ECE PhD Thesis Defense: Timothy Lim

ECE PhD Thesis Defense: Timothy Lim

Title: Power Scaling and Nonlinear Conversion in Fiber-based Sources at Novel Wavelengths

Presenter: Timothy Lim

Advisor: Professor Michelle Sander

Chair: TBA

Committee: Professor Michelle Sander, Professor Siddharth Ramachandran, Professor Enrico Bellotti, Professor Jerry Chen

Google Scholar Link: https://scholar.google.com/citations?user=BtaXSJIAAAAJ&hl=en

Abstract: Power-scaling of fiber-based laser sources is an attractive research area as they can enable numerous different applications in medicine, industry, and defense security due to their high spatial beam quality, compact size, and robustness. Energy scaling of silica-based fiber sources is limited to specific wavelengths regions by typical used rare-earth dopants such as ytterbium (Yb) around 1 µm, erbium (Er) with emission around 1.55 µm, thulium (Tm) with a gain emission from 1.7 to 2.0 um, and holmium (Ho) with an emission from 2.05 to 2.1 µm. Extending the wavelength range of high-power silica-based fiber-based sources beyond the wavelength regions of available rare-earth-doped gain fibers through nonlinear conversion processes can push the current silica-based fiber laser technology and enable further applications in imaging, communication, and material processing.

This dissertation focuses on different ways to increase the power of fiber-based laser systems at novel wavelengths outside the typical gain emission spectrum of rare-earth gain dopants. The first part of this thesis focuses on extending mature Yb fiber-based technology into the visible regime through degenerate four-wave mixing in photonic crystal fibers (PCFs). The relative strength of four-wave mixing generation is strongly dependent on the uniformity of various fiber parameters along the fiber length to maximize phase matching and nonlinear pulse generation. However, current commercially available PCFs can feature up to 10% fluctuation in core diameter uniformity. Therefore, the dispersion and nonlinear coefficient of custom-drawn and commercially available photonic crystal fibers through white light interferometry and femtosecond nonlinear pulse propagation. The custom-drawn photonic crystal fiber demonstrated a 5x improvement in dispersion uniformity and greater spectral similarity between forward and backward pulse propagation. The higher fiber parameter uniformity can enable higher conversion efficiency into the visible regime, enabling various applications such visible light communication and bio-medical imaging.

The second part of this thesis focuses on the design and optimization of a high-energy (394 nJ) Tm-doped chirped-pulse-amplification fiber laser system operating at 1.9 μm. Despite the broad gain spectrum of Tm-doped fibers, power scaling of Tm-doped ultrafast fiber lasers below 1920 nm is generally limited due to signal reabsorption. Optimization of a frequency-doubling system, ultrafast (390 fs) pulses at 950 nm are generated with the highest pulse energy (138 nJ) of any fiber-based source around this wavelength to date. The presented high-energy fiber laser system at 1.9 µm is an attractive source for applications in material processing and biomedical surgery. Additionally, the frequency-doubled pulses are a strong candidate for increasing the field-of-view and imaging speeds of two-photon microscopy with spatiotemporal multiplexing.