Low Loss Integrated Photonic Devices and Their Applications in Astrophotonics and Quantum Information

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The silicon nitride (Si3N4) integration platform for photonics is becoming a low-loss and compact platform for various applications such as astrophotonics and quantum information. In this dissertation, the fabrication techniques for low loss integrated photonics devices is first discussed. Then, we present the design, fabrication, and characterization of an on-chip low-loss (1.39 dB on-chip loss) echelle grating spectrometer that can work in an ultra-broad band (potentially from visible to mid-infrared spectral range) with high resolution and low crosstalk (<-30dB). We demonstrate an on-chip add-drop filter working on arbitrarily spaced five channels based on the complex waveguide Bragg gratings and the multimode interferometer (MMI). A more than 40 dB extinction ratio is achieved in all five channels and the device is shown to be low loss (<1dB on chip loss) and broadband (>200nm 3-dB bandwidth). We also study a Fabry-Pérot Bragg grating cavity enhanced four wave mixing process on a Si3N¬4/SiO2 platform, from an analytical treatment to an experimental demonstration. A unique grating loss reduction and a dispersion compensation technique is also demonstrated. Multiple loss reduction approaches are demonstrated to further reduce the loss and improve the four-wave mixing efficiency, including depositing thicker Si3N4 film, using different etch masks, and different etching gases. Using a ring resonator, the waveguide loss has been extracted and reduced from 0.33 dB/cm to 0.18 dB/cm. Moreover, we explore aluminum nitride (AlN) on sapphire as a new platform for integrated photonics devices. A special fabrication process for the AlN-on-sapphire chip is developed and the loss is measured to be 1.95 dB/cm based on a ring resonator approach. Lastly, we demonstrate a high extinction ratio Mach-Zehnder interferometer (MZI) with MMI for exoplanet discovery. The MMI-MZI features an extinction ratio of 58 dB without thermal tuning and 50 dB with thermal tuning. A TM noise reduction technique is also shown to help realize the maximum extinction ratio.