Soliton States in On-Chip Fabry-Perot Bragg Grating Microresonators for Frequency Comb Generation

Abstract

Frequency comb technology has revolutionized precision spectroscopy, timekeeping, distance measurements, and many other fields. Frequency combs enabled these revolutions by linking optical frequencies to radio/ microwave frequencies. Although the technological applications are vast, utilization outside the lab is challenging primarily due to the size of the lab equipment, the power supply needed, and the optical cavity stability necessary for frequency comb formation. The work presented in this thesis supports the incremental progress toward a robust and portable frequency comb-based device, specifically toward the challenge of device size.

Microresonators are designed to enable small-sized frequency comb optical cavities for frequency comb generation. This work focuses on a linear Fabry-Perot cavity geometry. Our design utilizes chirped Bragg gratings, enabling a frequency-dependent cavity length for dispersion management; dispersion is finely tuned by varying the Bragg grating length and chirp in fabrication. When the effects due to the Group Velocity Dispersion (GVD) and Self-Phase Modulation (SPM) are balanced, the optical waveform in the cavity experiences no net dispersion, enabling a waveform that does not vary through time or space. In this regime, the non-varying waveform is referred to as a soliton. Similar to atoms' organization in matter, multiple solitons can form stable formations called soliton molecules, crystals, or glasses (as well as others). Various soliton states can only be formed in specific dispersion profiles. The repetitive time domain profile from a cavity operating in a soliton state gives rise to a frequency comb.

This work presents three main categories of measurements and characterizations. First, I present a method for dispersion measurements of the individual Bragg gratings through the implementation of a white light interferometry process and the algorithm I developed for this data analysis. Next, I present characterizations of the full microresonator cavity across the entire reflection spectrum of the pair of Bragg gratings. These characterizations were done with low optical power to remain in the linear regime of the cavity’s material (Silicon Nitride). Finally, I present characterizations of the nonlinear response of these microresonators at high optical power and some of the soliton states generated. I conclude this work by tying dispersion measurements to the experimentally available nonlinearity as an analysis system for quickly finding a desired soliton state. An analysis system for quickly finding a desired soliton state supports incremental effort in creating robust, portable frequency comb devices.

This work was developed from experimental characterizations (matched with theoretical characterizations by my colleagues at the University of Maryland - Baltimore County) of the soliton glass state. As a novel state, only our group has published or presented its characterizations (as of the writing of this thesis, September 2024). The soliton glass state has many desirable characteristics for portable frequency comb devices: frequency comb 'teeth' are separated by one Free Spectral Range with low phase noise, and the total output power is the sum of the total number of solitons that make up the soliton glass. The work presented in this thesis supports an experimental understanding of producing a soliton glass state, which is a prime candidate for frequency comb generation at a portable scale.