176 research outputs found
Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry
We report the ultrafast laser fabrication and mid-IR characterization (3.39
microns) of four-port evanescent field directional couplers. The couplers were
fabricated in a commercial gallium lanthanum sulphide glass substrate using
sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed
using optimal fabrication parameters were found to exhibit propagation losses
of 0.8 dB/cm. A series of couplers were inscribed with different interaction
lengths, and we demonstrate power splitting ratios of between 8% and 99% for
mid-IR light with a wavelength of 3.39 microns. These results clearly
demonstrate that ultrafast laser inscription can be used to fabricate high
quality evanescent field couplers for future applications in astronomical
interferometry.Comment: 4 pages, 4 figure
M-lines characterization of selenide and telluride waveguides for mid-infrared interferometry
Nulling interferometry is an astronomical technique that combines equal
wavefronts to achieve a deep rejection ratio of an on-axis star, and that could
permit to detect Earth-like planets in the mid-infrared band 5 -- 20 microns.
Similarly to what is done in the near-infrared, high frequencies spatial
filtering of the incoming beams can be achieved using single-mode waveguides
operating in the mid-infrared. An appreciable reduction of the instrumental
complexity is also possible using integrated optics (IO) devices in this
spectral range. The relative lack of single-mode guided optics in the
mid-infrared has motivated the present technological study to demonstrate the
feasibility of dielectric waveguides functioning at longer wavelengths. We
propose to use selenide and telluride components to pursue the development of
more complex IO functions.Comment: accepted in OSA Optics Express, 11 pages, 4 figure
Integrated optics prototype beam combiner for long baseline interferometry in the L and M bands
In the last few years, integrated optics (IO) beam combiners have facilitated
the emergence of 4-telescope interferometers such as PIONIER or GRAVITY,
boosting the imaging capabilities of the VLTI. However, the spectral range
beyond 2.2microns is not ideally covered by the conventional silica based IO.
Here, we propose to consider new laser-written IO prototypes made of GLS
glasses, a material that permits access to the mid-infrared spectral regime.
Our goal is to conduct a full characterization of our mid-IR IO 2-telescope
coupler in order to measure the performance levels directly relevant for
long-baseline interferometry. We focus in particular on the exploitation of the
L and M astronomical bands. We use a dedicated Michelson-interferometer setup
to perform Fourier Transform spectroscopy on the coupler and measure its
broadband interferometric performance. We also analyze the polarization
properties of the coupler, the differential dispersion and phase degradation as
well as the modal behavior and the total throughput. We measure broadband
interferometric contrasts of 94.9% and 92.1% for unpolarized light in the L and
M bands. Spectrally integrated splitting ratios are close to 50% but show
chromatic dependence over the considered bandwidths. Additionally, the phase
variation due to the combiner is measured and does not exceed 0.04rad and
0.07rad across the band L and M band, respectively. The total throughput of the
coupler including Fresnel and injection losses from free-space is 25.4%. The
laser-written IO GLS prototype combiners prove to be a reliable technological
solution with promising performance for mid-infrared long-baseline
interferometry. In the next steps, we will consider more advanced optical
functions as well as a fiber-fed input and revise the optical design parameters
in order the further enhance the total throughput and achromatic behavior
Transmission behaviors of single mode hollow metallic waveguides dedicated to mid-infrared nulling interferometry
This paper reports the characterization of hollow metallic waveguides (HMW)
to be used as single-mode wavefront filters for nulling interferometry in the
6-20 microns range. The measurements presented here were performed using both
single-mode and multimode conductive waveguides at 10.6 microns. We found
propagation losses of about 16dB/mm, which are mainly due to the theoretical
skin effect absorption in addition to the roughness of the waveguide metallic
walls. The input and output coupling efficiency of our samples has been
improved by adding tapers to minimize the impedance mismatch. A proper
distinction between propagation losses and coupling losses is presented.
Despite their elevate propagation losses, HMW show excellent spatial filtering
capabilities in a spectral range where photonics technologies are only
emerging.Comment: This paper was published in Optics Express and can be found at
http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-26-1800
First fringes with an integrated-optics beam combiner at 10 um - A new step towards instrument miniaturization for mid-infrared interferometry
Observations at mas-resolution scales and high dynamic range hold a central
place in achieving, for instance, the spectroscopic characterization of
exo-Earths or the detailed mapping of their protoplanetary disc birthplace.
Ground or space-based multi-aperture infrared interferometry is a promising
technique to tackle these goals. But significant efforts still need to be
undertaken to achieve a simplification of these instruments if we want to
combine the light from a large number of telescopes. Integrated-optics appears
as an alternative to the current conventional designs, especially if its use
can be extended to a higher number of astronomical bands. This article reports
for the first time the experimental demonstration of the feasibility of an
integrated-optics approach to mid-infrared beam combination for single-mode
stellar interferometry. We have fabricated a 2-telescope beam combiner
prototype integrated on a substrate of chalcogenide glasses, a material
transparent from 1 to 14 um. We have developed laboratory tools to characterize
the modal properties and the interferometric capabilities of our device. We
obtain fringes at 10 um and measure a mean contrast V=0.981 \pm 0.001 with high
repeatability over one week and high stability over 5h. We show experimentally
- as well as on the basis of modeling considerations - that the component has a
single-mode behavior at this wavelength, which is essential to achieve
high-accuracy interferometry. From previous studies, the propagation losses are
estimated to 0.5 dB/cm for such components. We also discuss possible issues
that may impact the interferometric contrast. The IO beam combiner performs
well at 10. We also anticipate the requirement of a better matching between the
numerical apertures of the component and the (de)coupling optics to optimize
the total throughput. The next step foreseen is the achievement of wide-band
interferograms.Comment: Accepted in A&A; 7 pages; 7 figure
Impact of {\eta}earth on the capabilities of affordable space missions to detect biosignatures on extrasolar planets
We present an analytic model to estimate the capabilities of space missions
dedicated to the search for biosignatures in the atmosphere of rocky planets
located in the habitable zone of nearby stars. Relations between performance
and mission parameters such as mirror diameter, distance to targets, and radius
of planets, are obtained. Two types of instruments are considered: coronagraphs
observing in the visible, and nulling interferometers in the thermal infrared.
Missions considered are: single-pupil coronagraphs with a 2.4 m primary mirror,
and formation flying interferometers with 4 x 0.75 m collecting mirrors. The
numbers of accessible planets are calculated as a function of {\eta}earth. When
Kepler gives its final estimation for {\eta}earth, the model will permit a
precise assessment of the potential of each instrument. Based on current
estimations, {\eta}earth = 10% around FGK stars and 50% around M stars, the
coronagraph could study in spectroscopy only ~1.5 relevant planets, and the
interferometer ~14.0. These numbers are obtained under the major hypothesis
that the exozodiacal light around the target stars is low enough for each
instrument. In both cases, a prior detection of planets is assumed and a target
list established. For the long-term future, building both types of
spectroscopic instruments, and using them on the same targets, will be the
optimal solution because they provide complementary information. But as a first
affordable space mission, the interferometer looks the more promising in term
of biosignature harvest.Comment: Accepted by Ap
Evidence of a discontinuous disk structure around the Herbig Ae star HD 139 614
A new class of pre-main sequence objects has been recently identified as
pre-transitional disks. They present near-infrared excess coupled to a flux
deficit at about 10 microns and a rising mid-infrared and far-infrared
spectrum. These features suggest a disk structure with inner and outer dust
components, separated by a dust-depleted region (or gap). We here report on the
first interferometric observations of the disk around the Herbig Ae star HD
139614. Its infrared spectrum suggests a flared disk, and presents
pre-transitional features,namely a substantial near-infrared excess accompanied
by a dip around 6 microns and a rising mid-infrared part. In this framework, we
performed a study of the spectral energy distribution (SED) and the
mid-infrared VLTI/MIDI interferometric data to constrain thespatial structure
of the inner dust disk region and assess its possibly multi-component
structure. We based our work on a temperature-gradient disk model that includes
dust opacity. While we could not reproduce the SED and interferometric
visibilities with a one-component disk, a better agreement was obtained with a
two-component disk model composed of an optically thin inner disk extending
from 0.22 to 2.3 au, a gap, and an outer temperature-gradient disk starting at
5.6 au. Therefore, our modeling favors an extended and optically thin inner
dust component and in principle rules out the possibility that the
near-infrared excess originates only from a spatially confined region.
Moreover, the outer disk is characterized by a very steep temperature profile
and a temperature higher than 300 K at its inner edge. This suggests the
existence of a warm component corresponding to a scenario where the inner edge
of the outer disk is directly illuminated by the central star. This is an
expected consequence of the presence of a gap, thus indicative of a
pre-transitional structure.Comment: 14 pages, 6 figure
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