Widely-Tunable Optical Parametric Oscillation in chi(3) Microresonators

Abstract

In this thesis, we report a novel approach to enable optical sources with wideband wavelength tunability. By using c(3) microresonators, we are able to generate two new narrowband widely detuned optical frequencies via the nonlinear process of optical parametric oscillation. The wideband tunability is enabled by operating the resonator in condition of normal dispersion, in the presence of higher order dispersion. This allows us to phasematch unusually large frequency shift parametric oscillation. We first present an experimental demonstration of widely tunable optical parametric oscillation in silica (SiO2) microspheres. Through comprehensive theoretical analysis and experiments, we are able to demonstrate over 720 nm of discrete tunability using a low-power, continuous wave C-band pump laser. We find that the maximum tuning range attainable in this system was limited due to the high attenuation of fused silica above 1900 nm. We then show that magnesium fluoride (MgF2) microresonators can overcome the limitations experienced by silica-based resonators. We consider several different MgF2 microresonators to experimentally demonstrate over an optical octave of discrete tunable output from 1083 to 2670 nm. In addition, signatures of mid-infrared sidebands (out to 3860 nm) are also observed in this demonstration. By using the delayed selfheterodyne interferometer method, we find that these generated sidebands share similar spectral linewidths to the pump laser. Moreover, we demonstrate a small amount of continuous tunability of the parametric sidebands by leveraging the resonators intrinsic thermal nonlinearity. Finally, we investigate the formation of localized frequency combs located around the pump and the two widely detuned sidebands. In this investigation, we present an experimental demonstration of these clustered frequency combs in microresonators, as well as proposing a theory that explains their generation. Numerical simulations based on the Lugiato-Lefever equation are carried out to validate the proposed theory. Furthermore, we also look at, in detail, the coherence of the numerically simulated comb clusters.