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In long-baseline interferometry, over the last few decades integrated optics beam combiners have become at- tractive technological solutions for new-generation instruments operating at infrared wavelengths. We have investigated different architectures of discrete beam combiners (DBC), which are 2D lattice arrangement of channel waveguides that can be fabricated by exploiting the 3D capability of the ultrafast laser inscription (ULI) fabrication techniques. Here, we present the first interferometric on-sky results of an integrated optics beam combiner based on a coherent pupil remapper and 4 input/23 output zig-zag based DBC, both written monolith- ically in a single borosilicate glass. We show the preliminary results of visibility amplitudes and closure phases obtained from the Vega star by using the previously calibrated transfer matrix of the device.

The project NAIR "Novel Astronomical Instrumentation based on photonic light Reformating" is a DFG-funded collaboration to exploit the recognized potential of photonics solutions for a radically new approach to astronomical instrumentation for optical/infrared high precision spectroscopy and high angular resolution imaging. We present a project update, with the developments in our ULI waveguides and 3D printed structures for astronomical instrumentation and on sky testing results obtained at the WHT, Subaru (SCExAO) and LBT. This shows the NAIR project is helping to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation.

We report the ultrafast laser inscription (ULI) and characterization of 3 dB directional achromatic couplers for K-band between 2 and 2.4 μm. The couplers were fabricated in commercial Infrasil glass using 1030 nm femtosecond laser pulses. Straight waveguides inscribed using optimal fabrication parameters exhibit an average propagation loss of ∼1.21 dB over full range of K-band with a single-mode behavior for a length of 17 mm. Directional couplers with different interaction lengths and waveguide widths were fabricated and characterized. We demonstrate that 3 dB achromatic directional couplers for K-band can be fabricated using ULI. These results show that ULI can fabricate highquality couplers for future applications in astronomical interferometry. Our eventual aim is to develop a two-telescope K-band integrated optical beam combiner to replace JouFLU at CHARA.

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.

Stellar interferometry performed in integrated photonic devices allows to increase the angular resolution of a ground-based telescope. Here we present the fabrication and characterization of a low-loss polarization insensitive photonic circuit for astrophotonics, whose geometry was engineered to combine interferometrically up to eight input beams. The employed fabrication technique consisted in the femtosecond laser micromachining followed by a thermal annealing to reduce the birefringence of the waveguides. The fabricated device was characterized to validate its functioning in terms of polarization insensitivity, good transmission and proper beam combination, thus benchmarking its suitability with real on-sky observations.

Astrophotonics is an emerging tool for increasing the angular resolution in ground-based sky observations. Due to the unpolarized nature of celestial light, it is necessary to operate with fully polarization insensitive integrated devices. In this respect, here we show that a thermal annealing after the femtosecond laser writing of waveguides reduces their birefringence of more than order of magnitude, providing integrated circuits whose behaviour is insensitive to the polarization of the input light. As a validation of this technique, we present the successful fabrication of a low-loss integrated device for performing stellar interferometry of up to 8 input beams.

We will show the first results for a pupil remapping device with an integrated optics discrete beam combiner. Our expected monochromatic visibility functions are in good agreement with simulation and experiment. The device will be used for our upcoming on-sky tests at 4-m Willian-Herschel Telescope (WHT) in canary islands.

We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be ∼ 1.2 ± 0.5 d B across the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of > 90 % . This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.

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.

We will review the development in the last decade of discrete beam combiners (DBC), phase sensors based on the propagation of light in photonic lattices. The latest results on the development of DBC for astronomical applications will be presented, along with a new application for the complete tomography of modes at the tip of a multi-mode fiber. The possible use of the DBC in monitoring and controlling modal instabilities in high power lasers will be discussed.

The project "Novel Astronomical Instrumentation through photonic Reformatting" is a DFG-funded collaboration to exploit the recognized potential of photonics solutions for a radically new approach to astronomical instrumentation for optical/infrared high precision spectroscopy and high angular resolution imaging. We present a project overview and initial development results from our Adaptive Optics-photonic test bed, Ultrafast Laser Inscribed waveguides for interferometric beam combination and 3D printing structures for astronomical instrumentation. The project is expected to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation., 15 pages, 11 figures, SPIE Astronomical Telescopes + Instrumentation

By exploiting the interference of light coming from different telescopes it is possible to achieve high-resolution images of astronomical objects [1]. Astrophotonics, namely the realization of stellar interferometry in integrated photonic circuits, represents a promising platform in this field, due to the intrinsic interferometric stability of the integrated devices with respect to bulk optics [2]. However, since the celestial light is unpolarized, a fully polarization insensitive behaviour is required, and this can be very challenging to obtain in an integrated device, due to the intrinsic birefringence of waveguides. To obtain low-birefringence waveguides, it is possible to exploit femtosecond laser micromachining [3]. With this technology, different layouts were proposed to achieve this result [4]. Nevertheless, they place strict requirements on the geometry of the device, for example on the distance between the waveguides of a directional coupler. Moreover, these layouts may induce some polarization rotations.