Chirped pulse upconversion for femtosecond mid-infrared spectroscopy at 100 kHz.

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.

Nonlinear post-compression of a hybrid vortex mode in a gas-filled capillary.

We demonstrate nonlinear temporal compression of a vortex beam by propagation in a gas-filled capillary. Starting from an ytterbium-based laser delivering 700 µJ 640 fs pulses at a 100 kHz repetition rate, the vortex beam is generated using a spiral phase plate and coupled to a capillary where it excites a set of four modes that have an overlap integral of 97% with a Laguerre-Gauss LG10 mode. Nonlinear propagation of this hybrid, orbital angular momentum (OAM)-carrying mode results in temporal compression down to 74 fs at the output. Beam and pulse characterizations are carried out to determine the spatial profile and temporal duration of compressed pulses. This result in multimode nonlinear optics paves the way towards the generation of OAM-carrying few-cycle pulses, isolated attosecond XUV pulses, and tunable UV pulses through resonant dispersive wave emission.

Simple carrier-envelope phase control and stabilization scheme for difference frequency generation-based systems.

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.

Raman wavelength conversion in a multipass cell.

Positively chirped femtosecond pulses at 1030 nm are wavelength-converted using spontaneous and stimulated Raman scattering in a potassium gadolinium tungstate crystal inserted inside a multipass cell. Recirculation in the cell and the Raman material allows both a high conversion efficiency and good spatial beam quality for the generated Stokes beams. The converted pulses can be compressed to sub-picosecond duration. Multipass cells could be an appealing alternative to other Raman shifter implementations in terms of thermal effects, control of the Raman cascade, and overall output beam quality.

High-power sub-15 fs nonlinear pulse compression at 515 nm of an ultrafast Yb-doped fiber amplifier.

We present an efficient and robust scheme to produce energetic sub-15 fs pulses centered at 515 nm with a peak power exceeding 3 GW. Combining efficient second-harmonic generation of a 135 fs, 50 W Yb-doped fiber amplifier with a low-loss capillary-based visible pulse compression stage, we reach an overall efficiency higher than >20%. The system is also designed to take advantage of the repetition rate flexibility of the fiber amplifier, leading sub-15 fs pulse generation from 166 to 500 kHz with an average power exceeding the 10 watt level. The combined reduction of the laser wavelength and pulse duration is expected to highly improve the yield of high-order harmonic generation to provide high photon flux of ultrashort extreme ultraviolet radiation.

Spectral compression in a multipass cell.

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.

Simple Yb:YAG femtosecond booster amplifier using divided-pulse amplification.

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.

Supercontinuum-seeded few-cycle mid-infrared OPCPA system.

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.

Nonlinear compression of high energy fiber amplifier pulses in air-filled hypocycloid-core Kagome fiber.

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.

Coherent beam combining with an ultrafast multicore Yb-doped fiber amplifier.

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.

Hybrid high-energy high-power pulsewidth-tunable picosecond source.

A hybrid ytterbium-doped fiber-bulk laser source allowing the generation of 3 ps, 350 µJ, 116 MW peak power Fourier transform-limited pulses at 50 kHz repetition rate and 1030 nm wavelength is described. Pulse duration tunability is provided by an adjustable spectral compression-based seeder system. Energy scaling capabilities of the architecture by use of the divided-pulse amplification method are investigated. This source provides a robust, compact, and versatile solution for applications such as RF photocathode guns, x- and γ-ray generation by inverse Compton scattering, and optical parametric chirped-pulse amplification pumping.

High-energy chirped- and divided-pulse Sagnac femtosecond fiber amplifier.

We report on the generation of 1.1-mJ, 300-fs pulses at 50 kHz by implementing an amplifier architecture whereby four stretched pulse replicas are created in the temporal and spatial domains, allowing pulse energy scaling by the same factor. The whole spatiotemporal coherent combining geometry is passive, avoiding the need for active electronic stabilization loop systems. The combining efficiency remains above 90% at all power levels.

Spectral and spatial full-bandwidth correlation analysis of bulk-generated supercontinuum in the mid-infrared.

We report the measurement of spectral and spatial correlations in supercontinua generated by focusing microjoule pulses from a femtosecond ytterbium-doped fiber amplifier laser in bulk YAG. The measurement is full-bandwidth at a repetition rate of 1 MHz owing to the use of time-stretch dispersive Fourier transform technique. In contrast with fiber-based supercontinuum generation, our results show an excellent stability of the spectral and spatial properties of the output supercontinuum, with an essentially correlated behavior in the 1.4-1.7 µm wavelength range. These results provide strong ground for the development of supercontinuum-seeded ultrafast optical parametric amplifier systems in the mid-infrared using ytterbium lasers as pump sources.

Energy scaling of a nonlinear compression setup using passive coherent combining.

Passive spatial and temporal coherent combining schemes are implemented to scale the output energy of a nonlinear temporal compression setup. By generating 32 replicas of the incident femtosecond pulses, the output of a high-energy fiber chirped-pulse amplifier can be compressed using self-phase modulation in a large-mode-area rod-type fiber at peak-power levels well beyond the self-focusing power. We demonstrate the generation of 71 fs 7.5 µJ pulses at 100 kHz repetition rate, corresponding to a peak power of 86 MW.

Femtosecond fiber chirped- and divided-pulse amplification system.

We implement both chirped pulse amplification and divided pulse amplification in the same femtosecond fiber amplifier setup. This scheme allows an equivalent stretched pulse duration of 2.4 ns in a compact tabletop system. The generation of 77 W of compressed average power at 4.8 MHz, together with 320 fs and 430 µJ pulses at a repetition rate of 96 kHz, is demonstrated using a distributed mode-filtering, rod-type, ytterbium-doped fiber. Limitations in the temporal recombining efficiency due to gain saturation inside the fiber amplifier are identified.

Two-channel pulse synthesis to overcome gain narrowing in femtosecond fiber amplifiers.

We demonstrate spectral coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using active phase stabilization based on an electro-optic phase modulator, an average power of 10 W before compression and a high gain factor of 30 dB are obtained. At this gain value, 130 fs pulses with a spectral width of 19 nm can be generated, highlighting the strong potential of pulse synthesis for the reduction of the minimum duration of ultrashort pulses in fiber chirped-pulse amplifiers.