In this work the reduction of conversion efficiency due to spectral bandwidth of fiber laser radiation is investigated.
Subsequently, compensation optics to correct the spectral phase mismatching inside the nonlinear crystal is dimensioned
and tested. For the experimental study a laboratory fiber laser setup is used consisting of a seed diode and a three stage
fiber amplifier. The laser delivers an average output power of up to 100 W at 1 MHz. Even below the Raman threshold
the output is far away from Fourier limit, providing a nearly Lorentzian spectral shape and a temporal pulse width of
800 ps. As the bandwidth increases nearly linearly with the pump power of the third amplifier stage, this parameter could
be controlled for the experiments.
All conversion experiments are conducted with a moderate load of the nonlinear crystals, i.e. intensity less than
150 MW/cm2. Without compensation of the spectral phase mismatch, a maximum conversion efficiency of 15 % is
attained for a Type I configuration with a 20mm long LBO crystal. Using the compensation setup 27 W of green light are
obtained from 60 W infrared light at a bandwidth of 4.7 nm. Therefore the efficiency rises to 44% at the same load.
Diode lasers are frequently used for numerous applications demanding high cw average power in the kW region and
comparably low brightness. These applications include polymer welding, transformation hardening of metals and
medical applications. Compared to solid state lasers, diode lasers can not be efficiently q-switched due to their low
upper state life time. Therefore diode lasers are usually not suited for applications requiring high peak power like
marking and coating removal. To overcome this problem, we have developed a novel electrically pulsed diode laser
source. If the pulses are comparably short in the region of a few hundred nanoseconds, diode lasers can be operated with
a current five to ten times higher than the maximum cw current. This so called super-pulse mode of operation broadens
the field of applications of high power diode lasers towards applications usually reserved for q-switched solid state
lasers. To benefit from the improved brightness delivered by the super-pulsed diode lasers for materials processing, a
state of the art beam forming optics is required. In this paper, we will demonstrate the design of a super-pulsed diode
laser source consisting of four diode laser bars coupled into a 100 μm NA=0.2 optical fiber. This module is designed for
an output power of 500 W. To select diode laser bars appropriate for the super-pulse mode of operation, different diode
laser bars have been tested with peak currents up to ten times higher than the rated cw current. Material processing
results with super-pulsed diode lasers will be presented.
To realize a completely monolithic, pulsed, fiber laser without free space elements we describe a gain-switched
fiber laser pumped with a pulsed diode laser at about 965 nm with more than 30 μJ in a 200 ns pulse. For
best beam quality we use a single mode fiber with a 6 μm core diameter. We report lasing of an Yb-doped
double-clad fiber at 1080 nm and a pulse energy of about 8 μJ with a variable repetition rate from 1 - 50 kHz.
The experimental results are compared with the data of a time resolved simulation and basic analytically derived
High power fiber lasers deliver multiple kW laser power with a diffraction limited beam quality. A drawback for
some applications is the arbitrary polarization. We report on experimental and theoretical results of kW class
cw fiber lasers with linear polarization. A comparison of different concepts for generation of polarized high power
output will be presented. The most feasible design for kW class power scaling will be selected. In order to find
an empirical formula for calculating bend loss, additional measurements are carried out and are then compared
to the theoretical results.
An analytical approach for the thermal design of high-power-fiber laser components is presented. The modular structure
of the model allows adaption to different fiber designs and gives insight into the governing parameters of the heat
transport. Furthermore the analysis and analytic optimization of interacting effects of groups of layers is possible with
this method and is presented in this work. A previously suggested cooling scheme for a heat load of 120 W/m is
analyzed. Applying the analysis to air-clad-fibers is leading to results differing up 40 % from previous works. The FEM-analysis
of the cooling of splices shows that the cooling scheme suggested for the active fiber is not sufficient for splices
for a fiber resonator in the kW-range. Using a one-dimensional model it can be shown that if a small percentage of loss
in the splice is absorbed inside the recoat, it is necessary to reduce the recoat thickness.
Pumping fiber lasers is the driving force for the development of high brightness, mid power, passively cooled, fiber
coupled diode lasers. We compare concepts for providing 50 W in a 100 micron fiber at the optimum fiber laser pump
wavelength of 976 nm. The set up is experimentally demonstrated and compared to the optical analysis.
Three basic diode laser concepts are included into this comparison: single emitters, high density emitter arrays and low
density emitter arrays on bars.
Low density stacking in the horizontal direction with increasing the filling factor by a microlens array is the first concept.
For this concept two diode bars with low filling factor are fast and slow axis collimated. Beam transformation, shaping
and focusing are similar to the second concept.
In the second concept a diode laser array with high filling factor is regarded. An 800 μm diode laser bar consists of an
array of four or five emitters. Two bars are polarization coupled and collimated with single lenses. Beam symmetrization
is performed by the well known step mirror. A simple anamorphotic optic enables beam shaping and fiber coupling.
The third one, single emitters, represents optical beam combining of laser diodes that are high density stacked in the
vertical direction. Five emitters are placed in an optical stack, each one collimated with its own lens. Two optical stacks
are polarization coupled and focused on the fiber end. The three concepts are compared in terms of power efficiency and
complexity, and the results of prototype systems are presented.
Design, theoretical modeling, and experimental characterization of a widely tunable Ti:Sapphire laser with nanosecond pulses and high pulse peak power is presented. The laser provides a continuous tuning range of from 675 nm to 1025 nm with no exchange of optics required. At a pulse rate of one kilohertz it delivers pulse energies of up to 2.5 mJ, pulse durations of around 20 ns, a spectral bandwidth of 10GHz and an almost diffraction-limited beam quality of M<sup>2</sup><1.2 with a smooth characteristic of these parameters over the full wavelength range. This clearly exceeds the performance data published so far with our previous designs. Effects, which tent to provoke spectral gaps in the past, are totally understood and definitely suppressed by a modified resonator design. The presentation contains a detailed description and discussion of performance determining design aspects, i.e. pump scheme and pump beam shaping, resonator design and the comparison of different tuning elements. As a main prerequisite of an appropriate resonator design thermal lensing in Ti:Sapphire crystals is discussed on the basis of experimental and theoretical results. This includes the wavelength dependency of the focal length, the astigmatism in end-pumped Ti:Sapphire crystals with Brewster-cut end faces, the influence of the pump-light distribution and different cooling schemes.