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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.