Events Calendar

Title: Power scaling and nonlinear conversion in fiber-based sources at novel wavelengths

Presenter: Timothy Lim

Advisor: Professor Michelle Sander

Chair: Professor Siddharth Ramachandran

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

Abstract: High-power fiber-based laser sources are desired for scientific, medical, and defense applications due to their compact size and high spatial beam quality. Power scaling of silica-based fiber sources is limited to specific wavelength regions by available rare-earth-doped dopants such as ytterbium (Yb) and thulium (Tm) operating around wavelengths of 1 μm and 1.7 – 2.0 μm, respectively. This thesis focuses on different ways to extend fiber-based sources to novel wavelengths through nonlinear conversion: degenerate four-wave mixing (FWM) in photonic crystal fibers (PCFs) along with mature Yb fiber technology to extend into the visible regime, and a high-energy frequency-doubled Tm-doped chirped-pulse amplification system for near-infrared light generation around 950 nm.

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 commercially available and custom-drawn PCFs are characterized through white light interferometry and femtosecond nonlinear pulse propagation. The custom-drawn PCF demonstrates a 5x improvement in dispersion uniformity and greater spectral similarity between forward and backward propagation. The higher fiber parameter uniformity can enable higher conversion efficiency into the visible regime, enabling various applications such underwater visible light communication.

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 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.