Continuous cost reduction, improved reliability and modular platform guide the design of our next generation heatsink
and packaging process. Power scaling from a single device effectively lowers the cost, while electrical insulation of the
heatsink, low junction temperature and hard solder enable high reliability. We report on the latest results for scaling the
output power of bars for optical pumping and materials processing. The epitaxial design and geometric structures are
specific for the application, while packaging with minimum thermal impedance, low stress and low smile are generic
features. The isolated heatsink shows a thermal impedance of 0.2 K/W and the maximum output power is limited by the
requirement of a junction temperature of less than 68oC for high reliability. Low contact impedance are addressed for
drive currents of 300 A. For pumping applications, bars with a fill factor of 60% are deployed emitting more than 300 W
of output power with an efficiency of about 55% and 8 bars are arranged in a compact pump module emitting 2 kW of
collimated power suitable for pumping disk lasers. For direct applications we target coupling kilowatts of output powers
into fibers of 100 μm diameter with 0.1 NA based on dense wavelength multiplexing. Low fill factor bars with large
optical waveguide and specialized coating also emit 300 W.
High energy solid state lasers are being developed for fusion experiments and other research applications where high energy per pulse is required but the repetition rate is rather low, around 10Hz. We report our results on high peak power diode laser stacks used as optical pumps for these lasers. The stacks are based on 10 mm bars with 4 mm cavity length and 55% fill factor, with peak power exceeding 500 W per bar. These bars are stacked and mounted on a cooler which provides backside cooling and electrical insulation. Currently we mount 25 bars per cooler for a nominal peak power of 12.5 kW, but in principle the mounting scheme can be scaled to a different number of devices depending on the application. Pretesting of these bars before soldering on the cooler enables us to select devices with similar wavelength and thus we maintain tight control of the spectral width (FWHM less than 6 nm). Fine adjustments of the centroid wavelength can be done by means of temperature of the cooling fluid or bias current. The available wavelength range spans from 880 nm to 1000 nm, and the wavelength of the entire assembly of stacks can be controlled to within 0.5 nm of the target value, which makes these stacks suitable for pumping a variety of gain media. The devices are fast axis collimated, with over 95% power being collimated in 6 mrad (full angle). The slow axis divergence is 9° (full angle) for 95% power content.
For many applications a frequency stabilized beam source with high output power and a good beam quality is
needed. Tapered lasers and amplifiers can provide a high output power, whereas they have a slightly lower beam
quality than ridge lasers. In a single mode fiber (SMF) coupled module, the beam quality provided by the
module is predetermined by the fiber. The technological progress of tapered lasers should allow a high enough
coupling efficiency to give SMF coupled modules using a tapered laser or amplifier the potential for a higher
output power than modules using a ridge laser.
It will be shown how this potential can be exploited by using different coupling systems for example with
cylindrical lenses either crossed or in combination with rotational lenses. The advantages, problems and coupling
results of those systems will be illustrated.
Two approaches of frequency stabilization will be shown. To stabilize a tapered amplifier the external cavity has
been set up by a fiber bragg grating on the backside of the amplifier. A volume holographic grating, which is
written in the fast axis collimation lens of the coupling system, was used to stabilize a tapered laser.
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.
High-power laser bars with emission in the red spectral range (635 - 660 nm) are of great interest for several applications
such as display and projection solutions or pumping of Cr:LiSAF solid state lasers. Another field of application is a
medical use of red lasers. The German funded project ROLAS combines medical and technical aspects of photodynamic
therapy (PDT). One special approach under investigation is a laser bar based multi-port PDT system: To allow the
optimum treatment of widespread, complex-shaped tumors a PDT laser system with 8 independently operable fiber
outputs is designed, based on two laser bars with 652 nm emission and independently addressable emitters.
The necessity of individually addressable emitters leads to a more complex and allows a thermally less optimized
package design. In combination with conductive cooling - which is a must for most medical applications - the
possibilities for low-temperature operation of the laser bars are severely constricted. Especially for high-power laser bars
in the 635-660 nm range operation under the expected unfavourable thermal conditions constitutes an additional
challenge: These devices by principle exhibit a strong temperature dependence of their performance due to the
comparably weak carrier confinement in the InGaAlP material system.
In this paper, based on detailed measurements, an analysis of the temperature dependence of the laser bar performance is
carried out and the consequences for mounting and application of the laser bars are shown. The measurements illustrate
the significant progress that has been achieved during the last two years in terms of temperature stability by applying
specific design measures.
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.
Laser modules for single mode fiber (SMF) coupling of frequency stabilized diode lasers are so far mainly
realized with ridge lasers due to their good beam quality. Tapered lasers are beam sources with a beam quality
which is nearly as good as that of a ridge laser but with a higher optical output power. Therefore they have the
potential for a higher SMF-coupled power than ridge lasers. It will be shown how the radiation of a tapered laser
or amplifier can be frequency stabilized and coupled into a SMF in a compactly build module.
To couple a tapered laser different coupling systems, using cylindrical lenses either crossed or in combination
with rotational lenses are possible. The advantages, problems and coupling results of those systems will be
For many applications it is necessary to stabilize the frequency of the laser. This can be achieved for example by
a fiber bragg grating, written in the SMF in which the laser is coupled or by a volume holographic grating,
applied to a lens in the coupling system. Another possibility is the use of a tapered amplifier, which is stabilized
by a fiber bragg grating on the backside of the amplifier.
The common wavelength regime for high-power diode laser modules is the range between 800 nm and
1000 nm. However, there are also many applications that demand for a wavelength of around 2 &mgr;m.
This wavelength range is extremely interesting for applications such as the processing of plastics,
medical applications as well as environmental analytics. The interest in lasers with this wavelength is
based on the special absorption characteristics of different types of material: Numerous plastics
possess an intrinsic absorption around 2 &mgr;m, so that the use of additives is no longer necessary. This is
of great value especially for medical-technical products, where additives require a separate approval.
Furthermore the longer wavelength allows the processing of plastics which are clear and transparent at
the visible. In addition, water, which is an essential element of biologic soft tissue, absorbs radiation at
the wavelength about 2 &mgr;m very efficiently. As radiation of this wavelength can be guided by glass
fibers, this wavelength may be very helpful for laser surgery.
Currently available lasers at the spectral range about 2 &mgr;m are solid-state lasers based on Ho- and Tmdoped
crystals. These systems suffer from high purchase costs as well as size and weight. In contrast
to this, diode lasers can be built more compact, are much cheaper and more efficient.
For this background, GaSb based high-power laser diodes for the wavelength regime of 1.9 - 2.3 &mgr;m
are developed at the Fraunhofer Institute for Solid State Physics (IAF). At the Fraunhofer Institute for
Laser Technology (ILT), fiber-coupled laser diode modules based on these laser bars are designed and
realized. A first module prototype uses two laser bars with a wavelength of 1.9 &mgr;m to provide an
output power of approx. 15 W from a 600 &mgr;m, NA 0.22 fiber. The module setup as well as the
characteristics of the laser bars at 1.9 &mgr;m wavelength are described in this paper.
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.
Diode laser systems have been established for material processing and pumping solid state lasers in the recent
years, due to flexibility, efficiency and lifetime. In the meantime, diode laser bars with an output power of more
than 120 W and a beam parameter product less than 70 mm mrad are available (see fig. 1). Depending on the
optical system an energy density in focus of more then 10<sup>6</sup> Wcm<sup>-2</sup> can be achieved. But for several applications
like hardening metal surfaces or welding thin blanks/plates the output power is insufficient. To increase optical
output power several diode laser bars are arranged vertically and/or horizontally. With these so called stacks
an optical output power of more than 4 kW can be achieved. Due to the incoherent beam coupling the beam
parameter product is increased at the same rate. But the energy density or intensity in focus is rather less than
constant. Other applications, e. g. welding or marking, require higher intensities, which can not be achieved
with diode lasers. For these applications diode pumped solid state laser are mostly applied.
During the last few years high power diode laser arrays have become well established for direct material processing due to their high efficiency of more than 50%. But standard broad-area waveguide designs are susceptible to modal instabilities and filamentations resulting in low beam qualities. The beam quality increases by more than a factor of four by using tapered laser arrays, but so far they suffer from lower efficiencies. Therefore tapered lasers are mainly used today as single emitters in external resonator configurations. With increased output power and lifetime, they will be much more attractive for material processing and for pumping of fiber amplifiers.
High efficiency tapered mini bars emitting at a wavelength of 980 nm are developed, and in order to qualify the bars, the characteristics of single emitters and mini bars from the same wafer have been compared. The mini bars have a width of 6 mm with 12 emitters. The ridge waveguide tapered lasers consist of a 500 μm long ridge and a 2000 μm long tapered section.
The results show very similar behavior of the electro-optical characteristics and the beam quality for single emitters and bars. Due to different junction temperatures, different slope efficiencies were measured: 0.8 W/A for passively cooled mini bars and 1.0 W/A for actively cooled mini-bars and single emitters. The threshold current of 0.7 A per emitter is the same for single emitters and emitter arrays. Output powers of more than 50 W in continuous wave mode for a mini bar with standard packaging demonstrates the increased power of tapered laser bars.
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.
During the last years high power diode lasers have become increasingly established for direct material processing. The advantages are the high efficiency (more than 50%) and long lifetime of more than 10.000h. An important factor believed to be responsible for the aging of diode lasers is the thermo-mechanical stress. High stress levels arise from the packaging process. The mismatch between the thermal expansion coefficient of the heat sink (typically copper 16.5x10-6 K-1) and the laserbar (GaAs 6.7x10-6 K-1) cause high mechanical stress. The change in length during the cooling process of a 10mm wide laserbar is more than 10μm. If a hard solder is used, the stress is much higher, because hard solder typically has a higher melting point and stress can not be reduced by relaxation.
Typically material with lower thermal expansion coefficient have a lower thermal conductivity than copper. This increases the thermal load of the laserbar, which decreases the life-time in this sense. The expansion-matching and the lower thermal conductivity of this material are working against each other. In order to find a good compromise, different active cooled expansion matched heat sinks are simulated. Very promising heat sinks have been fabricated and characterized. Also the solder selection has influence on the long term stability. A very soft solder is more critical in terms of long term stability. A higher diffusion takes place, so that the properties of the solder change during the lifetime of the diode-laser. Hard solder, especially AuSn, are well tested solders with a very high long term stability. (No changes of the intermetallic structure even at higher temperature.) The disadvantage of the hard solder is the incapability to reduce the mechanical stress through relaxation. Different solders are being used and investigated.
The main challenge to address single emitters in a high-power diode-laser-bar is the thermal and electrical management to avoid crosstalking. Especially p-side up assembly leads to increasing thermal influence of neighbouring emitters due to the low thermal conductivity of GaAs. Electro-magnetic fields inside and outside the laser-bar, for example caused by high frequency modulation (10 MHz) at a high current (up to 1 A), induce voltages into neighbouring electric circuits, hence the output power of neighbouring emitters can be affected.
In order to optimize the soldering process of laserbars onto heatsinks with Indium solder, several investigations have been made. First the growth of Indium oxide film is examined.
With this knowledge four different reduction materials are selected. Formic acid as a wet chemical reduction, a plasma activated Hydrogen/Argon gas, a gas enriched with formic acid, and a protective layer of Gold were investigated and compared for an optimized reduction of the oxide film of the Indium solder.
A cross section of the solder interface after the soldering process is made in order to see the distribution of the metals. High diffusion of the solder with its contact partners is a sign of a good connection. Enough pure Indium has to be available after the soldering process in order to use its creek properties to reduce the mechanical stress in the laserbar.
High beam quality can be achieved by accurate adjustment of the mechanical and micro-optical components in the manufacturing process of high power diode laser stacks. A charaterization device which can determine these parameters by automatically measuring the radiation properties of high-power diode-laser stacks has been developed. The result is a mechanically robust, easy to use characterization device of high reliability suited for applications in quality control and product optimization.
During the recent years the performance of high power diode lasers in terms of output power and lifetime has increased significantly. However, for many applications not only a high output power but also a good beam quality is necessary -- in other terms, high brightness is required. While the beam quality of classical broad area-type high power diode lasers is poor, special laser structures have been developed to achieve an improved beam quality. Examples are the tapered laser, the alpha-DFB-laser and -- the latest development -- the so-called "z-laser." The z-laser uses internal total reflection for the suppression of higher-order modes. The effectiveness of this working principle was first demonstrated by performing extensive numerical simulations. During the last year the first z-laser structures have been processed and characterized. The experimental results of these first test lasers are compared with the predictions from the numerical simulations and show a very good agreement. With these first lasers, approximately 500 mW output power at 6-times diffraction limited beam quality have been demonstrated. Nevertheless, there are also some not well understood features of the z-laser to be investigated, like a reduced conversion efficiency and untypical characteristic curves showing kinks. Understanding these features, demonstrating the reproducibility of the structure and further performance improvements are the goals of current rsearch.
One of the key topics in today's semiconductor laser development activities is to increase the brightness of high-power diode lasers. Although structures showing an increased brightness have been developed specific draw-backs of these structures lead to a still strong demand for investigation of alternative concepts. Especially for the investigation of basically novel structures easy-to-use and fast simulation tools are essential to avoid unnecessary, cost and time consuming experiments. A diode laser simulation tool based on finite difference representations of the Helmholtz equation in 'wide-angle' approximation and the carrier diffusion equation has been developed. An optimized numerical algorithm leads to short execution times of a few seconds per resonator round-trip on a standard PC. After each round-trip characteristics like optical output power, beam profile and beam parameters are calculated. A graphical user interface allows online monitoring of the simulation results. The simulation tool is used to investigate a novel high-power, high-brightness diode laser structure, the so-called 'Z-Structure'. In this structure an increased brightness is achieved by reducing the divergency angle of the beam by angular filtering: The round trip path of the beam is two times folded using internal total reflection at surfaces defined by a small index step in the semiconductor material, forming a stretched 'Z'. The sharp decrease of the reflectivity for angles of incidence above the angle of total reflection leads to a narrowing of the angular spectrum of the beam. The simulations of the 'Z-Structure' indicate an increase of the beam quality by a factor of five to ten compared to standard broad-area lasers.
A high power semiconductor laser with a novel lateral design using angular filtering by total reflection for increased brightness is demonstrated. In this so called `Z-Laser' two inner surfaces guide the laser beam by total reflection in a Z-shaped path through the laser. Higher order laser modes with larger divergence angles are suppressed because of a smaller reflectivity. This results in a reduced far-field angle. Simulations based on a 2D steady state wave equation solved by using the Pade approximation, an 1D carrier diffusion equation and a logarithmic gain model have been performed to design the device.
High-power diode lasers serve as small and highly efficient laser-beam sources. Their output power has been increased steadily over the recent years. Nowadays, the output power of one diode laser bar is sufficient for many different applications such as pumping of solid state lasers, direct material processing, medical applications or printing. The successful use of high-power diode lasers depends on their high reliability in combination with a long lifetime. For a further increase in the quality of high-power diode lasers the properties of semiconductor lasers have to be improved as well as the cooling and mounting techniques for these bars onto specially designed heat sinks.