Nulling interferometry for exoplanet detection: using a multimode interference coupler in a two dimensional chalcogenide glass platform
Creators: name: Kenchington Goldsmith, Harry-Dean
Publisher: name: The Australian National University
Year: 2021

description: Astronomy has begun an uptake in photonic technologies to replace cumbersome, heavy optics, to achieve lighter systems that are robust to environmental and instrument noise. An application of one such change, from optics to photonics, is for beam combination in multi-telescope systems, and specifically; direct imaging of exoplanets using interferometry. Nulling interferometry achieves both a high-resolution image and a deep null depth (by using destructive interference over the star). Its resolution is limited by the distance between the farthest telescopes (the longest baseline). So that the contrast between the exoplanet and star is minimised this work is constrained to the astronomical L'-band where young, and thus hot, exoplanets have their blackbody spectrum peak. The focus of this thesis was to design and construct a beam combiner that segregates the star and exoplanet light - the nulling interferometer. Using established ultraviolet lithography methods and plasma etching, a two-beam combiner on a photonic chip was constructed. These chips were fabricated using a combination of two chalcogenide glasses, made from networks of Ge, As, and Se for the core and Ge, As, and S for the cladding. From the start, a multimode interference coupler was chosen as the beam combiner for this work. This is a device that is unaffected by common fabrication errors and as such was shown to be reliable in various materials. The in-depth study shown in this work proved this to be the case for this material too. In addition, a version of the MMI with a broad bandwidth splitting ratio, over 400 nm, was discovered. This MMI was investigated thoroughly to understand the relationship between the excited light-modes in the slab and the increase in bandwidth. An investigation into whether the width of the MMI has any influence on the breadth of the bandwidth was undertaken, showing little relationship with a larger width. With smaller width MMI limited by the number of modes in the slab to generate this broad bandwidth special case. The computed nulling interferometer for this work was made using a single multimode interference coupler as the two-beam (or telescope) combiner that was simulated to have a 40 dB (10^-4) null over a 400 nm bandwidth. Experimentally, using a single input and calculating the expected null performance, achieved 30 dB (10^-3) over a 300 nm bandwidth, centred at 4000 nm. Less than predicted but a benchmark for nulling interferometers in the MIR. Subsequent direct measurements of extinction, using two MMIs in a Mach-Zehnder interferometer configuration, could not ascertain such a null depth. These measurements were limited by scattered light from the beam combiner as well as imperfect fabrication larger than predicted, especially in the MZI arms. The issue was related to the small size of the waveguides used and increasing them to multimode waveguides, for a short period to limit multimode excitation, would provide a much more robust system. Taking these limitations, and the solution, into account these structures should be able to produce the predicted deep null over the specified bandwidth once these issues have been addressed. To the author's knowledge, the extinction measurements in this thesis are the first of their kind and are a significant step towards measuring an exoplanet never before seen using nulling interferometry in the mid-infrared. The experiments on the beam combiners as 3 dB splitters show the expected null to be as deep as predicted, within error and highlight the difficulties in ascertaining exact measurements of extinction with modern technology and current fabrication techniques. Fabrication errors as they stand are the key issue that will need to be overcome in future work but are achievable with current technology.