Thin Optic Constraint

Mireille AkilianAmir Torkaman

Space Nanotechnology LaboratoryDecember 10, 2003

Outline

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

Problem StatementForces 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 arcsecLateral: 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

StrategiesAir pressure

optic

Vacuum

optic

Vacuum preloaded air

bearings

Double-Sided air bearings

optic

Double-Sided flexures

Concepts DevelopedFunctional 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 stageHold optic vertically

Back surface actuationOptic front surface clear

Small bearing OD

Metal bearings

Optimum feeding parameter

Flexure lengthConstrain up to 1.6 mm thick optic

Monolithic flexuresMinimum centerline misalignment

Flexure geometryAccount for 1°C temperature change

Double-Sided Air BearingsDesign Analysis

Y-translationconstraintsOpposed, inherently

compensated bearings(x6)

Vacuum preloadedbearings

Optic

Set Set AAoo: constrained by geometry: constrained by geometry

Choose hChoose hChoose AChoose Aii//AAoo

0.6 <Λξoptimum < 1.1

Choose Ps

Calculate load capacityand stiffness

khAo di Aido WPsΛξmm mm2 NN/mm2 N/µmmm mm2 µm

38.5 0.13 10 1.5 3.17 0.2 0.376 0.172

Double Side Air BearingsExperiment, Results, and Conclusions

Load vs. air gap

0

1

2

3

4

0 5 10 15 20Air gap ( m)

Load

(N)

Operating region

Theoretical load capacity

µ

No friction and thermal mismatch between optic and deviceNo contact deformationAcceptable 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 FlexuresDesign Analysis

Wire EDM-ed, Monolithic flexures

42 mm

ϕ = 2 mm ruby balls

0.8 mm

6.75 mm0.6 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 FlexuresFlexure 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 Design2

3 2

3

2.47

3 2

3.52

buckle z

z z

x zFx

axial

xlateral

M

nflx

EIF FL

M X F

F L M LEI EIEAkLFk

EIm 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 ExperimentsAutocollimator Experiments:

• Flexure tilt stage: achieves desired range & accuracy (2 µrad)

• RepeatabilityPitch: 1.2 µradYaw: 11 µrad

Surface Metrology ResultsShack-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 of 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