On-chip Supercontinuum Generation towards Mid-Infrared.

The new era of the smart world is coming, with urgent requirements for advanced information sensing technologies for applications ranging from intelligent manufacturing, autonomous vehicles and environment monitoring to disease diagnostic and smart home. Sensing technology using light, especially with the mid-infrared wavelengths from 2 µm to 10 µm which overlaps the spectral signature of the majority chemical bonds, can provide real-time and precise detection and analysis of the objects, including the chemical components or the dynamic positions. Many applications involving optical sensing, like spectroscopy or imaging, require coherent and broadband light sources. One way of generating a broadband spectrum is supercontinuum generation (SC). A supercontinuum is generated by the extreme nonlinear interactions between a pump laser and the nonlinear medium, it is known as a white laser because of its broad spectral bandwidth, high coherence and high intensity. While supercontinuum generation has been intensively researched in bulk media and fibers for many years, the generation of a supercontinuum in waveguides on a chip may be preferred in many applications for the sake of low cost, small instrument volume and low power consumption. However, although many on-chip nonlinear waveguides have been developed for supercontinuum generation, most of them are operating in the near infrared wavelengths, and it is still challenging to extend the source to the midinfrared (MIR). This challenge is mostly because of the lack of a versatile nonlinear waveguide platform, which can confine light enough at long wavelengths and have low
loss. This thesis presents a study of supercontinuum generation in on-chip waveguides. The main focus is spectral broadening towards the MIR beyond 3 µm, while pumping at near infrared (NIR) wavelengths. Supercontinuum in waveguides with both
quadratic and cubic nonlinearity has been explored. A single-mode nonlinear analytical envelope equation (NAEE) in a quasi-phase-matched (QPM) waveguide has been derived as the mathematical model of spectral broadening by cascading quadratic nonlinearity joint with cubic nonlinearities. The supercontinuum generation, with solitons formed in the normal dispersion regime by a negative total Kerr-like nonlinearity, phase matching to four-wave-mixing (FWM), as well as three-wave-mixing (TWM) dispersive waves (DW), is presented, with experimental and numerical results in on-chip periodically poled lithium niobate (PPLN) waveguides. By poling the waveguides with different periods, several different quasi-phase-matching (QPM) conditions have been explored and different-frequency-generation (DFG) dispersive waves have been achieved in MIR from 4 to 4.6 µm, when pumped with a commercial mode-locked fiber laser. Additionally, numerical investigations on the supercontinuum generated with the cascading quadratic nonlinearity find DFG DWs to have low noise. The tunability and high coherence of the DFG DW makes it a good candidate
for various applications. We have also achieved and investigated supercontinuum generation in nano-fabricated TiO2 waveguides with both all-normal dispersion (ANDi) and anomalous dispersion for a broad wavelength range around the pump wavelength (1550 nm). One octavespanning supercontinuum has been realized in both cases. In the all-normal-dispersion waveguides, a supercontinuum with very flat spectrum has been observed. Furthermore, since the spectral broadening is caused by purely coherent self-phase modulation (SPM) and optical wave breaking (OWB), the supercontinuum generated in the ANDi waveguides shows high coherence and intensity stability. With the results demonstrated in this thesis, it is envisioned that, if fabrication breakthrough could enable stronger confinement at MIR wavelengths, this TiO2 nonlinear waveguide platform could generate on-chip supercontinuum above 3 µm and find applications in the areas mentioned in the head of this abstract. Hopefully, this thesis can inspire future work on on-chip, high quality supercontinuum generation covering the MIR, by using advanced cascading nonlinear effects and novel or improved nonlinear waveguide platforms.