Within this work we demonstrate the efficient nonlinear temporal compression of mJ pulses emitted by an ultrafast thulium-doped fiber laser system. For spectral broadening, a krypton and helium filled Herriott-type multi-pass cell with broadband dielectric mirrors is employed. The input pulses with 1,78 mJ and 85 fs are spectrally broadened and subsequently compressed utilizing fused silica plates revealing a pulse duration below 29 fs while featuring an overall transmission of 91%. In addition to the preservation of the input beam quality, the system exhibits a shot-to-shot noise ratio of less than 1.2% as well as an excellent long-term power stability with fluctuations below 1% over a time span of 2 hours.
The presented results demonstrate the advantageous properties of the multi-pass cell approach: High efficiency and high transversal beam quality at high average power, not only for conventional ultrafast ytterbium-based laser systems at 1 µm wavelength, but also in the mid-infrared regime. We believe that this system, delivering an average power above 162 W and sub-5-cycle pulse duration, provides a promising working point for following secondary source experiments like THz- or high harmonic generation.
We present a novel, robust and efficient scheme for nonlinear pulse compression at highest average powers, which is based on self-phase modulation in a bulk nonlinear medium placed inside a multi-pass-cell (MPC). The scheme is suitable for the compression of sub-ps pulses with peak powers exceeding the threshold for catastrophic self-focusing of the nonlinear medium. Experimentally, we compress the output pulses from an Yb:YAG-Innoslab amplifier from 850 fs to <170 fs at 375 W of output power, 37.5 μJ pulse energy and almost diffraction-limited beam quality using fused silica as the nonlinear medium. The efficiency of the compression unit exceeds 90%.
We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this
beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type
pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at
repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is
M2 = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium
Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the
conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into
account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam.
To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we
performed extensive absorption measurements of all crystals used for conversion experiments. These measurements
provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath
Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The
absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior
at both occurring wavelengths.
We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is M² = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam. To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we performed extensive absorption measurements of all crystals used for conversion experiments. These measurements provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior at both occurring wavelengths.
More than 370 W average power at 515 nm were achieved by frequency doubling of sub-picosecond pulses of an
Innoslab based Yb:YAG MOPA system. At 50 MHz repetition rate this corresponds to a pulse energy of 7 μJ in the
green. The pulse duration of 680 fs and the transformation limit of the infrared source could be maintained, thus the laser
provides more than 10 MW pulse power. The beam quality at 280 W was M2 < 1.4, at more than 370 W it was measured
as M2 < 1.7. Extensive numerical modeling was applied to design the conversion stage.
High precision and high throughput material processing using ultrashort pulsed laser radiation of high average power
requires a detailed understanding of the laser matter interaction on ultrafast time scales. In this paper, we will focus on
energy transport mechanisms based on the two-temperature-model and the resulting ablation regimes for single pulses.
Heat accumulation at high pulse repetition rates and spatial pulse overlap will be discussed. Additional, a novel nonthermal
ablation mechanism for graphite and corresponding materials will be presented.
Laser sources of high average power are essential to transfer femtosecond technology to industrial applications. We demonstrate
a compact diode-pumped Yb:KGW femtosecond oscillator-Yb:YAG Innoslab amplifier MOPA with nearly
transform and diffraction limited 636 fs pulses at 620 W average output power and 20 MHz repetition rate. By cascading
two amplifiers an average output power of 1.1 kW and peak power of 80 MW is achieved in a single, linearly polarized
beam. The MOPA is operated at room temperature and no CPA technique is used. The specific properties of Innoslab
MOPAs are compared with fibers and thin-disks.
A flexible ultrafast laser amplifier system based on Ytterbium Innoslab technology with an average power exceeding
200W is presented. The pulse duration of the system can be continuously tuned between 500fs and 6ps, limited only by
the amplification bandwidth of Yb:YAG and the stretcher of the seed source. The repetition rate can be varied from
26.6MHz down to 1MHz. For the ps-regime more than 200μJ and for the fs-regime more than 50μJ are demonstrated
without the need of temporal compression of the high power beam after the amplifier. Spectral bandwidth is close to the
transform limit of the shortest measured pulses. Beam quality is measured to be near the diffraction limit (M2<1.3).
The Innoslab design, already established for neodymium doped laser materials, was applied to ytterbium doped laser
materials. Recent progresses in brightness of high power diode lasers facilitate efficient pumping of quasi-three-level
laser materials. A compact diode-pumped Yb:YAG Innoslab fs-oscillator-amplifier system, scalable to the kilowatt
range, was realized. Nearly transform and diffraction limited 682 fs pulses at 400W average output power and 76 MHz
repetition rate at room temperature and without using CPA technique have been achieved so far.
The self-amplitude modulation of Kerr-lens mode-locked colquiriite lasers was maximized by pumping scheme, resonator geometry and group-delay dispersion. The realized compact, maintenance-free and battery-powered Cr:LiSGaF laser achieves 13.5fs pulse-width and 102mW output power.
Future commercial applications of fs-lasers require compact, reliable beam sources with low effort of operating expense. We report for the first time to our knowledge on a real all solid state master oscillator power amplifier system delivering pulse energies in the range of regenerative Ti:Sapphire amplifier systems (> 100 μJ) with sub 100 fs pulselength. The use of colquiriite crystals requires a pump source of high luminosity in the red spectral range. We realized such a pump source with red diode laser bars. Direct diode pumping and actively controlled thermoelectric cooling of diode bars and laser crystals ensure stable operation of oscillator and regenerative amplifier. For simplicity of the system we investigate stretcher-free CPA exploiting intracavity dispersion of the amplifier resonator. Due to low amplification of colquiriite crystals many roundtrips are required resulting in a stretched seed pulse. For pulse-compression a 6-prism-compressor was designed minimizing third order dispersion. Remaining dispersion is pre-compensated with chirped resonator mirrors. The presented laser system has been used for ablation of ear bones.
Decreasing film thicknesses and sizes of microstructures require an ultraprecise removal of the material and a reduction of the heat-affected zone. For these applications picosecond laser pulses seem to offer new challenges. Because of the short pulse length higher power densities can be reached and rapid heating can possibly lead to an earlier evaporation of the material and to a reduction of the molten zone which resolidifies after the end of the laser pulse at the edges of the processed area. The removal of different materials was investigated using laser pulses with a pulse length of 40 ps produced by a diode-pumped mode-locked Nd:YAG-laser in combination with a regenerative amplifier. The laser radiation was focused to a 7 micrometers spot diameter, yielding power densities up to 5 X 1012 W/cm2. Pump and probe investigations were used to study the interaction of intense ultrashort laser beams with matter. By this technique ultrashort processes with time resolution determined by the pulse length of pump and probe pulses can be photographed. The measurements allow a detailed characterization of the material removal including melting, vaporization and fast resolidification and the feedback of the surrounding atmosphere to the processed microstructures. The single-shot removal threshold fluence and the removal rate per pulse for 40 ps laser pulses and a wavelength of 1064 nm were determined for Si3N4-ceramics. Different materials like metals, semiconductors and ceramics were microstructured by picosecond laser radiation yielding structural dimensions smaller than 20 micrometers .
We report the successful implementation of Gires-Tournois and chirped mirrors in a diode-pumped, Kerr-lens mode-locked Cr:LiSGaF laser. The laser delivered 30 mW of 79 fs, nearly transform limited pulses at 855 nm and 90 MHz repetition rate. The mirror-dispersion controlled cavity is compared to our prism setup and pulse width limitations in diode pumped Cr:LiSGaF/Cr:LiSAF lasers are identified. Mode matching calculations of pump beam and cavity mode are presented to optimize low threshold, highly efficient fs-operation. Following this analysis a compact prismless design of roughly shoe box size is suggested, which incorporates an additional high n2 element to enhance stability.
Picosecond laser pulses were used for microstructuring metals, ceramics and diamond. Using 40 ps laser pulses line patterns with a minimum line width in the range of 7 micrometer were formed in diamond without destroying the crystallinity. In Si3N4-ceramics holes were drilled with diameters smaller than 6 micrometer. Ultrathin copper and gold films of 1 micrometer thickness were removed from fused silica and silicon (100) without influencing the substrates. The ablation process was investigated by high-speed photography and pump-&- probe-measurements with a time resolution of 40 picoseconds. The measurements allow a detailed characterization of the melting, vaporization and solidification processes. Shock waves in the ambient atmosphere were detected and the energy in the shock wave was determined.
The expansion of the laser induced plasma during material processing of Cu, Au, Si, and Si3N4 with ultrashort laser pulses is investigated using streak photography and high speed photography using delayed 35 ps laser pulses. Pulses of a diode-pumped, modelocked Nd:YAG laser which are amplified by a regenerative amplifier having pulse durations of 35 ps and a maximal energy of 0.1 mJ are used. Experiments are performed using both the fundamental wavelength of 1064 nm and by frequency doubling the wavelength of 532 nm. The onset of plasma formation is during the leading edge of the pulse. The spatial expansion of the plasma corresponds to the self-similar motion of a spherical plane wave in a gas. Independent of the processed material in ambient atmosphere the energy content of the shock wave is about one fifth of the optical energy of the laser pulse.
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