We present a laser source providing up to 18 W and 1.5 mJ at a wavelength of 3 μm. The output is generated by frequency conversion of randomly polarized multimode radiation at 1064 nm of an Nd:YAG laser in a two-stage conversion setup. The frequency converter comprises an optical parametric oscillator and a subsequent optical parametric amplifier using PPLN as nonlinear medium in both stages. To implement fiber-based beam delivery for materials processing, we coupled the output at 3 μm to a multimode ZrF<sub>4</sub>-fiber. This source was then used to remove epoxy resin from the surface of CFRP samples.
Laser radiation of 3 μm wavelength was generated by frequency conversion of an industrial IR laser and applied in the context of CFRP bonding pre-treatment. Reinforced and non-reinforced epoxy resins were treated with this radiation varying the relevant parameters such as laser power or treatment time. The interaction between laser radiation of 3012 nm and 1064 nm wavelength and matrix resin was analyzed mechanically (e.g. ablation depth), optically (such as fiber exposure) and chemically (e.g. contamination removal). The results gathered show that, even with the small achievable pulse fluences, a sufficient treatment of the specimens and a sensitive removing of the contaminated layers are possible.
The active alignment of fast axis collimator lenses (FAC) is the most challenging part in the manufacturing process of optical systems based on high power diode laser bars. This is due to the high positioning accuracy in up to 5 degrees of freedom and the complex relations between FAC misalignment and properties of the resulting power density distribution. In this paper an experimental approach for FAC alignment automation is presented. The alignment algorithm is derived from a beam propagation model based on wave optics. The model delivers explicit relations between FAC misalignment and properties of the distorted power density distribution in the near and far field. The model allows to calculate the FAC misalignments and to correct them in one or multiple steps. The alignment algorithm is tested with a demonstrator system. The demonstrator contains an optical system which allows a real time analysis of the near field and far field power distribution of individual emitters. For the tests two different types of FAC lenses and high power diode laser bars are used. The FAC lenses are prealigned within a range of ±50 μm and 0.5 degree around the suitable position. During the automated alignment process the translational and rotational remaining misalignment and the properties of the far field power density distribution are recorded. The experimental results are evaluated regarding reliability and flexibility of the presented FAC alignment algorithm.
We present a compact High-Power DenseWavelength Division Multiplexer (HP-DWDM) based on Volume Bragg Gratings (VBGs) for spectrally stabilized diode lasers with a low average beam quality <i>M<sup>2</sup></i> ≤50. The center wavelengths of the five input channels with a spectral spacing of 1.5 nm are 973 nm, 974.5 nm, 976 nm, 977.5 nm and 979 nm. Multiplexing efficiencies of 97%±2% have been demonstrated with single mode, frequency stabilized laser radiation. Since the diffraction efficiency strongly depends on the beam quality, the multiplexing efficiency decreases to 94% (<i>M<sup>2</sup></i> = 25) and 85%±3% (<i>M<sup>2</sup></i> = 45) if multimode radiation is overlaid. Besides, the calculated multiplexing efficiency of the radiation with <i>M<sup>2</sup></i> = 45 amounts to 87:5 %. Thus, calculations and measurements are in good agreement. In addition, we developed a dynamic temperature control for the multiplexing VBGs which adapts the Bragg wavelengths to the diode laser center wavelengths. In short, the prototype with a radiance of 70GWm<sup>-2</sup> sr<sup>-1</sup> consists of five spectrally stabilized and passively cooled diode laser bars with 40Woutput after beam transformation. To achieve a good stabilization performance ELOD (Extreme LOw Divergence) diode laser bars have been chosen in combination with an external resonator based on VBGs. As a result, the spectral width defined by 95% power inclusion is < 120pm for each beam source across the entire operating range from 30 A to 120 A. Due to the spectral stabilization, the output power of each bar decreases in the range of < 5 %.
In this paper we present the development of a compact, thermo-optically stable and vibration and mechanical shock
resistant mounting technique by soldering of optical components. Based on this technique a new generation of laser
sources for aerospace applications is designed. In these laser systems solder technique replaces the glued and bolted
connections between optical component, mount and base plate. Alignment precision in the arc second range and
realization of long term stability of every single part in the laser system is the main challenge.
At the Fraunhofer Institute for Laser Technology ILT a soldering and mounting technique has been developed for high
precision packaging. The specified environmental boundary conditions (e.g. a temperature range of -40 °C to +50 °C)
and the required degrees of freedom for the alignment of the components have been taken into account for this technique.
In general the advantage of soldering compared to gluing is that there is no outgassing. In addition no flux is needed in
our special process. The joining process allows multiple alignments by remelting the solder. The alignment is done in the
liquid phase of the solder by a 6 axis manipulator with a step width in the nm range and a tilt in the arc second range. In a
next step the optical components have to pass the environmental tests. The total misalignment of the component to its
adapter after the thermal cycle tests is less than 10 arc seconds. The mechanical stability tests regarding shear, vibration
and shock behavior are well within the requirements.
In this work, we present high-wall plug efficiency (WPE) diode lasers at 975 nm, which are based on an Al-free active
region. On a 2 mm x 100 μm laser, we have obtained a high maximum wall-plug efficiency of 69% at 10°C CW. Based
on the same structure, we have realised a 1-cm bar, mounted on an active submount, and which delivers 70 W CW,
together with 67% wall-plug efficiency. By improving the laser structure, we have obtained a higher WPE of 70% on an
uncoated 2 mm x 100 μm broad area laser. We also present a new structure with a reduced fast-axis far-field of only 34°
The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the
increasing output power of the diode laser bars the cooling performance of the packaging becomes more important.
Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications.
The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications
where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high
All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts
can be neglected.
N-contact cooling is typically used to achieve new records of optical output power in the labs.
These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on
both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets.
Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are
investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also.
The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and
differences of different package styles in comparison to bar geometries variations. The second part of the studies
characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced
stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are
The reliability of high-power diode laser bars is limited by the thermo-mechanical stress occurring during the packaging
process and operation. The stress is caused by the mismatch of the thermal expansion coefficients between heat sink and
laser bar. In general the stress influence grows with the bar size. The development of tapered laser bars leads to higher
cavity lengths so the thermo-mechanical stress in the longitudinal direction becomes more important. In this work the
packaging influences on different sized laser bars are compared. At first thermal and thermo-mechanical influences are
evaluated in FEM-simulations. Afterwards laser bars of different lengths and widths are mounted and characterized. The
occurring strain is analyzed by electroluminescence using the correlation between stress and polarization properties of
the laser bar radiation. Because of the correlation between temperature and wavelength, a thermal analysis of the
mounted laser bars can be done by emitter resolved spectra scanning. The influence on reliability is analyzed in an aging
study with intermediate characterization steps.
Laser diodes and diode laser bars are key components in high power semiconductor lasers and solid state laser systems.
During manufacture, the assembly of the fast axis collimation (FAC) lens is a crucial step. The goal of our activities is to
design an automated assembly system for high volume production. In this paper the results of an intermediate milestone
will be reported: a demonstration system was designed, realized and tested to prove the feasibility of all of the system
components and process features. The demonstration system consists of a high precision handling system, metrology for
process feedback, a powerful digital image processing system and tooling for glue dispensing, UV curing and laser
operation. The system components as well as their interaction with each other were tested in an experimental system in
order to glean design knowledge for the fully automated assembly system. The adjustment of the FAC lens is performed
by a series of predefined steps monitored by two cameras concurrently imaging the far field and the near field intensity
distributions. Feedback from these cameras processed by a powerful and efficient image processing algorithm control a
five axis precision motion system to optimize the fast axis collimation of the laser beam. Automated cementing of the
FAC to the diode bar completes the process. The presentation will show the system concept, the algorithm of the
adjustment as well as experimental results. A critical discussion of the results will close the talk.
Thermo-mechanical stress occurring during the packaging process and during operation limits the reliability of high-power
diode laser bars. The stress is caused by the mismatch of the thermal expansion coefficients between the heat sink and laser bar material. A soft solder layer can partially reduce the stress by relaxation. A convenient approach for reducing the stress is the matching of the thermal expansion of the heat sink to the laser bar material. The disadvantage of most expansion-matched heat sinks is a higher thermal resistance so that the device temperature increases and the
lifetime decreases. For the development of thermal and strain optimized diode laser packages an analysis of both the thermal and strain distribution is reasonable. In this work the strain is analyzed by electroluminescence using the correlation between stress and the polarization properties of the laser bar radiation. This method allows a qualitative emitter resolved strain mapping along the slow-axis. Because of the correlation between temperature and wavelength a thermal analysis of mounted laser bars can be done by
an emitter resolved spectral mapping. Irregularities in the thermal contact between laser bar and heat sink such as defects
in the solder layer become visible by irregular emitter spectra.
The work shows examples for the optimization of the package. The analysis of the thermal and strain distribution shows the advantages and disadvantages of the particular approaches, like variations of solder thickness or expansion matched packages.
The field of applications for diode laser bars is growing continuously. The reasons for this are the growing width of
available wavelengths and the increasing optical output power. In parallel to this the requirements for packaging for the
high power diode laser bars increase and are more manifold. Expansion matched, non corrosive, non erosive, low
thermal resistance and high thermal conductivity are some of the keywords for the packaging in the near future.
Depending on the thermal power density, two different types of heat sinks are used: active and passive. The active heat
sinks can further be subdivided in micro- or macro-channel heat sinks.
The development of macro-channel heat sinks was necessary because of the limited lifetime of the common micro-channel
heatsink. The bigger channels reduce especially erosion and corrosion effects. By taking the increasing
resonator length of the laser bars into account the cooling performance of the macro-channel heatsink will be sufficient
for many applications. In cases of high thermal power densities there are still no alternatives to micro-channel heat
sinks. New material combinations shall minimize the erosion and corrosion effects.
New raw materials such as diamond composite materials with a higher thermal conductivity than copper and matched
thermal expansion will find their working field at first in the passively cooling of laser bars. The next generation of
active heat sinks will also be partly made out of the high performance materials. The point of time for this improvement
depends on machining behavior, availability and price of the raw material.