first steps of the development of a cophasing sensor for synthetic aperture optics applications

19 February 2009 Cophasing sensor for synthetic aperture optics applications

First steps of the development First steps of the development of a cophasing sensor for of a cophasing sensor for synthetic aperture optics synthetic aperture optics

applicationsapplications

Géraldine GUERRIGéraldine GUERRI

Post-Doc ARC @ CSL

19 February 2009Géraldine GUERRICophasing sensor for synthetic aperture optics applications

Framework : Extremely Large Telescopes (ELT)

• Ground-Based Large telescopes projects :

• Space telescopes projects :– JWST : 18 segments 6.5m aperture, 25 kg/m² density– Increasing demand for larger apertures : 20m

diameter, 6 kg/m² density

E-ELT(Europe)

GMT(USA)

TMT(Europe)

42 m diameter1000 segments




Large lightweight telescope in

space • Technological need :

space mirrors

large diameter deployable lightweight cheap

• My work and CSL concern : development a demonstrator breadboard of a cophasing sensor for space segmented mirrors made with 3 or 7 segments

• Critical questions :

• How manufacturing this kind of mirror ?

• How controling the mirror wavefront error ?

• How aligning coherently the sub-apertures between each other?


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


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

• At longer term use of integrated optical components

Piston measurement

Range: ± 1 mm

Accuracy: 50 nm

Tip/tilt measurement

Range: 100 µrad

Accuracy: 0.5 µrad


Work plan

Survey of state of the art of cophasing sensor

Sensor techniques selection

Validation by numerical simulations

Experimental validation

Study and Design of a space-compatible breadboard

Feasibility demonstrator of the cophasing of 3 sub-apertures with standard optical

components


Review of the state of art of cophasing sensor

Pupil plane detection sensor

Slope measurement :

Shack-Hartmann sensor,

Pyramidal sensor

Curvature sensor

Focal plane detection sensor

Dispersed fringe sensor,

Phase shifting interferometer

Phase retrieval/Phase diversity algorithm

• Trade-off criteria :Trade-off criteria :

• best compliance with the requirements

• sensor maturity

• breadboard feasibility within a short term

• Survey of 15 different principles :Survey of 15 different principles :


Cophasing sensor : methods selection

PISTON TIP-TILT

Coarse phasingDispersed fringe sensing (CSL : Roose et al, 2006)

Shack-Hartmann Sensor(Shack & Platt, 1971)

Fine cophasing

Error < λ/2

Phase retrieval real-timealgorithm

(Baron et al., 2008)

Shack-Hartmann Sensor

or

Phase diversity real-time algorithm

(Mocoeur et al., 2008)


• 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

Phase retrieval algorithm


Numerical validation of the phase retrieval

algorithm for piston estimation

3 sub-aperture pupil

PSFOTF

Modulus OTF

Phase

Without Piston error

With Piston error

Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF


-400 -300 -200 -100 0 100 200 300 400-300

-200

-100

0

100

200

300

Valeur de piston introduite (nm)

Val

eur d

e pi

ston

obt

enue

(nm

)

• Algorithm validation

Phase retrieval algorithm numerical validation

• Algorithm Computation time (MATLAB) : 0.4s

• Test of the sensor linearity

Valeurs des pistons introduits (nm)-----------------------------------------------p1 : -50p2 : 0p3 :100

Différence de piston calculées (nm)------------------------------------------------p1-p2 : -50

p1-p3 : -150

p2-p3 : -100


Phase retrieval demonstrator set-up

Laser diode

λ=633nm

Focusing Lens

f=300mm

Pupil mask

CCD Camera

Beam expander

Pinhole

Collimating Lens

f=50mm

Implementation in laboratory in progress ….

Window of known thickness


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-tiltmeasurement


Outlook

• Tests of the preliminary sensor performances in open & closed loop

• Study and design of a compact and space-compatible sensor with fibered and integrated optics

• Implementation, validation and performance assessment of this cophasing sensor


Thanks for your attention


Différence de piston calculées (nm)------------------------------------------------p1-p2 : -50

p1-p3 : -150

p2-p3 : -100


Phase retrieval demonstrator breadboard

PHOTO

CCD Camera Atmel :

• 2048x2048 pixels

• 7.4 µm x 7.4 µm pixels

• 10 bits dynamics

Shack Hartmann Sensor :

• 101 x 101 MicroLens

• λ/10 resolution

Implementation in progress ….


Camera + source

Microlenses

S/W

Shack-Hartman system

Piston Sensor

DSP controller

Tip tilt controller

Tip Tilt actuator

AMOS

CSL

ULB

Thales

ULB

CSL

ULB


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


Valeurs des pistons introduits (nm)-----------------------------------------------p1 : -50p2 : 0p3 :100


Framework

• Today’ s astronomy needs extremely large telescope (High FOV, high resolution) with huge diameter >30m

• Technological solutions– Large segmented telecopes– Multiple aperture telescopes


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


Numerical simulations

• Validation of two algorithms :– Dispersed speckle piston sensor .. in progress

Problems with sensor linearity

– Real-time phase retrieval algorithms


Deformable mirror

preliminary design

WP 1000

ULB

Deformable mirror

Breadboard detailled design

WP 2000CSL

Procurement and bread

board manufacturing

WP 3000AMOS

Review the state-of-the-art of cophasing methods and

selectionWP 1100

CSL

Review the state-of-the-art in

piezo actuator control methods

and selectionWP 1200

Thales

Detailled design of the

cophasing and WFS sub systems

WP 2100CSL

Detailled design of the piezo control sub systems

WP 2200Thales

ThalesManagement and Reporting

WP 0300Thales

Overal Project

Management and reporting

WP 0100CSL

ULB Management and Reporting

WP 0200ULB

Management, reporting and

support WP 0000

CSL

AMOS Management and Reporting

WP 0400AMOS

Wafer technology review and preliminary

conceptWP 1300

ULB

Preliminary opto

mechanical concept WP 1400

AMOS

Wafer detailled design

WP 2300ULB

Opto mechanical

detailled design

WP 2400AMOS

Manufacturing of the piezo control sub

systems WP 3200Thales

Manufacturing of Wafer

subsystem WP 3300ULB

Manufacturing of Opto

mechanical subsystemWP 3400AMOS

Manufacturing of the

cophasing and WFS sub systems

WP 3100CSL

Rigid mode control and

control strategies overviewWP 1500ULB

Rigid mode control and

control strategies

detailled designWP 2500ULB

Control algorithm

developmentWP 3500

ULB

AIV and testing

WP 4000CSL

Test Results and Synthesis

Report WP 4300ULB

Establish Guidelines WP 5100XXX

Functional and

performance testing

WP 4200CSL

Guidelines and

Recommendations for

Future WorkWP 5000 ULB

AIVWP 4100AMOS

Lightweight space deformable mirror : project work plan

Critical issue : the manufacturing of the sub-system dedicated to cophasing and the wavefront sensor of the mirror


Cophasing sensor selection

Complexity is transferred from hardware to software

• 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

Piston measurement


• Shack-Hartmann Wavefront sensor available at CSL

• Analytical and real time Phase retrieval algorithm

Tip-Tilt measurement



Piston – Tip/Tilt definition

X

Y

Z

Piston : Change of poistion along the Z axis (λ or nm)

Tip : Rotation of the surface around the Y axis (rad or arsec)

Tilt : Rotation of the surface around the X axis (rad or arsec)


Review of the state of art of cophasing sensor

Pupil plane detection sensors Focal plane detection sensors

Parameter/ Method

Shack-Hartmann Sensor Curvature Sensor Pyramid 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 YES

Tip-tilt measurement ? YES YES YES NO YES YES

YES but with restrictions with combined piston YES

#DOF MULTI3DOF for each segment

3 DOF for each 61 segments : 183 >11 >11 High

Algorithm N/A N/A N/A

Keck narrowband Algorithm (Chanan et al., 2000) N/A

Gradient-based iterative method

Analytic Least-Square approach

Gradient-based iterative method

Data processing Slope integration Poisson's equation Slope integration FFT 2D FFT 2D 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 2D on the zone of interest

Time to compute 1 FFT 2D on the zone of interest

Time to compute 100N FFT 2D on the zone of interest for N diversity focal images

#Apertures High High High 12 High High TBC already ? 12 High

H/W sourceMonochromatic or Polychromatic

Monochromatic or Polychromatic Visible or NIR Polychromatic

2 superluminescent diodes at 834.6 nm & 859.6 nm High flux needed

White light source + 1 narrow band filter (λc = 650 nm, Δλ = 40 nm) High flux needed

H/W detector CCD

Avalanche photodiode (bulky & expensive) or CCD CCD 1024x1024 CCD 4 CCD

Standard CCD Flux of

105 e-/frame

Standard CCD ; Flux

of 105 e-/frameDefocus generator + Standard CCD

Optical complexity Medium Medium Medium Medium High Simple Simple

Simple if only 2 phase diversity images are needed

Range up to 1500λ|Piston| < λ/2 ; High for tilt

|Piston| < λ/2 ; High for tilt 1 µm to 16 µm

Piston: ± 7.2 µm ; Tip-tilt: ± 250 µrad

[-λ/2 ; λ/2] for piston and tilt

λ/2 < Piston < λ/2 ; |Tilt| < 0.3λ

[-λ/2 ; λ/2] for piston and tilt

Linearity Close to 1 TBD TBD 0.98 TBD0,99 for Piston ; 0.93 for Tilt

0,96 for piston ; 1,15 for Tilt 0.99 for piston and tilt

Measurement accuracy From λ/100 to λ/1000 less than 15 nm rms λ/30 rms 90 nm better than λ/13 rms TBD TBD TBD

Repeatability < λ/200 rms TBD TBD TBDPiston: 0.48 nm rms ; Tip-tilt: 74 nrad rms

Piston : 0.75 nm for

5.105 photoe- ; Tilt :

1.21 nm for 3.3 104

photoe-



1.21 nm for 1.6 105

photoe-



1.21 nm for 1.1 104

photoe-

Particularities

Cylindrincal microlenses on the E-ELT sensor to measure piston steps

Restricted to spherical shapes that allow operation without gigantic beam expander

Excellent for phase reconstruction but time consuming

Need for diluted noncentrosymmetric pupil ; Small phase aberrations assumption (<2π rad)

Excellent for phase reconstruction but time consuming

Type of achievable phasing : Cophasing (fine) or coherencing coarse Both Both Both Coarse Phasing Fine Phasing Both Fine Phasing BothType of telescopes that can be controlled All Segmented Segmented Segmented Segmented Multi-aperture ; SegmentedMulti-aperture

Multi- Aperture ; Segmented

Maturity

Well know and manufactured in mass ; The most used sensor

Preliminary laboratory results and upcoming on-sky tests on the VLT

Preliminary laboratory results and upcoming on-sky tests on the VLT & LBT

Testbed results ; On sky test on Keck

Preliminary laboratory results and upcoming on-sky tests on the VLT

Laboratory tests in progress



Instrument controlled by this method or envisaged to be

Keck, VLT/NACO, Gemini North and South, MMT, Palomar, Lick, E-ELT, Grantecan

Subaru, VLT/MACAO, Gemini South/NICI, Grantecan, E-ELT ? LBT, WHT ?, E-ELT ? JWST ; Keck E-ELT

JWST ; Point-source and extended scenes observations

Darwin, SOTTISE (Earth Observation)

JWST ; Earth observation

Main references Platt & Shack, 1971 Roddier, 1988 Ragazzoni, 1996

Shi et al., Applied Optics 2004

Fogale Nanotech: Wilhem et al., Applied Optics 2008

F. Baron Thesis, ONERA, 2005 ; Baron et al., JOSA 2008

F. Baron Thesis, ONERA 2005 ; Baron et al., JOSA 2008

F. Baron Thesis, ONERA, 2005 ; Delavacquerie et al., 2008

Sensor type

Trade-off criteria


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



Complexity is transferred from hardware to software

• 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

– Multiple aperture piston/tip/tilt/more (DWARF)– Multiple aperture piston with extended scenes

Piston measurement

first steps of the development of a cophasing sensor for synthetic aperture optics applications

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