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EXTRA-SOLAR PLANET IMAGING WITH A SPACE TELESCOPE AND A NULLING INTERFEROMETRIC CORONAGRAPH

Michael Shao, B. Martin Levine, Duncan Liu, J. Kent Wallace, and Benjamin Lane*Jet Propulsion Laboratory/California Institute of Technology

4800 Oak Grove DrivePasadena, CA 91109-8099

ABSTRACTThis paper describes a space mission for the direct detection and spectroscopy of Jupiter-like and Earth-like extrasolar planets in visible light using a modest aperture (1-4m) space telescope with a nulling interferometer based coronagraphic instrument. This concept is capable of satisfying the scientific objectives of the Terrestrial Planet Finder mission at a fraction of the complexity and at less cost than previous concepts. We discuss the key features of our mission design, and we present latest results of the technology developments needed for achieving a ten billion to one star light suppression ratio required.

INTRODUCTIONWith a flux ratio in the optical of ~10-9-10-10 between a planet and its star, the hardest problem in imaging extra solar planets is that of contrast suppression, and achieving a very low background against which to detect a planet requires control of both scattered and diffracted light. The Hubble Space Telescope (D=2.4m) can detect a V = 30 object, so a 27 magnitude object takes much less than 1 hr of integration. In terms of resolution the orbit of a Jupiter-like planet at 10 parsec subtends an angle approximately 0.5 arc seconds, which requires a diffraction limited telescope of only 30cm or greater (at 0.75µm wavelength), and an earth-like planet at 0.1as can be resolved with a 1.5m diameter aperture.A nulling interferometer, however, can be used to suppress both diffraction and scattering, and an imaging instrument can be located behind a modest sized singleaperture to resolve an extrasolar planet (Shao, 1990). In principle, a nulling interferometer effectively cancels the starlight and has 100% transmission for planet light when the optical path from the planet is λ/2 different from the star. For a modest sized aperture, about D=1m, a Jupiter-like planet could be resolved by synthesizing an interferometer with a 30 cm baseline, and at D=4m, an earth-like planet can be resolved with a 1.5m baseline.This paper describes a instrument for direct planet detection that we call the nulling coronagraph. The schematic system is shown in Figure 1. It synthesizes a four element nulling interferometer from the telescope pupil to suppress the diffraction from a central star. After nulling, an array of coherent single mode optical fibers is used to negate the effects of residual stellar leakage (scattering) due to imperfections in the telescope optics and optical train. A simple imaging system after this array forms the final extrasolar planet image, or a spectrometer can measure spectra for signs of life. This concept combines all the advantages of a

nulling interferometer with the simplicity of a modest size, diffraction limited single aperture telescope. Advances in nulling technology enable this approach(Wallace, Shao, Levine and Lane, 2003). A further key element of the nulling approach is the use of single mode fiber spatial filter in conjunction with the nulling interferometer (Liu, Levine, and Shao, 2003) . The progress toward demonstration of these subsystems isall presented below. This combination makes very deep nulling possible without the requirement to achieve and maintain extreme (λ/4000) wavefront quality over a (large) full aperture of the space telescope.

IMAGING PROPERTIES OF THE VISIBLE NULLING CORONAGRAPH

A nulling interferometer interferes the light from two apertures, destructively. This is shown in the figure below as a two telescope interferometer, (Shao 2002). Light that is “on axis” is destructively interfered, but planet light “off axis” passes through the nuller and is detected. Behind the interferometer we can place a camera to image the field of view. The use of a camera for a visible nulling coronagraph is in contrast to an IR nulling interferometer where a single pixel detector is used. It’s important to understand what the nuller does to the image. The nuller effectively projects a transmission grating on the sky. The camera images the sky but the transmission of the camera/nuller depends on the angular position of the object. In this way the nulling coronagraph is similar to a Lyot Coronagraph where the transmission of the coronagraph is less when the light is blocked by the coronagraphic stop.

Single Aperturetelescope

Pointing/Tracking control:

Diffraction Control:Achromatic Nulling

Scattered Light Control: Fiber-optic Spatial Filter Array

Imaging System / Low Resolution Spectrometer

Figure 1: Schematic of an imaging extrasolar planets with a shearing interferometer based instrument behind a single aperture telescope.

Space 200323 - 25 September 2003, Long Beach, California

AIAA 2003-6303

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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Required null depth N Whereas a conventional 2 beam interferometer with a projected baseline, b=bx=by=s (see Figure 2), and with wave number, k= 2π/λ produces a nulled fringe intensity proportional to θ2, where θis the angle of the planet from the optical axis,

( ) 220

2

00 cos θφφφ ksIeAeAI xx ii ≈−= −

a 4 subaperture interferometer produces an intensity proportional to θ4 thus deepening the null from the star and enhancing the contrast between the nulled star and the reflected planetary light (Angel and Woolf, 1997, Mennesson and Mariotti, 1997). A plot of these two nulls are shown below in Figure 3.

( ) 424

0

2

0000

2cos2

θφ

φφφφ

≈

−+−= −−

kbI

eAeAeAeAI yyxxiiii

Imaging sensitivity The depth in the four aperture null is also needed to suppress the starlight from extended objects. For example, a star the size of our sun subtends a width of 1mas. Thus when the light from the star is integrated over the width of the nulled fringe pattern, this scales the peak of the resulting airy disk due to the residual starlight. This is shown in the next figure (Figure 4). The required contrast is obtained by going out a small portion of the Airy disk. It is from this property that we say that the nulling coronagraph has the property that it

can resolve a planet within the first lobe of its Airy disk. This is in comparision to traditional Lyot coronagraphs (see inset) wihich requires detection past 3 Airy side lobes.The integration time varies as a function of required signal to noise, telescope througput, null depth, bandwidth, and planetary parameters of brighness, distance from the star, and distance to the telescope (Levine et.al. 2002, Menneson et. al. 2002b). Shown in Figure 5 are the results of integration time as a function of aperture diameter (minus the shear, s) for a Jupiter-like planet (left), and an earth-like planet (right). This assumes an SNR of 5, 15% througput, null depth of 10-

10, and 20% bandpass. These results show that the aperture diameter need not be large to detect planets (1.5m for Jupiters and 4m for earths).

Figure 3: Intensity response of a two and a four element nulling inteterferometer. Left, fringe pattern over a 0.1as field of view. Right, nulling depth over same field but with a 4 element nuller

Imaging camera

λ/s

Figure 2: Imaging with a nulling interferometer. Right, Image with superimposed fringe patternblocking the star light while transmitting the planet.

s

λ/2s

Figure 4: Light suppression from a nulling coronagraph (A similar plot is given for the Lyot coronagraph is in the inset).

Figure 5: Integration time and aperture size requirements for a visible nulling coronagraph for an earth-like extra-solar planet. (The inset is a simulated image of our solar system for a nominal (4m) diameter telescope.

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INSTRUMENT CONCEPTStarlight suppression is accomplished by two nulling interferometers coupled together with a 3 mirror periscope (Figure 6). Each nuller shears the pupil and provides the null in one of two orthogonal directions. The periscope mirrors rotate the sheared pupil by 90 degrees to feed the second nulling interferometer which provides the shear in the second dimension. Thereresulting 4 aperture shear synthesizes an Angel Cross (Angel, 19990, Shao 1992). The nulled pupil is then followed by an array of microlenses to spatially filter

the light via single mode optical fibers (Shao, 1990). Recollimation by a similar lens array provides a corrected exit pupil, and imaging by an imaging cameraproduces the final nulled image of the planetary system. The basic idea and major advantage to this design is that no active wavefront correction is required for spatial scales smaller than the size of the lenslet. This is due to using single mode optical fiber. On the otherhand, using an array of subaperture fibers guarantees an extended field of view for nulling imaging. The main figure of merit for a nulling interferometer is its rejection rate. Even with perfect pointing and perfect wavefront quality, the interferometric transmission is only zero on the line of sight, so that a star of finite angular size will always produce leaks limiting the rejection rate. In practice, null depths as low as 107 are necessary to compensate for the large contrast ratio between stellar and planetary visible fluxes. The remaining three orders of magnitude suppression are obtained by the fiber-optic array spatial filter. The key concept is that after the filter, the residual nulled light is incoherent with the planet light. Thus when the filtered pupil is projected into the far-field by the imaging system, the residual starlight forms random speckles over the field view. If there are 1000 subapertures in the spatial filter array, then the effective null depth will be reduced by a factor of 1000 to 10-10, the required

contrast level between a star and an earth-like planet. Further description of the nulling interferometer and the spatial filter array are given in (Shao et. al., 2002, Menneson et..al. 2002b, and Levine et. al. 2002). The nulling interferometer concept is shown in the right hand portion of the figure. The beam splitters are reversed so that light in both arms of the interferometertraverse equal optical paths, and to minimize noncommon path error. Dispersive plane dielectric plates are included in both optical paths to broaden the null over the widest possible bandpass (Morgan, 2000).

INSTRUMENT DESIGNNulling interferometerThe layout for one of the two nulling interferometers is shown below in Figure 7. This configuration is capable of both amplitude and phase control as well as spectral control. The shutters in each arm can be closed individually to balance the amplitude of light that is detected at the output. In addition, the ‘out of band’ camera serves as a control input to find the deepest null. The concept is to detect an intensity signal out of the bandpass of the nulling interferometer that relates to the null inside the bandpass. Once detected, the laser metrology system uses that set point to maintain this deep null over the required integration time. One surface of one of the roof top mirrors will eventually be a segmented deformable mirror with tip, tilt, and piston degrees of freedom (upper mirror in figure). Each segment is matched to one subaperture of the fiber array, so that a piston movement on the mirror corrects for phase, and the tip and tilt motions can ‘detune’subaperture coupling to adjust amplitude. This designalso minimizes polarization mismatches by changing the angle of incidence on the interferometer to 30 degrees instead of 45 degrees, and by careful alignment it is also capable of nulling the incident beam in both polarizations.

Turning/Rotation Mirrors

Collimated beam fromtelescope

X shear, sx

Y shear, sy

Image plane(real image)

fiber-optic array spatial filter

Figure 6: Visible nulling coronagraph instrument concept. Left, the major components of instrument. Right, a concept for single input symmetric nulling interferometer.

Pupil Input Bright output

Variable Delay

Dispersive Component For Achromatic Nulling

Variable Pupil Shear

Null output

4 Pupil Overlap

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- +

+

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DM

Out-of-bandPupil Camera

Input

“Dark”Output

Phase Plates

Phase Plates

TrackingCamera

Piston/ShearingRooftop

DMRooftop

MetrologyInjection

Input Pupil Output Pupil DESCRIPTION

ISSUED: 01/13/03REV: A

VTPF Nuller 1

4 m

TPF ~ 0.5 m

“Bright” OutputMetrology

Local Oscillator

Shutter

Shutter

~1m

Figure 7: Layout of nulling interferometer. Also included are an out of band camera for null control and a laser metrology system for maintaining long and stable nulls.

Nulling PerformanceWe measured the performance of the nulling interferometer before integration of the laser metrology system. Using a laser diode at 635nm and with dual polarization, this system shown in Figure 8 was able to achieve steady state nulls of 7x10-6 and transient nulls as low as 6x10-7 (Figure 9). Note that when integrated with the 1000 channel fiber array, it is equivalent to 7x10-9 and 6x10-10, repectively, the latter less than a factor of 50 for the TPF mission!With the laser metrology system, we obtain steady state null depths of 5x10-6 (5x10-9 equivalent) (Figure 9bottom). This null depth is consistent with the performance of the nulling metrology system shown in Figure 10. The left most figure below (Figure 10a) is

the power spectral density of the laser metrology system and the right hand figure is the cumulative power spectral density (PSD) or the resultant varianceas the last value on the curve. The cumulative psd has proven to be a valuable tool for isolating electrical and mechanical noise sources in our experiment. For example the peak near 120 Hz represents an unknown transient, that causes the cumulative psd to suddenly rise. From the height of this step, we estimate that this causes an equivalent of 1.4Å (rms) toward the final variance in the null depth. The lowest values of the spectrum represent the noise floor of our system, and under vacuum conditions represents the best performance we could hope to achieve. Note that when integrated over the psd bandpass, the noise floor is of the order of 10-7 which is the null value required foreach of the 1000 subapertures, indicating that this set up will be sufficient for advanced studies on achieving deep achromatic nulls. Further details about the performance of this nulling interferometer can be found in the paper by Wallace et. al. (2003).

Input Light sourc

Detector fiberfeed

Shear /Piston ‘Roof Top’

‘Roof Top’ reflector

Symmetric beam

splitters Shutters in front of each

roof to measure intensity

equalization for each arm

Figure 8: Nulling interferometer experiment prior to integration of laser metrology system.

0 20 40 60 80 100 120 140 160 18010

-7

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dataset 11

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Figure 9: Steady state null from the nulling interferometer. Top, Nulling result prior to implementation of laser metrology. Bottom, Nulling result under control of laser metrology.

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Coherent Fiber Optic ArrayThe architecture of this coherent array is comprised of an array of single mode optical fibers coupled by an array of lenses at either end (Figure 11). Each fiber in the array supports only a single mode of propagation, hence it filters any spurious or high spatial frequency

signal. All scattered light is dissipated leaving only the reflected light from the planet. Star light has already been cancelled from the nulling interferometer. Single-mode fibers basically trade all local amplitude and phase mismatches (besides piston) between the recombined wavefronts against global intensity mismatches. Intensity mismatches are of secondary effect on the null depth (Mennesson et. al. 2002b, Shao et. al. 2002). Both the lens array and the optical fibers are commercial products. The difficulty in making the fiber array component is having all the individual fibers the same length. Details on the construction of theses arrays can be found in Liu et. al. (2003), but basically they are constructed from stacking equal length fibers at the intersection of two dove prisms which have been cemented on top of a

rectangular slab of optical quality glass (Figure 12). The fibers in the stack are bonded together with a lowviscosity fluid such as epoxy. The stack is locked into place cementing another dove prism whose top has been beveled to accommodate the size of the fiber array. The array is polished at both ends to equalize the length of each fiber.Presented are the results from two prototype test arrays. The first is made from 61 conventional step index fibers with an approximate mode field diameter of 3.5µm. The main purpose of making this step-index single-mode fiber array is to gain knowledge of a reliable assembly process. An image of fiber array is shown in Figure 13. The fibers are bonded between the prisms and the plate with a low-viscosity, low-hardness, UV-cured epoxy. After assembly, the two ends of the fiber array are polished optically to have a reflected rms wavefront error < 1/10 wave and parallel to each other within 20 arc seconds.The fibers in the first 10 rows also from the bottom arealigned well, but the last 3 top rows are not. We believe that we can minimize this problem with more careful placement procedure that uses uniform pressure on the fibers to help with regular alignment. We have aligned a lens array to the fiber array, and an image of

IndexMatchingBondingMaterial

Fiber Array

Lens ArrayLens Array

Figure 11: Components of a Coherent Single Mode Fiber Array.

Null limit ~2.5x10-6 (2.5x10-9)

Null limit ~ 4x10-6 (4x10-9)

Null limit ~2.5x10-6 (2.5x10-9)Null limit ~2.5x10-6 (2.5x10-9)

Null limit ~ 4x10-6 (4x10-9)

Figure 10: Performance of laser metrology system. a). Left hand plot is the power spectral density of the laser metrology system. b). The right hand plot is the cumulative psd. The sudden steps in the cumulative psd are due to spikes in the psd.

a b

12

3

LengthHeight

4

Figure 12: Construction concept for a hexagonal packed fiber optic array using 3 prisms on a rectangular slab of optical quality glass. The fiber pattern is displayed by the inset.

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the far field pattern of the coupled channel is shown in Figure 14. By placing a mirror in contact with with the other end of the fiber array, a reflected, double passed beam is measured interferometrically.. A measurement of approximately 15 channels gives a measurement of approximately λ/15 rms. From separate interferometric measurements of the lens array alone, we estimate that the total error on a complete array would be approximately λ/10 rms.The second array is fabricated using Photonic Crystal Fibers (PCF) with a 12mm mode field diameter. Thelarger mode field diameter is expected to reduce the alignment tolerance between the individual fibers and the elements in the lens array. The utility of this approach is illustrated by our first attempt to build a PCF array (Figure 15). The figure shows the two ends of the polished fiber array with guided white illumination light visible in the core region.

Surrounding air holes are also visible in the cladding region. Polishing may have caused the fiber edges irregular on one side of fiber array (top). This is improved when the other side of the array was polished. The most possible reason for the forming of the visible gaps between fibers on both ends is that the middle flat-

top prism was not pushed down enough during epoxy curing. Future assemblies will be expected to be more regular with fewer gaps. Even with these imperfections, however, we were able to couple 12 fiber channels by aligning a lens array to the fiber array. The wavefront quality of these 12channels was measured interferometrically to λ/20 rms(λ= 0.6328µm) which is better than with the conventional step-index fiber array. We expect to implement a procedure to measure the regularity of future arrays in order to quantify the effectiveness of our assembly procedure. And although we have demonstrated the advantage of using a large mode field diameter fiber array using the off-the-shelf PCF, it may be more advantageous to use a custom large MFD conventional step-index single-mode fiber in the future to ease the handling and polishing of the fiber array. This would eliminate any requirement for

precision cleaving and air-hole sealing. Also it has better coupling efficiency in white light than PCF (Liu et. al., 2003).

Figure 13: Macrophotograph of a polished end from a step index fiber array

Figure 14: Far field spot pattern from the coupled channels in the step index array.

Figure 15: Summary of progress toward fabrication of a coherent single mode fiber array using Photonic Crystal Fibers. Macrophotographs of both polished ends of the array. d) Far-field pattern of laser after being focused into the fiber array by the lens array.

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SUMMARYThe detection of planets outside of our solar system with a space telescope can be accomplished with modestly sized telescope by using a nulling interferometer in the pupil plane of the telescope followed by a coherent array of fiber optic spatial filters. Simulation experiments show that a Jupiter like planet can be detected with a 1.5m diameter telescope, and an Earth like planet can be detected with a 4m telescope. The difficulty in full aperture wavefront control has been transferred to the nulling interferometer, and to a coherent array of single mode optical fibers. A symmetric nulling interferometer adjustable in spatial shear along with a laser metrology system has been built and tested. In a laboratory environment, null depths have been demonstrated at the 5x10-6 (5x10-9) using a laser diode source. Futureexperiments seek to demonstrate nulling in white light with 10-20% bandwidths. A number of prototype optical fiber arrays have been built, and large mode field diameter fibers have been observed to relax requirements on the irregularity in spacing between fibers. Future work will concentrate on characterizing the regularity of future fiber arrays to quantify improvements in fabrication.

ACKNOWLEDGEMENTSThe authors would like to acknowledge contributions and discussions with E. Serabyn, B. Mennesson, T. Velusamy, R. Morgan, S. Shaklan, and V. Ford. This work was preformed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.* present address: MIT Center for Space Research, 70 Vassar Street, Room 37-664h, Cambridge, MA 02139

REFERENCES AND BIBLIOGRAPHYAngel, R., (1990), “Use of a 16-m Telescope to Detect Earthlike Planets,” Proceedings of the Workshop on The Next Generation Space Telescope, P. Bely and C. Burrows, eds., Space Telescope Science Institute, pp. 81–94.Angel, J.R.P, and Woolf, N.J., (1997),”An Imaging Nulling Interferometer To Study Extrasolar Planets,”Astrophysical Journal, v475, pp. 373-379.B.M. Levine, Michael Shao, C.A. Beichman, B. Mennesson, R. Morgan, G. Orton, E. Serabyn, S.

Unwin, and T. Velusamy, (2002), ‘A Visible Light Terrestrial Planet Finder --Planet Detection and Spectroscopy by Nulling Interferometry with a Single Aperture Telescope’, SPIE v4852.Liu, D. Levine, B.M., and Shao, M., (2003), ‘Design and fabrication of a coherent array of single-mode optical fibers for the nulling coronagraph’, SPIE 5170-22.Malbet, F.,Yu, J., and Shao, M.(1995), Pub Astron Soc Pacific, V107, pp. 386–398, April 1995.Menneson, B., and Mariotti, J.M., (1997), “Array Configurations for a Space Infrared Nulling Interferometer Dedicated to the Search for Earthlike Extrasolar Planets”, Icarus, v128, pp. 202–212.Mennesson B.,Ollivier M., and Ruilier C., (2002a), “On the use of single-mode waveguides to correct the optical defects of a nulling interferometer”, J. Opt. Soc. Am. A, Feb 2002.Mennesson, B.P., Shao, M., Levine, B.M., Wallace, J.K., Liu, D.T., Serabyn, E., Unwin, S.C., Beichman, C.A., (2002b), ‘Optical Planet Discoverer: how to turn a 1.5m class space telescope into a powerful exo-planetary systems images’, SPIE, v4860.Morgan, R., Burge, J., and Woolf, N., (2000), “Nulling Interferometric Beam Combiner Utilizing Dielectric Plates” Experimental Results in the Visible Broadband SPIE v4006.Serabyn, E., Wallace, J.K., Hardy, G.J., Schwindthin, E.G.H., and Nguyen, (1999), “Deep Nulling of Visible LASER Light”, Appl. Opt., v38, p7128.Serabyn, E. and Colavita, M.M., (2001),’Fully Symmetric Nulling Beam Combiners’, Applied Optics, v40, pp. 1668–1671.Shao, M.,(1991), “Hubble Extra Solar Planet Interferometer”, SPIE v1494.Shao, M., Serabyn, E., Levine, B.M., Mennesson, B.P., and Velusamy, T, (2002), ‘Visible nulling coronagraph for detecting planets around nearby stars’, SPIE v4860.Wallace, K., Hardy, G, and Serabyn, E., (2000), “Deep and stable interferometric nulling of broadband light with implications for observing planets around nearby stars”, Nature, v406.Wallace, J.K., Shao, M., Levine, B.M., and Lane, B., ‘Experimental Results from the Optical Planet Detector Interferometer’, (2003), SPIE 5170-21.