ULTRA-LOW-LOSS SILICON NITRIDE WAVEGUIDE GRATINGS AND THEIR APPLICATIONS IN ASTROPHOTONICS
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Recent progresses in silicon photonics have enabled many exciting applications in data communications, sensing, quantum information, and astrophotonics. Astrophotonics is an emerging research field which aims to apply the fast-evolving photonics technology to astronomy. Compared with the silicon-on-insulator (SOI) based silicon photonics, silicon nitride (SiN) based silicon photonics inherits many prominent characteristics such as CMOS compatibility and fabrication flexibility. Furthermore, SiN-based photonics excels in applications strongly associated with low loss level and wide transparent window. All these features are all very attractive for astronomical instrumentation. Typical applications of astrophotonic components are photonic lanterns, frequency combs, highly selective optical filters, and on-chip spectroscopy. Specifically, the scope of this dissertation covers the astrophotonic filters and spectroscopy, from the design, fabrication to characterization. The photonic components which they are based on are ultra-low-loss SiN waveguide and waveguide gratings. The fabrication techniques of ultra-low-loss SiN photonic devices will be first discussed. I will demonstrate several methods to reduce the waveguide and grating losses, including the optimization of SiN deposition, e-beam lithography, etching, cladding oxide deposition, and thermal annealing. In the third chapter, an efficient waveguide characterization approach is developed for measuring losses in on-chip waveguides. This approach is based on measuring the transmission of a Fabry-Perot Bragg grating cavity formed by two highly reflective and low loss Bragg grating mirrors. In the fourth chapter, I will discuss on the design and characterization of a high performance integrated arbitrary filter from 1450 nm to 1640 nm. The filter’s target spectrum is chosen to suppress the night-sky OH emission lines, which is critical for ground-based astronomical telescopes. To reduce the device footprint, the designed 50-mm-long 55-notch filter is mapped to a compact spiral waveguide. The last topic of this dissertation is on-chip spectroscopy with arrayed waveguide grating (AWG). Different with conventional AWG used in WDM telecommunication applications, this astrophotonic spectroscopic AWG particularly needs a large free spectral range (FSR) and a flat focal-plane for the following up free-space cross disperser. The basic principle and preliminary experimental results of AWG will be first presented, followed by discussions of two AWG designs with flat output-plane.