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ECE PhD Dissertation Defense: Shutao Xu

Title: Thulium-doped Ultrafast fiber laser system designs and dynamics

Presenter: Shutao Xu

Advisor: Professor Michelle Y. Sander

Chair: Professor Sahar Sharifzadeh

Committee: Professor Michelle Y. Sander, Professor Anna Swan, Professor Roberto Paiella, Professor Abdoulaye Ndao, Professor Jerry Chen

Google Scholar Link: https://scholar.google.com/citations?user=zIQa4OkAAAAJ&hl=en

Abstract: Thulium (Tm)-doped ultrafast fiber lasers with emission wavelengths around 2 μm are desirable sources for scientific, industrial, medical, and environmental applications and flexible testbeds for investigating nonlinear pulse dynamics. Although exceptional research attention has been drawn by Tm-doped ultrafast fiber lasers in recent years, their designs and dynamics are significantly less explored compared to other fiber laser systems. Despite the broad emission spectrum of Tm-doped fibers, power scaling of Tm-doped ultrafast fiber lasers has been limited at shorter wavelengths of their emission spectrum (

This dissertation focuses on developing various novel Tm-doped ultrafast fiber laser systems with unprecedented performance: High-energy operation is demonstrated at less-exploited wavelengths and unique waveforms are generated with their nonlinear dynamics investigated in real-time. First, a high-energy (394-nJ) Tm-doped chirped-pulse-amplification fiber laser system is designed and optimized for operation at the wavelength of 1900 nm and supports the generation of 950-nm ultrashort (390-fs) pulses via frequency-doubling. The system represents the highest pulse-energy (138 nJ) in the femtosecond regime for any fiber-based systems around this wavelength to date, which can be highly attractive for two-photon microscopy with spatiotemporal-multiplexing.

To gain deeper insights into the operation of ultrafast Tm-doped fiber lasers, various new nonlinear dynamics are investigated by a home-built real-time characterization setup based on dispersive Fourier transform for 2 μm pulses: A new mode-locking regime is demonstrated which can deliver both up-chirped and close-to-chirp-free dissipative pulses with a 10-fold difference in their pulse energies/durations, providing a versatile source that can switch between different pulse profiles. Following that, soliton molecules with unique partial spectral modulation patterns are synthesized based on two dissimilar pulses from the same cavity, which represent an interesting analogy to ‘heteronuclear’ chemical molecules and hold great potential for optical information processing. Further, mode-locking evolution between dissimilar coherent pulses are studied in Tm-doped ultrafast fiber lasers. Finally, combining both high-energy operation and novel waveform-generation, we present a Tm-doped fiber laser source delivering amplified (~ 200 nJ) noise-like pulses without requiring any feedback mechanism.