Chirped Pulse Oscillators: Generating microjoule femtosecond pulses at megahertz repetition rate

The maximum energy achievable directly from conventional
Ti:sapphire oscillators has been limited by the onset of
instabilities such as cw-generation and pulse splitting because of
the high intensity in the laser medium. Generation of microjoule
pulses at megahertz repetition rates are of special interest in
many areas of science and technology. The main subject of this
thesis is the development of high energy Ti:sapphire oscillators at
megahertz repetition rate. The main concept that was applied to
overcome the difficulties pointed out above was to operate the
laser in the positive dispersion regime. By operating the laser in
this regime, intracavity picosecond pulses are generated that can
be externally compressed down to femtosecond pulse durations. The
long pulse duration inside the laser offers an elegant way to
reduce pulse instabilities by decreasing the intracavity intensity
via pulse stretching. Drawing on this concept, Ti:sapphire
chirped-pulse oscillators delivering sub-50-fs pulses of 0.5 uJ and
60 nJ energy are demonstrated at average power levels of 1 and 4 W
(repetition rate: 2 MHz and 70 MHz), respectively. The 0.5 uJ
pulses have a peak power in excess of 10 MW. By locking a 76-MHz
chirped-pulse oscillator to a femtosecond-enhancement cavity 7.8 uJ
pulses with 55 fs pulse duration were achieved. High harmonic
generation was demonstrated in a Xenon target placed close to the
enhancement cavity focus.