We report on ultrashort pulse generation from a passively mode-locked erbium fiber laser operating in the highly positive dispersion regime. Highly-chirped pulses with 5.3 ps duration and spectral bandwidth of 8.3 nm are generated. They are extra-cavity compressed down to 757 fs. Numerical simulations confirm the experimental results and show that these pulses could be interpreted as dissipative solitons.

High-repetition-rate sources are very attractive for high-order harmonic generation (HHG). However, due to their pulse characteristics (low energy, long duration), those systems require a tight focusing geometry to achieve the necessary intensity to generate harmonics. In this Letter, we investigate theoretically and experimentally the optimization of HHG in this geometry, to maximize the extreme UV (XUV) photon flux and improve the conversion efficiency. We analyze the influence of atomic gas media (Ar, Kr, or Xe), gas pressure, and interaction geometries (a gas jet and a finite and a semi-infinite gas cell). Numerical simulations allow us to define optimal conditions for HHG in this tight focusing regime and to observe the signature of on-axis phase matching. These conditions are implemented experimentally using a high-repetition-rate Yb-doped fiber laser system. We achieve optimization of emission with a recorded XUV photon flux of 4.5×10(12) photons/s generated in Xe at 100 kHz repetition rate.

We report on a passively mode-locked erbium-doped fiber laser, using a high nonlinear modulation depth saturable absorber mirror, in a Fabry-Perot cavity. A segment of dispersion compensation fiber is added inside the cavity in order to build a high-positive dispersion regime. The setup produced highly chirped pulses with an energy of 1.8 nJ at a repetition rate of 33.5 MHz. Numerical simulations accurately reflect our experimental results and show that pulse-shaping in this laser could be interpreted as producing 'dissipative solitons'.

An Er(3+) fiber laser passively mode locked by a resonant saturable absorber mirror achieves more than 130 mW average power at 1560 nm from a Fabry-Perot cavity. The pulsed regime is self-starting from the CW regime without any Q-switch transition. The output pulse has a duration of 10.2 ps and can be extracavity dechirped with 42% efficiency down to 614 fs, which represents 1.2 times the Fourier limit imposed by the spectrum. To date, this corresponds to the highest averaged power directly extracted at such a wavelength from a fiber laser mode locked with a saturable absorber mirror.

We perform a post-compression of high energy pulses by using optical-field ionization of low pressure helium gas in a guided geometry. We apply this approach to a TW chirped-pulse-amplification based Ti:Sapphire laser chain and show that spectral broadening can be controlled both with the input pulse energy and gas pressure. Under optimized conditions, we generate 10 fs pulses at TW level directly under vacuum and demonstrate a high stability of the post compressed pulse duration. These high energy post-compressed pulses are thereafter used to perform high harmonic generation in a loose focusing geometry. The XUV beam is characterized both spatially and spectrally on a single shot basis and structured continuous XUV spectra are observed.

We generate high-order harmonics with a 50 W, Yb femtosecond fiber laser system operating at 100 kHz in a tight focusing configuration. We achieve a high photon flux even with pulses longer than 500 fs. We collect the diverging XUV harmonic beam in a 35 mrad wide solid angle by using a spectrometer designed to handle the high thermal load under vacuum and refocus the XUV beam onto a detector where the beam is characterised or can alternatively be used for experiments. This setup is designed for a 50 eV XUV bandwidth and offers the possibility to perform XUV-IR pump probe experiments with both temporal and spectral resolution. The high-order harmonics were generated and optimized at 100 kHz by using several gas target geometry (a gas jet and a semi-infinite gas cell) and several gases (Argon, Krypton, Xenon) that provide XUV beams with different characteristics. After the spectrometer and for HHG in Xenon, we detect more than 4x10 10 photons per second over 4 harmonics, that is a useful XUV power on target of 0.1 µW. This corresponds to the emission of more than 1µW per harmonic at the source and we achieved similar flux with both the semi-infinite cell and the jet. In addition, we observe a strong spectral selectivity when generating harmonics in a semi-infinite gas cell as few harmonics clearly dominate the neighbouring harmonics. We attribute this spectral selectivity to phase matching effects.

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