KEYWORDS: Calibration, Data modeling, Signal to noise ratio, Education and training, Nulling interferometry, Equipment, Point spread functions, Wavefronts, Sensors, Optical path differences
Photonic technologies have enabled a generation of nulling interferometers, such as the guided light interferometric nulling technology instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterization of habitable planet atmospheres. But even with cutting-edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control and its accuracy is only as good as the instrument’s calibration. Here, we present an approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method and can also be used in real time for fringe tracking. This is achieved using a deep neural network to learn the true in-situ complex transfer function of the instrument and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-real-time approach is used instead of the statistical methods used in other techniques (such as null self calibration, or NSC) and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
KEYWORDS: Data modeling, Calibration, Wavefronts, Signal to noise ratio, Point spread functions, Nulling interferometry, Adaptive optics, Telescopes, Sensors, Machine learning
Photonic technologies have enabled a new generation of nulling interferometers such as the GLINT instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterisation of habitable planet atmospheres. But even with cutting edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control, and its accuracy is only as good as the instrument’s calibration. Here we present a new approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method, and can also be used in realtime for fringe tracking. This is achieved by using a deep neural network to learn the true in-situ complex transfer function of the instrument, and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-realtime approach is used instead of the statistical methods used in other techniques (such as numerical self calibration, or NSC), and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
Nulling interferometry is one of the most promising techniques for imaging exoplanets at solar system scales as it simultaneously meets the stringent requirements for contrast and angular resolution. The GLINT instrument, operating at Subaru telescope behind the SCExAO extreme adaptive optics system, has delivered significant advances in performance, paving the way for a science-ready instrument. Results from previous commissioning runs have confirmed that integrated optics and self-calibration methods yield a robust pathway to detecting exoplanets below the diffraction limit of the telescope. Furthermore, limitations to the current performance have been identified. The most critical are the compensation of remaining phase fluctuations and delivering deep achromatic nulls. Specifically, one technology that offers enormous promise to resolve these issues is the use of tricouplers to perform simultaneous nulling and fringe tracking, or wavefront characterization and correction free of non-common path aberrations. By adding a broadband π radian phase-shifter, fringe tracking and path length stabilization is performed around a deep achromatic null, providing excellent high contrast performance at small angular scales. In this paper, we present the commissioning of GLINT, its current challenges and describe the modelling of devices to overcome them. We discuss the expected performance of a nuller based on these principles of phase control and null depth as implemented within the GLINT instrument. We further present laboratory characterization of 3D-written tricouplers made using ultrafast laser inscription.
Post Extreme Adaptive-Optics (ExAO) spectro-interferometers design allows high contrast imaging with an inner working angle down to half the theoretical angular resolution of the telescope. This regime, out of reach for conventional ExAO imaging systems, is obtained thanks to the interferometric recombination of multiple sub-apertures of a single telescope, using single mode waveguides to remove speckle noise. The SCExAO platform at the Subaru telescope hosts two instruments with such design, coupled with a spectrograph. The FIRST instrument operates in the Visible (600-800nm, R~400) and is based on pupil remapping using single-mode fibers. The GLINT instrument works in the NIR (1450-1650nm, R~160) and is based on nulling interferometry. We present here how these photonic instruments have the unique capability to simultaneously do high contrast imaging and be included in the wavefront sensing architecture of SCExAO.
With thousands of confirmed exoplanets, the era of discovery is giving way to that of characterization. Direct imaging is crucial, but extremely difficult due to high star-to-planet contrasts and high angular resolutions. Nulling interferometry, which suppresses contaminating starlight via destructive interference, aims to meet this challenge. A pathfinder of this technique is the GLINT nuller: a 6-baseline, spectrally dispersed pupil-remapping interferometer deployed at the Subaru telescope, in which a single photonic chip performs all the critical optical processes. We present the instrument, novel data processing based on self-calibrating methods, laboratory characterization and the latest on-sky results.
Ultrafast laser inscription is a technique to create low-loss three dimensional optical circuits within bulk dielectrics that is compatible with a wide range of optical materials. Its unique capabilities and the ability to rapid prototype and quickly iterate through different designs has made it exceptionally attractive for astrophotonics. This paper will summarize the basic aspects of ultrafast laser inscription and review recent progress in its application to astrophotonics, such as stellar interferometry.
As confirmed exoplanets climb into the thousands, the era of exoplanets discovery is giving way to exoplanet characterization. The most desirable scenario is one where the exoplanet can be directly imaged. Direct imaging not only delivers orbital parameters, but also yields the chemical composition of the atmosphere. The potential for habitable zone exoplanets to exhibit biosignatures in such data from a visionary future instrument drives intense interest. However, this requires to simultaneously reach extremely high star-to-planet contrast (from 104 to 108) and extremely high angular resolution (around and below the diffraction limit). Accomplishing all this through the atmosphere blurred by turbulence remains a critical challenge, yet it is one that nulling interferometry in combination with extreme adaptive optics aims to meet. This technique overcomes the contrast problem by removing the starlight with destructive interference, permitting the faint light coming from the planet to remain. In this paper, we present the latest evolution of nulling interferometry instrumentation: the integrated- photonic nuller. It allows spatial filtering, multiple simultaneous baselines, simultaneous photometric channels and simultaneous measurement of the "nulled" signal (the light emitted from the planet after cancelling the starlight) as well as the "anti-nulled" signal (the channel containing the redirected starlight). Exploiting these fundamental optical principles, the delivery of imaging and differential spectroscopy of exoplanetary systems becomes possible. This paper describes a pathfinder that has implemented these ideas into a robust and compact photonic-chip platform known as the GLINT (Guided-Light Interferometric Nulling Technology) project.
This paper is one of a three-part series of papers on photonics-based mid-IR interferometry. Here, we put the emphasis on the challenges of operating integrated optics over a broad wavelength range, a natural condition in the field of Astrophysics. We report on the recent advancements made in obtaining high interferometric contrast (> 90%) through 2-telescope combiners in the mid-IR and give an outlook on more advanced functions and 4-telescope combiners.
With many thousands of exoplanets discovered one of the important next steps in astronomy is to be able to characterise them. This presents a great challenge and calls for new observational capabilities with both high angular resolution and extreme high contrast in order to efficiently separate the bright light of a host star to that of a faint companion. Glint South is an instrument that uses photonic technology to perform nulling interferometry. The light of a star is cancelled out by means of destructive interference in a photonic chip. One of the challenges is the star light injection into the chip. This is done by a unique active system that optimises the injection and provide low order correction for the atmospheric turbulence. We are reporting on the latest progress following several tests on the Anglo Australian Telescope.
In this contribution, we review the results of the ALSI project (Advanced Laser-writing for Stellar Interferometry), aimed at assessing the potential of ultrafast laser writing to fabricate mid-infared integrated optics (IO) devices with performance compatible with an implementation in real interferometric instruments like Hi5 or PFI. Waveguides for the L, L' and M bands with moderate propagation losses were manufactured in Gallium Lanthanum Sulfide and ZBLAN glasses and used to develop photonic building blocks as well as a full mid-IR 4-telescope beam combiner. We discuss the advantages and disadvantages of the tested combiners and discuss a possible roadmap for the continuation of this work.
In this paper, we propose a multilayer arrayed waveguide gratings (AWG) for spectro-interferometry applications which were fabricated using the femtosecond laser direct writing technique. Individually these devices consisted on an array of 19 single mode waveguides, were designed for operation at 633nm, and have a measured free spectral range of 28nm and a resolution of 4 nm. The aim of this work is to show the potential of arrayed waveguides gratings (AWG) stacked in a vertical layered structure, in order to simultaneously achieve spectral dispersion and multi-telescope interferometry for astrophotonic applications (visibility vs baseline as a function of wavelength). In particular, with the fabrication of three independent AWG, we can address closure phase studies, an important tool for exoplanet detection in astronomy.
RHEA is a single-mode ´echelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. The instrument has a novel fiber feed with an integral field unit injecting into a grid of single-mode fibers reformatted to form a pseudo-slit, increasing throughput and enabling highspatial resolution observations when operating behind Subaru and the SCExAO adaptive optics system. The past 18 months have seen a replacement cable constructed for the instrument to address modal noise caused by closely packed fibers with similar path lengths. Here we detail the cable fabrication procedure, design improvements, increased precision in meeting the required sub-micron optical tolerances, throughput gains, and known remaining issues.
The Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph is being developed to serve as a basis for multiple copies across a network of small robotic telescopes. The spectrograph operates at the diffraction-limit by using a single-mode fiber input, resulting in a compact and modal-noise-free unit. The optical design is mainly based on off-the-shelf available components and comprises a near-Littrow configuration with prism cross-disperser. The échelle format covers a wavelength range of 430-650 nm at R=75,000 resolving power. In this paper we briefly summarize the current status of the instrument and present preliminary results from the first on-sky demonstration of the prototype using a fully automated 16" telescope, where we observe stable and semi-variable stars up to V=3.5 magnitude. Future steps to enhance the efficiency and passive stability of RHEA are discussed in detail. For example, we show the concept of using a multi-fiber injection unit, akin to a photonic lantern, which not only enables increased throughput but also offers simultaneous wavelength calibration.
Using single-mode fibres in astronomy enables revolutionary techniques including single-mode interferometry and spectroscopy. However, injection of seeing-limited starlight into single mode photonics is extremely difficult. One solution is Adaptive Injection (AI). The telescope pupil is segmented into a number of smaller subapertures each with size ~ r0, such that seeing can be approximated as a single tip / tilt / piston term for each subaperture, and then injected into a separate fibre via a facet of a segmented MEMS deformable mirror. The injection problem is then reduced to a set of individual tip tilt loops, resulting in high overall coupling efficiency.
The RHEA Spectrograph is a single-mode echelle spectrograph designed to be a replicable and cost effective method of undertaking precision radial velocity measurements. Two versions of RHEA currently exist, one located at the Australian National University in Canberra, Australia (450 - 600nm wavelength range), and another located at the Subaru Telescope in Hawaii, USA (600 - 800 nm wavelength range). Both instruments have a novel fibre feed consisting of an integral field unit injecting light into a 2D grid of single mode fibres. This grid of fibres is then reformatted into a 1D array at the input of the spectrograph (consisting of the science fibres and a reference fibre capable of receiving a white-light or xenon reference source for simultaneous calibration). The use of single mode fibres frees RHEA from the issue of modal noise and significantly reduces the size of the optics used. In addition to increasing the overall light throughput of the system, the integral field unit allows for cutting edge science goals to be achieved when operating behind the 8.2m Subaru Telescope and the SCExAO adaptive optics system. These include, but are not limited to: resolved stellar photospheres; resolved protoplanetary disk structures; resolved Mira shocks, dust and winds; and sub-arcsecond companions. We present details and results of early tests of RHEA@Subaru and progress towards the stated science goals.
Integrated optics (IO) has proven to be a competitive solution for beam combination in the context of astronomical interferometry (e.g. GRAVITY at the VLTI). However, conventional silica-based lithographic IO is limited to wavelengths shorter than 2.2μm. We report in this paper the progress on our attempt to extend the operation of IO to longer wavelengths. Previous work has demonstrated the suitability of chalcogenide devices in the MID-IR in the N band and monochromatically at 3.39 μm. Here, we continue this effort with the manufacturing of new laser written GLS IO as beam combiners designed for the astronomical L band and characterized interferometrically at 3.39 μm. In the era of multi-telescope interferometers, we present a promising solution to strengthen the potential of IO for new wavelength ranges.
The Australian Astronomical Observatory is currently investigating the use of adaptive optics technologies for the 3.9m Anglo-Australian Telescope at Siding Spring Observatory. It might be that ground-layer or multi-object adaptive optics is beneficial for the Anglo-Australian Telescope (seeing ∼1.5"). Key to achieving this goal is an adaptive optics test-bench developed for laboratory experiments and on-sky demonstration. The test-bench provides a facility to demonstrate on-sky natural guide star adaptive optics as well as second stage correction with active injection into single mode waveguides. The test-bench provides wide field access of up to 20 arcminutes for testing our plug-plate distributed wavefront sensors. Data has been collected in a range of seeing conditions where closed-loop corrections were performed. We present the design, results and plans for the adaptive optics on-sky demonstrator.
SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.
The advent of 30 m class Extremely Large Telescopes will require spectrographs of unprecedented spectral resolution in order to meet ambitious science goals, such as detecting Earth-like exoplanets via the radial velocity technique. The consequent increase in the size of the spectrograph makes it challenging to ensure their optimal environmental stabilization and precise spectral calibration. The multimode optical fibers used to transport light from the telescope focal plane to the separately housed environmentally stabilized spectrograph introduces modal noise. This phenomena manifests as variations in the light pattern at the output of the fiber as the input coupling and/or fiber position changes which degrades the spectrograph line profile, reducing the instrument precision. The photonic lantern is a guided wave transition that efficiently couples a multimode point spread function into an array of single modes. If arranged in a linear array at the input of the spectrograph these single modes can in principle provide a diffraction-limited mode noise free spectra in the dispersion axis. In this paper we describe the fabrication and throughput performance of the hybrid reformatter. This device combines the proven low-loss performance of a multicore fiber-based photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect that performs the reformatting function to a diffraction-limited pseudo-slit. The device provided an in laboratory throughput of 65 ± 2% at 1550 ± 20 nm and an on-sky throughput of 53 ± 4% at 1530 ± 80 nm using the CANARY adaptive optics system at the William Herschel Telescope.
Tightly focused femtosecond laser pulses can be used to alter the refractive index of virtually all optical glasses. As the
laser-induced modification is spatially limited to the focal volume of the writing beam, this technique enables the
fabrication of fully three-dimensional photonic structures and devices that are automatically embedded within the host
material. While it is well understood that the laser-material interaction process is initiated by nonlinear, typically
multiphoton absorption, the actual mechanism that results in an increase or sometimes decrease of the refractive index of
the glass strongly depends on the composition of the material and the process parameters and is still subject to scientific
studies.
In this paper, we present an overview of our recent work aimed at uncovering the physical and chemical processes that
contribute to the observed material modification. Raman microscopy and electron microprobe analysis was used to study
the induced modifications that occur within the glass matrix and the influence of atomic species migration forced by the
femtosecond laser writing beam. In particular, we concentrate on borosilicate, heavy metal fluoride and phosphate glasses.
We believe that our results represent an important step towards the development of engineered glass types that are ideally
suited for the fabrication of photonic devices via the femtosecond laser direct write technique.
This paper reports on the modal noise characterisation of a hybrid reformatter. The device consists of a multicore-fibre photonic lantern and an ultrafast laser-inscribed slit reformatter. It operates around 1550 nm and supports 92 modes. Photonic lanterns transform a multimode signal into an array of single-mode signals, and thus combine the high coupling efficiency of multimode fibres with the diffraction-limited performance of single-mode fibres. This paper presents experimental measurements of the device point spread function properties under different coupling conditions, and its throughput behaviour at high spectral resolution. The device demonstrates excellent scrambling but its point spread function is not completely stable. Mode field diameter and mode bary-centre position at the device output vary as the multicore fibre is agitated due to the fabrication imperfections.
Due to their high efficiency and broad operational bandwidths, volume phase holographic gratings (VPHGs) are often
the grating technology of choice for astronomical instruments, but current VPHGs exhibit a number of drawbacks
including limits on their size, function and durability due to the manufacturing process. VPHGs are also generally made
using a dichromated gelatine substrate, which exhibits reduced transmission at wavelengths longer than ~2.2 μm,
limiting their ability to operate further into the mid-infrared.
An emerging alternative method of manufacturing volume gratings is ultrafast laser inscription (ULI). This technique
uses focused ultrashort laser pulses to induce a localised refractive index modification inside the bulk of a substrate
material. We have recently demonstrated that ULI can be used to create volume gratings for operation in the visible,
near-infrared and mid-infrared regions by inscribing volume gratings in a chalcogenide glass. The direct-write nature of
ULI may then facilitate the fabrication of gratings which are not restricted in terms of their size and grating profile, as is
currently the case with gelatine based VPHGs.
In this paper, we present our work on the manufacture of volume gratings in gallium lanthanum sulphide (GLS)
chalcogenide glass. The gratings are aimed at efficient operation at wavelengths around 1 μm, and the effect of applying
an anti-reflection coating to the substrate to reduce Fresnel reflections is studied.
We report on the development and testing of the building blocks of a possible compact heterodyne setup in the mid-infrared,
which becomes particularly relevant for flight instrumentation. The local oscillator is a Quantum Cascade Laser
(QCL) source at 8.6 μm operable at room temperature. The beam combination of the source signal and the local
oscillator will occur by means of integrated optics for the 10 μm range, which was characterized in the lab. In addition
we investigate the use of superlattice detectors in a heterodyne instrument. This work shows that these different new
components can become valuable tools for a compact heterodyne setup.
In this paper we report the fabrication and mid-infrared characterization (λ = 3.39 μm) of evanescent field directional couplers. These devices were fabricated using the femtosecond laser direct-writing technique in commercially available Gallium Lanthanum Sulphide (GLS) glass substrates. We demonstrate that the power splitting ratios of the devices can be controlled by adjusting the length of the interaction section between the waveguides, and consequently we demonstrate power splitting ratios of between 8% and 99% for 3.39 μm light. We anticipate that mid-IR beam integrated-optic beam combination instruments based on this technology will be key for future mid-infrared astronomical interferometry, particularly for nulling interferometry and earth-like exoplanet imaging.
Here we demonstrate a new generation of photonic pupil-remapping devices which build upon the interferometric framework developed for the Dragonfly instrument: a high contrast waveguide-based device which recovers robust complex visibility observables. New generation Dragonfly devices overcome problems caused by interference from unguided light and low throughput, promising unprecedented on-sky performance. Closure phase measurement scatter of only ~0.2° has been achieved, with waveguide throughputs of > 70%. This translates to a maximum contrast-ratio sensitivity (between the host star and its orbiting planet) at 1λ /D (1σ detection) of 5.3×10−4 (when a conventional adaptive-optics (AO) system is used) or 1.8×10−4 (for typical ‘extreme-AO’ performance), improving even further when random error is minimised by averaging over multiple exposures. This is an order of magnitude beyond conventional pupil-segmenting interferometry techniques (such as aperture masking), allowing a previously inaccessible part of the star to planet contrast-separation parameter space to be explored.
Spectroscopy is a technique of paramount importance to astronomy, as it enables the chemical composition, distances
and velocities of celestial objects to be determined. As the diameter of a ground-based telescope increases, the pointspread-
function (PSF) becomes increasingly degraded due to atmospheric seeing. A degraded PSF requires a larger
spectrograph slit-width for efficient coupling and current spectrographs for large telescopes are already on the metre
scale. This presents numerous issues in terms of manufacturability, cost and stability.
As proposed in 2010 by Bland-Hawthorn et al, one approach which may help to improve spectrograph stability
is a guided wave transition, known as a “photonic-lantern”. These devices enable the low-loss reformatting of a
multimode PSF into a diffraction-limited source (in one direction). This pseudo-slit can then be used as the input to a
traditional spectrograph operating at the diffraction limit. In essence, this approach may enable the use of diffractionlimited
spectrographs on large telescopes without an unacceptable reduction in throughput.
We have recently demonstrated that ultrafast laser inscription can be used to realize “integrated” photoniclanterns,
by directly writing three-dimensional optical waveguide structures inside a glass substrate. This paper presents
our work on developing ultrafast laser inscribed devices capable of reformatting a multimode telescope PSF into a
diffraction-limited slit.
We report the fabrication and characterization of prototype femtosecond-laser direct-written integrated photonic lanterns
for operation in the mid-infrared (mid-IR). The devices were inscribed inside the bulk of a commercial gallium
lanthanum sulphide (GLS) chalcogenide glass substrate and the characterization was performed using monochromatic
light with a wavelength of 3.39 μm. We demonstrate that these proof-of-concept devices are capable of coupling specific
multimode states of light into an array of single-modes, and vice-versa, with low-loss. In the future, instruments that
utilize the single-moded output of such components may find applications in areas such as heterodyne spectroscopy,
interferometry and remote sensing.
A key requirement for astronomical instruments in next generation Extremely Large Telescopes (ELTs) is the
development of large-aperture Integral Field Units (IFUs) that enable the efficient and spatially contiguous sampling of
the telescope image plane for coupling stellar light onto a spectrometer. Current IFUs are complex to fabricate and suffer
from stray light issues, which limits their application in high-contrast studies such as exoplanet imaging. In this paper,
we present our work on the development of freeform microlens arrays using the rapidly maturing technique of ultrafast
laser inscription and selective wet chemical etching. Using the focus spot from a femtosecond laser source as a tool with
an essentially unrestricted “tool-path”, we demonstrate that it is possible to directly write the surface of a lenslet in three
dimensions within the volume of a transparent material. We further show that high surface quality of the lenses can be
achieved by using an oxy-natural gas flame to polish the lens surface roughness that is characteristic of the post-etched
structures. Using our technique, the shape and position of each lenslet can be tailored to match the spatial positioning of
a two-dimensional multimode fiber array, which can be monolithically integrated with the microlens array.
Since the discovery, that a tightly focused femtosecond laser beam can induce a highly localized and permanent refractive index change in a wide range of dielectrics, ultrafast laser inscription has found applications in many elds due to its unique 3D and rapid prototyping capabilities. These ultrafast laser inscribed waveguide devices are compact and lightweight as well as inherently robust since the waveguides are embedded within the bulk material. In this presentation we will review our current understanding of ultrafast laser - glass lattice interactions and its application to the fabrication of inherently stable, compact waveguide lasers and astronomical 3D integrated photonic circuits.
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