Brahim Arezki

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.

First fringes with an integrated-optics beam combiner at 10 μm - A new step towards instrument miniaturization for mid-infrared interferometry

Context. Observations of milliarcsecond-resolution scales and high dynamic range hold a central place in the exploration of distant planetary systems in order to achieve, for instance, the spectroscopic characterization of exo-Earths or the detailed mapping of their protoplanetary disc birthplace. Multi-aperture infrared interferometry, either from the ground or from space, is a very powerful technique to tackle these goals. However, significant technical efforts still need to be undertaken to achieve a simplification of these instruments if we wish to recombine the light from a large number of telescopes. Integrated-optics concepts appear to be a suitable alternative to the current conventional designs, especially if their use can be extended to a higher number of astronomical bands. Aims. This article reports, for the first time to our knowledge, the experimental demonstration of the feasibility of an integrated-optics approach to mid-infrared beam combination for single-mode stellar interferometry. Methods. We fabricated a two-telescope beam combiner prototype integrated on a substrate of chalcogenide glass, a material transparent from ∼1 μ mt o∼14 μm. We developed laboratory tools to characterize in the mid-infrared the modal properties and the interferometric capabilities of our device. Results. We obtain interferometric fringes at 10 μm and measure a mean contrast V = 0.981 ± 0.001 with high repeatability over one week and high stability over a time-period of ∼5 h. We show experimentally – as well as on the basis of modeling considerations – that the component has a single-mode behavior at this wavelength, which is essential to achieve high-accuracy interferometry. From previous studies, the propagation losses are estimated to be 0.5 dB/cm for this type of component. We also discuss possible issues that may impact the interferometric contrast. Conclusions. The IO beam combiner performs well at the tested wavelength. We also anticipate the requirement of a closer matching between the numerical apertures of the component and the (de)coupling optics to optimize the total throughput. The next step foreseen is the achievement of wide-band interferograms.

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 laser light nulling experiment using single-mode conductive waveguides

Aims: In the context of space interferometry missions devoted to the search of exo-Earths, this paper investigates the capab ilities of new single mode conductive waveguides at providing modal fil tering in an infrared and monochromatic nulling experiment; Methods: A Michelson laser interferometer with a co-axial beam combination scheme at 10.6 µm is used. After introducing aphase shift using a translating mirror, dynamic and static measurements of the nulling ratio are performed in the two cases where modal filtering is implemented and suppressed. No additional active control of the wavefront errors is involved. Results: We achieve on average a statistical nulling ratio of 2.5×10 −4 with a 1-� upper limit of 6×10 −4 , while a best null of 5.6×10 −5 is obtained in static mode. At the moment, the impact of external vibrations limits our abilit y to maintain the null to 10 to 20 seconds.; Conclusions: A positive effect of SM conductive waveguide on modal filtering has been observ ed in this study. Further improvement of the null should be possible with proper mechanical isolation of the setup.

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.

Modal filtering for nulling interferometry First single-mode conductive waveguides in the mid-infrared

We have investigated the manufacturing and characterization of first single-mode conductive waveguides to be used as modal filters for nulling interferometry in the mid-infrared range [4−20 µm]. As a very high dynamic range is mandatory for the detection of Earth-like planets, modal filtering is a crucial instrumental aspect. The hollow metallic waveguides (HMW) presented here are manufactured using micro-machining techniques. Single-mode behavior has been investigated in the laboratory through polarization analysis and transmission features have been measured using relative flux comparison. Single-mode behavior was assessed at λ = 10.6 µm for rectangular waveguides with dimensions a = 10 µ ma ndb ≤ 5.3 µm with an accuracy of ∼2.5%. The tests showed that a single-polarization state can be maintained in the waveguide. A comparison with results on multi-mode HMW is given. Excess losses of 2.4 dB (∼58% transmission) have been measured for a singlemode waveguide. In particular, the importance of coupling conditions in the waveguide is emphasized here. The goal of manufacturing and characterizing the first single-mode HMW for the mid-infrared has been achieved. This opens the road to the use of integrated optics for interferometry in this spectral range.

Increasing the imaging capabilities of the VLTI using integrated optics

Several scientific topics linked to the observation of extended structures around astrophysical sources (dust torus around AGN, disks around young stars, envelopes around AGBs) require imaging capability with milli-arcsecond spatial resolution. The current VLTI instruments, AMBER and MIDI, will provide in the coming months the required high angular resolution, yet without actual imaging. As a rule of thumb, the image quality accessible with an optical interferometer is directly related to the number of telescopes used simultaneously: the more the apertures, the better and the faster the reconstruction of the image. We propose an instrument concept to achieve interferometric combination of N telescopes (4 ≤ N ≤ 8) thanks to planar optics technology: 4 x 8-m telescopes in the short term and/or 8 x 1.8-m telescopes in the long term. The foreseen image reconstruction quality in the visible and/or in the near infrared will be equivalent to the one achieved with millimeter radio interferometers. Achievable spatial resolution will be better than the one foreseen with ALMA. This instrument would be able to acquire routinely 1 mas resolution images. A 13 to 20 magnitude sensitivity in spectral ranges from 0.6 to 2.5 μm is expected depending on the choice of the phase referencing guide source. High dynamic range, even on faint objects, is achievable thanks to the high accuracy provided by integrated optics for visibility amplitude and phase measurements. Based on recent validations of integrated optics presented here an imaging instrument concept can be proposed. The results obtained using the VLTI facilities give a demonstration of the potential of the proposed technique.

Advances in the Development of Mid-Infrared Integrated Devices for Interferometric Arrays

This article reports the advances on the development of mid-infrared integrated optics for stellar interferometry. The devices are fabricated by laser writing techniques on chalcogenide glasses. Laboratory characterizaton is reported and analyzed.

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 waveguides 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.

Ultrafast laser inscription of mid-IR integrated optics for astronomy

The development of integrated optical (IO) circuits for mid-IR radiation (wavelengths from ∼3 to ∼30 µm) is of significant interest for applications in bio-sensing and astronomy [1]. With specific reference to astronomy, the mid-IR spectrum is of particular interest since this is the region where objects such as planets, at earth-like temperatures, can be probed. It is also the region where biomarkers such as H<inf>2</inf>O, CO<inf>2</inf> and O<inf>3</inf> can be identified.