71 research outputs found
400 kHz repetition rate THz-TDS with 24 mW of average power driven by a compact industrial Yb-laser
We demonstrate a high average power terahertz time-domain (THZ-TDS)
spectrometer based on optical rectification in the tilted-pulse front geometry
in lithium niobate at room temperature, driven by a commercial, industrial
femtosecond-laser operating with flexible repetition rate between 40 kHz - 400
kHz. The driving laser provides a pulse energy of 41 uJ for all repetition
rates, at a pulse duration of 310 fs, allowing us to explore repetition rate
dependent effects in our TDS. At the maximum repetition rate of 400 kHz, up to
16.5 W of average power are available to drive our THz source, resulting in a
maximum of 24 mW of THz average power with a conversion efficiency of ~ 0.15 %
and electric field strength of several tens of kV/cm. At the other available
lower repetition rates, we show that the pulse strength and bandwidth of our
TDS is unchanged, showing that the THz generation is not affected by thermal
effects in this average power region of several tens of watts. The resulting
combination of high electric field strength with flexible and high repetition
rate is very attractive for spectroscopy, in particular since the system is
driven by an industrial, compact laser without the need for external
compressors or other specialized pulse manipulation
Spectral broadening of 2 mJ femtosecond pulses in a compact air-filled convex-concave multi-pass cell
Multi-pass cell (MPC) based temporal pulse compressors have emerged in the
last years as a powerful and versatile solution to the intrinsic issue of long
pulses from Yb-based high-power ultrafast lasers. However, the spectral
broadening of high-energy (typically more than 100 uJ) pulses has only been
realized in complex setups, i.e., in large and costly, pressure-controlled
vacuum chambers to avoid strong focusing, ionization, and damage on the
mirrors. Here, we present spectral broadening of 2 mJ pulses in a simple and
compact (60 cm long) multi-pass cell operated in ambient air. Instead of the
traditional Herriott cell with concave-concave (CC/CC) mirrors, we use a
convex-concave (CX/CC) design, where the beam stays large at all times allowing
both to minimize damage and operate in ambient air. We demonstrate spectral
broadening of 2.1 mJ pulses at 100 kHz repetition rate (200 W of average power)
from 2.1 nm (pulse duration of 670 fs) to a spectral bandwidth of 24.5 nm,
supporting 133 fs pulses with 96% transmission efficiency. We show the
compressibility of these pulses down to 134 fs, and verify that the spectral
homogeneity of the beam is similar to previously reported CC/CC designs. To the
best of our knowledge, this is the first report of a CX/CC MPC compressor,
operated at high pulse energies in air. Because of its simplicity, small
footprint and low cost, we believe this demonstration will have significant
impact in the ultrafast laser community.Comment: The following article has been submitted to Optics Letters. 4 pages,
4 figure
Temperature-Dependent THz Properties and Emission of Organic Crystal BNA
As high-average power ultrafast lasers become increasingly available for
nonlinear conversion, the temperature dependence of the material properties of
nonlinear crystals becomes increasingly relevant. Here, we present
temperature-dependent THz complex refractive index measurements of the organic
crystal BNA over a wide range of temperatures from 300 K down to 80 K for THz
frequencies up to 4 THz for the first time. Our measurements show that whereas
the temperature-dependent refractive index has only minor deviation from room
temperature values, the temperature-dependent absorption coefficient decreases
at low temperature. We additionally compare these measurements with conversion
efficiency and spectra observed during THz generation experiments in the same
temperature range and using the same crystal, using an ultrafast Yb-laser for
excitation. Surprisingly, the damage threshold of the material does not improve
significantly upon cooling, pointing to a nonlinear absorption mechanism being
responsible for damage. However, we observe a significant increase in THz yield
at lower temperatures, which is most likely due to the reduced THz absorption.
These findings will be useful for future designs of high average power pumped
organic-crystal based THz-TDS systems
Broadband THz-TDS with 5.6 mW average power at 540 kHz using organic crystal BNA
We demonstrate efficient optical rectification in the organic crystal BNA
(N-benzyl-2-methyl-4-nitroaniline), driven by a temporally compressed,
commercially available industrial Yb-laser system operating at 540 kHz
repetition rate. Our THz source reaches 5.6 mW of THz average power driven by
4.7 W, 45 fs pulses and the resulting THz-TDS combines a very broad bandwidth
of 7.5 THz and a high dynamic range of 75 dB (in a measurement time of 70 s).
The conversion efficiency at maximum THz power is 0.12%. To the best of our
knowledge, this is the highest THz power so far demonstrated with BNA, achieved
at a high repetition rate, and enabling to demonstrate a unique combination of
bandwidth and dynamic range for THz-spectroscopy applications.Comment: 5 pages, 5 figures. Submitted to APL Photonic
Frequency comb offset dynamics of SESAM modelocked thin disk lasers
We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of SESAM modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M2 value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators. We demonstrate CEO frequency detection with > 30 dB signal-to-noise ratio with a resolution bandwidth of 100 kHz from a SESAM modelocked Yb:YAG TDL delivering 140 W average output power with 748-fs pulses at 7-MHz pulse repetition rate. We compare with a low-power CEO frequency stabilized Yb:CALGO TDL delivering 2.1 W with 77-fs pulses at 65 MHz. For both lasers, we perform a complete noise characterization, measure the relevant transfer functions (TFs) and compare them to theoretical models. The measured TFs are used to determine the propagation of the pump noise step-by-step through the system components. From the noise propagation analysis, we identify the relative intensity noise (RIN) of the pump diode as the main contribution to the CEO frequency noise. The resulting noise levels are not excessive and do not prevent CEO frequency stabilization. More importantly, the laser cavity dynamics are shown to play an essential role in the CEO frequency dynamics. The cavity TFs of the two lasers are very different which explains why at this point a tight CEO frequency lock can be obtained with the Yb:CALGO TDL but not with the Yb:YAG TDL. For CEO stabilization laser cavities should exhibit high damping of the relaxation oscillations by nonlinear intra-cavity elements, for example by operating a SESAM in the roll-over regime. Therefore the optimum SESAM operation point is a tradeoff between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point
50-W average power Ho:YAG SESAM-modelocked thin-disk oscillator at 2.1 um
Ultrafast laser systems operating with high-average power in the wavelength
range from 1.9 um to 3 um are of interest for a wide range of applications for
example in spectroscopy, material processing and as drivers for secondary
sources in the XUV spectral region. In this area, laser systems based on
holmium-doped gain materials directly emitting at 2.1 um have made significant
progress over the past years, however so far only very few results were
demonstrated in power-scalable high-power laser geometries. In particular, the
thin-disk geometry is promising for directly modelocked oscillators with high
average power levels that are comparable to amplifier systems at MHz repetition
rate. In this paper, we demonstrate Semiconductor Saturable Absorber Mirror
(SESAM) modelocked Ho:YAG thin-disk lasers (TDLs) emitting at 2.1 um wavelength
with record-holding performance levels. In our highest average power
configuration, we reach 50 W of average power, with 1.13 ps pulses, 2.11 uJ of
pulse energy and ~1.9 MW of peak power. To the best of our knowledge, this
represents the highest average power, as well as the highest output pulse
energy so far demonstrated from a modelocked laser in the 2 um wavelength
region. This record performance level was enabled by the recent development of
high-power GaSb-based SESAMs with low loss, adapted for high intracavity power
and pulse energy. We also explore the limitations in terms of reaching shorter
pulse durations at high power with this gain material in the disk geometry and
using SESAM modelocking, and present first steps in this direction, with the
demonstration of 30 W of output power, with 692 fs pulses in another laser
configuration.Comment: 12 pages, 10 figure
Low-noise, 2-W average power, 112-fs Kerr-lens mode-locked Ho:CALGO laser at 2.1 um
We report on an in-band pumped soft-aperture Kerr-lens mode-locked Ho:CALGO
bulk laser at 2.1 um, generating 2 W of average power with 112 fs pulses at
91-MHz repetition rate. To the best of our knowledge, this is the highest
average power from a 100-fs class mode-locked laser based on a Tm3+ or Ho3+
doped bulk material. We show that the laser has excellent noise properties with
an integrated relative intensity noise of 0.02% and a timing jitter of 950 fs
(RMS phase noise 0.543 mrad) in the integration interval from 10 Hz to 10 MHz.
The demonstrated combination of high average power, short pulses, and low-noise
make this an outstanding laser source for spectroscopy and many other
applications at 2.1 um.Comment: 4 pages, 6 figure
GHz repetition rate, sub-100-fs Ho:CALGO laser at 2.1 um with watt-level average power
We report on a GHz fundamental repetition rate Kerr-lens mode-locked Ho:CALGO
laser emitting at 2.1 um. The laser employs a ring-cavity to increase the
fundamental repetition rate to 1.179 GHz and can be made to oscillate in both
directions stably with nearly identical performance: for counterclockwise
oscillation, it generates 93-fs pulses at 1.68 W of average power, whereas 92
fs and 1.69 W were measured for clockwise operation. Our current results
represent the highest average power from a 2-um GHz oscillator and the first
sub-100-fs pulse duration from a Ho-based oscillator.Comment: 4 pages, 6 figure
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