Search

Australian Research Council [DP120103751]; Research School of Astronomy and Astrophysics; Australian National University

Project PANOPTES (http://www.projectpanoptes.org) is aimed at establishing a collaboration between professional astronomers, citizen scientists and schools to discover a large number of exoplanets with the transit technique. We have developed digital camera based imaging units to cover large parts of the sky and look for exoplanet transits. Each unit costs approximately $5000 USD and runs automatically every night. By using low-cost, commercial digital single-lens reflex (DSLR) cameras, we have developed a uniquely cost-efficient system for wide field astronomical imaging, offering approximately two orders of magnitude better etendue per unit of cost than professional wide-field surveys. Both science and outreach, our vision is to have thousands of these units built by schools and citizen scientists gathering data, making this project the most productive exoplanet discovery machine in the world.

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 echelle 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.

High-order wavefront correction is not only beneficial for high-contrast imaging, but also spectroscopy. The size of a spectrograph can be decoupled from the size of the telescope aperture by moving to the diffraction limit which has strong implications for ELT based instrument design. Here we present the construction and characterization of an extremely efficient single-mode fiber feed behind an extreme adaptive optics system (SCExAO). We show that this feed can indeed be utilized to great success by photonic-based spectrographs. We present metrics to quantify the system performance and some preliminary spectra delivered by the compact spectrograph.

We have been developing focal-plane phase-mask coronagraphs ultimately aiming at direct detection and characterization of Earth-like extrasolar planets by future space coronagraph missions. By utilizing photonic-crystal technology, we manufactured various coronagraphic phase masks such as eight-octant phase masks (8OPMs), 2nd-order vector vortex masks, and a 4th-order discrete (32-sector) vector vortex mask. Our laboratory experiments show that the 4th-order vortex mask reaches to higher contrast than the 2nd-order one at inner region on a focal plane. These results demonstrate that the higher-order vortex mask is tolerant of low-order phase aberrations such as tip-tilt errors. We also carried out laboratory demonstration of the 2nd-order vector vortex masks in the High-Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL), and obtained 10-8-level contrast owing to an adaptive optics system for creating dark holes. In addition, we manufactured a polarization-filtered 8OPM, which theoretically realizes achromatic performance. We tested the manufactured polarization-filtered 8OPM in the Infrared Coronagraphic Testbed (IRCT) at the JPL. Polychromatic light sources are used for evaluating the achromatic performance. The results suggest that 10-5- level peak-to-peak contrasts would be obtained over a wavelength range of 800-900 nm. For installing the focal-plane phase-mask coronagraph into a conventional centrally-obscured telescope with a secondary mirror, pupil-remapping plates have been manufactured for removing the central obscuration to enhance the coronagraphic performance. A result of preliminary laboratory demonstration of the pupil-remapping plates is also reported. In this paper, we present our recent activities of the photonic-crystal phase coronagraphic masks and related techniques for the high-contrast imaging.

Contrast limit for the direct imaging of extrasolar planets from ground based adaptive optics (AO) observations is set by the presence of static and slow-varying aberrations in the optical path that lead to the science instrument. To complement the otherwise highly successful angular differential imaging (ADI) technique toward small angular separation, we propose to employ additional wavefront control to modulate the diffraction. This flexible approach introduces enough diversity to discriminate genuine structures of the observed target from spurious diffraction features in the image. One possible implementation of such form of coherence differential imaging (CDI) is a speckle nulling algorithm that iteratively suppresses diffraction features inside a region constrained by the number of active elements of the deformable mirror modulating the wavefront, and the coronagraph. This paper presents on-sky results obtained with this approach, on the Subaru Coronagraphic Extreme AO (SCExAO) instrument.

Current AO observations rely heavily on the AO188 instrument, a 188-elements system that can operate in natural or laser guide star (LGS) mode, and delivers diffraction-limited images in near-IR. In its LGS mode, laser light is transported from the solid state laser to the launch telescope by a single mode fiber. AO188 can feed several instruments: the infrared camera and spectrograph (IRCS), a high contrast imaging instrument (HiCIAO) or an optical integral field spectrograph (Kyoto-3DII). Adaptive optics development in support of exoplanet observations has been and continues to be very active. The Subaru Coronagraphic Extreme-AO (SCExAO) system, which combines extreme-AO correction with advanced coronagraphy, is in the commissioning phase, and will greatly increase Subaru Telescope’s ability to image and study exoplanets. SCExAO currently feeds light to HiCIAO, and will soon be combined with the CHARIS integral field spectrograph and the fast frame MKIDs exoplanet camera, which have both been specifically designed for high contrast imaging. SCExAO also feeds two visible-light single pupil interferometers: VAMPIRES and FIRST. In parallel to these direct imaging activities, a near-IR high precision spectrograph (IRD) is under development for observing exoplanets with the radial velocity technique. Wide-field adaptive optics techniques are also being pursued. The RAVEN multi-object adaptive optics instrument was installed on Subaru telescope in early 2014. Subaru Telescope is also planning wide field imaging with ground-layer AO with the ULTIMATE-Subaru project.

A key avenue to improving the precision of radial velocity measurements is by using photonic devices to collect the light from the focal plane and delivering the beams to the slit of spectrograph via a single-mode fiber. Single-mode fibers have the favorable property that they allow light to propagate in a single energy distribution characterized by a Gaussian shape with a flat wavefront which is temporarily stable and independent of changes to the injection. These properties mean that the point spread function delivered to the input slit of a spectrograph is highly stable with time and independent of changes to the injection which is currently a key limitation to precision radial velocity measurements and known as "Modal Noise". Further light delivery via single-mode fibers is the key requirement to realize long baseline interferometers such as the Optical Hawaiian Array for Nanoradian Astronomy. Injecting into single-mode fibers efficiently is inherently difficult because it requires closely matching the intensity and wavefront of the focused beam to that supported by the fiber. The atmosphere is currently the key roadblock to efficient injection. However, extreme adaptive optics systems such as Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system currently being commissioned will enable high order wavefront correction and make efficient coupling into single-mode fibers possible. In addition, pupil apodization optics used for coronagraphy, known as phase induced amplitude apodization lenses also present in the SCExAO instrument, allow for close matching of the intensity distributions. We report on the progress and lessons learnt on developing an efficient single-mode injection unit within the SCExAO instrument. As part of the PANDORA project we aim to use this injection and combine it with several other photonic technologies to enable high precision radial velocity measurements in new and innovative ways.

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.

VAMPIRES is a high-angular resolution imager developed to directly image planet-forming circumstellar disks, and the signatures of forming planets that lie within. The instrument leverages aperture masking interferometry - providing diffraction-limited imaging despite seeing - in combination with fast-switching differential polarimetry to directly image structure in the inner-most regions of protoplanetary systems. VAMPIRES will use starlight scattered by dust in such systems to precisely map the disk, gaps, knots and waves that are key to understanding disk evolution and planet formation. It also promises to image the dusty circumstellar environments of AGB stars. This instrument perfectly compliments coronagraphic observations in the near-IR, and can operate simultaneously with a coronagraph, as part of the SCExAO extreme-AO system at the Subaru telescope. In this paper the design of the instrument will be presented, along with an explanation of the unique data analysis process and the results of the first on-sky tests.

The Panoptic Astronomical Networked OPtical observatory for Transiting Exoplanets Survey (PANOPTES, www.projectpanoptes.org) project is aimed at identifying transiting exoplanets using a wide network of low-cost imaging units. Each unit consists of two commercial digital single lens reflex (DSLR) cameras equipped with 85mm F1.4 lenses, mounted on a small equatorial mount. At a few $1000s per unit, the system offers a uniquely advantageous survey eficiency for the cost, and can easily be assembled by amateur astronomers or students. Three generations of prototype units have so far been tested, and the baseline unit design, which optimizes robustness, simplicity and cost, is now ready to be duplicated. We describe the hardware and software for the PANOPTES project, focusing on key challenging aspects of the project. We show that obtaining high precision photometric measurements with commercial DSLR color cameras is possible, using a PSF-matching algorithm we developed for this project. On-sky tests show that percent-level photometric precision is achieved in 1 min with a single camera. We also discuss hardware choices aimed at optimizing system robustness while maintaining adequate cost. PANOPTES is both an outreach project and a scientifically compelling survey for transiting exoplanets. In its current phase, experienced PANOPTES members are deploying a limited number of units, acquiring the experience necessary to run the network. A much wider community will then be able to participate to the project, with schools and citizen scientists integrating their units in the network.

Princeton University is building the Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS), an integral field spectrograph (IFS) for the Subaru telescope. CHARIS is funded by the National Astronomical Observatory of Japan and is designed to take high contrast spectra of brown dwarfs and hot Jovian planets in the coronagraphic image provided by the Coronagraphic Extreme Adaptive Optics (SCExAO) and the AO188 adaptive optics systems. The project is now in the build and test phase at Princeton University. Once laboratory testing has been completed CHARIS will be integrated with SCExAO and AO188 in the winter of 2016. CHARIS has a high-resolution characterization mode in J, H, and K bands. The average spectral resolution in J, H, and K bands are R82, R68, and R82 respectively, the uniformity of which is a direct result of a new high index material, L-BBH2. CHARIS also has a second low-resolution imaging mode that spans J,H, and K bands with an average spectral resolution of R19, a feature unique to this instrument. The field of view in both imaging modes is 2.07x2.07 arcseconds. SCExAO+CHARIS will detect objects five orders of magnitude dimmer than their parent star down to an 80 milliarcsecond inner working angle. The primary challenge with exoplanet imaging is the presence of quasi-static speckles in the coronagraphic image. SCExAO has a wavefront control system to suppress these speckles and CHARIS will address their impact on spectral crosstalk through hardware design, which drives its optical and mechanical design. CHARIS constrains crosstalk to be below 1% for an adjacent source that is a full order of magnitude brighter than the neighboring spectra. Since CHARIS is on the Nasmyth platform, the optical alignment between the lenslet array and prism is highly stable. This improves the stability of the spectra and their orientation on the detector and results in greater stability in the wavelength solution for the data pipeline. This means less uncertainty in the post-processing and less overhead for on-sky calibration procedures required by the data pipeline. Here we present the science case, design, and construction status of CHARIS. The design and lessons learned from testing CHARIS highlights the choices that must be considered to design an IFS for high signal-to-noise spectra in a coronagraphic image. The design considerations and lessons learned are directly applicable to future exoplanet instrumentation for extremely large telescopes and space observatories capable of detecting rocky planets in the habitable zone.

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.

We discuss the development of multi-core fiber Bragg gratings (FBGs) to be applied to astrophotonics, more specifically to near-infrared spectroscopy for ground-based instruments. The multi-core FBGs require over 100 notches to reject the OH lines in a broad wavelength range (160 nm). The number of cores of the fiber should correspond to the mode number in the multi-mode fibers and should be large enough to be able to capture a sufficient amount of light from the telescope. A phase-mask based technique is used to fabricate the multi-core FBGs.

In this paper we present theoretical and laboratory results on integrated directly-written photonic lanterns with varying taper lengths. These lanterns convert seeing-limited light into multiple diffraction limited signals, in other words, a multimode signal into multiple single-mode signals. We investigated 19-channel structures which were written within a 30-mm-long glass block and designed to operate at 1550 nm. A single structure consisted of a multimode waveguide which transitioned into an array of single-mode waveguides and then back to a multimode waveguide utilizing cosine taper transitions. Based on simulations we found that transition lengths of 6 mm were sufficient to obtain throughput at a level of ~95%. Fabricated devices showed losses (coupling and transition losses) at the level of 30% for injection F/# < 5 and taper lengths < 5 mm. We believe that such devices show great promise for future use in astronomy.

The size of the optical elements (gratings, mirrors, lenses) in traditional astronomical spectrographs scales with telescope diameter (unless the instrument is operating at the diffraction limit). For large telescopes, this leads to spectrographs of enormous size and implied cost. The integrated photonic spectrograph offers the potential to break this scaling law and allow massively multiplexed instruments. One proposed format for such a spectrograph recently demonstrated on-sky employs the arrayed-waveguide grating, which creates dispersion using interference between a series of waveguides with precisely defined length increments. Arrayed-waveguide gratings fabricated via planar techniques are used extensively in the telecommunications industry as optical (de)multiplexers. Current commercial devices are not directly applicable for astronomical use, and several design modifications are thus required. Here we investigate the potential capabilities and limitations of arrayed-waveguide grating technology to provide massively multiplexed spectroscopy for astronomy. In particular, we examine the dependence of the arrayed-waveguide grating design parameters (such as focal length, device order, array spacing, array length increment, refractive index contrast, chip size, number and structure of input modes, and configuration of output imaging or cross-dispersive optics) on the characteristics of the device output (operating wavelength, free spectral range, spectral resolution, multiplexing capacity, and number of required detector pixels).

We present results from a laboratory characterization of integrated photonic arrayed-waveguide grating chips, which are a modified version of commercial arrayed-waveguide grating multiplexors, for the purposes of creating an integrated photonic spectrograph. Using a robust probing setup we measure the peak total efficiency of the chips to be ~75%. We measure the spectral resolution full-width half maximum to be 0.22 ± 0.02 nm, (giving R = λ/δλ = 7000 ± 700 at 1500 nm). For our device we find the free-spectral range is ~60 nm and slightly larger than the full-width half-maximum of the efficiency profile (53 nm). Finally, we briefly discuss the importance of an integrated cross-dispersion component for the new integrated photonic spectrograph prototype.

In this publication we present the results of a detailed study into directly written multimode waveguides for astronomical applications. We show that waveguides up to 100 im across can be inscribed with the cumulative heating form of this technique. The waveguides have 2 concentric guiding regions which are elliptical; a core that has an average ellipticity of 1.1±0.1 and an outer cladding with an ellipticity of 0.15±0.03. It was demonstrated that the ellipticity of the waveguides could be reduced by creating "structured" waveguides which consist of several waveguides stacked together. The 7 mm long waveguides demonstrated insertion losses at 800 nm as low as 39% when light was launched and collected by a standard multimode fibre (50 im core diameter and numerical aperture of 0.12), which is representative of the fibres currently used on astronomical installations. More importantly, we show for the first time that structured waveguides designed to have outer cladding regions which match the dimensions of the core of the launch and collection fibers, have lower insertion losses than structured waveguides designed to have matching core dimensions. It is believed that by moving to longer wavelengths of operation and exploring other structuring and beam shaping techniques it may be possible to reduce the losses even further and make these waveguides of practical use for astronomy.

Interest in pupil-remapping interferometry, in which a single telescope pupil is fragmented and recombined using fiber optic technologies, has been growing among a number of groups. As a logical extrapolation from several highly successful aperture masking programs underway worldwide, pupil remapping offers the advantage of spatial filtering (with single-mode fibers) and in principle can avoid the penalty of low throughput inherent to an aperture mask. However in practice, pupil remapping presents a number of difficult technological challenges including injection into the fibers, pathlength matching of the device, and stability and reproducibility of the results. Here we present new approaches based on recently-available photonic technologies in which coherent three-dimensional waveguide structures can be sculpted into bulk substrate. These advances allow us to miniaturize the photonic processing into a single, robust, thermally stable element; ideal for demanding observatory or spacecraft environments. Ultimately, a wide range of optical functionality could be routinely fabricated into such structures, including beam combiners and dispersive or wavelength selective elements, bringing us closer to the vision of an interferometer on a chip., 9 pages, 6 figures, SPIE 2010

Temporal spectral astronomy or time resolved astronomy is the study of astrophysical phenomena that show spectral variability on very short timescales. These timescales are often too short to be resolved by current astronomical equipment. The lack of detailed observations in this area keeps important theoretical descriptions of astronomical events unclear or incomplete. To resolve this, instruments with very high spectral resolution and fast read-out speeds are needed. Photonic devices such as fibre Bragg gratings (FBGs) offer potential advantages. The use of Bragg gratings in optical fibres allows for very high spectral resolution and the stability and precision needed for the observation of the fast variation of one particular spectral line, with the potential to observe multiple spectral lines at once. Here, we present the concept for a fibre Bragg grating based instrument specifically for temporal spectral astronomy and we discuss the different profiles of FBGs for such applications.

Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large Mode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, we demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs can be used in few mode LMA fibers to suppress reflections into higher order modes.

The ability to tune the Bragg wavelength of a fibre-Bragg grating (FBG) in an all-fibre laser can offer added functionality such as laser wavelength tunability, polarization selectivity,1 and Q-switching.2 Compared to current techniques which rely on mechanically straining the FBG to achieve Bragg-wavelength tunability, an all-optical technique for tuning an FBG offers potentially faster switching speeds and a more robust and simple cavity. All-optical tuning of the Bragg wavelength of an FBG has been demonstrated previously by resonant optical pumping; however this technique has only been applied to passive systems for switching applications. 3 In this work, we have further investigated this optical-tuning process, experimentally identifying three time-scale regimes, and optimised it for application to active systems. Furthermore, we constructed an erbium all-fibre laser cavity consisting of an outputcoupler FBG and an optically-tunable, high-reflector FBG. The cavity pumping and the optical tuning of the FBG were kept independent. By repetitively tuning the high-reflector FBG on- and off-resonance with the output-coupler FBG, we actively Q-switched the erbium fibre laser at repetition rates up to 35 kHz, limited only by our diode driver. We show that grating tuning at >300 kHz is possible with the existing embodiment, and discuss further potential to operate at MHz rates.

A well-known side-effect from fibre Bragg grating UV-fabrication is short wavelength attenuation, where irradiation with laser light, usually in the UV, generates both defect-induced absorption and scattering. These losses are especially problematic for high power optical fibre lasers operating at shorter wavelengths where resonant assisted coupling into the glass matrix through the rare earth ions can take place (e.g. Yb 3+ ). In this, work we present a study of the relative magnitude of short wavelength attenuation in gratings written by the point-by-point method using a Ti-sapphire femtosecond laser operating at 800 nm. Such gratings are very stable and have been used as the feedback elements in fibre lasers with powers exceeding 100 W. We show that the scattering properties responsible for the attenuation are analogous to those associated with type II gratings written with UV lasers.