Thin Optic Constraint Mireille Akilian Amir Torkaman Space - - PowerPoint PPT Presentation

Thin Optic Constraint

Mireille Akilian Amir Torkaman

Space Nanotechnology Laboratory December 10, 2003

Outline

• Problem Statement
• Functional Requirements
• Strategies
• Concepts developed
• Detail design
• Results and Conclusions

Problem Statement

Forces that lead to optic surface warp:

• Gravity (weight sag at angle θ, effect linear with θ; θ < 70 arcsec)
• Friction (2-3 µm error)
• Thermal expansion mismatch between optic and device (100’s of

microns) Optics flatness on one side < 0.5 µm peak to valley

Functional Requirements

• Reduced effects of thermal expansion and friction
• Gravity pitch accuracy 36 arcsec
• Placement repeatability:

Pitch: 72 arcsec Lateral: 1.5 mm

• Ease of inserting optic into device
• Optic front surface clear during metrology

Three constraint points on surface of rectangular and circular optics

Optic

Strategies

Air pressure

• ptic

Vacuum

• ptic

Vacuum preloaded air bearings Double-Sided air bearings

• ptic

Double-Sided flexures

Concepts Developed

Functional Requirements Design Parameters

Double-Sided Air Bearings Double-Sided Flexures Air gap variation < 3 µm Air temperature drop < 0.5°C Acceptable stiffness Inclinometer & tilt stage Hold optic vertically Back surface actuation Optic front surface clear Small bearing OD Metal bearings Optimum feeding parameter Flexure length Constrain up to 1.6 mm thick optic Monolithic flexures Minimum centerline misalignment Flexure geometry Account for 1°C temperature change

Double-Sided Air Bearings

Design Analysis

Y-translation constraints Opposed, inherently compensated bearings (x6) Vacuum preloaded bearings Optic

Set Set A Ao

• : constrained by geometry

: constrained by geometry Choose h Choose h Choose A Choose Ai

i/

/A Ao

• 0.6 <Λξoptimum < 1.1

Choose Ps Calculate load capacity and stiffness

k h Ao di Ai do W Ps Λξ

mm mm2 N N/mm2 N/µm mm mm2 µm

38.5 0.13 10 1.5 3.1 7 0.2 0.376 0.172

Double Side Air Bearings

Experiment, Results, and Conclusions

Load vs. air gap

1 2 3 4 5 10 15 20

Air gap ( m) Load (N)

Operating region

Theoretical load capacity

µ

No friction and thermal mismatch between optic and device No contact deformation Acceptable load capacity and stiffness Design and assembly complexity Moderate to High cost

Double-Sided Flexure Assembly

Double- sided flexures Bottom and side flexures Reference Block Flexure tilt stage

Double-Sided Flexures

Design Analysis Wire EDM-ed, Monolithic flexures

42 mm ϕ = 2 mm ruby balls 0.8 mm 6.75 mm 0.6 mm 21 mm

2 mm

• 1. Vertical flexures

Allow for optic insertion/removal Provide preload (klateral = 2.45E-4 N/µm)

• 2. Horizontal flexures

Accommodate for thermal expansion

(klateral = 0.024 N/µm) Material Aluminum 6061 T651

Double-Sided Flexures

Flexure Deflection and Stress

Lateral displacement due to optic thickness After optic placement: Horizontal displacement 283 µm Vertical displacement 6.6 µm Vertical parasitic motion

Bottom/Side Flexure Design

2 3 2 3

2.47 3 2 3.52

buckle z z z x z Fx axial x lateral M n flx

EI F F L M X F F L M L EI EI EA k L F k EI m L δ δ ω = > = ∆ × = + = = =

Outer Diameter (mm) 0.635 Inner Diameter (mm) 0.508 Length (mm) 50 Buckling Force (N) 0.93 Axial Stiffness (N/µm) 0.456 Lateral Stiffness (N/µm) 2.22E-05

Vertical Reference Flat

• Front surface flatness: 0.1um

– Optically polished Nickel coated Aluminum block – 90 deg Angle +/- 1 arcsec

• Base has tilt adjustment

– Resolution: 2 µrad

• Inclinometer resolution: 14 arcsec

Flexure Tilt Stage Design

• Allows for pitch / yaw

adjustments (2 ± 0.0005°)

• Actuation Mechanism:

– Fine-thread (#¼-100) screws

• Preload Mechanism:

– Springs or Belleville washers

System Assembly

Preliminary Experiments

Autocollimator Experiments:

• Flexure tilt stage:

achieves desired range & accuracy (2 µrad)

• Repeatability

Pitch: 1.2 µrad Yaw: 11 µrad

Surface Metrology Results

Shack-Hartman Surface Metrology Results:

• No local deformations at

flexure/optic interface

• Placement repeatability

55 nm

Conclusion

• Identified and analyzed optic deformation forces

– Gravity – Friction – Thermal expansion

• Theoretically and experimentally proved the ideal concept
• f using air bearings
• Designed and built an optic holding device using flexures
• Future Improvements:

– Reference flat must be polished – Systematic and repeatable optic placement procedure – Further surface metrology experiments