19 February 2009 Cophasing sensor for synthetic aperture optics applications First steps of the development of a cophasing sensor for synthetic aperture optics applications Géraldine GUERRI Post-Doc CSL

19 February 2009Géraldine GUERRI Cophasing 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 diameter 1000 segments 25 m diameter 7 segments 30 m diameter 492 segments

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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?

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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 : best compliance with the requirements sensor maturity breadboard feasibility within a short term Survey of 15 different principles : Survey of 15 different principles :

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Cophasing sensor : methods selection PISTONTIP-TILT Coarse phasing Dispersed fringe sensing (CSL : Roose et al, 2006) Shack-Hartmann Sensor (Shack & Platt, 1971) Fine cophasing Error < λ/2 Phase retrieval real-time algorithm (Baron et al., 2008) Shack-Hartmann Sensor or Phase diversity real-time algorithm (Mocoeur et al., 2008)

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Numerical validation of the phase retrieval algorithm for piston estimation 3 sub-aperture pupil PSF OTF 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Algorithm validation Phase retrieval algorithm numerical validation Algorithm Computation time (MATLAB) : 0.4s Test of the sensor linearity Valeurs des pistons introduits (nm) p1 : -50 p2 : 0 p3 :100 Différence de piston calculées (nm) p1-p2 : -50 p1-p3 : -150 p2-p3 : -100

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Thanks for your attention

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Différence de piston calculées (nm) p1-p2 : -50 p1-p3 : -150 p2-p3 : -100

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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 ….

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Valeurs des pistons introduits (nm) p1 : -50 p2 : 0 p3 :100

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Framework Today’ s astronomy needs extremely large telescope (High FOV, high resolution) with huge diameter >30m Technological solutions –Large segmented telecopes –Multiple aperture telescopes

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Numerical simulations Validation of two algorithms : –Dispersed speckle piston sensor.. in progress Problems with sensor linearity –Real-time phase retrieval algorithms

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Shack-Hartmann Wavefront sensor available at CSL Analytical and real time Phase retrieval algorithm Tip-Tilt measurement Cophasing sensor selection

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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)

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications Review of the state of art of cophasing sensor Sensor type Trade-off criteria

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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

19 February 2009Géraldine GUERRI Cophasing sensor for synthetic aperture optics applications 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 –Multiple aperture piston/tip/tilt/more (DWARF) –Multiple aperture piston with extended scenes Piston measurement