Fiber-connectorized ultrafast-laser-inscribed K-band integrated optics beam combiner for the CHARA telescope array.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1d B over the 2-2.3 µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Thermal imaging of dust hiding the black hole in NGC 1068.

In the widely accepted 'unified model'1 solution of the classification puzzle of active galactic nuclei, the orientation of a dusty accretion torus around the central black hole dominates their appearance. In 'type-1' systems, the bright nucleus is visible at the centre of a face-on torus. In 'type-2' systems the thick, nearly edge-on torus hides the central engine. Later studies suggested evolutionary effects2 and added dusty clumps and polar winds3 but left the basic picture intact. However, recent high-resolution images4 of the archetypal type-2 galaxy NGC 10685,6, suggested a more radical revision. The images displayed a ring-like emission feature that was proposed to be hot dust surrounding the black hole at the radius where the radiation from the central engine evaporates the dust. That ring is too thin and too far tilted from edge-on to hide the central engine, and ad hoc foreground extinction is needed to explain the type-2 classification. These images quickly generated reinterpretations of the dichotomy between types 1 and 27,8. Here we present new multi-band mid-infrared images of NGC 1068 that detail the dust temperature distribution and reaffirm the original model. Combined with radio data (J.F.G. and C.M.V.I., manuscript in preparation), our maps locate the central engine that is below the previously reported ring and obscured by a thick, nearly edge-on disk, as predicted by the unified model. We also identify emission from polar flows and absorbing dust that is mineralogically distinct from that towards the Milky Way centre.

Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability, and diversity.

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind's most fundamental questions: "How unique is our Earth?"

First stellar photons for an integrated optics discrete beam combiner at the William Herschel Telescope.

We present the first on-sky results of a four-telescope integrated optics discrete beam combiner (DBC) tested at the 4.2 m William Herschel Telescope. The device consists of a four-input pupil remapper followed by a DBC and a 23-output reformatter. The whole device was written monolithically in a single alumino-borosilicate substrate using ultrafast laser inscription. The device was operated at astronomical H-band (1.6 µm), and a deformable mirror along with a microlens array was used to inject stellar photons into the device. We report the measured visibility amplitudes and closure phases obtained on Vega and Altair that are retrieved using the calibrated transfer matrix of the device. While the coherence function can be reconstructed, the on-sky results show significant dispersion from the expected values. Based on the analysis of comparable simulations, we find that such dispersion is largely caused by the limited signal-to-noise ratio of our observations. This constitutes a first step toward an improved validation of the DBC as a possible beam combination scheme for long-baseline interferometry.

Astrophotonics: introduction to the feature issue.

Astrophotonics is an emerging field that focuses on the development of photonic components for astronomical instrumentation. With ongoing advancements, astrophotonic solutions are already becoming an integral part of existing instruments. A recent example is the €60M ESO GRAVITY instrument at the Very Large Telescope Interferometer, Chile, that makes heavy use of photonic components. We envisage far-reaching applications in future astronomical instruments, especially those intended for the new generation of extremely large telescopes and in space. With continued improvements in extreme adaptive optics, the case becomes increasingly compelling. The joint issue of JOSA B and Applied Optics features more than 20 state-of-the-art papers in diverse areas of astrophotonics. This introduction provides a summary of the papers that cover several important topics, such as photonic lanterns, beam combiners and interferometry, spectrographs, OH suppression, and coronagraphy.

Chromatic response of a four-telescope integrated-optics discrete beam combiner at the astronomical L band.

We show the results of simulation and experimental study of a 4-telescope zig-zag discrete beam combiner (DBC) for long-baseline stellar interferometry working at the astronomical L band (3 - 4 µm) under the influence of a narrow bandwidth light source. Following Saviauk et al. (2013), we used a quasi-monochromatic visibility-to-pixel matrix (V2PM) for retrieving the complex coherence functions from simulated and experimentally measured power at the output of the device. Simulation and coefficient of determination (R2) measurements show that we are able to retrieve the visibility amplitudes with >95 % accuracy of our chromatic model source up to a bandwidth of 100 nm centred at 3.5 µm. We characterized a DBC manufactured by 3D ultra-fast laser inscription (ULI) written on gallium lanthanum sulphate (GLS). Experimental results showed retrieval of visibility amplitude with an accuracy of 80-90 % at 69 nm bandwidth, validating our simulation. The standard deviation of experimental phase residuals are between 0.1-0.4 rad, which shows that the retrieval procedure is sufficient to get good quality images, where phase perturbations of less than 1 rad are expected under good seeing conditions for astronomical applications.

Towards 3D-photonic, multi-telescope beam combiners for mid-infrared astrointerferometry.

In the past two decades high precision optical astronomical interferometry has benefited from the use of photonic technologies. Today, near-infrared interferometric instruments deliver high-resolution, hyperspectral images of astronomical objects and combine up to 4 independent telescopes at a time thanks to integrated optics (IO). Following the success of IO interferometry, several initiatives aim at developing components which could combine simultaneously more telescopes and extend their operation beyond the near-infrared bands. Here we report on the development of multi-telescope IO beam combiners for mid-infrared interferometry exploiting the three-dimensional (3D) structuring capabilities of ultrafast laser inscription. We characterise the capability of a 2-telescope and a 4-telescope beam combiner to retrieve the visibility amplitude and phase of monochromatic light fields at a wavelength of 3.39 µm. The combiner prototypes exploit different 3D architectures and are written with a femtosecond laser on substrates of Gallium Lanthanum Sulfide. Supporting numerical simulations of the performance of the beam combiners show that there is still room for improvement and indicate a roadmap for the development of future prototypes.

Ultrafast laser inscription in ZBLAN integrated optics chips for mid-IR beam combination in astronomical interferometry.

Astronomical interferometry is a unique technique that allows observation with angular resolutions on the milliarcsec scale by combining the light of several apertures hundreds of meters apart. The PIONIER and GRAVITY instruments at the Very Large Telescope Interferometer have demonstrated that silica-based integrated optics (IO) provide a small-scale and highly stable solution for the interferometric beam combination process. Yet, important science cases such as exoplanet hunting or the spectroscopic characterization of exoplanetary atmospheres are favorable for observation in the mid-IR, namely the atmospheric windows L and L' band (3-4 µm), a wavelength range that is not covered by conventional silica-based IO. Here, we propose laser-inscribed IO 2×2 couplers in ZBLAN and experimentally assess the critical properties of the component for broadband mid-IR interferometry. We measure the splitting ratio over the 2.5 to 5.0 µm range and find excellent broadband contrast over the L (3.1-3.6 µm) and L' (3.6 - 4.0 µm) bands. Furthermore, we quantify the dispersion properties of the coupler and find a phase variation as low as 0.02 rad across the L and L' band, respectively. By optimizing the NA of our injection beam, we measured a very high total throughput of 58% over the L band including Fresnel reflection and coupling losses. We also compare our findings to recent advances in mid-IR IO in GLS and discuss its advantages and disadvantages for the implementation in future mid-IR interferometers.

Modeling the response of mesospheric sodium to pulsed-laser excitation.

A simulation modeling excitation of the sodium D2 line by nanosecond time scale pulsed lasers is described. By numerically integrating transition rates in the sodium hyperfine structure, the return flux per sodium atom is predicted as a function of laser power. The simulation should be useful for studies of mesospheric sodium and adaptive optics. Applications include the estimation of sodium column density from lidar return flux, and of laser guide star brightness for different pulsed laser formats. The simulation assumes that the pulse repetition frequency is sufficiently low (smaller than a few kilohertz) that atomic collisions restore local thermodynamic equilibrium between pulses. It is also assumed that the pulse length is short compared to the Larmor precession time scale. The numerical results are well-approximated by a simple analytic model for a three-level atom. The number of emitted photons is found to be primarily dependent on the product of the length of the laser pulse and the energy density.

Interferometric nulling of four channels with integrated optics.

Nulling interferometry has been identified as a competitive technique for the detection of extrasolar planets. In its basic form, the technique consists of combining out-of-phase a single pair of telescopes to effectively null the light of a bright star and reveal the dim glow of the companion. However, in order to mitigate the effect of the stellar leaks through the interferometer, a broad angular central null is required. The hierarchical combination of several pairs of telescopes can accomplish this task. We have manufactured and tested with monochromatic light an integrated optics component, which combines a linear array of four telescopes in the nulling mode envisaged by Angel and Woolf [Astroph. J.475, 373-379 (1997).10.1086/apj.1997.475.issue-1ASJOAB0004-637X]. By simulating in the laboratory the motion of a star in the sky, we could measure the expected angular transmission of the four-telescope nuller. Moreover, the tests have demonstrated a broad nulling scaling as the fourth power of the baseline delay.

Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry.

We report the ultrafast laser fabrication and mid-IR characterization (3.39 µm) of four-port evanescent field directional couplers. The couplers were fabricated in a commercial gallium lanthanum sulfide glass substrate using sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed using optimal fabrication parameters were found to exhibit propagation losses of ∼0.8 dB·cm(-1). A series of couplers were inscribed with different interaction lengths, and we demonstrate power-splitting ratios of between 8% and 99% for mid-IR light with a wavelength of 3.39 µm. These results clearly demonstrate that ultrafast laser inscription can be used to fabricate high-quality evanescent field couplers for future applications in astronomical interferometry.

Three-dimensional mid-infrared photonic circuits in chalcogenide glass.

We report the fabrication of single-mode buried channel waveguides for the whole mid-IR transparency range of chalcogenide sulphide glasses (λ ≤ 11 µm), by means of direct laser writing. We have explored the potential of this technology by fabricating a prototype three-dimensional three-beam combiner for future application in stellar interferometry that delivers a monochromatic interference visibility of 99.89% at 10.6 µm and an ultrahigh bandwidth (3-11 µm) interference visibility of 21.3%. These results demonstrate that it is possible to harness the whole transparency range offered by chalcogenide glasses on a single on-chip instrument by means of direct laser writing, a finding that may be of key significance in future technologies such as astrophotonics and biochemical sensing.

Single mode mid-infrared silver halide asymmetric flat waveguide obtained from crystal extrusion.

A flat waveguide for the middle infrared was made by co-extrusion of two silver halide crystals of different chemical compositions. The transmission of the waveguide and its modal behavior was studied using a Fourier Transform Spectrometer and a dedicated optical bench. Analyzing this spectrum, we were able to obtain the cut-off wavelength of the waveguide. We observed a single mode behavior for wavelengths longer than 8.83mum, in good agreement with the theoretically expected values. This novel procedure is ideal for tailoring the properties of the waveguide for specific applications, in particular the spectral range where it exhibits a single-mode behavior. It can thus be applied to achieve modal filtering for mid-IR astronomical interferometers (e.g. beam combiners, nullers, etc.).

Mid-infrared guided optics: a perspective for astronomical instruments.

Research activities during the last decade have shown the strong potential of photonic devices to greatly simplify ground based and space borne astronomical instruments and to improve their performance. We focus specifically on the mid-infrared wavelength regime (about 5-20 microm), a spectral range offering access to warm objects (about 300 K) and to spectral features that can be interpreted as signatures for biological activity (e.g. water, ozone, carbon dioxide). We review the relevant research activities aiming at the development of single-mode guided optics and the corresponding manufacturing technologies. We evaluate the experimentally achieved performance and compare it with the performance requirements for applications in various fields of astronomy. Our goal is to show a perspective for future astronomical instruments based on mid-infrared photonic devices.

Transmission behaviors of single mode hollow metallic waveguides dedicated to mid-infrared nulling interferometry.

This paper reports the characterization of hollow metallic waveguides (HMW) to be used as single-mode wavefront filters for nulling interferometry in the 6-20microm range. The measurements presented here were performed using both single-mode and multimode conductive waveguides at 10.6microm. We found propagation losses of about 16dB/mm, which are mainly due to the theoretical skin effect absorption in addition to the roughness of the waveguide's metallic walls. The input and output coupling efficiency of our samples has been improved by adding tapers to minimize the impedance mismatch. A proper distinction between propagation losses and coupling losses is presented. Despite their elevate propagation losses, HMW show excellent spatial filtering capabilities in a spectral range where photonics technologies are only emerging.

M-lines characterization of selenide and telluride thick films for mid-infrared interferometry.

Nulling interferometry is an astronomical technique that requires to combine extremely flat wavefronts to achieve a deep rejection ratio in order to detect Earth-like planets in the mid-infrared band [5 - 20 microm]. Similarly to what is done in the near-infrared, high spatial filtering of the incoming beams can be achieved using single-mode waveguides operating in the mid-infrared. An appreciable reduction of the instrumental complexity is also possible using integrated optics (IO) devices in this spectral range. The lack of single-mode guided optics in the mid-infrared has motivated the present technological study to demonstrate the feasibility of dielectric waveguides functioning at longer wavelengths. We propose to use selenide and telluride components to pursue the development of more complex IO functions.