Overcoming the effects of the Earth's atmosphere on astronomical observations with 3D integrated photonic technologies
thesisposted on 28.03.2022, 01:20 by Izabela Spaleniak
The earth's atmosphere prevents telescopes, astronomical instruments and astrophotonic devices from working at their full capabilities. Atmospheric turbulence introduces wavefront distortions, while hydroxyl molecules in the ozone layer produce strong emission lines in the infrared part of the spectrum. Most techniques to address these problems are inefficient and expensive, forcing the instrumentation to be of large size and cost. By using a femtosecond laser direct-write technique, 3D optical circuitry called photonic lanterns were fabricated. The devices enable the creation of compact optical instruments and integrated photonic devices on ground-based telescopes. The objective of this thesis was to optimise the design of photonic lanterns to increase their efficiency, and create an on-sky prototype instrument to demonstrate the feasibility of the approach. A detailed optimisation of the three main building blocks of photonic lanterns and slit reformatting devices was performed. After optimising design for multimode waveguides, the mode evolution along a transition section between the multimode and isolated single-mode waveguides sections was analysed, and three types and lengths of transition to find the adiabatic regime were studied. An optimal design was identified for which back-to-back photonic lanterns and slit reformatting devices are >90% efficient. In the second part of the thesis, fully integrated photonic lanterns with multiple waveguide Bragg gratings were created. Multiple gratings were placed in the waveguide using the point-by-point femtosecond laser inscription technique. Devices which filter out one, two, three or four wavelengths were fabricated. The best performance was demonstrated by the device with three gratings for wavelengths of 1545 nm, 1552nm and 1559 nm, featuring grating strengths of 5.12 dB, 5.60 dB, and 2.87 dB, respectively. In the third part of the thesis, a slit reformatting device and a single-mode high resolution spectrograph were used to create a prototype on-sky demonstrator instrument and conduct a proof-of-concept test. The astronomical data were acquired on the 0.4m telescope at the Macquarie University Observatory, Sydney. The spectrum of Antares revealed multiple CO band heads in the astronomical H band, as well as sharp atmospheric water absorption lines demonstrating the feasibility of the technique -- abstract.