The Nulling Coronagraphâ€”Using a Nulling Interferometer for

Terre

stria

l Pla

net F

inde

r Mis

sion

A NASAOriginsMission

The Nulling Coronagraph—Using a Nulling Interferometer for Planet Detection in Visible Light

with a Single Aperture Telescope

Michael Shao, B. Martin Levine, Duncan Liu, J. Kent Wallace, and Benjamin Lane

Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA 91007

Presented at the Workshop on Innovative Designs for the Next Large Aperture UV/Optical Telescope (NHST),

10 April 2003

(Acknowledgements: E. Serabyn, B. Mennesson, T. Velusamy, and S. Shaklan)

Terre

stria

l Pla

net F

inde

r Mis

sion


TPF Problem

10-10

• Detection of planets around closest 150 stars and characterization (spectroscopy) for signs of life

– ~50-120mas IWD angular separation– ~mv=5 (F, K,G-type Star) brightness– TPF Biomarker Report:

• 0.7 to 1.0 µm (minimum)• 0.5 to ≈1.1 µm (preferred) • 0.75 µm unique Oxygen band not

found in the IR

• 10-10 starlight suppression required– Diffraction control requirement– Scattered light control requirement

Terre

stria

l Pla

net F

inde

r Mis

sion


General Overview

• Why another coronagraph concept?– Nulling interferometer/coronagraph belongs in a class of small

inner working distance devices• Use 4~5m telescopes instead of 8~10 m telescopes for the TPF

mission.• If a 4m telescope is $XX dollars, and if the cost of a telescope goes

as D2.5 the cost of a 8m telescope is very (very) high.– Orders of magnitude easier to control scattered light, at 10-10 per

airy spot• Description of the basic concept

– 4 beam nulling interferometer– Single Mode Fiber Array

• Experimental demonstrations– Deep and Stable Nulling for starlight suppression– Fiber Array Progress

Terre

stria

l Pla

net F

inde

r Mis

sion


Large Apertures are NOT Needed in Principle to Detect and

Characterize Extra-solar Planets

• A modest sized aperture telescope can theoretically resolve an extra-solar planet – Jupiters at 10 pc D > 0.3m (λ=0.75mm)– Earths at 10 pc D > 1.5 m

• The HST (at 2.4m diameter) is capable of imaging mv=30 stars

• The major issue is overcoming the contrast between star and planet (10-9-10-10)

Terre

stria

l Pla

net F

inde

r Mis

sion


Nulling Introduction λ /b

CCD Transmission Pattern of NullerOn the sky. (Star is at the center)

λ /2b

b

•When the light from two pupils are combined, the output can be imaged. •The image is an Airy function with diameter 2.44λ/D where D is the telescope diameter. •But the intensity of that image is modulated by the fringe pattern (on the sky) where b is the baseline between the pupils.

•if the star is at a null, the star’s image has 0 intensity. If the planet is at the peak, the planet’s light is unattenuated.

•A nulling interferometer that works with a single aperture telescope is different than one that combines light from 2+ telescopes

Terre

stria

l Pla

net F

inde

r Mis

sion


2-arm vs. 4-arm Nulling Interferometersy

( ) 424

0

2

0

2

0000

2cos2

sin2

coscos2

cos4

θφ

φθφθ

φφφφ

=

−

=

−+−= −−

kbII

kbkbII

eAeAeAeAI yyxx iiii

( )[ ] ( ) 2200

20

2

00

coscoscos12

cos2

sin4

θφφθ

φθφφ

ksIksII

ksIeAeAI xx ii

≈−=

=−= − b

x

s

Terre

stria

l Pla

net F

inde

r Mis

sion


Null Depth• Stellar sources have a finite diameter

• e.g. our sun @ 10pc~ 1mas• Any cancellation scheme must

account for this effect

2 Arm Stellar Leakage (Integrated Null Depth)= 8.2x10-5

4 Arm Stellar Leakage (Integrated Null Depth)=6.7x10-10

Terre

stria

l Pla

net F

inde

r Mis

sion


• Four-arm interferometer leakage below 10-10 at θ~λ/D

Ideal performance of Visible Nulling Coronagraph

Terre

stria

l Pla

net F

inde

r Mis

sion


Image of Solar SystemSimulation of 4m nulling interferometerimage after rotation.

Integration time for an Solar system at 10pc

SNR=5 on Earth in 5.1 hrsSNR=10 on each of 20 spectralChannels in 200 hrs

+ EV

J

S

M

Terre

stria

l Pla

net F

inde

r Mis

sion


OTA+FSM +Beam Compressor 1REVISIONS

REV DESCRIPTION DATE BY

A Initial Release 01/13/02 JKW

Stop

Primary:Diameter: 4.0 mROC: 17.20 mConic: -1.0Parent Focal Length: 8.60 mParent f/#: f/2.15Off-axis Distance: 3.0 mEffective Focal Length: 8.86 m

Exit Pupil:Diameter: 0.40 m

Secondary:Diameter: ~0.40 mROC: -1.72 mConic: -1.0Parent Focal Length: -0.886 mOff-Axis Distance: 0.30 m

7.74 m

8.50 m

Exit Diameter: 0.05 m

Compressor Primary:Diameter: ~0.40 mROC: 4.0 mConic: -1.0Parent Focal Length: 4.0 mParent f/#: f/5Off-axis Distance: 0.40 mEffective Focal Length: 2.02 m

Compressor Secondary:Diameter: 0.05 mROC: 0.5 mConic: -1.0Parent Focal Length: 0.04 mParent f/#: f/5Off-axis Distance: 0.05 mEffective Focal Length: 0.2525m

0.86 m

DESCRIPTION

ISSUED: 01/04/02REV: A

VTPF Telescope

Terre

stria

l Pla

net F

inde

r Mis

sion


X shear MMZ

Y shear MMZ

Visible Nulling System Concept

Diffraction limitedimaging system (λ/10)

Lenslet and fiber-optic array spatial filter

s

bxby

Beam with X and Y shear,θ4 null output

Image plane(real image)~(64 x 64)

Telescope Pupil

θ4 Null in Pupil Overlap Area

+ -

Baseline is √2 x shearSingle Mode Fiber array enables:

10-9 suppression achieved with 10-7 nuller and 100 lenslets10-10 suppression achieved with 10-7 nuller and 1000 lensletsMultiple sub-apertures make the detection less susceptible to Exo-Zodiacal Dust

Residual background is incoherent with planet imagePreserves field of view

- +

Turning/Rotation Mirrors

Terre

stria

l Pla

net F

inde

r Mis

sion


Achromatic Nulling Interferometer Demonstration (Shao and Serabyn)

•Single pupil input•Symmetric design•Preserves pupil orientation and polarization•Pupil shear adjustable–variable null baseline•Dielectric plates provide achromatic null

+ -

DispersiveComponents For AchromaticNull

Variabledelay

Variable shear, s

Null output

Bright outputPupil Input

Symmetric Beam Splitters

DispersiveComponents For AchromaticNull

s

Terre

stria

l Pla

net F

inde

r Mis

sion


Refractive Dispersion Correction (Morgan)

•Broadband null--Calculation for nulling depth for common optical glasses, BK7 and Fused Silica.

•Broadband null optimization•Calculation for initial laboratory demonstration

• ∆t,BK7 = 391.687 +/- 0.052 µm• ∆t, F.S. = 479.189 +/- 0.057 µm

Terre

stria

l Pla

net F

inde

r Mis

sion


DM

Out-of-bandPupil Camera

Input

“Dark”Output

Phase Plates

Phase Plates

TrackingCamera

Piston/ShearingRooftop

DMRooftop

MetrologyInjection

Input Pupil Output Pupil DESCRIPTION


VTPF Nuller 1

4 m

TPF ~ 0.5 m

“Bright” OutputMetrology

Local Oscillator

Shutter

Shutter

Nuller #1

Terre

stria

l Pla

net F

inde

r Mis

sion


Proof of Concept Nulling Interferometer

• First Laboratory Results--White Light Fringe Scan

– Optical bench ‘isolated’ with pads and doors, otherwise ambient laboratory conditions

• The starlight leakage only depends on the amplitude and phase mismatch inside the fiber.

Spectral Density

0

500

1000

1500

2000

2500

3000

600 620 640 660 680 700 720 740Wavelength (nm)

Inte

nsity

Input Light source

Detector feed

Shear ‘Roof Top’

reflector

Phase ‘Roof Top’

reflector

Symmetric beam

splittersShutters in front

of one roof to measure intensity

equalization for each arm

DM

Out-of-bandPupil Camera

Input

“Dark”Output

Phase Plates

Phase Plates

TrackingCamera

Piston/ShearingRooftop

DMRooftop

MetrologyInjection

Input Pupil Output Pupil DESCRIPTION


VTPF Nuller 1

4 m

TPF ~ 0.5 m

“Bright” OutputMetrology

Local Oscillator

Shutter

Shutter

Terre

stria

l Pla

net F

inde

r Mis

sion


Deep Nulls from Monochromatic Light

0 20 40 60 80 100 120 140 160 18010-7

10-6

10-5

10-4

10-3

10-2

10-1

100dataset 11

time sec

•Laser diode source ~1nm bw•Intensity matched to 0.1~0.2% •Polarization aligned•Small dither imposed, sync’d with scope display. Dither signal demodulated by “eye”•OPD control by hand on pzt voltage knob.•Closed loop control currently being integrated

•Sustained null, Average leakage 7x10-6 (7x10-9 per airy spot)

•Best transient null 6x10-7 (6x10-10

per airy spot), 40msec sampling ~ Late Nov/Early Dec 2002

Terre

stria

l Pla

net F

inde

r Mis

sion


Laser Metrology Nulling Detector Performance

Null ~2.5x10-5

Null ~3.7x10-6

Noise floor = 10pm/√Hz

Terre

stria

l Pla

net F

inde

r Mis

sion


Summary

• Single Aperture Telescope with Visible Nulling based Interferometer System Capable of Imaging and Spectroscopy of Planets (D=4-5m)– Mission Concept and Instrument Defined

• Starlight Suppression is the Most Challenging Technology– Steady State Null @ 6x10-9, and Transient 7x10-10 Demonstrated– Laser Metrology System Integrated– Near Term Work

• Deep Laser Null• White Light Null (25-30% band pass)• Demonstration of Single Mode Coherent Fiber Array

– Long Term Experiments:• Use of DM to control amplitude and phase• 100-300 channel SMF array• System Demonstration on Test Bed

Terre

stria

l Pla

net F

inde

r Mis

sion


Back up Slides

Terre

stria

l Pla

net F

inde

r Mis

sion


Coherent Fiber Array Nulling Requirements

Final Imagingoptic

CCD

lensletarray

CoherentFiberarray

In a visible TPF based on a nulling interferometer

For Earth Detection:Fiber array has ~ 1000 fibers

Final image plane has a field of view ~1000 airy spots (~30x30)

Average null of 1e-7 means that 1e-7 light spreadover 1000 airy spots, or 1e-10 scattered lightper airy spot.

Nulling requirement (Vis TPF/Earth) is1e-7 for Q=1, planet flux = scattered light flux3e-7 for Q=0.3

Light from Nuller

Requirement for Jupiter Imager ~10~100x easier

Terre

stria

l Pla

net F

inde

r Mis

sion


Single Mode Fiber Array Design and Characterization

• Commercial (preliminary) fiber array results– Irregular gaps between fibers

• Machine tolerance on metal housing issue– Useful for development of:

• lens array to fiber alignment • array characterization procedures

• Next Generation Array Design– Use large mode field diameter Photonic Crystal

Fiber (PCF) to relax alignment error requirement– Use V-groove type fiber array construction for

flexible fiber and lenslet spacing or custom self assembly fixture

– Custom lens array with focal plane at substrate• Precision mapping of lenslet to fiber positions

IndexMatchingBondingMaterial

Fiber Array

Lens Array

Lens Array

• Assemble Using index matching bonding material to relax fiber array polishing requirement

Terre

stria

l Pla

net F

inde

r Mis

sion


Phase Map—Single Mode Fiber Array

Terre

stria

l Pla

net F

inde

r Mis

sion


Key Components• Deformable mirror, for amplitude

and phase control.– The MEM’s DM is far from

perfect but the use of single mode fibers is very tolerant of DM imperfections

– Need a “custom” DM, with both piston and tip/tilt control of segmented subapertures

• Coherent fiber array– Currently looking single mode

fibers with large mode diameters, to ease alignment of lenslet array to fiber array.

– Fibers only need to be coherent to ~λ/10, allow ~10um kinks.

Terre

stria

l Pla

net F

inde

r Mis

sion


Summary

• Visible Nulling Interferometer Summary– Achieved sustained null of 7x10-6, and transient null of 6x10-7

• with a 1000 fiber bundle, implies scattered light in Visible TPFwould be 7x10-9 and 6x10-10 per Airy spot, respectively.

– Closed-loop metrology systems has been integrated into the interferometer

– Closed loop testing in progress

• Only a factor of 2~6 away from what is needed in a visible TPF with a 4m primary, to detect Earths around solar like stars at 10pc.

• Single Mode Fiber Array Summary– Lenslet array center-to-center spacing measured to ±1.33µm

• Repeatability is ± 0.22 µm– In process to build 20 and 80 SMF arrays

• 5x decrease in position tolerance on lenslet array and alignment

Documents

The Nulling Coronagraphâ€”Using a Nulling Interferometer for