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Photonics integrated circuits are a promising solution for the growing demands of data transmission and future system-on-chip technologies. Within this context, nonlinear optical interactions offer unique opportunities for all-optical processing, sampling, and sensing on-chip. This thesis focused on the study of silicon nitride Si3N4 waveguides for nonlinear frequency conversion, specifically continuous-wave four-wave mixing (FWM), due to their ultra-low propagation loss, amenability to high-quality fabrication, broad transparency window, and ability to operate efficiently with watt-level pump powers. These features make them an attractive choice for on-chip FWM applications.In addition to conversion efficiency (CE), the conversion bandwidth and spectral reach of FWM also play a crucial role in determining the limits of the applications. Therefore, this work focuses on efficient broadband on-chip wavelength conversion. The analysis starts with the polarization-selective dispersion engineering in Si3N4 waveguides enabling broadband wavelength conversion around 1.6 µm and 2.0 µm wavelength. Two-sided 3 dB conversion bandwidths of 160 nm and 120 nm are observed by leveraging polarization-dependent dispersion between TM to TE polarizations, respectively. The dispersion of the waveguides can be also utilized to satisfy the phase-matching at far-detuned wavelengths through higher-order dispersion terms. Such polarization-leveraged distant phase-matching has been observed in the waveguides reaching up to 2.6 µm wavelengths and 1.3 µm wavelengths utilizing common erbium and thulium-band.Recent record high CW CE are obtained thanks to the use of low-loss long waveguides. However, increasing the effective length typically comes at the expense of bandwidth. Efficient-broadband conversion is investigated in various waveguides, which are engineered to maintain the broadest bandwidth at different lengths that can enhance the CE up to 2.9 dB. The obtained CE and bandwidth of the wavelength conversion schemes shown in this thesis are compared with state-of-the-art wavelength converters, proving their potential.Yet, several challenges were observed during this analysis: strong sensitivity of the dispersion to the waveguide dimensions, reduced CE due to higher-order mode excitation, and optical power fluctuations with mode-mixing. The origin of higher-order modes is identified as the transition of the different segments of the curvature. Temperature tuning is also analyzed to characterize and partially mitigate mode-mixing.Finally, while polarization-sensitivity can be leveraged, it can also be a hurdle for applications requiring polarization-insensitivity. Polarization-insensitive wavelength conversion can be obtained by employing a depolarized pump at the cost of CE. The CE drop is analyzed and a minimum of 2.55 to a maximum of 4.1 dB drop depending on the depolarization scheme is observed. The theoretical calculations are experimentally tested in chalcogenide photonic crystal fibers, selected due to their excellent nonlinearity.In conclusion, Si3N4 waveguides show huge potential for on-chip optical processing and integrated photonics. While challenges persist, there is ample room for enhancing CE and bandwidth. While the unique polarization-selective nature can serve specific applications, depolarized FWM provides a pathway for on-chip PI parametric conversion.
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