Universität zu Köln

Ahmed, Sanny ORCID: 0000-0002-4504-6382 (2024). Towards the Spectroscopy of Giant Exoplanets with Nulling Interferometry: Fabrication & Characterisation of the Mid-Infrared 4-Telescope Photonic Beam Combiner of NOTT/VLTI. PhD thesis, Universität zu Köln.

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Abstract

Nulling Observations of exoplaneTs and dusT, or in short ‘NOTT’, is the first long-baseline self-calibrated nulling interferometer based on a photonic beam combiner as part of the upcoming visitor instrument ASGARD at the VLTI (Paranal, Chile). The NOTT instrument aims to detect and characterise young exoplanets in the mid-infrared astronomical L’ band using nulling interferometry. The photonic or integrated optic beam combiner for NOTT is fabricated using Ultrafast Laser Inscription (ULI) in chalcogenide glass. ULI uses an ultrashort pulsed laser to form single-mode waveguides for the mid-infrared region within the bulk of the substrate. The integrated optics beam combiner follows the “Double-Bracewell” architecture and consists of three cascaded directional couplers with 50/50 achromatic splitting. The first two directional couplers produce a null signal each, which are then coherently combined by the central directional coupler. This central directional coupler creates two nulled signals with mirrored transmission maps, where their difference produces a self-calibrated astrophysical null. The beam combiner’s unique 3D architecture allows each individual telescope beam to first pass through a side-step to suppress uncoupled stray light. Asymmetric Y-junctions then sample the corresponding flux to provide simultaneous photometric information for each input enabling the most accurate estimation of the null depth. This thesis presents the step-by-step development and characterisation of directional couplers, Y-junctions, a 2-telescope beam combiner and up to NOTT’s 4-telescope beam combiner, all fabricated in Gallium-Lanthanum-Sulphide (GLS) glass using ULI It includes the first detailed mid-infrared laboratory characterisation of such a photonic device using 4 beams across a wavelength band between 3.65 and 3.85 µm. Detailed characterisation and optimisation of the directional couplers has resulted in achromatic splitting ratio close to 50/50 across the L’ band with good reproducibility. These directional couplers feature null depths in the range of 10´2 using polychromatic light from a supercontinuum source without any polarisation control. Y-junctions have been engineered for achromatic 50/50 splitting for the 2-telescope beam combiner and achromatic 20/80 splitting to meet the requirements for NOTT’s 4-telescope beam combiner. For the 4-telescope beam combiner, a self-calibrated null of 10^-4 is demonstrated within the limitation of the interferometric test bench at room temperature. The beam combiner will be installed in the NOTT instrument during the first deployment phase. Although the current 4T-Nuller falls slightly short of the ideal specifications, a second-generation beam combiner will eventually have to be fabricated to achieve all the instrument’s science goals. This beam combiner will be building on the groundwork established by this thesis.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Ahmed, Sanny bigganobish@gmail.com orcid.org/0000-0002-4504-6382 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-743325
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Ehemalige Fakultäten, Institute, Seminare > Faculty of Mathematics and Natural Sciences
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language interferometry English nulling interferometry English NOTT-ASGARD-VLTI English ultrafast laser inscription English integrated optics English nuller English astrophotonics English
Date of oral exam:	31 May 2024
Referee:	Name Academic Title Eckart, Andreas Prof. Dr. Labadie, Lucas Prof. Dr. Braden, Markus Prof. Dr. Harris, Robert Ast. Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/74332