
CCD camera
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.