5,985 research outputs found
Phase Noise of the Radio Frequency (RF) Beatnote Generated by a Dual-Frequency VECSEL
We analyze, both theoretically and experimentally, the phase noise of the
radio frequency (RF) beatnote generated by optical mixing of two orthogonally
polarized modes in an optically pumped dual-frequency Vertical External Cavity
Surface Emitting Laser (VECSEL). The characteristics of the RF phase noise
within the frequency range of 10 kHz - 50 MHz are investigated for three
different nonlinear coupling strengths between the two lasing modes. In the
theoretical model, we consider two different physical mechanisms responsible
for the RF phase noise. In the low frequency domain (typically below 500 kHz),
the dominant contribution to the RF phase noise is shown to come from the
thermal fluctuations of the semicondutor active medium induced by pump
intensity fluctuations. However, in the higher frequency domain (typically
above 500 kHz), the main source of RF phase noise is shown to be the pump
intensity fluctuations which are transfered to the intensity noises of the two
lasing modes and then to the phase noise via the large Henry factor of the
semiconductor gain medium. For this latter mechanism, the nonlinear coupling
strength between the two lasing modes is shown to play an important role in the
value of the RF phase noise. All experimental results are shown to be in good
agreement with theory
Comparison of coronagraphs for high contrast imaging in the context of Extremely Large Telescopes
We compare coronagraph concepts and investigate their behavior and
suitability for planet finder projects with Extremely Large Telescopes (ELTs,
30-42 meters class telescopes). For this task, we analyze the impact of major
error sources that occur in a coronagraphic telescope (central obscuration,
secondary support, low-order segment aberrations, segment reflectivity
variations, pointing errors) for phase, amplitude and interferometric type
coronagraphs. This analysis is performed at two different levels of the
detection process: under residual phase left uncorrected by an eXtreme Adaptive
Optics system (XAO) for a large range of Strehl ratio and after a general and
simple model of speckle calibration, assuming common phase aberrations between
the XAO and the coronagraph (static phase aberrations of the instrument) and
non-common phase aberrations downstream of the coronagraph (differential
aberrations provided by the calibration unit). We derive critical parameters
that each concept will have to cope with by order of importance. We evidence
three coronagraph categories as function of the accessible angular separation
and proposed optimal one in each case. Most of the time amplitude concepts
appear more favorable and specifically, the Apodized Pupil Lyot Coronagraph
gathers the adequate characteristics to be a baseline design for ELTs.Comment: 12 pages, 6 figures, Accepted for publication in A&
Interaction Grammars
Interaction Grammar (IG) is a grammatical formalism based on the notion of
polarity. Polarities express the resource sensitivity of natural languages by
modelling the distinction between saturated and unsaturated syntactic
structures. Syntactic composition is represented as a chemical reaction guided
by the saturation of polarities. It is expressed in a model-theoretic framework
where grammars are constraint systems using the notion of tree description and
parsing appears as a process of building tree description models satisfying
criteria of saturation and minimality
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
SPHERE: the exoplanet imager for the Very Large Telescope
Observations of circumstellar environments to look for the direct signal of
exoplanets and the scattered light from disks has significant instrumental
implications. In the past 15 years, major developments in adaptive optics,
coronagraphy, optical manufacturing, wavefront sensing and data processing,
together with a consistent global system analysis have enabled a new generation
of high-contrast imagers and spectrographs on large ground-based telescopes
with much better performance. One of the most productive is the
Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE)
designed and built for the ESO Very Large Telescope (VLT) in Chile. SPHERE
includes an extreme adaptive optics system, a highly stable common path
interface, several types of coronagraphs and three science instruments. Two of
them, the Integral Field Spectrograph (IFS) and the Infra-Red Dual-band Imager
and Spectrograph (IRDIS), are designed to efficiently cover the near-infrared
(NIR) range in a single observation for efficient young planet search. The
third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to
look for the reflected light of exoplanets and the light scattered by debris
disks. This suite of three science instruments enables to study circumstellar
environments at unprecedented angular resolution both in the visible and the
near-infrared. In this work, we present the complete instrument and its on-sky
performance after 4 years of operations at the VLT.Comment: Final version accepted for publication in A&
Apodized pupil Lyot coronagraphs for arbitrary apertures. V. Hybrid Shaped Pupil designs for imaging Earth-like planets with future space observatories
We introduce a new class of solutions for Apodized Pupil Lyot Coronagraphs
(APLC) with segmented aperture telescopes to remove broadband diffracted light
from a star with a contrast level of . These new coronagraphs provide
a key advance to enabling direct imaging and spectroscopy of Earth twins with
future large space missions. Building on shaped pupil (SP) apodization
optimizations, our approach enables two-dimensional optimizations of the system
to address any aperture features such as central obstruction, support
structures or segment gaps. We illustrate the technique with a design that
could reach contrast level at 34\,mas for a 12\,m segmented telescope
over a 10\% bandpass centered at a wavelength 500\,nm. These
designs can be optimized specifically for the presence of a resolved star, and
in our example, for stellar angular size up to 1.1\,mas. This would allow
probing the vicinity of Sun-like stars located beyond 4.4\,pc, therefore fully
retiring this concern. If the fraction of stars with Earth-like planets is
\eta_{\Earth}=0.1, with 18\% throughput, assuming a perfect, stable wavefront
and considering photon noise only, 12.5 exo-Earth candidates could be detected
around nearby stars with this design and a 12\,m space telescope during a
five-year mission with two years dedicated to exo-Earth detection (one total
year of exposure time and another year of overheads). Our new hybrid APLC/SP
solutions represent the first numerical solution of a coronagraph based on
existing mask technologies and compatible with segmented apertures, and that
can provide contrast compatible with detecting and studying Earth-like planets
around nearby stars. They represent an important step forward towards enabling
these science goals with future large space missions.Comment: 9 pages, 6 figures, ApJ accepted on 01/04/201
Photometric characterization of exoplanets using angular and spectral differential imaging
The direct detection of exoplanets has been the subject of intensive research
in the recent years. Data obtained with future high-contrast imaging
instruments optimized for giant planets direct detection are strongly limited
by the speckle noise. Specific observing strategies and data analysis methods,
such as angular and spectral differential imaging, are required to attenuate
the noise level and possibly detect the faint planet flux. Even though these
methods are very efficient at suppressing the speckles, the photometry of the
faint planets is dominated by the speckle residuals. The determination of the
effective temperature and surface gravity of the detected planets from
photometric measurements in different bands is then limited by the photometric
error on the planet flux. In this work we investigate this photometric error
and the consequences on the determination of the physical parameters of the
detected planets. We perform detailed end-to-end simulation with the CAOS-based
Software Package for SPHERE to obtain realistic data representing typical
observing sequences in Y, J, H and Ks bands with a high contrast imager. The
simulated data are used to measure the photometric accuracy as a function of
contrast for planets detected with angular and spectral+angular differential
methods. We apply this empirical accuracy to study the characterization
capabilities of a high-contrast differential imager. We show that the expected
photometric performances will allow the detection and characterization of
exoplanets down to the Jupiter mass at angular separations of 1.0" and 0.2"
respectively around high mass and low mass stars with 2 observations in
different filter pairs. We also show that the determination of the planets
physical parameters from photometric measurements in different filter pairs is
essentialy limited by the error on the determination of the surface gravity.Comment: 13 pages, 7 figures, 4 tables. Accepted for publication in MNRA
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