Ultrafast Lasers and Amplifiers based on Yb-doped Gain Materials
The development and advancement of high powered, sub-picosecond laser sources have been crucial to the progression of ultrafast optical studies in the visible and near-infrared spectrum. Highly energetic, broadband pulses can be used to probe the electronic and structural dynamics of some of the fastest possible chemical and biological processes. This work outlines the development of a novel ultrafast laser system operating in the 1μm wavelength regime, using a chirped-pulse amplification scaling scheme. The system, based solely on ytterbium-doped fiber amplifiers, delivers pulses up to 400 μJ and average powers up to 100 W. An extremely stable fiber-based oscillator is presented, delivering 8 nJ pulses at a repetition rate of 38 MHz. Employing a stretched-pulse cavity design, an adjustable dispersive element enables operation in both positive and negative dispersion regimes. Compressed pulses as short as 43 fs were measured from the oscillator. A custom pulse-stretching fiber is used in lieu of a traditional grating compressor, and two single mode preamplifiers complete the front-end of the system, generating 100 mW output at rates ranging from 0.1 MHz to 1 MHz. Two diode-pumped, large-mode-area, ytterbium-doped fibers provide high power amplification. A large-core fiber amplifier delivers single mode output up to 10W average power and a photonic-crystal rod amplifier provides final amplification up to 100 W. The beam quality remains excellent, even at high power operation. At 200 kHz the amplifier produced 400 μJ pulses which can be compressed to 350 fs, which represents among the highest peak power generated from this class of amplifier. In addition to this complete laser system, two other ytterbium-based ultrafast lasers were developed: a Yb:KGW crystal regenerative amplifier, and an all-fiber oscillator. The regenerative amplifier is seeded by the front-end discussed. It operates at lower repetition rates, 1 kHz to 10 kHz, outputting 400 μJ pulses compressible to 250 fs. Further development of this amplifier should enable mJ-level output. The single- diode-pumped, all-fiber oscillator is based on a nonlinear amplifying loop mirror. It outputs 1 nJ pulses at 13 MHz. The oscillator may serve as an extremely stable and compact seed laser for a future high power system.
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