Tobias Flöry

Mid-infrared laser filaments in the atmosphere

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 μm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers

We have developed the first (to our knowledge) femtosecond Tm-fiber-laser-pumped Ho:YAG room-temperature chirped pulse amplifier system delivering scalable multimillijoule, multikilohertz pulses with a bandwidth exceeding 12 nm and average power of 15 W. The recompressed 530 fs pulses are suitable for broadband white light generation in transparent solids, which makes the developed source ideal for both pumping and seeding optical parametric amplifiers operating in the mid-IR spectral range.

Post-filament self-trapping of ultrashort laser pulses.

Laser filamentation is understood to be self-channeling of intense ultrashort laser pulses achieved when the self-focusing because of the Kerr nonlinearity is balanced by ionization-induced defocusing. Here, we show that, right behind the ionized region of a laser filament, ultrashort laser pulses can couple into a much longer light channel, where a stable self-guiding spatial mode is sustained by the saturable self-focusing nonlinearity. In the limiting regime of negligibly low ionization, this post-filamentation beam dynamics converges to a large-scale beam self-trapping scenario known since the pioneering work on saturable self-focusing nonlinearities.

Hollow-core-waveguide compression of multi-millijoule CEP-stable 3.2 μm pulses

We present the extension of high-power pulse compression deeper into the challenging IR spectral range around 3.2 μm wavelength, where the effects of material absorption, dispersion, and free electron disturbance on nonlinear propagation become increasingly limiting parameters. 5 mJ, 80 fs pulses from a KTA parametric amplifier were spectrally broadened in a large-core hollow fiber with argon as the nonlinear medium. Subsequent compression through anomalous dispersion in CaF2 yielded 2.5 mJ close-to-transform-limited two-cycle pulses exhibiting a passively stabilized carrier envelope phase (CEP). Furthermore, we outline the feasibility of generating sub-two-cycle pulses with good spatial and temporal characteristics.

Direct carrier-envelope phase control of an amplified laser system

Here we demonstrate a direct method to stabilize the CEP of an Yb:KGW MOP A laser system, resulting in sub-100 mrad stability. To the best of our knowledge, this value surpasses most previously measured CEP jitter for Ti:sapphire CPA systems as well as any previous attempt to stabilize Yb-based oscillator and amplifier systems. This is a crucial step in widening the utility of amplifier femtosecond Yb sources, which are easily scalable in average power and pulse energy but could not be meaningfully employed in attosecond optics until now, given the difficulty of their CEP control.

High peak-power monolithic femtosecond ytterbium fiber chirped pulse amplifier with a spliced-on hollow core fiber compressor

We demonstrate a monolithic Yb-fiber chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor. For an output energy of 77 nJ, 220 fs pulses with 92% of the energy contained in the main pulse, can be obtained with minimal nonlinearities in the system. 135 nJ pulses are obtained with 226 fs duration and 82 percent of the energy in the main pulse. Due to the good dispersion match of the stretcher to the hollow core photonic bandgap fiber compressor, the duration of the output pulses is within 10% of the Fourier limited duration.

Suppression of population-lifetime-determined energy instability in a femtosecond kHz Yb CPA

Energy saturation is achieved by seeding 1-kHz Yb:CaF2 amplifier by μJ pulses from a monolithic Yb-fiber MOPA leading to 6-mJ output. The saturation is possible at any rep-rate with appropriate seed and optical loss levels.

High-energy pulse stacking via regenerative pulse-burst amplification

Here we present a coherent pulse stacking approach for upscaling the energy of a solid-state femtosecond chirped pulse amplifier. We demonstrate pulse splitting into four replicas, amplification in a burst-mode regenerative Yb:CaF2 amplifier, designed to overcome intracavity optical damage by colliding pulse replicas, and coherent combining into a single millijoule level pulse. The thresholds of pulse-burst-induced damage of optical elements are experimentally investigated. The scheme allows achieving an enhancement factor of 2.62 using a single-stage stacker cavity and, potentially, much higher enhancement factors using cascaded stacking.

Generation of a single UV pulse from a near-IR pulse burst

High harmonic generation (HHG) requires high peak-power laser sources. Most of the well-known high peak power lasers are operating in the near-IR wavelength region. Recently it was demonstrated that HHG can be effectively phase matched in the soft X-ray region by using very high intensity UV lasers and multiply charged ions [1]. High average and high peak power UV sources operating around and below 280 nm are required for many other applications, such as ablation in ophthalmology, materials processing and photoelectron spectroscopy. Due to lack of ultrafast high peak power lasers operating in UV, generation of ultrashort UV pulses is possible by up-converting frequency of near-IR laser. This can be done by cascaded harmonic generation in nonlinear crystals with efficiency higher than 40% [2]. However, to obtain high pulse energies in UV region, high energy IR pump is necessary. This becomes increasingly difficult for femtosecond laser pulses because of the optical damage problem in CPA systems. Very high pulse stretching rates in the CPA become unfeasible due to the limited size of dispersive optics. Alternatively, the intensity in the laser cavity can be decreased by using a pulse burst which effectively increases the pulse duration. Therefore, this approach is also suitable for increasing energy throughput in fiber delivery and fiber post compression schemes [3].

High repetition rate fs pulse burst generation using the Vernier effect

We demonstrate pulse burst generation method based on the Vernier effect. The pulse burst with controllable amplitudes and phases is formed using a femtosecond oscillator and regenerative amplifier cavity that have slightly different round trip times. This operation mode can be used for the purposes of coherent pulse stacking, rapid material miroprocessing and rapid scan spectroscopy.