Dispersion Engineered Highly-Doped Silica Glass Waveguide for Broadband Nonlinear Applications

Abstract

Recent advancements in complementary metal-oxide semiconductor (CMOS) compatible high-index contrast integrated optical waveguide platforms have not only bolstered the traditional advantages of integration but have also expanded the functionalities of planar lightwave circuits (PLC) beyond conventional linear applications, such as wavelength filtering, optical switching, and routing in fiber-optic communication networks, to encompass more intricate nonlinear applications. The robust optical confinement achievable in these high-index contrast waveguides amplifies their nonlinear effects, facilitating applications such as all-optical signal processing and switching, optical frequency comb (OFC) and supercontinuum (SC) generation, and quantum photonics. While Silicon (Si) photonics circuits were among the first platforms investigated owing to Si's exceptionally high nonlinearity - manifested by a Kerr coefficient approximately 100 times greater than silica, its operation within the popular telecom band has been significantly curtailed due to two-photon absorption. A nonlinear optical platform that has demonstrated notable success in nonlinear optics is the highly-doped silica glass (HDSG) waveguide platform. The CMOS-compatible HDSG platform boasts an adjustable refractive index between 1.45 and 1.9 in the C-band. Despite its nonlinearity being an order of magnitude lower than Si, it exhibits negligible linear and nonlinear losses in the telecom wavelengths, resulting in a remarkably high nonlinear figure-of-merit. Moreover, its mature fabrication process and the availability of a wide array of supporting optical structures position this platform as a prime candidate for the commercialization of integrated nonlinear photonics circuits.

This thesis elucidates the dispersion engineering of the HDSG waveguide dispersion profile tailored for nonlinear applications, specifically focusing on the generation of broad OFC and SC spectra. We scrutinize the impact of incorporating a lower index oxide layer within a HDSG channel waveguide to optimize the dispersion profile at telecom wavelengths. Through meticulous control over the thickness of the oxide slot and the dimensions of the waveguide, we showcase the ability to design key dispersion characteristics, including the flatness of the zero-dispersion wavelength, phase-matching location, and overall dispersion curve flatness. Given that OFC and SC generations necessitate different waveguide elements, we develop two distinct dispersion measurement systems to experimentally validate the dispersion of fabricated waveguide structures against simulated designs. For OFC generation, we employ a laser sweeping system comprising a hydrogen cyanide gas cell and a Mach-Zehnder interferometer to extract resonance locations of the microring resonator (MRR) for calculating the dispersion profile. For SC generation, we introduce the phase fitting method to determine the best-fitted dispersion curve from the measured spectrum, alongside the equilibrium wavelength method to extract dispersion from equilibrium wavelength shifts as a function of time delay.

To affirm the effectiveness of the dispersion-engineered MRR in OFC generation, we compare OFC spectra from MRRs with and without the lower index slot using both open-loop and closed-loop pumping schemes. Our findings reveal that MRRs with the lower index slot produce wider OFC spectra compared to those without the slot, with the spectrum broadened by approximately 50% in the open-loop scheme and almost 100% in the closed-loop scheme. For SC generation, the introduction of a 0.1 μm oxide slot enables the attainment of a low and flat anomalous dispersion region around the C-band, facilitating SC generation with a spectrum spanning over 1.5 octaves, ranging from 817 nm to 2183 nm when pumped at 1560 nm, with a high degree of coherence. We verify our experimental results through numerical simulations based on a generalized nonlinear Schrödinger equation model, demonstrating a close correspondence between dispersive wave locations and specific spectral features.

Date of Award	26 Jun 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sai Tak CHU (Supervisor)