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.