NOTT (formerly Hi-5) is the L’-band (3.5-4.0μm) nulling interferometer of Asgard, an instrument suite in preparation for the VLTI visitor focus. The primary scientific objectives of NOTT include characterizing (i) young planetary systems near the snow line, a critical region for giant planet formation, and (ii) nearby mainsequence stars close to the habitable zone, with a focus on detecting exozodiacal dust that could obscure Earthlike planets. In 2023-2024, the final warm optics have been procured and assembled in a new laboratory at KU Leuven. First fringes and null measurements were obtained using a Gallium Lanthanum Sulfide (GLS) photonic chip that was also tested at cryogenic temperatures. In this paper, we present an overall update of the NOTT project with a particular focus on the cold mechanical design, the first results in the laboratory with the final NOTT warm optics, and the ongoing Asgard integration activities. We also report on other ongoing activities such as the characterization of the photonic chip (GLS, LiNbO3, SiO), the development of the exoplanet science case, the design of the dispersion control module, and the progress with the self-calibration data reduction software.
Asgard/NOTT is an ERC-funded project hosted at KU Leuven and is part of a new visitor instrumental suite, called Asgard, under preparation for the Very Large Telescope Interferometer (VLTI). Leveraging nulling capabilities and the long VLTI baselines, it is optimized for high-contrast imaging of the snow line region around young nearby main-sequence stars. This will enable the characterization of the atmosphere of young giant exoplanets and warm/hot exozodiacal dust with spectroscopy in the L’-band (3.5-4.0μm). In this work, we present the first lab assembly of the instrument done at KU Leuven and the technical solutions to tackle the challenge of performing nulling in the mid-infrared despite the thermal background. The opto-mechanical design of the warm optics and the injection system for the photonic chip are described. The alignment procedure used to assemble the system is also presented. Finally, the first experimental results, including fringes and null measurements, are given and confirm the adequacy of the bench to test and optimize the Asgard/NOTT instrument.
Fringe stability and tracking are a determining aspect for the performance of current interferometric observations. While the theory predicts that the aperture of large telescopes such as the VLTI UT should yield smoothed-out piston perturbations that could be compensated using a slow fringe tracker running at a few tens of Hz, this is far from the current experimental reality. In practice, the optical path variations observed with the GRAVITY fringe tracker still contain high frequency components that limit the fringe-tracking exposure time and therefore its precision and limiting magnitude. Most of these perturbations seem to come from mechanical vibrations in the train of mirrors, leading to the instrument, and in particular from the mirrors of the telescope. With this work, and as part of the GRAVITY+ efforts, accelerometers were added to all the mirrors of the coudé train, including the coming M8, to complement the existing instrumentation of M1, M2, and M3, and compensate in real-time the optical path using the main delay lines. We show how the existing architecture, while optimal for the first mirrors, is not suitable for the vibration content found in the new mirrors, and we opt instead for narrow-band filters based on phase-locked-loop filters (PLL). Thanks to this architecture, we were able to reclaim up to 50nm of OPD RMS from vibrations peaks between 40 and 200Hz. We also outline the avenues to push this approach further, through the upgrade of the deformable mirrors and the beam-compressor differential delay lines (BCDDL) as part of GRAVITY+, paving the way to obtaining better than 100nm RMS fringe tracking, even on faint targets.
Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L′-band (3.5 to 4.0) μm, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of 10−3 in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements of beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss <6.4% of the best efficiency for the injection and a null depth of ∼2.10−3, mainly from optical path delay errors outside the scope of this work.
European Southern Observatory (ESO)’s Very Large Telescope Interferometer (VLTI), Paranal, Chile, is one of the most proficient observatories in the world for high angular resolution astronomy. It has hosted several interferometric instruments operating in various bandwidths in the infrared. As a result, the VLTI has yielded countless discoveries and technological breakthroughs. We propose to ESO a new concept for a visitor instrument for the VLTI: Asgard. It is an instrumental suite comprised of four natively collaborating instruments: High-Efficiency Multiaxial Do-it ALL Recombiner (HEIMDALLR), an all-in-one instrument performing both fringe tracking and stellar interferometry with the same optics; Baldr, a Strehl optimizer; Beam-combination Instrument for studying the Formation and fundamental paRameters of Stars and planeTary systems (BIFROST), a combiner whose main science case is studying the formation processes and properties of stellar and planetary systems; and Nulling Observations of dusT and planeTs (NOTT), a nulling interferometer dedicated to imaging young nearby planetary systems in the L band. The overlap between the science cases across different spectral bands yields the idea of making the instruments complementary to deliver sensitivity and accuracy from the J to L bands. Asgard is to be set on the former AMBER optical table. Its control architecture is a hybrid between custom and ESO-compliant developments to benefit from the flexibility offered to a visitor instrument and foresee a deeper long-term integration into VLTI for an opening to the community.
Hi-5 is an ERC-funded project hosted at KU Leuven and a proposed visitor instrument for the VLTI. Its primary goal is to image the snow line region around young planetary systems using nulling interferometry in the L’ band, between 3.5 and 4.1 μm, where the contrast between exoplanets and their host stars is very advantageous. The breakthrough is the use of a photonic chip based beam combiner, which only recently allowed the required theoretical raw contrast of 10−3 in this spectral range. The VLTI long baseline interferometry enables to reach high angular resolution (4.2 mas at 3.8 μm wavelength with the Auxiliary Telescopes (ATs)), while high contrast detection is achieved using nulling interferometry. This polarisation requires a high degree of optical symmetry between the four pupils of the VLTI, only possible with precise phase, dispersion and intensity control systems. The instrument is currently in its design phase. In this paper, the warm optics design and the injection system up to the photonic chip are presented. The different properties of the design are presented including the optics used, the characteristics of the four beams and the current drawbacks. Particular attention is devoted to the optical alignment and the tolerance analysis in order to estimate the precision required for the alignment procedure and therefore to choose adapted optical mountings.
The Hi-5 instrument, a proposed high-contrast L' band (3.5-4.0 μm) nulling interferometer for the visitor focus of the Very Large Telescope Interferometer (VLTI), will characterize young extra-solar planetary systems and exozodiacal dust around nearby main-sequence stars. Thanks to VLTI's angular resolution (λ=B = 5 mas for the longest UT baseline), it will fill the gap between young giant exoplanets discovered by ongoing single-aperture direct imaging surveys and exoplanet populations discovered by radial velocity surveys. In this paper, we investigate the exoplanet detection yield of Hi-5. First, we present the latest catalog of stars identified as members of young stellar associations within 150 pc of the Sun thanks to the BANYAN algorithm and other searches for young moving group members. Realistic exoplanet populations are then generated around these stars and processed with the SCIFYsim tool, the end-to-end simulator for the Hi-5 instrument. Then, two formation models are used to estimate the giant planet's luminosity. The first is the New Generation Planetary Population Synthesis (NGPPS), also known as the Bern model, and the second is a statistical model based on gravitational instability (hot-start model - AMES-Dusty model). We show that Hi-5 is insensitive to cold-start planets but can detect giant hot-start planets. With ATs, more than 40 planets could be detected assuming 20 nights of observations. With its unique capabilities, Hi-5 is also able to constrain in mass the observed systems. Hi-5 is sensitive to planets with a mass > 2 Mjup around the snow line.
The vibrations affecting the telescopes and downstream optics largely contribute to the optical path length error in interferometry and constitute an important limitation for fringe tracking sensitivity and high-contrast imaging. At the VLTI-UT, vibrations are measured and compensated with the MANHATTAN-II system. This system currently measures the acceleration in the first three mirrors of the Coud´e train with piezo-electric accelerometers and sends the compensation error to the delay lines. The GRAVITY+ program and the next generation of the VLTI planet-finding instruments require the current Optical Path Distance (OPD) residual of approximately 200 nm rms in the UTs to be smaller than 100 nm, and consequently an extension of the MANHATTAN-II system is envisaged to all the Coud´e train mirrors that are known to be affected by high-frequency vibrations. In this work, we present the planned hardware upgrade of the Manhattan system and an analysis of the self-intrinsic noise of MANHATTAN-II. Piezo accelerometer noise depends on parasitic electric charges, generated for example from long cables. The noise theoretical model is experimentally tested with a dummy accelerometer reproducing the equivalent circuit, and by a three-sensors coherence analysis. We find that the intrinsic-noise for M2-UT1 is of the order of 2 μg/Hz0.5 in the amplifier bandwidth 1 Hz – 25 kHz. Accounting for all sensors in every mirrors in the extended configuration, the Manhattan-II self-intrinsic noise corresponds to an overall equivalent OPD rms of 166 nm for the shortest possible cable lengths configuration, before the fringe tracker. We suggest that sensors with sub- μg/Hz0.5 noise could be necessary to approach the ultimate limit of the atmospheric piston calculated from a Kolmogorov model.
A small-footprint 4-telescope photonic beam combiner is at the heart of the Hi-5 instrument, the high-contrast VLTI visitor instrument focusing on the detection and characterization of young exoplanets in the mid-infrared L’ band. Hi-5 implements the technique of nulling interferometry to efficiently suppress the strong stellar radiation of the central source and enhance the detection of the nearby faint planetary signal. Based on the “Double Bracewell” architecture, the photonic nulling beam combiner is designed around three cascaded achromatic directional couplers with 50/50 coupling ratios. This allows the nulled signals of the first two couplers to be cross-combined with a third central combiner, which produces two conjugated asymmetric transmission maps projected onto the sky. Each individual telescope beam passes first through a side-step to suppress uncoupled stray-light. The corresponding flux is then sampled by an asymmetric Y-junction to provide a simultaneous photometric channel for the estimation of the self-calibrated nulls. We report here on the prototyping phase of the Hi-5 4-telescope photonic beam combiner that is manufactured by ultrafast laser inscription in a Gallium-Lanthanum-Sulphide (GLS) glass substrate, which exhibits high transparency in the L’ band of interest. Using our 2-beam spectro-interferometric lab bench, we measure the throughput of the beam combiners, the chromatic and broadband coupling ratios in the 3.6-3.9 μm range for the couplers and the Y-junctions, as well as the broadband interferometric properties of these 4-telescope mid-infrared photonic beam combiners.
Precise control of the optical path differences (OPD) in the Very Large Telescope Interferometer (VLTI) was critical for the characterization of the black hole at the center of our Galaxy - leading to the 2020 Nobel prize in physics. There is now significant effort to push these OPD limits even further, in-particular achieving 100nm OPD RMS on the 8m unit telescopes (UT’s) to allow higher contrast and sensitivity at the VLTI. This work calculated the theoretical atmospheric OPD limit of the VLTI as 5nm and 15nm RMS, with current levels around 200nm and 100nm RMS for the UT and 1.8m auxiliary telescopes (AT’s) respectively, when using bright targets in good atmospheric conditions. We find experimental evidence for the f−17/3 power law theoretically predicted from the effect of telescope filtering in the case of the ATs which is not currently observed for the UT’s. Fitting a series of vibrating mirrors modelled as dampened harmonic oscillators, we were able to model the UT OPD PSD of the gravity fringe tracker to <1nm/ √Hz RMSE up to 100Hz, which could adequately explain a hidden f−17/3 power law on the UTs. Vibration frequencies in the range of 60-90Hz and also 40-50Hz were found to generally dominate the closed loop OPD residuals of Gravity. Cross correlating accelerometer with Gravity data, it was found that strong contributions in the 40-50Hz range are coming from the M1-M3 mirrors, while a significant portion of power from the 60-100Hz contributions are likely coming from between the M4-M10. From the vibrating mirror model it was shown that achieving sub 100nm OPD RMS for particular baselines (that have OPD∼200nm RMS) required removing nearly all vibration sources below 100Hz.
The Very Large Telescope Interferometer is one of the most proficient observatories in the world for high angular resolution. Since its first observations, it has hosted several interferometric instruments operating in various bandwidths in the infrared. As a result, the VLTI yields countless discoveries and technological breakthroughs. We introduce to the VLTI the new concept of Asgard: an instrumental suite including four natively collaborating instruments: BIFROST, a stellar interferometer dedicated to the study of the formation of multiple systems; Hi- 5, a nulling interferometer dedicated to imaging young nearby planetary systems in the M band; HEIMDALLR, an all-in-one instrument performing both fringe tracking and stellar interferometry with the same optics; Baldr, a fibre-injection optimiser. These instruments share common goals and technologies. Thus, the idea of this suite is to make the instruments interoperable and complementary to deliver unprecedented sensitivity and accuracy from J to M bands. The interoperability of the Asgard instruments and their integration in the VLTI are the main challenges of this project. In this paper, we introduce the overall optical design of the Asgard suite, the different modules, and the main challenges ahead.
Hi-5 is the L’-band (3.5-4.0 μm) high-contrast imager of Asgard, an instrument suite in preparation for the visitor focus of the VLTI. The system is optimized for high-contrast and high-sensitivity imaging within the diffraction limit of a single UT/AT telescope. It is designed as a double-Bracewell nulling instrument producing spectrally-dispersed (R=20, 400, or 2000) complementary nulling outputs and simultaneous photometric outputs for self-calibration purposes. In this paper, we present an update of the project with a particular focus on the overall architecture, opto-mechanical design of the warm and cold optics, injection system, and development of the photonic beam combiner. The key science projects are to survey (i) nearby young planetary systems near the snow line, where most giant planets are expected to be formed, and (2) nearby main sequence stars near the habitable zone where exozodiacal dust that may hinder the detection of Earth-like planets. We present an update of the expected instrumental performance based on full end-to-end simulations using the new GRAVITY+ specifications of the VLTI and the latest planet formation models.
Hi-5 is a proposed L' band high-contrast nulling interferometric instrument for the visitor focus of the Very Large Telescope Interferometer (VLTI). As a part of the ERC consolidator project called SCIFY (Self-Calibrated Interferometry For exoplanet spectroscopY), the instrument aims to achieve sufficient dynamic range and angular resolution to directly image and characterize the snow line of young extra-solar planetary systems. The spectrometer is based on a dispersive grism and is located downstream of an integrated optics beam-combiner. To reach the contrast and sensitivity specifications, the outputs of the I/O chip must be sufficiently separated and properly sampled on the Hawaii-2RG detector. This has many implications for the photonic chip and spectrometer design. We present these technical requirements, trade-off studies, and phase-A of the optical design of the Hi-5 spectrometer in this paper. For both science and contract-driven reasons, the instrument design currently features three different spectroscopic modes (R=20, 400, and 2000). Designs and efficiency estimates for the grisms are also presented as well as the strategy to separate the two polarization states.
The very short life-time of the excited carriers is a very important intrinsic property of quantum well photodetectors (QWIPs), based on III-V semiconductor materials, that have not been exploited for performances yet. Its typical value is in the order of a ps [7], which leads to two important consequences: the detector frequency response can be up to 100 GHz and its saturation intensity is extremely high in the in the order of 10e7 W/cm2. These two figures are ideal for a heterodyne detection scheme, where a powerful local oscillator (LO) can drive a strong photocurrent, higher than the detector dark current, that can coherently mix with a signal shifted in frequency with respect to the LO. Notably, these unique properties are unmatched in infrared interband detectors based on mercury-cadmium-telluride alloys, which have a much longer carrier lifetime and therefore intrinsic low-speed response. Yet, the performances of all photonic detectors are limited by the high dark current which originates from thermal emission of electrons from the wells, rising exponentially with temperature and imposing cryogenic operation (~ 80K) for high sensitivity.
In the present work we show that the intrinsic limitation of QWIP detectors can be overcome by the use of a photonic metamaterial made of metallic resonators. The absorbing region of the detector consists of a 5 period GaAs/AlGaAs QWIP operating at a wavelength 8.9µm (139meV) which has been designed according to an optimized bound to continuum structure. The absorbing region is inserted in an array of double-metal patch resonators [ref], which provide sub-wavelength electric field confinement and act as antennas. The resonant wavelength is fixed by the patch size s through the expression lambda = 2 s neff, with neff = 3.3 the effective index. This allows us to tune the cavity mode in resonance with the absorption peak of the bare detector for values of s between 1.3µm-1.4µm. The detectors obtained in this way have high detectivity up at room temperature, in the order of 3-4 10e7 cmHz1/2/W. Given this his high detectivity we could calibrate the detector using a black body emitting only few µW. Up to date high sensitivity at room temperature, with values comparable to those we are reporting, have been demonstrated, only in the 3-5µm wavelength range, using quantum cascade detectors (QCDs) [9-11] and with MCT standard detectors.
To the best of our knowledge this is the best results ever reported for a photoconductive detector at 9µm at room temperature. Since the detector has an extremely fast response of several GHz we built an heterodyne setup tp beat two quantum cascade lasers. We obtained heterodyne signal at high frequencies up to 3GHz, with NEP in the pW range at 300K.
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