We demonstrate a simple method for spectral broadening and compression of laser pulses at megahertz repetition rates
by self-phase modulation in a large mode area (LMA) fiber. In order to avoid the currently limiting factor of damage by
self-focusing, we positively chirp the input pulse, which allows coupling of significantly more energy into the fiber,
while maintaining the same spectral bandwidth and compression as compared to the Fourier-limited case at lower
energy. Using a commercial chirped pulse Ti:Sa oscillator (Femtolasers, Femtosource XL) with 55fs, 400nJ pulses at
5MHz and an LMA fiber with 25μm core diameter, we generate 16fs, 350nJ pulses, which is a factor of 4 more energy
than possible with unchirped input pulses. Good stability has been measured over at least 1 hour for the chirped case and
unchirped case. Furthermore, with a 5μm core diameter LMA fiber we generated compressed pulses with 6fs and 18nJ
output energy. This would allow a carrier-to-envelope phase stabilization of the laser system by external selfstabilization
via acoustic difference-frequency modulation. The compact size and its simplicity makes the combination
of a chirped pulse oscillator with chirped-fiber-broadening an attractive option for ultrafast spectroscopy at MHz
We report on our latest results in the development of high-energy, long-cavity Chirped Pulse Oscillators (CPOs). Our
concept allows the generation of ultrashort laser pulses at MHz repetition rates with a pulse duration of less then 50fs
and an energy approaching the μJ-level directly out of an oscillator. Thus, our unique approach completely avoids the
need for any additional amplification stages. This paper is, to the best of our knowledge, the first demonstration that
laser pulses with a peak power exceeding 18MW can be generated directly out of a femtosecond oscillator.
220-nJ, 42-fs, 5.25-MHz pulses from a long-cavity Ti:Sapphire chirped pulse oscillator were spectrally broadened by
nonlinear propagation in a Sapphire plate. The chirp was subsequently compensated with dispersive mirrors. After farfield
spatial filtering the compressor delivered 80-nJ, sub-15-fs pulses at 5.25 MHz.
A novel 500-nJ Oscillator has been developed in order to investigate the energy-scaling potential of this compression
scheme. 16-fs 130-nJ compressed pulses were obtained with this source. A second compression stage has been
calculated and designed in order to reduce the pulse duration down to < 10 fs.