The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems
emitting ultrashort pulses. In this contribution the experimental results of a coherently combined femtosecond fiber CPA
laser system consisting of 2 amplifiers is presented. Combining efficiencies as high as 89% and compressed pulse
energies of up to 3mJ were achieved. Additionally, the system showed excellent long-term stability.
Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a
coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the
nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical
path length difference between the pulses have to be considered. In this paper the impact of these effects on the
combining process has been investigated and simple analytical equations for the evaluation of this impact have been
obtained. These formulas provide design guidelines for laser systems using coherent combining.
We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses
from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently
combined after amplification. The splitting and combination process is based on the polarization combining technique
using polarization cubes. A Hansch-Couillaud detector measures the polarization state of the combined beam. The error
signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a
piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in
a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were
carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to
all ultrafast amplification schemes independent of the architecture of the gain medium.
We experimentally demonstrate phase-shaping in fiber CPA-systems, providing pulse-energies at the mJ-level. The
applied method is based on an analytical model describing the impact of SPM in CPA-systems. Using this phase-shaping
technique nearly transform limited pulses are produced at B-integrals up to 10 rad. Compared to a nonlinear CPAsystem
with the best performance being achieved by adjusting the compressor, operation of the same system using the
phase-shaping method permits peak-power enhancement by a factor better than 2.
We report on the generation of 830 W compressed average power at 78 MHz pulse repetition frequency and 640 fs pulse
duration. We discuss further power scaling including the issue of transversal spatial hole burning. Therefore, we
describe a low-nonlinearity fiber design capable of producing fundamental mode radiation at ultra high average powers
from short length (range of 1m) and large mode field diameter (>50μm) fibers. In conventional large mode area fiber
most of the core is typically uniformly doped. As a consequence gain factors for the fundamental mode and the next
higher order modes are comparable. Furthermore, the fundamental mode extracts inversion only in the central part of the
core according to its intensity profile, leading at high pump and signal power levels to high and unused inversion density
with a strong overlap with higher order transversal modes. In experiments this leads to a threshold-like onset of mode
instability, originating from mode competition. Finally, this effect avoids further power scaling. The presented fiber
features an optimized doping profile to prefer the amplification of the fundamental mode. In addition non-extracted
inversion is minimized avoiding the issue of transversal spatial hole burning. As a consequence ultrafast fiber laser
systems with novel performance are in reach, i.e. systems delivering simultaneously >1GW peak power and >1kW
average power. In a first iteration a ROD-type fiber with 60μm MFD and 1.7m length was used in a CPA system to
produce pump power limited 355 W of average power at 1 MHz.
In this work we present a new method for peak-power scaling in nonlinear CPA-systems. By clipping the tails of the
spectrum we demonstrate pulse quality enhancement and an increase of peak-power at the output of the CPA-system. A
theoretical model allows us to determine an optimal ratio between the spectral clipping bandwidth and the pulse
bandwidth at a certain B-integral. Additionally, a simple redesign of a grating based stretcher unit, applying our new
spectral clipping technique, would significantly increase the output peak-power of such nonlinear CPA-systems by a
factor up to six due to the higher stretching ratio.
We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered
during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system,
the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only
pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is
experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing
threshold can be enhanced.
We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate
corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.
Pulse-contrast degradation at the CPA-system's output is analyzed. If Kerr-nonlinearity is present, weak initial spectral
phase and amplitude modulations are responsible for the decrease. The pulse is split into several sub-pulses. Bessel-functions
describe the intensities of the side-pulses relative to the principal pulse. We provide the governing physical quantities.