Integrated photonics for mid-infrared stellar interferometry
thesisposted on 28.03.2022, 14:44 by Thomas Gretzinger
Nulling-interferometry is a powerful tool to advance beyond the resolving power of ground based observatories with the prospect of directly detecting exoplanets. By suppressing the radiation of the host star through destructive interference, the emission from a young planet can be observed. A favourable contrast between a planet and a star as well as reduced atmospheric disturbances are found centered around 4 µm wavelength. For high stability and robustness, it is preferable to deploy integrated optics devices based on waveguide technology. Their advancement however is hindered in the mid-infrared wavelength range due to the lack of suitable host materials as well as compatible manufacturing techniques to fabricate low loss photonic devices. This thesis details the development of mid-infrared optical waveguides and the key components for an integrated nulling interferometer chip in gallium-lanthanum-sulphur glass, utilising femtosecond laser direct-writing. By combining the multiscan technique with the cumulative heating fabrication, single-mode waveguides with a propagation loss of 0.22±0.02 dB/cm at 4 µm were realised. Evidence of structural changes and ion migration in these positive refractive index waveguides are presented using Raman spectroscopy and electron probe micro-analysis (EPMA), respectively. 2-D Raman maps revealed full-width at half maximum variations and a peak shift in the symmetric vibrations of the GaS4 main Raman band. The 2-D spectral map of the Boson peak band was used to understand and identify the material densification profile in a high refractive index glass waveguide. EPMA provided evidence of sulphur ion migration and the observation of an anion (S2) migration causing material modification. These low-loss waveguides were the foundation of S-bends with a negligible bending loss and Y-splitters with a 50/50 power division across a 600 nm wavelength window. Directional couplers were developed from symmetric into asymmetric directional couplers with a 50/50power splitting ratio over a broad wavelength range (3.8 - 4.05 µm). Furthermore, multimode interference couplers are presented with an even splitting capability between 3.75 - 4.25 µm with no polarisation dependency. Both types of couplers feature a high broadband extinction ratio. These main building blocks are developed to create a future compact nulling interferometer with a total projected intrinsic loss of < 1 dB, a value that is sufficient to perform future on-sky experiments in relatively short observation runs on ground based telescopes.