Robert Fleischhaker

Group velocity control in the ultraviolet domain via interacting dark-state resonances

The propagation of a weak probe field in a laser-driven four-level atomic system is investigated. We choose mercury as our model system, where the probe transition is in the ultraviolet region. A high-resolution peak appears in the optical spectra due to the presence of interacting dark resonances. We show that this narrow peak leads to the superluminal light propagation with strong absorption, and thus by itself is only of limited interest. But if in addition a weak incoherent pump field is applied to the probe transition, then the peak structure can be changed such that both sub- and superluminal light propagation or a negative group velocity can be achieved without absorption, controlled by the incoherent pumping strength. A suitable choice of laser propagation directions allows us to preserve these results under Doppler averaging.

Nonlinear effects in pulse propagation through Doppler-broadened closed-loop atomic media

Nonlinear effects in pulse propagation through a medium consisting of four-level double-

Compact gigawatt-class sub-picosecond Yb:YAG thin-disk regenerative chirped-pulse amplifier with high average power at up to 800 kHz

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Non-infrared femtosecond lasers: status and prospects

-type systems are studied theoretically. We apply three continous-wave driving fields and a pulsed probe field such that they form a closed interaction loop. Due to the closed loop and the finite frequency width of the probe pulses the multiphoton resonance condition cannot be fulfilled, such that a time-dependent analysis is required. By identifying the different underlying physical processes we determine the parts of the solution relevant to calculate the linear and nonlinear response of the system. We find that the system can exhibit a strong intensity dependent refractive index with small absorption over a range of several natural linewidths. For a realistic example we include Doppler and pressure broadening and calculate the nonlinear selfphase modulation in a gas cell with Sodium vapor and Argon buffer gas. We find that a selfphase modulation of

Hollow core fiber delivery of sub-ps pulses from a TruMicro 5000 Femto edition thin disk amplifier

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Industrial grade fiber-coupled laser systems delivering ultrashort high-power pulses for micromachining

is achieved after a propagation of few centimeters through the medium while the absorption in the corresponding spectral range is small.

Industry-grade high average power femtosecond light source

The paper reports on a sub-picosecond Yb:YAG thin-disk regenerative chirped-pulse amplifier , reaching gigawatt intensities and up to 160 W average power with a simplified and much more compact laser system (laser head dimensions 100 cm x 60 cm).

A Maxwell–Schrödinger solver for quantum optical few-level systems ☆

The unique properties of ultrafast laser pulses pave the way to numerous novel applications. Particularly lasers in the sub-pico second regime, i.e. femtosecond lasers, in the last decade arrived at a level of reliability suitable for the industrial environment and now gain an increasing recognition since these pulse durations combine the advantages of precise ablation with higher efficiency especially in the case of processing metallic materials. However, for some micro processing applications the infrared wavelength of these lasers is still a limiting factor. Thus, to further broaden the range of possible applications, industrial femtosecond lasers should combine the advantages of femtosecond pulses and shorter wavelengths. To that extend, we present results obtained with a frequency doubled TruMicro 5000 FemtoEdition. We show that depending on the processed material, the higher photon energy as well as tighter focusing options of the shorter wavelength can open up a new regime of processing parameters. This regime is not accessible by infrared light, leading to a wider range of possible applications.

Four-wave mixing enhanced white-light cavity

We report on the fiber-based transmission of sub-ps single-mode pulses with an average power of 50 W at a wavelength of 1030 nm generated by a TruMicro Series 5000 Femto Edition thin disk amplifier. The air-filled hollow-core Kagométype delivery fiber exhibits a hypocycloid core wall and is tailored to offer very low dispersion and nonlinearity at 1030 nm. It minimizes the mode overlap with the glass components to obtain a sufficiently high damage threshold. With propagation losses of only 20 dB/km and an optimized mode matching and coupling by means of a telescope and a 5- axes table we achieve an overall transmission efficiency of more than 80% with a resulting M2 of 1.15. Our laser source offers the selection of repetition rates from 200 to 800 kHz which translates to pulse energies between 60 and 250 μJ. The pulse duration of 900 fs is maintained at the fiber exit, while the spectral width broadens to 20 nm due to self phase modulation in the air core, which could be used to further compress the pulses temporally. Using a fiber-based beam transport allows for mechanical decoupling of the processing head from the laser source, increasing flexibility for applications in the field of material processing with ultra-short pulsed lasers.

Phase-controlled pulse propagation in media with cross coupling of electric and magnetic probe field component

We report on an industrial fiber-delivered laser system producing ultra-short pulses in the range of a few picoseconds down to a few hundred femtoseconds with high average power suitable for high-precision micromachining. The delivery fiber is a hollow-core photonic crystal fiber with a Kagomé shaped lattice and a hypocycloid core wall enabling the guiding of laser radiation over several meters with exceptionally low losses and preservation of high beam quality (M2<1.3). The mode-matching and coupling optics are integrated into the laser head providing a compact footprint without the need for external boxes. The laser head is carefully designed regarding its thermo-mechanical properties to allow a highly reliable coupling stability. The exchangeable delivery fiber is packaged using Trumpf’s well established LLK-D connectors which offer a very high mechanical precision, the possibility to add water cooling, as well as full featured safety functions. The fiber is hermetically sealed and protected by a robust but flexible shield providing bend protection and break detection. We show the linear and nonlinear optical properties of the transported laser radiation and discuss its feasibility for pulse compression. Measurements are supported by simulation of pulse propagation by solving the nonlinear Schrödinger equation implementing the split-step Fourier method. In addition, mode properties are measured and confirmed by finite element method simulations. The presented industrial laser system offers the known advantages of ultra-short pulses combined with the flexibility of fiber delivery yielding a versatile tool perfectly suitable for all kinds of industrial micromachining applications.