ELP-OA: status report of the setup of the demonstrator of the Polychromatic Laser Guide Star at...

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ELP-OA : status report of the setup

of the demonstrator of the

Polychromatic Laser Guide Star at

Observatoire de Haute-Provence

Renaud Foy1,2, Nicolas Meilard1, Michel Tallon1, EricThiebaut1, Pierre-Eric Blanc2, Michel Boer2, Andree Laloge2,Auguste Le Van Suu2, Sandrine Perruchot2, Pierre Richaud2,

Alain Petit3, Thierry Fusco4

1CRAL - Observatoire de Lyon, France

2Observatoire de Haute-Provence, France

3Laboratoire de Spectroscopie et de l’Interaction laser-matiere,DEN/DPC, CEA, France

4DOTA - ONERA, France

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Adaptive optics requires a phase reference.

Isoplanatic patch : θ ≈ 30” @ 2.2µm, 5” @ 0.55µmIf no NGS within θ

=⇒ LGS=⇒ little hope to rely on NGS for the tilt @ V !

=⇒Polychromatic Laser Guide Star : retrievesthe tip-tilt from the LGS.ELP-OA ( Etoile Laser Polychromatique pourl’Optique Adaptative ) R&D programme at OHP

3Why do ELTs need LGS aided AO ?Topic Spat. res. RangePLANETSExoplanets 1-2 mas 0.6-1.4µmSolar System 5mas @ 1µm vis - ther. IRSTARS AND DISKSProbing birth places 2-10 mas @ NIR 1-5 µmNormal and peculiar stars DL @ Hα HαChemical comp. : chronometry ”high” 300-700 nmBHs in GCs ≈1 mas K, CaII, vis.STARS & GALAXIESResolved Stellar Populations DL V - KResolved Stars in Clusters 3 mas 400-600nmStellar Kinematic Archaeology DL V - KIntracluster Stellar Population DL J & KCosmic SFR from SNs S ≈0.5 J-H-KYoung, Massive Star Clusters 30-40 mas >0.8 µmBlack holes in GN DL . 1 µm Opt. & NIR

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PLGS principle

Air refraction index :

n(λ, P , T ) − 1 = f (λ) × g(P , T )

=⇒θλ3= ∆θλ1,λ2

× (nλ3− 1)/∆nλ1,λ2

. (1)

The tip-tilt at λi expresses in terms of the tip-tiltdifference between any two λs.

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4D5/2

578 nm

3S1/2

569 nm 75 ns

3P3/2

150 ns 2338 nm4P3/2

4S1/2

589 nm 16 ns

1140 nm 40 ns

2207 nm160 ns

1/23P3P3/2

330 nm 320 ns1.6 M Hz

589 nm 16 ns10 M Hz

13 M Hz

589.6 nm 32 ns

Energy levels of neutral sodium atom.

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Laser-Na interaction optimizationBased on BEACON code (Bellanger et al., 2004) : resolution ofoptical Bloch equations.

Atom model : quantumLaser field model : classical

BEACON computes the density matrix evolution =

f(hamiltonian(isolated atom) + hamiltonian(µ−→E ))

Rate equation models not suitable : ∂−→

E

∂t6= 0.

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BEACON input parameters

◮ laser power density (W/cm2) : 2 × 22W,

◮ pulse time profile : gaussian,

◮ pulse FWHM δt : variable, 40 ns,

◮ time to reach maximum peak power : 2δt,

◮ polarization : variable, circular-circular,

◮ spectral shift : function of phase

modulation,

◮ phase modulation parameters : number of

functions, amplitude and frequency.

8Optimization of pulse length

120 ns

80 ns100 ns

60 ns50 ns40 ns

16 ns

10 20 30 40 500.000

0.002

0.004

0.006

0.008

0.010

Power density @ 589nm (Watt/cm 2)

Ret

urn

flux

(pho

ton/

atom

/pul

se/s

r)

2−photon excitation of Na Gaussian pulses. Both laser beams:

circular polarization, 2 sine modulation functions

Take into account technological constraints : P andFR not easily tunable, efficiency of dyes. Also : spotcharacteristics, ...2-photon excitation is a non linear process ! !

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Modulation function

0 20 40 60 80 100 120

0.

1.

2.

3.

4.10−4

Pulse length (ns)

Pho

ton/

atom

/pul

se/s

r2−photon excitation of Na: return flux at 330 nm

Intensity at 589 and at 569 nm : 1 W/cm2

Backscattered flux at 330 nm versus pulse length.Black dots : an optimized phase modulation fuction.Red crosses : modulation used for PASS-2 experimentat CEA/Pierrelatte.

10Is this return flux large enough ?

σ(θ) ∝ FWHM/√

N

Answer # 1 : with a classicalprojector, NOIncreasing the projector diameter produces specklenoise. If AO precompensates for the upward beam=⇒ saturation of Na absorption. No benefit !Answer # 2 : with an interferometricprojector, YES

Cramer-Rao criterion : σ(θ) limited not by FWHM

but by the smallest resolved features in the laserspot image, as produced by e.g. 3 apertureinterferometer producing a pattern in themesosphere.

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Specs of the interferometricprojector

◮ fringe pattern > λ/D ≈ 77 mas @ 569 nm

◮ no speckle : single aperture diameter d < r0

◮ pattern translation invariant =⇒ at most 1phase closure relation =⇒ at most 3 apertures

◮ need of an AO at the master telescope focus,down to 330 nm

◮ no center of gravity =⇒ 2 and soon 3Dcrosscorrelations

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220 240 260 280 300

220

240

260

280

300

Figure 1 :Projecteur in pupil plane

pli, pup_proj_569

0 100 200 300 400 500 0

100

200

300

400

500

Figure 7 :Laser guide star image by telescope

pli, tel_569_tab(1,,)

0 50 100 150 200 0

50

100

150

200

Figure 9 :Criterion function

13Strehl ratio produced by BOA (ONERA) at 330nm

14End-to-end modelInputs :

◮ Lasers : P , FR , modulation, polarization ...◮ Projector : pupil function, transmission (≈80%)◮ Atmosphere : Kolmogorov phase screen,

transmission (80%)◮ Mesosphere : Na column density (4 × 1013

atoms/m2), altitude (92 km)◮ Atmosphere : Kolmogorov phase screen,

transmission(30% @ 330nm, 80% @ 2.3µm)◮ Master telescope : pupil, transmission (≈77%)◮ Adaptive optics : Strehl ratio, transmission◮ Detector : EMCCD, ρ330 ≈ 60%, ρ589 ≈ 80%◮ Data processing : phase restoration or cross

correlations

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Interferometric projectoroptimization See also poster Meilard et al.

4 6 8 100.10

0.15

0.20

0.25

0.30

0.35

0.40

Aperture diameter (cm)

Str

ehl @

550

nm

ELP−OA interferometric projector <P589> = <P569> = 22 W at the mesosphere r0 = 10cm. Baseline = 80 cm. 3 apertures

55 60 65 70 75 80 850.20

0.25

0.30

0.35

0.40

Baseline (cm)S

treh

l @ 5

50 n

m

ELP−OA interferometric projector <P589> = <P569> = 22 W at the mesosphere

r0 = 10cm. Aperture diameter = 5 cm. 3 apertures

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Goal Stilt ≈ 30 − 40% @ 550 nm

17Ongoing construction of thedemonstratorELP-OA layout

��

��

Seismometer

Telescope

OHP1.52 m

330 − 569 nm

Measurement

Tilt

FOCAL INSTRUMENTATION

Tip−tilt mirror

Vibrations corrections

Tilt correction green / red

Science

channel

Adaptive Optics

GSM seeing monitor

Sei

smom

eter

Bea

ms

Transport

Laser chains

589 nm

+

569 nm

MesosphereMesosphere ∗∗

Laun

ch te

lesc

ope

Na m

onitor telescope

∗

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Auxiliary equipments :

◮ GSM from Nice

◮ Na monitor, from 1.2m telescope

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Time schedule• Laser rooms commissioning July 2009

• 3 NdYAGs pump lasers delivered

August 2009

• integration of dye circulators end onJanuary 2010

• laser chains integration end on April 2010

• beam transport to the dome end on April2010

• first launch at the mesosphere end April2010

• full experiment 2011

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Analysis and roadmap from the ELT AdaptiveOptics Working Group (2006)

... the polychromatic LGS development should bemonitored and as for other novel concepts proposedin this document encouraged. Should the pendingtheoretical issues and simulations be positive, itshould be encouraged for on-sky demonstration.

Acknowledgments : ANR, PACA Region, DGA, CNRS/INSU

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