h.j.tiziani, university of stuttgart · advanced optics by aspherical elements processing and...
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Advanced Optics by Aspherical Elements
Processing and Metrology of aspheric SurfacesH.J.Tiziani, University of Stuttgart
Content
Introduction, Historical Background
Processing of aspheric surfaces
Metrology of aspheric surfaces
Summary and Future trends
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Advanced Optics by Aspherical Elements Processing
Considerable progress has been made the last 30 years
Timeline 1638 Decartes; Shape copying machine principle
1920Mackensen Realisation of Shape copying
1976 Jones, Aspen, Bajuk; Computer Controlled Polishing
1976 Computer controlled grinding
1850 Manual polishing of aspheres
1980s Fluid Jet, MRF, IBF; Correction
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Progress in generating and polishing processes (Historical)H.J.Tiziani
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Principle of generating orpolishing spherical surfaces
Principle of a copy machine for aspheres
Grinding and Polishing of aspherical mirrors
Advanced Optics by aspherical Elements Processing
Grinding and Polishing of Aspheric Surfaces3 H.J.Tiziani
Generating Polishing/ smoothing Local correction
MetalIR material
Diamond turning
GrindingMilling
CCP
Fluid jet
Pitch polishing
Speed polishing
MRF
IBF
Fabrication
Classical Hybrid Moulding
Different fabrication processes
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Advanced Optics by Aspherical Elements Processing
Diamond turning Precision grinding Zonal polishing Precision optics moulding
Some manufacturing technologies5 H.J.Tiziani
Advanced Optics by aspherical Elements Grinding
Disc wheel grinding
The disc-shaped grinding tool is moved with precise contour-controlled axes (2 linear) to generate the aspherical surface.
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Advanced Optics by Aspherical Elements Grinding
Cup wheel grinding
The cup-shaped grinding tool is tilted with a constant angle to the surface normal. With this angle a point contact between tool and lens is achieved and the aspherical surfaceis generated with precise contour-controlled axes (1 swivel and 2 linear).
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Advanced Optics by Aspherical Elements Grinding
Roughness control by Grinding8 H.J.Tiziani
Advanced Optics by Aspherical Elements Computer controlled processing
Principle and set-up of one of the first computer controlled grinding machinesbuilt by Zeiss (courtesy Carl Zeiss)
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Advanced Optics by Aspherical Elements Computer controlled polishing
dtds
ALCp
dtdz
⋅⋅=
where
Thickness change over time or removal rate [m/s];
Cp: Preston coefficientL: Load = Total normal force [N]; A: Surface area, where the removal takes place [m²]
: Relative velocity of work piece to tool [m/s]dtds
Polishing:The removal of material by polishing is a function of tool pressure,therelative velocity between tool and optical element and the polishing time.
Removal by polishing :Preston equation(1927)
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≈ few µm
work pieceFilled with slurry
grain
Polishing pad
≈ few µm
work pieceFilled with slurry
grain
Polishing pad
cracks
brittle mode
surface
plastic zone
diamond
cracks
brittle mode
surface
plastic zone
diamondbrittle mode
surface
plastic zone
diamond
R
c(x): tool function
x
nominal shape
real shape
R(x)
0 x
Corrective Polishing(CCP) SmoothingGenerating
( ) ( ) ( )∫ −= ''' dxxsxxcxR
Tool function Real form measured
Desired removal
Computer controlled Form polishing11 H.J.Tiziani
Advanced Optics by Aspherical Elements Corrective pol.
Final Form Polishing Process12 H.J.Tiziani
Advanced Optics by Aspherical Elements
Zonal polishing
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Advanced Optics by Aspherical Elements CCP
tool
optical element
Computer controlled polishing process of the NTT ESO 3.5m mirror (left). Principle of the CCP process (right).
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Advanced Optics by Aspherical Elements Corrective polishing
Principle structure of ALPS Courtesy Schwarzhans Swiss Optic
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Advanced Optics by Aspherical Elements Corrective polishing
Form Corrective Polishing
Computer suported ultraprecision finishing of aspheric surfaces with Zonal Polishing is iterative because of variations of the tool function but can be economical by appropriate metrology.
Fluid jet, Magneto Reological Finishing or Ion beam correction polishing show no tool wear. The patented method of Magneto Rheological Finishing (MRF) has beeninvented in the late 80s in Minsk/Belarus. It has been further developed by a team around W. Kordonski and the Center for Optics Manufacturing (COM) in Rochester/USA for a deterministic fabrication of optical surfaces in particular aspheres). In 1998, QED Technologies introduced the first commercial product using MRF. The complete process chain with MRF for a production of aspherical surfaces was first presented in 2000 at Optatec in conjunction with Schneider Optic machines.
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Principle of the MRF Process. The material is removed by the sheared MR fluid (W. Kordonsky, QED Technologies)
Advanced Optics by Aspherical Elements MRF Process
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Advanced Optics by Aspherical Elements IBF
Principle of Ion Beam Figuring (IBF)(left). Outside view of an IBF System (right).
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Guiding cylinder
spacer
Precision gob
MasterInjection molding
Hot embossing
Injection embossing
Different kinds of plastic moulding (left), the blank press method for glass (right)
Moulding temperature
Moulding method
Mould material
Advanced Optics by Aspherical Elements Moulding
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Advanced Optics by Aspherical Elements Micro lens fabrication
Wafer-based microlens manufacturing technology
Reactive ion etching (RIE) transfer process of resist micro lenses in fused silica. A correction of the lens slope is obtained by changing the etch rate between the resist and fused silica during the etching process.
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Advanced Optics by Aspherical Elements Metrology for optical systems
Metrology
Measuring principles for optical system performance
•Optical Transfer Function (OTF), The Modulation Transfer Function, MTF, as well as the a phase transfer function PTF•Interferometry•Wavefront Sensor such as the Shack-Hartmann wavefront sensor.•Foucault Test •The star test is used in astronomy and microscopy; the image of a pinhole is analysed.•Polarization properties are useful for stress analysis in the material for instance or stress induced by cementing or mounting of lenses or crystalline optical materials used for very short wavelengths •Transmission and stray light measurement.
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Punktbild Hubble-Weltraumteleskop
Reason for limited resolution:Enlarged spread function (point image)
Hubble-Telescope
With sphericalAberration
WithoutSph. Ab.
Advanced Optics by Aspherical Elements Spread function/OTF
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Advanced Optics by Aspherical Elements Surface classification
Form ( shape), lateral dimension
Waviness, Defects,
Roughness,
mm1≥Λ
mmm 120 ≤Λ≤µ
mµ20≤Λ
Surface Classification and Surface Measurement
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Metrology for optical surfaces
Surface form and roughness measurement of grinded and lapped surfaces •Stylus instrument
•Confocal instrument
•White light interferometry
Surface form measurement of polished optical surfaces•Interferometry is the best known and the most precise measuring technique for measuring the surface form.•The Shack-Hartmann wavefront sensor is an alternative•A stylus instrument is an another technique for measuring individual surface forms•Confocal instrument (scanning)•White light interferometry (scanning) •Deflectometry •Polarization optical methods
Roughness and waviness measurement of polished surfaces
•Stylus instrument ,White light interferometry, Confocal.
•Atomic force instrument
•Scattering: ARS, TIS24 H.J.Tiziani
Advanced Optics by Aspherical Elements Taktile measurement
Diamond tip in tactile measurements
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Advanced Optics by Aspherical Elements Point measurement, Foc. Det.
White light Interferometer
Mirau-Objective 50 x
Measuring field 0,384 x 0,286 mm²
NA (Objective) 0,55
Working distance ca. 0,5 mm
Lateral Resolution ca. 1 µm
z-Resolution 10 nm
0 1 2 3 4 5
Z [µm]
100
150
I
Lightsource
Referencemirror
Object
Tubuslens
Partly reflectingmirror
Mirau-objective
Camera
Lightsource:•White light•LED•Superlum. diode•Laserdiode (antireflex)
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PinholeBeamsplitter
f
I
z
z
Object
z=f
αλ
cos.FWHM
−⋅
=1450
α
FWHM
Advanced Optics by Aspherical Elements Point measurement, Foc. Det.
Pointdetector
Principle of Confocale Measurement27 H.J.Tiziani
Advanced Optics by Aspherical Elements Surface characterization by Power Spectral Density, PDS
Micromap 2,5-fach
Micromap 20-fach
AFM
Micromap 2,5-fach
Micromap 20-fach
AFM
Power spectral density plot and related surface topography for a diamond turned master.The characteristic structures from diamond turning are clearly visible in the PSD plot
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Topography, 5µm*5µm500 * 500 pixeltip radius < 5nm :
Roughness:0.34nm RMS
2D-Fourier Transforme :Spatial Frequency Spectrum
Spatial Frequency Filtered,Low-Pass : |kgr| = 1/λ | λ=500nm
kx /µm-1
k y/µ
m-1
Fourier-filteredTopography :
Roughness0.18nm RMS
0
0
25
-25 25
Influence of the low pass filter on the roughness measurement
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Advanced Optics by Aspherical Elements Surface characterization , Roughness measurement, ARS
Schematic of operation for microstructure testing(Duparre)30 H.J.Tiziani
Advanced Optics by Aspherical Elements Surface measurement methods
50 mm – 2m4 - 80,01 mm – 1 mmMacroscopic Fringe Projection
o.1mm-100mm0.1nm-10nmScattering
scanningpointwise0.05nm-0.1nmAtomic force microscope
1mm-100mmscanning speed0.1mm/sec0.5nm-400nmStylus instrument
1mm - 30 mm4 - 80,1 µm - 10 µmMicroscopic Fringe Projection
50 µm - 5 mm100 - 10001 nm -10 nmWhite Light Interferometry
1 µm – 30 mmparallel proc50 - 20010nm - 10 µmConfocale Principle
(Microscope)
Diameter of Measuring field
Camera Images inpixels(typical)
Height Resolution(typical)
Measuring Principle
Comparison of 3D-microscopic surface measuring methods 31 H.J.Tiziani
Advanced Optics by Aspherical Elements Interferometry for surface Form measurement
Laser
M=F‘
KS Obj.
PS
Camera
TPL3
L2
L1
Twyman Green interferometer for system or surface testing with typical fringe patterns
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Advanced Optics by Aspherical Elements Interferometry for Form measurement (asph.surface)
Element under Test
Beamsplitter
Beamsplitter
Mirror
Mirror
ΣRef+ ΣObj
ΣObj
Adaptive Optics
ΣRef
Master
Comparison Measurement using Mach Zehnder- Interferometer
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Advanced Optics by Aspherical Elements Aspheric surface Form measuring
Problem:
• Wavefront departure from a best fit sphere leads to high local fringe density in the interferogram
Reason:
• slope of aspheric wavefront
Effect:
• Interferogram cannot be unwrapped• (Violation of Nyquist-theorem)• Vignetting and caustic of the wavefront
Moiré 1.Order
Interferogram
Problems by Testing Aspherical Surfaces
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Advanced Optics by Aspherical Elements Form measurement of AS
Methods for Testing Aspheric Surfaces
Wavefront shaping systems + null test + partial compensation
staticdiffractive
refractive
adaptivemembrane mirror
LCD
dynamic subapertures
dynamic microlens array
static microlens array
Multiple wavelength interferometry
(Moiré techniques)
Shack-Hartmann sensor
Alternative methods
Other methods: Stilus instruments(pointwise), Stitching(lateral, along opt.axis),Deflectometry
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Advanced Optics by Aspherical Elements Interferometry for Form measurement of AS
Reference
Asphereunder test
CGH
CCD
Objective
Spatial filter
Reference
Aspheric system under test
CGH
CCD
Spatial filter
Testing aspherical surfaces, AS, and systems with Null lens(CGH)
Test of refractive Null lens with CGH in reflection
Testing an aspherical surface in reflection; CGH in transmission
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Advanced Optics by Aspherical Elements Null lens testing
PV = 133,1 nm / RMS = 21,5 nm
-100 -50 0 50 100
x [mm]
-100
-50
0
50
100
y [m
m]
-20020406080
[nm
]
PV = 53,9 nm RMS = 16,5 nm
-100 -50 0 50 100
x [mm]
-100
-50
0
50
100
y [m
m]
010203040
[nm
]
PV = 126,9 nm RMS = 13,6 nm
-100 -50 0 50 100
x [mm]
-100
-50
0
50
100
y [m
m]
-60-40-200204060
[nm
]
Refraktives K-System
CGHRefractivecompensationsystem
CGH: νmax = 500 Lp/mmpmin = 2 µmd = 220 mm
CGH as Aspheric Master (Null Mirror)
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Advanced Optics by Aspherical Elements CGH as Null lens
Parameters to be considered in CGH-Null Testing– Interferometer type and CGH Position – Chrome on glass vs. Phase type CGH
(Diffraction efficiency, Setup: application in single/double reflection/pass?)
– Inline vs. Off axis CGH (Separation of diffraction orders)
– CGH Specification and Feasibility(Wavefront aberration caused by a patterndistortion error depends on spatial frequencyin the CGH)
CGH Design
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Advanced Optics by Aspherical Elements Interferometry with CGH
Procedure for testing aspheric surfaces
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CGH-null: νmax = 35 Lp/mmpmin = 29 µmd = 86 mm
PV = 450,5 nmRMS = 79,7 nm
Alignment-CGH: νmax = 500 Lp/mmpmin = 2 µm
Testing of an asphere
Advanced Optics by Aspherical Elements Inerferometry with CGH
CGH as Null-Lens
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Advanced Optics by Aspherical Elements Interferometry with CGHAberrations caused by
pattern distortion
-20 -10 0 10 20
x [mm]
-20
-10
0
10
20
y [m
m]
-100
-50
0
50
100
[nm
]
1. Distortion of the hologrampattern
→ Writing errors
2. Surface figure errors of theCGH-substrate:
→ Surface figure errors, flexure
Surface figure errorsof a chrome blank
-40 -20 0 20 40
x [mm]
-40
-20
0
20
40
y [m
m]
-150-100-50050
[nm
]
CGH-Aberrations
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Advanced Optics by Aspherical Elements Interferometry with CGH
Cross –Comparison with different CGH‘s
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Advanced Optics by Aspherical Elements Interferometry with CGH
Calibration
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Advanced Optics by Aspherical Elements CGH generation
Technical data wavelength of exposition 488 nm intensity of exposition < 20 M W /cm 2 diam eter of substrate < 290 m m thickness of substrate < 24 m m spot size 0.8 µm resolution (radia l) 80 nm resolution (azim utal) 0.1"
CGH generation with a Photoplotter for testing aspheric surfaces
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Advanced Optics by Aspherical Elements CGH Typ
Amplitude CGH
Phase CGH
Copying Technique and Etching process45 H.J.Tiziani
Advanced Optics by Aspherical Elements Form measurement
Alternative Methods
• Wavefront adaption with membrane mirrors
• Wavefront adaptation with adaptive optics(liquid crystal displays)
• Multiple Wavelengths Interf.
• Shack Hartmann (adaptive)
• Stitching lateral and axial (Verifire Zygo)
• Reflectometry
• Pointwise measurement (scanning)
46 H.J.Tiziani
Advanced Optics by Aspherical Elements Deform. mirror
Wavefront adaptation in Astronomy by deformable Mirror47 H.J.Tiziani
Advanced Optics by Aspherical Elements Wavefront adaptation
• continuous shape• high reflectivity, dielectric
coatings possible(even used in laser resonators)
• fast (switching times below 1 ms)
Examples for electrode layouts
Hexagonal Structur Ringshaped Structur
Membrane Mirror48 H.J.Tiziani
λ/4pol. BS HVamp.
.Transmission sphere-
Aspheric surfaceunder test
CCD
Reference
HeNe
telescope
TGI Setup
Membranemirror
Asphärische WellenfrontdeformationenMembranspiegel #1
-1,0 -0,5 0 0,5 1,0rel. Apertur
-40
-20
0
z [µ
m]
Asphärische WellenfrontdeformationenMembranspiegel #1
-1,0 -0,5 0 0,5 1,0rel. Apertur
-40
-20
0
z [µ
m]
Asphärische WellenfrontdeformationenMembranspiegel #1
-1,0 -0,5 0 0,5 1,0rel. Apertur
-40
-20
0
z [µ
m]
Advanced Optics by Aspherical Elements Wavefront adaptation
Wavefront Adaptation by a Membrane Mirror49 H.J.Tiziani
Advanced Optics by Aspherical Elements Multi wave lengths Interferometry for Form measurement
Two wave lengths Interferometry
λ 1 λ 2
λ λ λλ λ
µsyn m=−
=1 2
2 1
66 7| |
,
822 nm
0 200 400 600 800X
0
200
400
600
800
Y
0
1E-5
2E-5
3E-5
4E-5
5E-5
6E-5
812 nm precompensation
asphere
LD2 812 nm
LD1 822 nm
monomode-fiber
piezo-translator
membranemirror asreference
digital camera
50 H.J.Tiziani
Advanced Optics by Aspherical Elements Adaptive optics for Shack Hartman Sensor
Shack Hartmann Sensor
Shack Hartmann Sensor measures the slopeof the wave front
Some limitations- are due to :
•Ambiguity, when the spot depasses the corresponding pixel
•Deformation or defocusing of the spot due to curvatures of the wavefront across the microlens used (large aberrations)
A solution is the use of adaptive microlenses
51 H.J.Tiziani
Advanced Optics by Aspherical Elements Shack Hartmann Sensor
principle:
Local wavefront slope:
f
dydx
dy
1δ/δxW(x,y) =
fδ/δy
Accuracy scales with f
Wavefront reconstruction by
• direct integration of wavefront slopes
• least square methods for error minimization
Principle of Shack Hartmann Sensors
52 H.J.Tiziani
Advanced Optics by Aspherical Elements Shack Hartmann sensor for AS
extreme slopes:
spots leave target areas,ambiguity occurs
extreme curvatures:
aberrated spots not suitablefor accurate determinationof positions
Problems with extreme wavefront shapes
53 H.J.Tiziani
programmablehigh resolution
LCD
replacedby
Advanced Optics by Aspherical Elements Active Adaptive optics
static, diffractivemicrolens array
Dynamic microlenses generated by LCDs
54 H.J.Tiziani
Advanced Optics by Aspherical Elements Shack Hartmann with adaptive optics
accuratedeterminationof spotpositionsImpossiblewithout localab.correction
Measurement with consideration of local wavefront aberrations:Object under test:progressive lens with an addition of 3dpt.55 H.J.Tiziani
Advanced Optics by Aspherical Elements Adaptive optics for tilted wavefronts
Detail for generating the tilted reference wave and the phase shifting
Principles of dynamic reference or object beam
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Advanced Optics by Aspherical Elements Interfermetry with tilted wavefronts
Single phase measurements of a defocused reference surface with5x5 tilts(left), the result after unwrapping of 5x5 interferograms (right)
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Advanced Optics by Aspherical Elements Deflectometry
Fringe-Projector
Groundglass
Aspheric Surface(Progressive Eyeglass)
DeformedFringe pattern
CCD-Camera
Deflectometry used for testing Progressive Eyeglasses
58 H.J.Tiziani
Advanced Optics by Aspherical Elements Processing and Metrology
Summary and Conclusion•A lot has been invested in machines and processing lately for high precision and economic production of spherical and aspherical surfaces.
•Computer supported processing together with process simulation and
Correction has been and will be further improved
•Precision manufacturing is only possible if the quality can be measured. In particular, metrology for Form measurement of aspheric surfaces needs to be further improved. Today, metrology is a cost driver.
•Aspheric surfaces will be used more frequently when the cost of an aspheric surface is not much higher than twice the cost of a spherical surface.
•In process metrology is required. New techniques are or will be developed. Some possibilities were discussed at least in principle.
•In future system performance may be improved by an aspheric corrector surface.
•Care needs to be taken in the mounting because aspherical surfaces and systems are usualymore sensitive to centring (tilt)59
H.J.Tiziani