UC Santa Cruz

The overall aim of this project was to design, develop and demonstrate the highest average and peak power, Ultrashort-Pulse, Deep Ultraviolet Fiber Laser system. Our objective was to combine the robustness of a fiber laser with the production of deep UV radiation by fourth harmonic generation. By producing >100 MW class pulses with a M2<1.3 at a repetition rate of >100 kHz, such a system provides a new and unique capability for nonlinear optics, femtosecond (non-thermal) laser machining and other nonlinear phenomena. The deep UV output enables a host of processes resulting from the ability to achieve high irradiance with 4.7 eV photons. The development, theoretical description, and performance of the entire laser system including operation at the fundamental, 2nd, and 4th harmonics is described. While we could achieve an average power exceeding 5.4 W, pulse broadening in the UV resulted in lower peak power. Our highest average power while maintaining a peak power above 100 MW was 4.8 W at 100 kHz with 400 fsec pulses at 262 nm. To our knowledge, this is the highest peak power fiber laser system operating below 300 nm ever reported.

Initial beam quality was excellent but would degrade after minutes of operation due to the formation of color centers. A number of limitations were observed including two-photon absorption (giving rise to color centers), self-phase modulation, continuum generation and filamentation. These phenomena are investigated and the impact on achieving an operational system are discussed.

The first two chapters of this thesis outline the system as a whole and give a comparison to solid state lasers using bulk gain material. Chapter 3 starts with the fiber seed oscillator that begins the chirped-pulse amplification (CPA) system. It presents all the starting parameters for modeling the pulse train and seed characteristics as well as spectral bandwidth etc. Design considerations at 1030nm vs 1047nm for the CPA system are discussed. Chapter 4 presents theory, modeling and simulation of fiber dispersion and amplification including the use of photonic crystal amplifiers. Chapter 5 discusses compressor design. This includes various compressor grating mounting schemes and analysis and comparison of different compressor options. Chapter 6 focuses on harmonic generation at 2w and 4w as well as crystal selection and phase matching analysis. Decisions between critical and noncritical phase matched harmonic generation is considered. Various crystals are also considered and an analysis of BBO, LBO and CLBO is presented for nanosecond and femtosecond pulses. The remaining chapters provide experimental measurements of pulse train, laser output power, pulse spectrum, pulse duration, etc. Experimental measurements are compared to numerical models throughout.