Physics and Applications of Compact Optical Frequency Comb

Citation

Gao, Maodong (2025) Physics and Applications of Compact Optical Frequency Comb. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ck74-m354. https://resolver.caltech.edu/CaltechTHESIS:08292024-234844580

Abstract

Optical frequency combs (OFC) have been vastly developing and were awarded half of the Nobel Prize in 2005. OFCs are series of optical signals with distinct and equally spaced frequencies. One reason why OFCs are essential for modern optics and photonics engineering is that OFCs serve as a bridge between optical frequencies (hundreds of THz) and frequencies within the electronic bandwidth (from MHz to GHz, which is the distance between adjacent comb teeth). In this thesis, I first introduce some physical principles of optical resonators, which are critical components for confining optical energy and generating OFCs. Then, in the main body of this thesis, I study the physics and applications of two types of compact OFCs: soliton microcombs and electro-optical frequency combs.

Microcombs are OFCs generated on integrated photonics devices. Here, I first develop a methodology to experimentally characterize two important physical properties (material absorption loss and optical nonlinearity) of integrated photonic materials. Next, I focus on a novel method to generate mode-locked soliton microcombs on ultra-low-loss Si₃N₄ material. It was considered challenging to support bright solitons due to its normal dispersion. This novel method involves two resonators that are partially coupled together, which can modify the dispersion through mode hybridization and feature symmetry breaking. Following this, I investigate two characteristics closely related to the symmetry breaking of this coupled-ring device: the observation of Kelly sidebands and multicolor bright soliton generation. Finally, I demonstrate bright soliton generation in Al_0.2Ga_0.8As resonators, which feature high nonlinearity but were considered difficult to support bright solitons at room temperature due to its high material loss. Here, we mitigate the effect of material loss by pulse-pumping operation.

Electro-optical frequency combs are OFCs generated by modulating a continuous wave laser using an external radio-frequency source. Taking advantage of low-noise radio frequency and stable continuous-wave laser frequency, this OFC can serve as a frequency reference for astronomical observation. In this thesis, I first introduce the physics and operating principle of electro-optical frequency combs in Chapter 1, then discuss developing and deploying the near-infrared laser frequency comb at the W.M. Keck Observatory in Chapter 7.

In summary, the thesis discusses the physics and applications of mode-locked bright soliton microcombs, which can generate radio frequencies by taking the beat note of this OFC. I also discuss the physics and applications of electro-optical frequency combs, which are stable OFCs used for astronomical frequency references generated by radio-frequency modulation of continuous wave lasers. The critical role of OFCs as a bridge between optical frequencies and frequencies within the electronic bandwidth (MHz to GHz) is demonstrated, and their potential to revolutionize various fields, including high-precision metrology, telecommunications, and astrophysics, is highlighted.