Brillouin- and Kerr-mediated parametric light sources in scalable photonic platforms
Yvan Klaver is a PhD student in the Department of Laser Physics & Nonlinear Optics. (Co)Promotors are prof.dr.ir. D.A.I. Marpaung and dr. P.J.M. van der Slot from the Faculty of Science & Technology.
Light sources are fundamental to photonic systems, serving as frequency references, data carriers, and illumination for sensing and imaging. Many advanced applications, such as coherent communication and high-resolution spectroscopy, require sources with exceptional spectral purity, defined by narrow linewidth and stable frequency emission.
To realize such high-purity light in scalable integrated platforms, this thesis investigates Brillouin- and Kerr-active light sources based on silicon nitride. Unlike conventional emitters that rely on laser transitions, these nonlinear optical sources exploit parametric conversion processes, enabling wavelength flexibility and ultra-low-loss propagation a key attributes for integrated, high-coherence oscillators.
We design waveguides that support both Brillouin scattering, a nonlinear optical process mediated by an acoustic wave, and the Kerr nonlinearity, an intensity-dependent refractive index. By forming these waveguides into optical microresonators, we demonstrate high spectral purity Brillouin lasers and broadband Kerr microcombs. The work includes a several-orders-of-magnitude enhancement of Brillouin interaction strength, demonstrations of multiple Brillouin photonic applications, and a theoretical study of the competition between Brillouin and Kerr gain within a single microcavity.
In Chapter 2, we establish the theoretical and technical background for the thesis. This includes fundamentals of integrated photonics and nonlinear optics, with an emphasis on concepts directly relevant to microcomb generation and Brillouin lasers. Key topics include waveguide design and microring resonators, Kerr nonlinearity and the role in microcomb generation, and the fundamentals of Brillouin scattering and the role stimulated Brillouin lasers.
In Chapter 3, we investigated the suitability of silicon nitride waveguides for nonlinear optics, focusing on stimulated Brillouin scattering (SBS) and the Kerr effect. Three geometries were compared, namely, thick silicon nitride, symmetric dual-stripe (SDS), and asymmetric dual-stripe (ADS). Among them, the SDS waveguide showed the strongest potential, leading to optimized designs for Brillouin lasers and microcomb sources, including devices capable of switching between these functionalities. Fabrication yielded propagation losses of 7 dB/cm, validating the design approach, and preliminary experiments confirmed Brillouin lasing and Brillouin-induced Kerr frequency comb operation.
In Chapter 4, we developed a numerical model to study the interplay between Brillouin and Kerr nonlinearities in microring resonators. Simulations revealed that Brillouin-induced microcombs can form in forward or backward directions, depending on the free spectral range (FSR) and pump power. The results highlighted the importance of acoustic detuning, showing that, when Brillouin and Kerr strengths are comparable, an FSR slightly smaller than the Brillouin shift is necessary to avoid suppression of Brillouin lasing by cross-phase modulation.
In Chapter 5, we addressed the challenge of achieving strong Brillouin scattering in scalable photonic platforms by combining silicon nitride with a layer of tellurite glass and coupling to surface acoustic waves (SAWs). This approach enhanced Brillouin gain by more than two orders of magnitude, enabling demonstrations of a 10-dB amplifier, a 2.2-MHz wide tunable microwave photonic notch filter, and a Brillouin phonon laser with an intrinsic acoustic linewidth of 7 Hz. These results highlight the potential of hybrid platforms for high-resolution signal processing and coherent phonon sources.
Overall, this thesis demonstrates that Brillouin and Kerr nonlinear sources can be successfully integrated in silicon nitride platforms, paving the way toward scalable, multifunctional photonic systems. The combination of device design, modelling, and experimental validation underscores the scientific and technological promise of these sources, particularly in the context of high-purity RF generation via optical frequency division and the broader field of integrated microwave photonics.
