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 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 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.
In this contribution, we review the results of the ALSI project (Advanced Laser-writing for Stellar Interferometry), aimed at assessing the potential of ultrafast laser writing to fabricate mid-infared integrated optics (IO) devices with performance compatible with an implementation in real interferometric instruments like Hi5 or PFI. Waveguides for the L, L' and M bands with moderate propagation losses were manufactured in Gallium Lanthanum Sulfide and ZBLAN glasses and used to develop photonic building blocks as well as a full mid-IR 4-telescope beam combiner. We discuss the advantages and disadvantages of the tested combiners and discuss a possible roadmap for the continuation of this work.
We present the results of the technology developments which lead to the manufacturing of the first integrated optics 2-telescope ABCD unit and 4-telescope DBC component for L-band, mid infrared stellar interferometry. All the samples were manufactured with ultrafast laser inscription in Gallium Lanthanum Sulfide and were characterized in near field with monochromatic light at the wavelength of 3.39 μm. The choice of the laser writing parameters and the control of long range stresses arising from the manufacturing process are discussed taking in consideration the measured interferometric calibration data.
This paper is one of a three-part series of papers on photonics-based mid-IR interferometry. Here, we put the emphasis on the challenges of operating integrated optics over a broad wavelength range, a natural condition in the field of Astrophysics. We report on the recent advancements made in obtaining high interferometric contrast (> 90%) through 2-telescope combiners in the mid-IR and give an outlook on more advanced functions and 4-telescope combiners.
In the era of large telescopes and RV/Transit planetary missions, nulling interferometry remains a competitive technique for the characterization of Earths and Super-earths around Sun analogs in the mid-IR (Léger 2015, ApJ 808, 194). This is a spectral range where a number of bio-signatures can be accessed from space. One challenge of nulling is to benefit from well-established and qualified infrared fibers and integrated optics capable of mitigating the instrumental constraints on the beam combination and wavefront filtering to reach high extinction ratios. Such photonics devices have reached high maturity in the near-IR range as in the case of the integrated optics (IO) beam combiner of GRAVITY at the VLTI, leading to unprecedented interferometric accuracy.
Driven by the need of next-generation interferometers, we expand the photonic approach towards longer wavelengths and develop IO combiners based on the ultrafast laser writing technique. We developed single-mode, low-loss evanescent couplers in gallium lanthanum sulfide with a 50/50 splitting behavior around 3.4 µm and characterized the intrinsic chromaticity by FTS. High monochromatic and broadband contrasts are measured with unpolarized light at 3.39µm (>98%), over the L band (>95%), and over the M Band (4.5-4.8µm) (>95%). Our analysis of the interferometric visibilities and phase shows a small differential birefringence in the component and negligible differential dispersion. This results points out the promising properties of mid-infrared laser writing integrated optics devices to serve as high quality beam combiners. The extension to a four-aperture architecture appears plausible, with care to be taken about the impact of the design on the total throughput.
Photonic technology has pushed the limits of astronomy ever more in recent years. Especially, integrated optics (IO) has led to new standards in accuracy and stability in the field of astronomical interferometry where several beams need to be coherently and simultaneously combined. We follow and extend the IO concept by writing mid-IR waveguides in gallium lanthanum sulfide (GLS) using Ultrafast Laser Writing (ULI). Here, we report on the monochromatic and broadband interferometric capabilities in the mid-IR of such combiners. Finally, we outline the way to a fiber-fed IO 4-telescope instrument for next-generation astronomical interferometers.
KEYWORDS: Waveguides, Information operations, Interferometry, Mid-IR, Integrated optics, Near field optics, Manufacturing, Wave propagation, Interferometers, Biosensors, Visibility, Signal to noise ratio, Quantum optics, Ultrafast lasers
We present the laboratory characterisation of the first working, mid-infrared, integrated optics, 4-channel interferometric beam combiner based on the properties of two-dimensional arrays of evanescently coupled waveguides. Potential applications of the component to astronomy, biology and quantum optics are proposed and discussed.
In this work we propose a new geometry of discrete beam combiners (DBC) for spectrally-resolved stellar interferometry which overcomes limitations of previous designs. The new beam combiner is based on an array of coupled waveguides arranged in zig-zag pattern. It has been numerically optimized for the combination of 4 telescopes and engineered to operate in the L-band. We manufactured a first sample by direct laser writing in Gallium Lanthanum Sulfide glass, a highly transmissive material in the mid-infrared (550 nm to 10 μm). Initial near-field characterization of the fabricated sample at a wavelength of 3.4 μm are encouraging, but highlighted the necessity of a better control of the polarization dispersion of individual waveguides, as well as induced stresses from manufacturing process.
Fringe tracking at longer wavelengths is advantageous for its larger Fried parameter (R0) and longer coherence time (τ0). The fringe trackers which are currently available at the VLTi (Finito, FSU, Gravity, etc.) tracks fringes at the near infrared wavelengths (H and K bands). In our work we try to explore the possibilities to track near and mid- infrared fringes using GLS based laser written integrated optics beam combiners. We simulate the atmospheric optical path difference (OPD) using Kolmogorov/Von-Karman atmospheric turbulence statistics. We also include the measured the piston noise generated due to the instrumental vibrations. Using the resulting OPD time series we can estimate the sensitivity of the fringe tracker at the L band.
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.
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