In this paper we present the next step on the roadmap “system scalability towards an output power above 100 kW”, first
time presented in 2014 .
To take a step forward the optical power of the fiber-coupled diode laser has been increased beyond the power level
40kW. The power conversion efficiency exceeds 40%. The laser contains modules with 4 different wavelengths (960nm,
1020nm, 1040nm, 1060nm) there are two modules for each wavelength polarization multiplexed. After the slow-axis
collimation these wavelengths are combined using dense wavelength coupling before focusing onto the fiber endface.
The delivery-fiber is an uncoated fiber with a diameter of 2 mm and NA 0.22 corresponding a BPP of 220 mm mrad.
In a stability test the laser delivered a constant maximum output power with less than ±0.5 % variation over 100h.
Further results of the optical properties of the laser will be presented in this paper.
This new laser is based on a turn-key industrial platform, allowing straight-forward integration into almost any industrial
application, like welding or large area heat treatment. As application examples laser welding of thick sheet metal and
pumping of an active fiber will be presented. The footprint of the complete system is 2.8 m² with a height below 1.8 m.
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.
The passive-alignment-packaging technique presented in this work provides a method for mounting tolerance-insensitive
optical components e.g. non-linear crystals by means of mechanical stops. The requested tolerances for the angle
deviation are ±100 μrad and for the position tolerance ±100 μm. Only the angle tolerances were investigated, because
they are more critical. The measurements were carried out with an autocollimator. Fused silica components were used
for test series. A solder investigation was carried out. Different types of solder were tested. Due to good solderability on
air and low induced stress in optical components, Sn based solders were indicated as the most suitable solders. In
addition several concepts of reflow soldering configuration were realized. In the first iteration a system with only the
alignment of the yaw angle was implemented. The deviation for all materials after the thermal and mechanical cycling
was within the tolerances. The solderability of BBO and LBO crystals was investigated and concepts for mounting were
There is an ongoing and increasing interest in using expansion matched micro-channel heat sinks for high-power diode
laser bars. In this new approach the heat sinks are produced by μ-metal injection molding (μ-MIM). Unlike conventional
heat sinks which are made of copper, these particular heat sinks are made of copper-tungsten because it combines two of
low coefficient of thermal expansion (CTE) and reasonable thermal conductivity. Manufacturing heat sinks with the μ-MIM process allows for an economic mass production of complex micro near net shape parts. Especially when
manufacturing over 10,000 parts with the μ-MIM process, manufacturing cost per part reduce considerably. The main
goal is to use the opportunities μ-MIM offers. That means producing complex parts, which have a matched CTE in this
case to gallium arsenide (GaAs). Therefore a material which combines high thermal conductivity behavior with a low
coefficient of thermal expansion is needed. An additional advantage of the μ-MIM process is that the needed green
bodies of the heat sink can be joint together in a co-sintering process.
In this paper the current status of production of heat sinks with microstructured surfaces by μ-MIM for thermal
management applications are presented. The range of operation and the limitations are outlined with special concern
regarding the materials and the minimal structure size. The implication and advantages of using ultrafine powders are
emphasized. Therefore sintering behavior, microstructure of sintered parts and characteristic properties as density, CTE,
thermal conductivity and electro-optical characterization are shown identified.
The green cw laser presented in this work is realized by means of a Pr:YLF crystal emitting at 523 nm that is pumped by
a blue GaN laser diode in an extremely short resonator. With a 500 mW-diode a laser has been achieved with M<sup>2</sup> = 1, a
slope of 40 % and an output power of 140mW with an absorbed pump power of 410 mW which results in an electrooptical
efficiency of 6.5 %. Despite the reduced overlap with a 1 W-diode the output power rises to 290 mW with an
absorbed pump power of 850 mW and the M<sup>2</sup> increases only slightly. Based on these results a compact laser package has
been accomplished using a monolithic micro optics for the beam shaping of the diode light and joining all components
with a low-shrinkage adhesive on a common base plate. In a first test of the alignment strategy a laser with an output
power of 92 mW has been achieved by means of the 500 mW pump power.
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.
High power diode laser bars require packages with a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the thermal resistance of the packaging becomes even more important. It is the key information about the cooling efficiency of a package and in particular of the heat sink. Besides the heat sink the thermal resistance depends also on the solder interface, packaging process, and bar structure such as fill factor and resonator length.
This work presents a thermal comparison of different packaging types and laser bar designs. Different package types are experimentally measured and analyzed by numerical calculations to obtain information about the influence of the different parameters: Conductively cooled and water cooled copper heat sinks as well as a new type of expansion matched micro-channel heat sink made out of Cu-AlN sandwich are investigated. In addition to the different packages, laser bars with different resonator lengths are mounted and analyzed regarding their thermal behavior; the dependency of the thermal resistance on the resonator length is a particular interest of the investigation. In parallel to the experiments thermal simulations of the same packages and laser bar geometries are performed. The boundary conditions chosen in the simulations are comparable to the experimental values and the same parameters are varied.
The relations between theoretical and experimental results are presented. The analysis shows the influencing factors, so that the optimum package can be chosen for a specific application.
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.
The lifetime of high-power diode lasers, which are cooled by standard copper heatsinks, is limited. The reasons are the aging of the indium solder normally employed as well as the mechanical stress caused by the mismatch between the copper heatsink (16 - 17ppm/K) and the GaAs diode laser bars (6 - 7.5 ppm/K). For micro - channel heatsinks corrosion and erosion of the micro channels limit the lifetime additionally. The different thermal behavior and the resulting stress cannot be compensated totally by the solder. Expansion matched heatsink materials like tungsten-copper or aluminum nitride reduce this stress. A further possible solution is a combination of copper and molybdenum layers, but all these materials have a high thermal resistance in common. For high-power electronic or low cost medical applications novel materials like copper/carbon compound, compound
diamond or high-conductivity ceramics were developed during recent years. Based on these novel materials, passively cooled heatsinks are designed, and thermal and mechanical simulations are performed to check their properties. The expansion of the heatsink and the induced mechanical stress between laser bar and heatsink are the main tasks for the simulations. A comparison of the simulation with experimental results for different material combinations illustrates the advantages and disadvantages of the different approaches. Together with the boundary conditions the ideal applications for packaging with these materials are defined. The goal of the development of passively-cooled expansion-matched heatsinks has to be a long-term reliability of several 10.000h and a thermal resistance below 1 K/W.