Towards high-resolution and high-contrast imaging in mid-infrared astronomy : Integrated optics beam combiners for astrointerferometry

Research in astronomical instrumentation is driven by open questions about the structure of our universe and its constituents, such as black holes, the interstellar medium, stars and planets. This work is focused on a particular observation technique called astronomical interferometry for the infrared. In contrast to conventional single telescopes, an astronomical interferometer consists of multiple individual telescopes whose light is combined. From the interferometric signals between pairs of telescopes, information about the spatial structure of the observed source can be extracted. The advantage of this technique is its superior angular resolution that is given by λ/(2B), with λ being the observed wavelength and B the separation of the telescopes, compared to λ/D for a single telescope with D being the diameter of the telescope. Therefore, current interferometers with baselines of up to a few hundred meters surpass the resolution capabilities of the largest optical single telescopes (D~10m) by more than an order of magnitude. Until now, astronomical interferometry has produced results with unprecedented resolution measuring the photospheres of stars, separations of binaries, the close environment of the black hole at the center of our galaxy and the birth regions of exoplanets. On the other hand, the astronomical interferometer is a highly complex apparatus that requires dedicated instrumentation efforts in order to combine and measure stabilized and finely calibrated interferometric signals arising from distant astronomical targets between telescopes more than a hundred meters apart. The precise measurement of the interferometric observables, visibility and phase, are the prerequisite in order to unambiguously reconstruct the morphology of the observed object.
	
	This work is concerned with instrumentation for mid-infrared (mid-IR) astrointerferometry, in the following specified as the 3-5µm wavelength range. The mid-IR is a region of high scientific interest as it allows to probe cooler regions than stars such as planet forming regions. In fact, this region is considered to be the sweet spot for exoplanet detection due to the planets' stronger emission at these wavelengths and the decreased stellar flux leading to a favorable contrast. For this wavelength region, we aim to develop integrated optics chips to combine the light from the individual telescopes to read out their interferometric signals. Integrated optics chips, similarly to electronic integrated circuits for electrons, can route, split and combine photons in a palm-size device providing a compact and stable instrumental transfer function. Compared to conventional bulk optics beam combination designs, integrated optics deliver much more accurately calibrated interferometric observables. Such photonic devices have not been available in the mid-IR, which is why current facilities until now had to rely on classical bulk optics beam combination schemes, degrading the potential scientific return of the interferometer. The goal of this thesis is the characterization of several integrated optics chips for the mid-IR range using different materials and beam combination designs, and testing their critical properties for astronomical applications.
	
	To this end, I set up an optical testbench in Cologne allowing the interferometric testing of integrated optics beam combiners in the mid-IR. In the first two publications, I characterize two-telescope integrated optics combiners and assess their relevant properties to astronomy such as transmission, modal behavior, splitting ratio as well as dispersion and polarization properties. Most importantly, I experimentally demonstrate for the first time that high interferometric contrasts (>93\%) in integrated optics can indeed be achieved in the mid-IR over broad wavelength ranges. This characterization was carried out for two different integrated optics platforms, a chalcogenide glass (GLS) in the first paper and a fluoride glass (ZBLAN) in the second paper, with the second paper putting an extra focus on the comparison between the two platforms. The ultimate goal is the on-chip combination of four or more telescopes. The third paper goes beyond classical two-telescope beam combiners and more advanced architectures such as so-called ABCD combiners and four-telescope discrete beam combiners are investigated. It is experimentally demonstrated that those couplers are suited for retrieving the visibilities between monochromatic input light fields. This proof-of-concept study paves the way towards a four-telescope combiner. Finally, on the basis of the experimental results, the feasibility and performance of a four-telescope integrated optics based beam combiner instrument is discussed.