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First steps of the development
of a cophasing sensor for
synthetic aperture optics
applications
Géraldine GUERRI
Post-Doc ARC @ CSL
Cophasing sensor for synthetic aperture optics
applications
19 February 2009
Framework : Extremely Large
Telescopes (ELT)
• Ground-Based Large telescopes projects :
E-ELT
(Europe)
42 m diameter
1000 segments
GMT
(USA)
25 m diameter
7 segments
TMT
(Europe)
30 m diameter
492 segments
• Space telescopes projects :
– JWST : 18 segments 6.5m aperture, 25 kg/m² density
– Increasing demand for larger apertures : 20m
diameter, 6 kg/m² density
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Large lightweight telescope in
space
• Technological need :
 large diameter
 deployable
 lightweight
 cheap
space mirrors
• Critical questions :
• How manufacturing this kind of mirror ?
• How controling the mirror wavefront error ?
• How aligning coherently the sub-apertures between each
other?
• My work and CSL concern : development a demonstrator breadboard
of a cophasing sensor for space segmented mirrors made with 3 or 7
segments
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Cophasing sensor
• Measurement the relative positioning of each
subaperture : determination of piston and tip-tilt errors
Piston : Translation along
the optical axis (λ or nm)
Tip/ Tilt : Rotation of the
sub-pupil perpendicular to
the optical axis (rad or
arsec)
• 2 phasing regimes to
consider :
– Coarse phasing in
open loop
– Fine phasing in
closed loop : error < λ/2
Increase sensor complexity
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Sensor requirements
•
•
•
•
•
•
Cophasing of 3 to 7 sub-apertures
Separate measurement of piston and tip/tilt
Low weight and Compacity
Real-time correction
Reduced hardware complexity
Linearity, High range and accuracy
Tip/tilt measurement
Piston measurement
Range:
± 1 mm
Range:
100 µrad
Accuracy:
50 nm
Accuracy:
0.5 µrad
• At longer term use of integrated optical components
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Work plan
Survey of state of the art of cophasing sensor
Sensor techniques selection
Validation by numerical simulations
Experimental validation
Feasibility demonstrator of the cophasing of 3
sub-apertures with standard optical
components
Study and Design of a space-compatible
breadboard
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Review of the state of art of
cophasing sensor
• Survey of 15 different principles :
Pupil plane detection sensor
Focal plane detection sensor
 Slope measurement :
Shack-Hartmann sensor,
Pyramidal sensor
 Dispersed fringe sensor,
Phase shifting interferometer
 Curvature sensor
 Phase retrieval/Phase diversity
algorithm
• Trade-off criteria :
• best compliance with the requirements
• sensor maturity
• breadboard feasibility within a short term
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Cophasing sensor : methods
selection
Coarse phasing
PISTON
TIP-TILT
Dispersed fringe sensing
(CSL : Roose et al, 2006)
Shack-Hartmann Sensor
(Shack & Platt, 1971)
Fine cophasing
Shack-Hartmann Sensor
Phase retrieval real-time
algorithm
or
Error < λ/2
(Baron et al., 2008)
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
Phase diversity real-time
algorithm
(Mocoeur et al., 2008)
19 February 2009
Phase retrieval algorithm
• Phase errors extracted from one simple focal image
• The problem to solve is highly non linear
• Classical Phase retrieval algorithm are iterative and time
consuming (~ 60 FFT computations)
• (Baron et al., 2008) : For fine cophasing (Piston < λ/2),
analytical and real-time solutions exists (only one FFT
computation)
• Based on Optical Transfert Function (OTF) Computation
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Numerical validation of the phase retrieval
algorithm for piston estimation
Differential Piston errors can
be determined from the
intensity of peaks of the
phase of the OTF
Without
Piston
error
PSF
OTF
Modulus
OTF
Phase
3 sub-aperture pupil
With
Piston
error
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Phase retrieval algorithm
numerical validation
• Algorithm validation
•
Test of the sensor linearity
300
Différence de piston calculées (nm)
-----------------------------------------------p1-p2 : -50
p1-p3 : -150
200
Valeur de piston obtenue (nm)
Valeurs des pistons introduits (nm)
----------------------------------------------p1 : -50
p2 : 0
p3 :100
100
0
-100
-200
p2-p3 : -100
-300
-400
-300
-200
-100
0
100
200
300
400
Valeur de piston introduite (nm)
• Algorithm Computation time (MATLAB) : 0.4s
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Phase retrieval demonstrator
set-up
Collimating Lens
Window of
known thickness
f=50mm
Laser diode
Beam
expander
Focusing Lens
f=300mm
λ=633nm
CCD
Camera
Pinhole
Pupil mask
Implementation in
laboratory in progress ….
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Future prospects
• Experimental feasibility tests of the PR
method
• Optimisation of the PR algorithm
• Study and design of a system to introduce
various and precise piston values
• Implementation of the coarse piston
sensor
• Design and implementation of the tip-tilt
measurement
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Outlook
• Tests of the preliminary sensor
performances in open & closed loop
• Study and design of a compact and spacecompatible sensor with fibered and
integrated optics
• Implementation, validation and
performance assessment of this
cophasing sensor
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Thanks for your attention
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Différence de piston calculées (nm)
-----------------------------------------------p1-p2 : -50
p1-p3 : -150
p2-p3 : -100
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Phase retrieval demonstrator
breadboard
Shack Hartmann
Sensor :
CCD Camera Atmel :
• 2048x2048 pixels
• 101 x 101
MicroLens
• 7.4 µm x 7.4 µm
pixels
• λ/10 resolution
• 10 bits dynamics
PHOTO
Implementation in progress ….
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
ULB
Tip tilt controller
AMOS
Tip Tilt
actuator
Thales
Piston Sensor
CSL
Microlenses
ULB
Shack-Hartman system
DSP controller
CSL
Camera
+ source
ULB
S/W
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Measuring steps
• Piston measurement :
– Phase retrieval (PR) setup
– Large amplitude piston : central fringe
identification from visibility estimation
– Small amplitude piston : accurate phase
measurement by PR
• Tip-tilt measurement :
– Shack-Hartmann Wavefront Sensor
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Valeurs des pistons introduits (nm)
----------------------------------------------p1 : -50
p2 : 0
p3 :100
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Framework
• Today’ s astronomy needs extremely large telescope (High FOV,
high resolution) with huge diameter >30m
• Technological solutions
– Large segmented telecopes
– Multiple aperture telescopes
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Project presentation
- How to build :
large diameter
deployable
mirrors
lightweight
cheap
– Collaboration between CSL, SCMERO Laboratory
(Brussels University), AMOS & Thales
– The goal of the project is to develop a demonstrator
with 3 (7 design goal) segments λ/10 mirror
– One of the critical issues is the control of the WFE of
the system
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Numerical simulations
• Validation of two algorithms :
– Dispersed speckle piston sensor .. in progress
Problems with sensor linearity
– Real-time phase retrieval algorithms
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Lightweight space deformable
mirror : project work plan
Management,
reporting and
support
WP 0000
CSL
Overal
Project
Management
and reporting
WP 0100
CSL
ULB
Management
and Reporting
WP 0200
ULB
Thales
Management
and Reporting
WP 0300
Thales
AMOS
Management
and Reporting
WP 0400
AMOS
AIV and
testing
WP 4000
CSL
AIV
WP 4100
AMOS
Functional
and
performance
testing
WP 4200
CSL
Deformable
mirror
preliminary
design
WP 1000
ULB
Deformable
mirror
Breadboard
detailled design
WP 2000
CSL
Procurement
and bread
board
manufacturing
WP 3000
AMOS
Review the
state-of-the-art
of cophasing
methods and
selection
Review the state
-of-the-art in
piezo actuator
control methods
and selection
Wafer
technology
review and
preliminary
concept
WP 1100
CSL
WP 1200
WP 1300
Thales
ULB
Detailled
design of the
cophasing and
WFS sub
systems
WP 2100
CSL
Manufacturing
of the
cophasing and
WFS sub
systems
WP 3100
CSL
Preliminary
opto
mechanical
concept
WP 1400
AMOS
Detailled
design of the
piezo control
sub systems
WP 2200
Thales
Wafer
detailled
design
WP 2300
ULB
Opto
mechanical
detailled
design
WP 2400
AMOS
Manufacturing
of the piezo
control sub
systems
WP 3200
Thales
Manufacturing
of Wafer
subsystem
WP 3300
ULB
Manufacturing
of Opto
mechanical
subsystem
WP 3400
AMOS
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
Rigid mode
control and
control
strategies
overview
WP 1500
ULB
Rigid mode
control and
control
strategies
detailled design
WP 2500
ULB
Control
algorithm
development
WP 3500
Guidelines
and
Recommenda
tions for
Future Work
WP 5000
ULB
Test Results
and Synthesis
Report
WP 4300
ULB
Establish
Guidelines
WP 5100
XXX
Critical issue : the
manufacturing of
the sub-system
dedicated to
cophasing and
the wavefront
sensor of the
mirror
ULB
19 February 2009
Cophasing sensor
selection
Piston measurement
• Focal-Plane WFS are very appealing:
– Single/multi- aperture, simple hardware
– Real-time algorithms exists (Baron et al., 2008
Mocoeur et al., 2008)
– Performance experimentaly demonstrated at ONERA
Complexity is transferred from hardware to software
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Cophasing sensor
selection
Tip-Tilt measurement
• Shack-Hartmann Wavefront sensor
available at CSL
• Analytical and real time Phase retrieval
algorithm
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Piston – Tip/Tilt definition
Piston : Change of
poistion along the Z axis
(λ or nm)
Z
Tip : Rotation of the
surface around the Y axis
(rad or arsec)
Tilt : Rotation of the
surface around the X axis
(rad or arsec)
Y
X
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Review of the state of art of
cophasing sensor
Sensor type
Pupil plane detection sensors
Parameter/ Method
Shack-Hartmann
Sensor
Curvature Sensor
Dispersed fringe
sensing
Dual Wavelength
instantaneous
Phase-shifting
interferometer for
close-loop control
"Classical" phase
retrieval
"Real-time" phase
retrieval
"Classical" phase
diversity
Piston measurement ?
NO (except for E-ELT)
NO (except for E-ELT) YES
YES
YES
YES
YES
Tip-tilt measurement ?
YES
NO
MULTI
YES
3 DOF for each 61
segments : 183
YES
#DOF
YES
3DOF for each
segment
YES
YES but with
restrictions with
combined piston
>11
>11
High
Pyramid Sensor
YES
Keck narrowband
Algorithm (Chanan et
al., 2000)
N/A
YES
Gradient-based iterative Analytic Least-Square Gradient-based
method
approach
iterative method
Algorithm
N/A
N/A
N/A
Data processing
Slope integration
Poisson's equation
Slope integration
FFT 2D
Time to compute 100N
FFT 2D on the zone of
interest for N diversity
focal images
High
FFT 2D
Speed
Fast : up to 950 Hz
(depends on
configuration)
Slow : up to 5 Hz
Fast : 400 Hz
Low
4.44 Hz
Time to compute
between 20 and 60 FFT Time to compute 1
2D on the zone of
FFT 2D on the zone of
interest
interest
#Apertures
High
High
High
12
High
High
TBC already ? 12
FFT 2D
Monochromatic or
Polychromatic
Monochromatic or
Polychromatic
Visible or NIR
Polychromatic
2 superluminescent
diodes at 834.6 nm &
859.6 nm
High flux needed
H/W detector
CCD
Avalanche photodiode
(bulky & expensive) or
CCD
CCD
1024x1024 CCD
4 CCD
Optical complexity
Medium
up to 1500λ
Medium
|Piston| < λ/2 ;
High for tilt
Medium
Range
Medium
|Piston| < λ/2 ;
High for tilt
Linearity
Close to 1
TBD
TBD
0.98
High
Simple
Piston: ± 7.2 µm ; Tip- [-λ/2 ; λ/2] for piston and
tilt: ± 250 µrad
tilt
0,99 for Piston ; 0.93 for
Tilt
TBD
Simple
λ/2 < Piston < λ/2 ;
|Tilt| < 0.3λ
0,96 for piston ; 1,15
for Tilt
Measurement accuracy
From λ/100 to λ/1000
less than 15 nm rms
λ/30 rms
90 nm
better than λ/13 rms
Repeatability
< λ/200 rms
TBD
TBD
TBD
TBD
Piston : 0.75 nm for
5.105 photoe- ; Tilt :
Piston: 0.48 nm rms ; 1.21 nm for 3.3 104
Tip-tilt: 74 nrad rms photoe-
TBD
Piston : 0.75 nm for
5.105 photoe- ; Tilt :
1.21 nm for 1.6 105
photoe-
Particularities
Cylindrincal microlenses
on the E-ELT sensor to
measure piston steps
Restricted to
spherical shapes that
allow operation
Excellent for phase
without gigantic beam reconstruction but time
expander
consuming
Need for diluted
noncentrosymmetric
pupil ; Small phase
Excellent for phase
aberrations
reconstruction but time
assumption (<2π rad) consuming
Fine Phasing
H/W source
Trade-off
criteria
Focal plane detection sensors
Type of achievable phasing :
Cophasing (fine) or
coherencing coarse
Type of telescopes that can
be controlled
1 µm to 16 µm
Standard CCD Flux of
105 e-/frame
White light source + 1
narrow band filter (λc =
650 nm, Δλ = 40 nm) High flux needed
Standard CCD ; Flux
of 105 e-/frame
Both
Both
Both
Coarse Phasing
Fine Phasing
Both
All
Segmented
Segmented
Segmented
Segmented
Multi-aperture ; Segmented
Multi-aperture
Maturity
Well know and
manufactured in mass ;
The most used sensor
Preliminary laboratory
Preliminary laboratory results and upcoming
results and upcoming on-sky tests on the VLT
on-sky tests on the VLT & LBT
Instrument controlled by this
method or envisaged to be
Keck, VLT/NACO,
Gemini North and South, Subaru, VLT/MACAO,
MMT, Palomar, Lick, E- Gemini South/NICI,
ELT, Grantecan
Grantecan, E-ELT ?
LBT, WHT ?, E-ELT ?
JWST ; Keck
Main references
Platt & Shack, 1971
Ragazzoni, 1996
Fogale Nanotech:
F. Baron Thesis,
Shi et al., Applied Optics Wilhem et al., Applied ONERA, 2005 ; Baron
2004
Optics 2008
et al., JOSA 2008
Cophasing sensor for synthetic aperture optics
applications
Roddier, 1988
Preliminary laboratory
results and upcoming
Testbed results ; On sky on-sky tests on the
Laboratory tests in
test on Keck
VLT
progress
Géraldine GUERRI
E-ELT
JWST ; Point-source
and extended scenes
observations
Laboratory tests in
progress
Defocus generator +
Standard CCD
Simple if only 2 phase
diversity images are
needed
[-λ/2 ; λ/2] for piston
and tilt
0.99 for piston and tilt
TBD
Piston : 0.75 nm for
2.105 photoe- ; Tilt :
1.21 nm for 1.1 104
photoe-
Both
Multi- Aperture ;
Segmented
Laboratory tests in
progress
Darwin, SOTTISE
(Earth Observation)
JWST ; Earth
observation
F. Baron Thesis,
F. Baron Thesis,
ONERA, 2005 ;
ONERA 2005 ; Baron Delavacquerie et al.,
et al., JOSA 2008
2008
19 February 2009
Plan
•
•
•
•
•
•
Framework and project presentation
State of the art of the cophasing sensors
Sensor selection
Numerical simulations of the selected sensors
Sensor Feasibility demonstration breadboard
Future propects
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009
Cophasing sensor
selection
Piston measurement
• Focal-Plane WFS are very appealing:
– Single/multi- aperture, simple hardware
– Real-time algorithms exists (Baron et al., 2008
Mocoeur et al., 2008)
– Performance experimentaly demonstrated at ONERA
Complexity is transferred from hardware to software
– Multiple aperture piston/tip/tilt/more (DWARF)
Cophasing sensor for synthetic aperture optics
applications
Géraldine GUERRI
19 February 2009