Dispersion engineering silicon nitride waveguides for broadband nonlinear frequency conversion

Abstract

In this thesis, we investigated nonlinear frequency conversion of optical wavelengths using integrated silicon nitride (Si3N4) waveguides. Two nonlinear conversion schemes were considered: seeded four-wave mixing and supercontinuum generation. The first—seeded four-wave mixing—is investigated by a numerical study and proposed as light source for coherent anti-Stokes Raman scattering (CARS). The compatibility of the silicon nitride-based integrated waveguide with microfluidic channels enables potential applications for on-chip CARS spectroscopy. Both four-wave mixing and supercontinuum generation require waveguides with a large core area to obtain the dispersion required for phase-matched nonlinear frequency conversion. A novel fabrication technique for manufacturing large-core Si3N4 waveguides was investigated. The main advantage of this novel technique is the ability to manufacture crack-free Si3N4 waveguides with sufficient thickness to phase match nonlinear optical processes, while simultaneously realizing a high device yield. To demonstrate that such waveguides can be dispersion engineered and, i.e., allow for phase matching for nonlinear frequency conversion, we experimentally investigated supercontinuum generation in these waveguides using two different pump wavelengths. For a pump wavelength of 1560 nm, the waveguide was dispersion engineered to have a zero-dispersion wavelength just above 1600 nm, such that the pump wavelength experiences anomalous dispersion. The generated supercontinuum spanned more than 700 nm (at -30 dB), limited by the available pump energy. Theoretical modeling showed an exceptionally good agreement with the measured spectrum, and that an octave-spanning supercontinuum is possible if the pump energy is increased. The pulse-to-pulse coherence was calculated for this case and we found that the supercontinuum was fully coherent over its bandwidth (-30 dB), showing that Si3N4 waveguides could be used to generate an optical frequency comb. For a pump wavelength of 1064 nm, the waveguide was designed to have a zero-dispersion wavelength just below 1000 nm to have, again, anomalous dispersion for the pump wavelength. At this pump wavelength, the generated supercontinuum spanned an ultrabroad-bandwidth of nearly 500 THz covering nearly the whole transparency window of Si3N4/SiO2 waveguides.