A&A 376, L31-L34 (2001)
DOI: 10.1051/0004-6361:20011035
J. P. Berger1,2 -
P. Haguenauer2,3 -
P. Kern2 -
K. Perraut2
-
F. Malbet2 -
I. Schanen4
-
M. Severi5 -
R. Millan-Gabet1 -
W. Traub 1
1 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138, USA
2 - Laboratoire d'Astrophysique de l'Observatoire de
Grenoble, BP 53, 38041 Grenoble Cedex 9, France
3 -
CSO mesure, 70 Av. des Martyrs,
38000 Grenoble, France
4 -
Laboratoire d'Électromagnétisme Microondes et Optoélectronique,
38016 Grenoble Cedex 1, France
5 -
CEA-LETI Département de Microtechnologies,
17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Received 16 May 2001 / Accepted 18 July 2001
Abstract
We present in this paper the astronomical validation of a
new approach to interferometric starlight combination.
Using integrated optics technologies developed by the
telecommunication industry, we have implemented optical circuits on
coin-size chips that combine two beams and provide simultaneous
photometric calibration signals. We report the first interferometric
observations of stars using such beam combiners at the Infrared
Optical Telescope Array (IOTA). This result opens
the way to a new generation of miniaturized, high performance, and
reliable instruments, dedicated to interferometric aperture
synthesis.
Key words: interferometry - integrated optics - instrumentation
Modern optical interferometry started in 1975 with the first interferometric combination between two separate telescopes (Labeyrie 1975). However, to date, only two facilities have achieved aperture synthesis image reconstruction with three or four separated telescopes: COAST and NPOI. Direct imaging requires interferometers with a large number of separated apertures. The VLTI, the Keck Interferometer and CHARA interferometer will respectively provide 7, 6 and 6 telescopes. One of the main issues that has to be solved is the difficulty to combine many stellar beams with limited photon loss, high interferometric contrast and sufficient optical stability to provide accurately calibrated measurements. The complexity of a classical bulk optics beam combiner, using beam-splitters and mirrors, increases dramatically with the number of telescopes to be combined. For example, it takes N(N-1)/2beam-splitters to combine N beams in a coaxial pairwise scheme. Each optical surface decreases the throughput and requires careful alignment. The layout must be symmetric to avoid differential effects that affect contrast and maintain an internal stability crucial for closure phase measurements.
The need for improved accuracy in the simultaneous combination of a high number of beams led us to look for alternative solutions to classical bulk optics. Since 1996 (Kern et al. 1996; Malbet et al. 1999; Berger et al. 1999; Haguenauer et al. 2000), we have been exploring integrated optics (IO), a technology developed by the telecommunication and micro-sensor industries. This technique opens a new way to interferometrically combine beams from separated telescopes using IO optical circuits (analogous to integrated chips in micro-electronics), with the beam combination taking place in an assembly of optical waveguides lying in a solid substrate of few centimeters long and few millimeters large.
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Figure 1: a) Optical layout of the experiment (credits: E. Stadler). The LEMO chip's three outputs (described in the text) are imaged onto a liquid nitrogen cooled infrared detector matrix. b) Integrated optics components, top: LETI beam combiner, bottom LEMO beam combiner. c) IOTA interferometer. d) Optical breadboard. |
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IO chips can be found today at every step of the light path in an
optical telecom network. Several technologies, ion exchange and
silica etching being the most developed ones, are key to the
manufacture of various functions in optical chips based on the
classical microphotolithography process used in micro-electronics.
Integrated optics has proved to work remarkably well at the
wavelengths used by telecom or micro-sensors, i.e. 0.8, 1.31 and 1.55
where low-cost laser sources and very transparent
fiber optics are available. The achievable functions provide not only all
the usual optical ones (divider or combiner) but also diffracting and
dephasing devices. Output beams from these planar guides can act as the
input slit of a spectrograph, avoiding complex anamorphic optics. An
important additional advantage is that single mode waveguides also
spatially filter the wavefronts, leading to excellent calibrating
properties in the presence of atmospheric turbulence.
For the sky validation reported here, we used two different IO chips
designed for two-telescope beam combination in H band
(see Figs. 1a and b). The component
labelled LEMO was designed and manufactured using the ion exchange
process (Benech 1996): Na+ ions from a glass substrate are
exchanged with Ag+ ions in a molten salt through a dedicated
mask. It combines two input beams injected by fibers in a direct
Y-junction, and calibrates the flux contribution from each beam using
two reverse Y-junctions located before the combining function. The
component labelled LETI was designed and manufactured using the silica
etching technique (Mottier 1996). Doped silica layers, a few
microns thick, are deposited on a silicon substrate, etched following
the mask drawing and covered by a silica layer. The component combines
two input beams in an asymmetric directional coupler giving two
interferometric outputs in phase opposition and calibrates the flux as
described above for the LEMO component using two Y junctions located
before the coupler. Both beam combiners were connected with two
equal-length 1 m optical fibers.
Following a complete laboratory characterization of the optical
properties of these components (Berger et al. 1999; Haguenauer et al. 2000), we
set up an experiment at the Infrared Optical Telescope Array
(IOTA, Traub 1998) at Mt Hopkins, Arizona), At the IOTA,
2 telescopes of 45 cm diameter (a 3rd one is currently being implemented)
may be configured in baselines of length ranging from 5 to 38 m
(see Fig. 1c). The two IOTA light beams are carried from
the telescopes to the beam combination table (see
Fig. 1d). Off-axis parabolic mirrors are used to couple
light into the fibers connected to the component which outputs are
imaged on a NICMOS 3 infrared camera (Millan-Gabet et al. 1999) using custom
optics (see Figs. 1a and d). The optical path in one arm
is sawtooth-modulated with a maximum optical path difference (OPD) of
85 m by a piezo-actuated mirror in one of the arms. The data
acquisition is synchronized with the piezoelectric displacement and
the piezo stroke is centered around the zero OPD position. For each
scan an interferogram is recorded in each of the interferometric
outputs while simultaneously recording the calibration photometric
outputs as shown in Fig. 2.
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Figure 2: Fringes obtained with an IO two-telescope beam combiner. Top: raw interferometric signal, middle: the two photometric signals showing flux variations, bottom: frequency filtered interferogram corrected from photometry. Each trace is vertically shifted for clarity. |
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This first observation run was mainly aimed at characterizing our two beam combiners under interferometer observing conditions. We observed 14 stars with known diameters between November 26-30, 2000. Each observation consists of a set of 100 scans, as described above, plus a corresponding measurement of the background signal. A few minutes later, an identical sequence on the calibrator is recorded. As an illustration of the technique we present here results for the Mira star U Ori. The data reduction procedure employed is similar to that used in previous guided optics instruments (Coudé du Foresto et al. 1997), namely.
Source | 119 Tau | U Ori |
Wavelength |
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Projected baseline | ![]() |
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Previous diameter |
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Calibrated visibility | -- | 0.34 ![]() |
Measured diameter | -- |
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1 Dyck et al. (1998), Richichi et al. (1998).
2 van Belle et al. (1996).
U Ori was observed during three nights providing a total of five batches of 100 interferograms, one with the LEMO beam combiner (night Nov. 26th), four with the LETI beam combiner (2 during night Nov. 28th, 2 during night Nov. 29th). Each batch leads to one average visibility. After calibration, we used these five visibilities to perform a least square fit of a uniform diameter model.
If we adopt a 119 Tau diameter of 9.65 0.5 mas we find a U Ori
diameter of
mas, fully compatible with a previous
determination of 11.08
0.57 in the K band
at the IOTA (van Belle et al. 1996). Table 1
summarizes our observations. Figure 3 displays our
calibrated data points obtained with both beam combiners and the best
uniform diameter model fit.
Each batch of 100 visibilities has an average standard deviation smaller than 5% which translates into a statistical precision of 0.5%. The uncertainty on the calibrator diameter dominates the final estimation. The flux ratios between the interferometric and photometric channels were found to be remarkably stable all over the observation run allowing a precise calibration of the beam combiner behaviour. We see no statistical difference between the LEMO and the LETI beam combiner measurements which are all compatible with the model fit (see Fig. 3) within a maximum of 2% of absolute visibility. Although the number of points is not sufficient here to test the night to night repeatability, we see no significative trend down to the same 2% precision.
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Figure 3: Visibility points measured with LEMO (triangle) and LETI (cross) beam combiner. The curve represents the best uniform diameter fit. |
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In addition to these observational tests we carried out several instrumental tests. A full description of the performances of the instrument will be included in a forthcoming paper.
These results have demonstrated, for the first time that telecom-based integrated optics components can be used to combine stellar beams collected by separated telescopes in an optical long-baseline interferometer. These beam combiners are very stable and lead to precise measurements, moreover, they are versatile and easy to handle. The number of optical aligment adjustments is reduced, which dramatically reduces the complexity of multiple-beam combination for aperture synthesis imaging.
This is not only vital for large ground-based interferometers under construction but also for upcoming space missions. This technology will likely find many applications in the field of optical interferometry. Several concepts for up to eight telescopes beam combination are already under study (Berger et al. 2000).
Our next goal is to combine three telescopes beams at IOTA using integrated optics components recently developed for closure phase measurements and imaging applications (Haguenauer et al. 2000). The spectral coverage is also currently being extended to longer wavelengths. Finally, we propose this technology as a solution to combine the 7 telescopes of the VLTI.
Acknowledgements
We thank J. D. Monnier, P. Benech, F. Reynaud, S. Gluck, G. Grand, Y. Magnard, E. Stadler and M. Joubert. We thank the fluor team for their kind support at the IOTA. We thank Dr. van Belle, the referee, for his comments. This work was funded by the Centre National de la Recherche Scientifique, the Centre National d'Études Spatiales, the Smithsonian Institution and NASA. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) through the Michelson fellowship program funded by NASA as an element of the Planet Finder Program. JPL is managed for NASA by the California Institute of Technology.