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We demonstrate that chirped pulse up-conversion (CPU), a method routinely used with systems based on 1-kHz Titanium:Sapphire lasers, can be extended to a repetition rate of 100 kHz with an Ytterbium diode-pumped femtosecond amplifier. Individual mid-infrared spectra can thus be measured directly in the near infrared using a fast CMOS linescan camera. After an appropriate Fourier processing, a spectral resolution of 1.1 cm -1 is reported, currently limited by our spectrometer. Additionally, we demonstrate the application of CPU to a pump-probe measurement of the vibrational relaxation in carboxy-hemoglobin, and we show that the combination of fast scanning and fast acquisition enables a straightforward removal of pump scattering interference.

We report about a setup for carrier-envelope phase (CEP) control and stabilization in passive systems based on difference frequency generation (DFG). The principle of this approach relies on the amplitude to phase modulation transfer in the white-light generation process. A small modulation of the pump laser intensity is used to obtain a DFG output modulated in CEP. This technique is demonstrated in a CEP-stable system pumped by an Yb-doped fiber amplifier. It is first characterized by measuring CEP modulations produced by applying arbitrary waveforms. The CEP actuator is then used for slow drifts correction in a feedback loop. The results show the capability of this simple approach for OPA/OPCPA CEP-stabilized setups.

Starting from a femtosecond ytterbium-doped fiber amplifier, we demonstrate the generation of near Fourier transform-limited high peak power picosecond pulses through spectral compression in a nonlinear solid-state-based multipass cell. Input 260 fs pulses negatively chirped to 2.4 ps are spectrally compressed from 6 nm down to 1.1 nm, with an output energy of 13.5 µJ and near transform-limited pulses of 2.1 ps. A pulse shaper included in the femtosecond source provides some control over the output spectral shape, in particular its symmetry. The spatial quality and spatio-spectral homogeneity are conserved in this process. These results show that the use of multipass cells allows energy scaling of spectral compression setups while maintaining the spatial properties of the laser beam.

We propose and demonstrate an OPCPA architecture emitting few-cycle pulses at 3070 nm and 1550 nm based on a high-energy femtosecond ytterbium-doped fiber amplifier pump. The short pump pulse duration allows direct seeding by a supercontinuum in the 1.4 - 1.7 µm signal range, generated in bulk YAG. It also allows a simplified dispersion management along the system and broad optical gain bandwidth. The dual output system delivers 20 µJ, 49 fs signal pulses at 1550 nm and 10 µJ, 72 fs idler pulses at 3070 nm. Power scaling limitations due to beam distortion in the last MgO:PPLN-based OPCPA stage are discussed and investigated.

A hybrid-system approach using a low-gain Yb:YAG single crystal booster amplifier behind a state-of-the-art industrial high-power femtosecond fiber system is studied to significantly increase the output pulse energy of the fiber amplifier. With this system, more than 60 W of average power is demonstrated at 100 kHz for pulse duration of 400 fs, corresponding to an energy per pulse of 600 µJ. Reducing the repetition rate, the energy is increased up to 2.5 mJ (before compression), which corresponds to the limitation due to laser damage threshold of the optical coatings. To scale further the energy, passive divided-pulse amplification is then implemented at the entrance of the bulk amplifier. Using this geometry, a safe nominal operating point is presented with output pulse energies of 3 mJ before and 2.3 mJ after compression and with a pulse duration of 520 fs, corresponding to a peak power of 4.4 GW.

We report on the generation of 34 fs and 50 µJ pulses from a high energy fiber amplifier system with nonlinear compression in an air-filled hypocycloid-core Kagome fiber. The unique properties of such fibers allow bridging the gap between solid core fibers-based and hollow capillary-based post-compression setups, thereby operating with pulse energies obtained with current state-of-the-art fiber systems. The overall transmission of the compression setup is over 70%. Together with Yb-doped fiber amplifier technologies, Kagome fibers therefore appear as a promising tool for efficient generation of pulses with durations below 50 fs, energies ranging from 10 to several hundreds of µJ, and high average powers.

Active coherent beam combination using a 7-non-coupled core, polarization maintaining, air-clad, Yb-doped fiber is demonstrated as a monolithic and compact power-scaling concept for ultrafast fiber lasers. A microlens array matched to the multicore fiber and an active phase controller composed of a spatial light modulator applying a stochastic parallel gradient descent algorithm are utilized to perform coherent combining in the tiled aperture geometry. The mitigation of nonlinear effects at a pulse energy of 8.9 µJ and duration of 860 fs is experimentally verified at a repetition rate of 100 kHz. The experimental combining efficiency results in a far field central lobe carrying 49% of the total power, compared to an ideal value of 76%. This efficiency is primarily limited by group delay differences between cores which is identified as the main drawback of the system. Minimizing these group delay issues, e.g. by using short and straight rod-type multicore fibers, should allow a practical power scaling solution for femtosecond fiber systems.