VERMILION is a VLTI visitor instrument project intended to extend the sensitivity and the spectral coverage of Optical Long Baseline Interferometry (OLBIn). It is based on a new concept of Fringe Tracker (VERMILIONFT) combined with a J band spectro-interferometer (VERMILION-J). The Fringe Tracker is the Adaptive Optics module specific to OLBIn that measures and corrects in real time the Optical Path Difference (OPD) perturbations introduced by the atmosphere and the interferometer, by providing a sensitivity gain of 2 to 3 magnitudes over all other state of the art fringe trackers. The J band spectro-interferometer will provide all interferometric measurements as a function of wavelength. In addition to a possible synergy with MATISSE, VERMILION-J, by observing at high spectral resolution many strong lines in J (Paβ-γ, HeII, TiO and other metallic monoxides), will cover several scientific topics, e.g. Exoplanets, YSOs, Binaries, Active Hot, Evolved stars, Asteroseismology, and also AGNs.
For the telescope number and size of the PFI proposal (20 x 8m), the Q-band (around 25 μm) was shown to be an optimal complement to submm- and optical interferometry, to detect substructures in proto-planetary disks. Here, broadband medium-resolution heterodyne would be an appropriate option, considering recent experimental findings on a quantum-limit heterodyne detection (HD) scheme. Under high thermal background and the very small beam filling factors in interferometry, HD was shown to be more sensitive than direct detection for the same bandwidth for wavelengths larger 3-8 micron, depending on the conditions. A new class of heterodyne mixers based on graphene could extend the instantaneous IF-bandwidth up to hundred GHz, for which photonic correlators may be developed since a resolution of R=30000 would be sufficient at 10μm. Such large IF-bandwidths could then even be multiplied in “dispersed heterodyne receivers” using optical frequency combs and photonic techniques derived from the DWDM technology.
Recent results for the cross-correlation signal of a newly proposed balanced correlation receiver at 1.5 μm pointed towards a possible bypassing of the standard quantum limit for the receiver noise-temperature hν⁄k in cross-correlation by a factor of 4-6. The only radiation source strong enough for a clear hot-cold measurement was a heavily attenuated fiber-coupled superluminant LED (SLED), because a multi-mode fiber-coupled thermal halogen lamp was difficult to control in polarization due to its weakness when coupled to a single-mode fiber. This peculiarity left some doubts regarding a possible “strange” quantum-mechanical behavior of the signal light from the SLED. Here we want to present the concept for more convincing measurements using a true thermal signal source.
The current work presents a fiber coupling tip-tilt controller developed for a three-telescope experimental prototype of an Astronomical Fiber-Based Near-Infrared Heterodyne Interferometer. It is based on a commercial magneto-mechanical compact-disk laser-beam actuator on which the fiber-ferrule is mounted. The actuator is driven by a two-axis controller electronics board which was developed by us based on digital processing in a dsPIC33EP device with analog periphery, which reads the quad-photodiode signals amplified by 109, and drives the actuator with two high-current outputs. While this realizes the very fine and relatively fast (up to 100 Hz) fiber-position control in the telescope focus, as a basis to this, a relatively coarse and slow auto-guiding is given by an amateur guiding camera. During first optical bench testing we obtained an average coupled power increase of up to 50% under certain perturbations.
A semi-classical theory was re-derived in a consistent form for properly comparing direct and heterodyne detection as a function of wavelength. Plots are shown for example cases. We show that heterodyne should be better than direct detection for wavelengths longer than 3 microns, even with a bandwidth disadvantage, since direct detection is more sensitive to ambient temperature background than heterodyne detection. For interferometry the advantage of heterodyne is more pronounced in this case due to the smaller beam filling factors. When we include even the effect of surpassing the noise temperature quantum limit with a novel correlation receiver architecture (see paper 10701-94), the advantage of heterodyne detection becomes irrefutable.
KEYWORDS: Receivers, Signal to noise ratio, Heterodyning, Interference (communication), Photodiodes, Sensors, Signal detection, Optical correlators, Temperature metrology, Electroluminescence
We present concept and first experimental lab results for a novel heterodyne correlation receiver architecture and demonstrate that it can surpass the standard quantum limit (SQL) for the noise temperature by “correlating out” the local oscillator shot noise made uncorrelated at both receivers due to replacing the laser shot noise by individual beam splitter noise. It is based on two balanced receivers, comprising in total of 4 mixers, and uses an 8-bit digitization FPGA-based 1GHz bandwidth digital correlation between the two receivers. The demonstrated prototype was built for 1550 nm using InGaAs balanced photodiodes. We present here a summary of the results described in detail in a paper accepted at IEEEAccess journal. The extra-sensitivity would lead to heterodyne being better than direct detection for wavelengths beyond 3 microns. We propose therefore this receiver architecture as a building block in a heterodyne technology to be developed for the future Planet Formation Imager Infrared Interferometer (PFI). This paper is a reduced version of a paper accepted at IEEE Access a week before the conference [1].
We present the concept and experimental development of a low-cost near-infrared heterodyne interferometer prototype based on commercial 1.55 μm fiber components. As the most crucial component of it we characterized a novel sub-shot noise correlation detection system. We are upgrading to a Reconfigurable Open Architecture Computing Hardware, 2nd Generation (ROACH-2) board with the capacity of four parallel 1.25 GHz bandwidth digitization, so that phase closure measurements will be possible. We extended the stabilization of the local oscillator phase between the telescopes to cover the whole acoustic range. For the telescope to single-mode fiber coupling under atmospheric perturbation, we developed a fiber actuator lock-loop for small telescopes and good seeing, and tested an adaptive optics approach for mediocre seeing and/or larger telescopes. We constructed also a frequency comb based laser synthesizer system to include tests on multi-frequency band measurements towards ultra-broad band dispersed" heterodyne detection systems finally useful for the Planet Formation Imager (PFI).
The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4 x 8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry.
We present concept and first experimental lab results for a low-cost near-infrared heterodyne interferometer based on commercial 1.55μm fiber components with relative phase-stabilization between both telescopes. After a demonstration with 14”-telescopes, the concept should be upgradable to larger numbers of mid- or large-class telescopes. Given that the employed fiber phase stabilization scheme should enable the operation of long baselines, we discuss the applicability of this concept for long-baseline, high telescope number systems (scalability of the concept) and mid-infrared wavelengths. This could finally result in contributions to the design of the large infrared Planet Formation Imager which is being proposed currently.
The Planet Formation Imager (PFI) project aims to image the period of planet assembly directly, resolving structures as small as a giant planet's Hill sphere. These images will be required in order to determine the key mechanisms for planet formation at the time when processes of grain growth, protoplanet assembly, magnetic fields, disk/planet dynamical interactions and complex radiative transfer all interact - making some planetary systems habitable and others inhospitable. We will present the overall vision for the PFI concept, focusing on the key technologies and requirements that are needed to achieve the science goals. Based on these key requirements, we will define a cost envelope range for the design and highlight where the largest uncertainties lie at this conceptual stage.
The Planet Formation Imager (PFI) Project has formed a Technical Working Group (TWG) to explore possible facility architectures to meet the primary PFI science goal of imaging planet formation in situ in nearby starforming regions. The goals of being sensitive to dust emission on solar system scales and resolving the Hill-sphere around forming giant planets can best be accomplished through sub-milliarcsecond imaging in the thermal infrared. Exploiting the 8-13 micron atmospheric window, a ground-based long-baseline interferometer with approximately 20 apertures including 10km baselines will have the necessary resolution to image structure down 0.1 milliarcseconds (0.014 AU) for T Tauri disks in Taurus. Even with large telescopes, this array will not have the sensitivity to directly track fringes in the mid-infrared for our prime targets and a fringe tracking system will be necessary in the near-infrared. While a heterodyne architecture using modern mid-IR laser comb technology remains a competitive option (especially for the intriguing 24 and 40μm atmospheric windows), the prioritization of 3-5μm observations of CO/H2O vibrotational levels by the PFI-Science Working Group (SWG) pushes the TWG to require vacuum pipe beam transport with potentially cooled optics. We present here a preliminary study of simulated L- and N-band PFI observations of a realistic 4-planet disk simulation, finding 21x2.5m PFI can easily detect the accreting protoplanets in both L and N-band but can see non-accreting planets only in L band. We also find that even an ambitious PFI will lack sufficient surface brightness sensitivity to image details of the fainter emission from dust structures beyond 5 AU, unless directly illuminated or heated by local energy sources. That said, the utility of PFI at N-band is highly dependent on the stage of planet formation in the disk and we require additional systematic studies in conjunction with the PFI-SWG to better understand the science capabilities of PFI, including the potential to resolve protoplanetary disks in emission lines to measure planet masses using position-velocity diagrams. We advocate for a specific technology road map in order to reduce the current cost driver (telescopes) and to validate high accuracy fringe tracking and high dynamic range imaging at L, M band. In conclusion, no technology show-stoppers have been identified for PFI to date, however there is high potential for breakthroughs in medium-aperture (4-m class) telescopes architecture that could reduce the cost of PFI by a factor of 2 or more.
We are presenting first experimental results for subsystems of a low-cost near-infrared heterodyne interferometer concept based on commercial 1.55μm fiber-components with relative phase-stabilization between both telescopes, a shot noise limited heterodyne scheme with ambient temperature operated photodiodes, an ultra-coherent fiber laser, and a ROACH-based correlator. After we worked on a first demonstration with two 14” amateur telescopes on Betelgeuse, the concept should be upgradable to connect mid- or large-class telescopes, also given that the employed fiber phase stabilization scheme will enable the operation of long baselines.
A central component of a sideband separating (2SB) receiver is the quadrature hybrid which splits the incoming radio frequency signal into two branches with a 90°-phase shift. Its fabrication, however, is one of the factors limiting the operational bandwidth and the maximum frequency at which a 2SB can be built. In this paper a 100%-photonic approach to produce this split is presented. In this way, wider operational bandwidth and a higher maximum operational frequency could be achieved. We also present the first experimental results of a proof-of-concept implementation at 55 MHz that studied the phase stability and controllability of this approach using a commercial 90-degree optical hybrid.
Sideband-separating receivers are usually preferred in the presence of high atmospheric noise. However, one of the most important figures of merit for this kind of receiver, the sideband ratio, is still low and typically around 10 dB. This is because keeping low amplitude and phase imbalances over large RF and IF frequencies is extremely difficult. However, by introducing a digital back-end that mimics the performance of an IF-hybrid, such imbalances can be calibrated out. We have recently presented a digital sideband-separating receiver, working at the W band, that can achieve sideband ratios well above 35 dB. Here we extend this work by demonstrating that it can also be applied to receivers that incorporate a second down-conversion stage with the same performance.
This paper describes the practical implementation and on the field test results of an Enhanced algorithm, based on Jones matrix Eigen analysis, to find the minimal polarization change points for the ALMA's line length corrector fiber stretcher. These points are complicated to find because they can be located at any point of the Poincare sphere and they also change after manipulation of fibers and with time, so the optimization of the fibers stretcher needs to be systematically checked and redone. The execution time of the original algorithm takes up to 20 minutes depending on the starting polarization state. This enhanced algorithm reduces this time in most of cases to less than a minute, with no intrinsic dependency on the starting point.
More efficient and powerful continuous-wave photonic mixers as terahertz sources are motivated by the need of more versatile local oscillators for submillimeter/terahertz receiver systems. Uni-Travelling Carrier (UTC) photodiodes are very prospective candidates for reaching this objective, but so far only have been reported as lumped-elements or as edge-illuminated optical-waveguide travelling-wave (TW) devices. To overcome the associated power limitations of those implementations, we are developing a novel implementation of the UTC photodiodes which combines a travelingwave photomixer with vertical velocity-matched illumination in a distributed structure. In this implementation called velocity-matched travelling-wave uni-travelling carrier photodiode, it is possible to obtain in-situ velocity matching of the beat-fringes of the two angled laser beams with the submm/THz-wave on the stripline. In this way, minimum frequency roll-off is achieved by tuning the angle between the two laser beams. A first design of these TW-UTC PDs from our Terahertz Photonics Laboratory at University of Chile has been micro-fabricated at the MC2 cleanroom facility at Chalmers Technical University.
We are presenting a low-cost near-infrared heterodyne interferometer based on 1.55ìm fiber-components, amateur
telescopes and a 3-GSPS-ROACH-based correlator. While first performance is estimated to be sufficient for the
brightest stars, we expect science-relevant astronomical performance given various improvements we are working on, as
stabilization of fiber coupling, detectors near the quantum limit requiring lowest possible local oscillator power, and
fiber line-length correction. These will enable the investigation of extremely long baselines, adaption of existing
medium-class telescopes, and testing at the pointing-telescope slots of ALMA, and/or the extension to three of more
baselines.
More efficient continuous-wave photonic nearinfrared
mixers as terahertz sources are investigated
with the motivation to develop a universal photonic
local oscillator for astronomical submillimeter/terahertz
receiver systems. For this, we develop new concepts for
vertically illuminated traveling-wave (TW)
photomixers, TW Uni-Travelling Carrier (UTC)
photodiodes. Device simulation/modeling and
optical/terahertz testing is being done in the new
terahertz photonics laboratory at the Electrical
Engineering Department of the University of Chile,
whereas device fabrication is performed at the MC2
cleanroom facility at Chalmers Technical University.
We report on first progress in this direction.
A new type of geometry for terahertz traveling-wave photomixers, vertically pumped free-space by two detuned continuous-wave diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured as a first result, which meets power requirements for superconducting heterodyne mixer devices.
We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency.
The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz.
We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.
We describe the receiver concept for KOSMA's planned second generation SOFIA instrument STAR (SOFIA Terahertz Array Receiver). The receiver will contain a 4 X 4 element heterodyne mixer array for the frequency range from 1.7 to 1.9 THz (158 to 176 microns). Its main scientific goal is large scale mapping of the 158 micron fine structure transition of singly ionized carbon. The design frequency range covers this line out to moderate red shifts and also allows to observe a variety of other spectral lines.
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