Direct diode laser systems gain importance in the fields of material processing and solid-state laser pumping. With increased output power, also the influence of strong optical feedback has to be considered. Uncontrolled optical feedback is known for its spectral and power fluctuation effects, as well as potential emitter damage. We found that even intended feedback by use of volume Bragg gratings (VBG) for spectral stabilization may result in emitter lifetime reduction. To provide stable and reliable laser systems design, guidelines and maximum feedback ratings have to be found. We present a model to estimate the optical feedback power coupled back into the laser diode waveguide. It includes several origins of optical feedback and wide range of optical elements. The failure thresholds of InGaAs and AlGaAs bars have been determined not only at standard operation mode but at various working points. The influence of several feedback levels to laser diode lifetime is investigated up to 4000h. The analysis of the semiconductor itself leads to a better understanding of the degradation process by defect spread. Facet microscopy, LBIC- and electroluminescence measurements deliver detailed information about semiconductor defects before and after aging tests. Laser diode protection systems can monitor optical feedback. With this improved understanding, the emergency shutdown threshold can be set low enough to ensure laser diode reliability but also high enough to provide better machine usability avoiding false alarms.
This paper investigates the feasibility of using a 2kW direct diode laser source for producing high-quality cuts in a variety of materials. Cutting trials were performed in a two-stage experimental procedure. The first phase of trials was based on a one-factor-at-a-time change of process parameters aimed at exploring the process window and finding a semi-optimum set of parameters for each material/thickness combination. In the second phase, a full factorial experimental matrix was performed for each material and thickness, as a result of which, the optimum cutting parameters were identified. Characteristic values of the optimum cuts were then measured as per BS EN ISO 9013:2002.
Cutting of metal sheets is a key application in material processing and high power diode lasers gain further importance in this field due to their exceptionally good current to light conversion efficiency. Both, power scaling and process optimization are under investigation to improve the performance of this application. We report first results of a laser system combining these approaches. The presented diode laser power scaling is realized by means of an asymmetric, noncircular beam shape. The beam parameter product of the laser light is manipulated accordingly. In addition, the power scaling of this approach allows the generation of spot geometries which inherently support the interaction processes of the laser light with materials as metals. The setup is based on conventional, highly reliable and well tested principles and components, as they are passively cooled laser diode bars, coarse wavelengths coupling and fiber delivery. First results of material processing with stainless steel of 6 mm thickness are presented.
High power diode laser line generators are nowadays industrial standard for applications like plastic processing, vision
inspection and drying. With increased beam quality, especially peak intensity and homogeneity, they also enable new
applications like hardening, annealing or cutting of various materials. All of these applications have in common that
simultaneous processing is limited by the scalability of the generated line length without changing process relevant
parameters of the line like working distance, peak intensity, homogeneity and depth of focus. Therefore, a patent pending
beam shaping concept is presented that enables the interconnection of an arbitrary number of nearly free selectable laser
sources to generate scalable laser lines with outstanding beam parameters. System design, experimental setup and results
of a laser line generator are shown. It is based on a stitching concept consisting of ten fibre coupled high power diode
lasers, which generates a 200mm long and 2mm wide laser line with a homogeneity level of 97% p-v over a depth of
focus of +/- 5 mm with an overall output power of up to 4.2 kW. The concept is discussed regarding industrial
applications and the options for even higher beam quality, especially the capability of generating lines with increased
power densities up to several kW/cm² and a line length of several meters.
High power laser sources are used in various production tools for microelectronic products and solar cells, including the
applications annealing, lithography, edge isolation as well as dicing and patterning. Besides the right choice of the laser
source suitable high performance optics for generating the appropriate beam profile and intensity distribution are of high
importance for the right processing speed, quality and yield.
For industrial applications equally important is an adequate understanding of the physics of the light-matter interaction
behind the process. In advance simulations of the tool performance can minimize technical and financial risk as well as
lead times for prototyping and introduction into series production. LIMO has developed its own software founded on the
Maxwell equations taking into account all important physical aspects of the laser based process: the light source, the
beam shaping optical system and the light-matter interaction.
Based on this knowledge together with a unique free-form micro-lens array production technology and patented micro-optics
beam shaping designs a number of novel solar cell production tool sub-systems have been built. The basic
functionalities, design principles and performance results are presented with a special emphasis on resilience, cost
reduction and process reliability.
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.
Beam shaping improvements of line generators based on high power diode lasers lead to new application fields as
hardening, annealing or cutting of various materials.
Of special interest is the laser treatment of silicon. An overview of the wide variety of applications is presented with
special emphasis of the relevance of unique laser beam parameters like power density and beam uniformity.
Complementary to vision application and plastic processing, these new application markets become more and more
important and can now be addressed by high power diode laser line generators.
Herewith, a family of high power diode laser line generators is presented that covers this wide spectrum of application
fields with very different requirements, including new applications as cutting of silicon or glass, as well as the beam
shaping concepts behind it. A laser that generates a 5m long and 4mm wide homogeneous laser line is shown with peak
intensities of 0.2W/cm2 for inspection of railway catenaries as well as a laser that generates a homogeneous intensity
distribution of 60mm x 2mm size with peak intensities of 225W/cm2 for plastic processing. For the annealing of silicon
surfaces, a laser was designed that generates an extraordinary uniform intensity distribution with residual
inhomogeneities (contrast ratio) of less than 3% over a line length of 11mm and peak intensities of up to 75kW/cm2.
Ultimately, a laser line is shown with a peak intensity of 250kW/cm2 used for cutting applications. Results of various
application tests performed with the above mentioned lasers are discussed, particularly the surface treatment of silicon
and the cutting of glass.
An increase in the performance of micro-optic beam shaping resulted in diode laser modules with more than 400W out
of 200 μm fibre based on Broad Area Laser Bars (BALB). The brightness of a 400 W laser module opened the door for
new applications in material processing such as temper marking of stainless steel and metal sheet cutting.
Further improvements of the light sources and the beam shaping for BALB's have increased the efficiency of the laser
modules.
Therefore we present an output power of 1200 W out of a 200 μm fibre (0.22 NA). This is achieved by further
sophistication of the coupling technique and four wavelength coupling. The beam parameter product is still 22
mm*mrad with a power density of 3800 kW/cm2 if focussed to a 200 μm spot. Furthermore, each of the four
wavelength modules are separately exchangeable and checkable.
The availability of a top-hat profile out of the fibre proves itself to be advantageous compared to the traditional
Gaussian beam profiles of fibre, solid-state and gas lasers. This leads to excellent laser cutting results with extremely
small cutting kerfs down to 200 μm and very plane cutting edges. Process speeds rise up to more than 10 m/min i.e. for
thin sheet stainless steel or titanium. In the near future, 600 W out of 200 μm based on BALB's with a beam compressor
is possible. With wavelength coupling, power levels with up to 2 kW out of 200 μm fibre will be reached. This will
result in a power density of more than 6 MW/cm2.
Beam shaping improvements of line generators based on high power diode lasers in combination with newly designed and produced high precision micro-optics lead to new applications such as hardening, metallization and annealing of different materials.
Two aspects are mainly needed to be focused on for getting the best results and throughput in these applications. The first one is the overall power content along the narrow axis of the line, namely the peak intensity in combination with the beam shape. The second one is the intensity homogeneity along the long axis of the line.
Herewith, a beam shaping concept that fulfils the desired requirements in a variable modality is presented. The concept
consists of macro-lenses and newly designed micro-optics and results in a passively cooled high power diode laser emitting at 808nm. The laser has an output power of 1000W. The generated line has a length of 13mm and a width of <100μm at a remarkably large working distance of about 80mm. We attained an intensity distribution along the line length with a peak power density >80kW/cm2 and uniformity >97%. To achieve such an extraordinary homogeneity level, several approaches based on cylindrical lens arrays were designed and tested. Methods to reduce inhomogeneities caused by diffraction effects and effects based on geometric optics are presented as well as their results. Additionally, the potential of this concept with regard to modularity, expandability and variability is reviewed.
Finally, an application example - crystallisation of a thin film of a-Si on a glass substrate - is presented.
We present newly developed high power diode laser modules which are performing at outstanding power densities and line uniformity. The combination of recently designed laser diode bars on passive heat sinks and optimized micro-optics results to laser modules with power densities > 100kW/cm2 in a line length of 12mm x 0.1mm. The usage of non periodic structured homogenizers leads to a homogeneity of less than 3% p/v which allows precise heating and annealing applications. The application for such laser lines are hardening, metallization and annealing of different materials. In the presentation we will show results of thin film Si-a annealing process with direct diode laser annealing.
Fiber-coupled diode lasers have become an established source for many industrial applications due to their high wall-plug
efficiency, minimal maintenance and cost per watt. To decrease system size and cost for cooler and driver, high
coupling efficiencies have become more and more important.
Recent developments in broad area laser diode bars (BALB) and beam shaping systems with micro-optical components
are leading to new highly efficient fiber coupling.
We present newly developed high power diode laser modules which are performing at outstanding efficiencies with
smallest package design. The combination of recently designed laser diode bars on passive heat sinks and optimized
micro-optics results in laser modules with up to 60W out of a 200μm fiber with a 0.22 NA and > 50% electro-optical
efficiency out of the fiber core, based on only one laser diode bar.
The applications for such laser diode modules range from pumping of fiber lasers and amplifiers, over materials
processing to medical applications.
The presentation of the technology will show a path to scale high-brightness laser systems to higher power levels and
efficiencies. The combination of different coupling techniques will allow laser modules with 100W out of 100μm fiber
core up to 1.6kW out of 400μm fiber core with electro-optical efficiencies of > 45%.
KEYWORDS: Beam shaping, High power diode lasers, Homogenization, Semiconductor lasers, Lenses, Laser sources, Manufacturing, Laser processing, High power lasers, Semiconductors
Micro-lens arrays are widely used for beam shaping, especially beam homogenization of various laser sources.
Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages in laser
applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical
micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and
application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMO's unique production
technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an
even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of
aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis.
Additionally, the opportunities in simultaneous illumination from numerous light sources to one target are extended by
just geometrical arrangement without the need for collinear beam alignment. First application results of such micro-lens
arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100
W laser with tilted illumination at an angle of 35° is shown. A multi-kW line generator based on the superposition of
over 50 diode laser bars under different illumination angles is demonstrated as well. Thus, laser material processing like
plastics welding, soldering or annealing becomes much more convenient and less demanding regarding beam steering.
So far, diode laser systems could not compete against CO2-lasers or DPSSL in industrial applications like marking or
cutting due to their lower brightness. Recent developments in high-brightness diode laser bars and beam forming
systems with micro-optics have led to new direct diode laser applications.
LIMO presents 400W output from a 200μm core fibre with an NA of 0.22 at one wavelength. This is achieved via the
combination of newly designed laser diode bars on passive heat sinks coupled with optimized micro-optical beam
shaping. The laser is water cooled with a housing size of 375mm x 265mm x 70mm.
The applications for such diode laser modules are mainly in direct marking, cutting and welding of metals and other
materials, but improved pumping of fibre lasers and amplifiers is also possible. The small spot size leads to extremely
high intensities and therefore high welding speeds in cw operation. For comparison: The M2 of the fibre output is 70,
which gives a comparable beam parameter product (22mm*mrad) to that of a CO2 laser with a M2 of 7 because of the
wavelength difference.
Many metals have a good absorption within the wavelength range of the laser diodes (NIR, 808nm to 980nm), which
permits the cutting of thin sheets of aluminium or steel with a 200W version of this laser. First welding tests show
reduced splatters and pores owing to the optimized process behaviour in cw operation with short wavelengths.
The availability of a top-hat profile proves itself to be advantageous compared to the traditional Gaussian beam profiles
of fibre, solid-state and gas lasers in that the laser energy is evenly distributed over the working area.
For the future, we can announce an increase of the output power up to 1200W out of a 200μm fibre (0.22 NA). This will
be achieved by further sophistication and optimisation of the coupling technique and the coupling of three wavelengths.
The beam parameter product will then remain at 22mm*mrad with a power density of 3.8 MW/cm2 if focussed to a
200µm spot. This leads to excellent laser cutting results with extremely small cutting kerfs down to 200μm and very
plane cutting edges. Process speeds rise up to more than 10m/min i.e. for thin sheets of stainless steel or titanium.
KEYWORDS: Semiconductor lasers, Laser systems engineering, High power lasers, Diodes, Surgery, Continuous wave operation, Micro optics, Near field optics, Fiber coupled lasers, Resistance
Current laser systems based on high-power laser diode bars need active cooling either water cooling or the use of
thermo-electric coolers to ensure an adequate operating temperature for a reasonable lifetime. Here is a solution with a
bonded fin heat sink and forced ventilation introduced, a diode laser bar with an improved efficiency and a low thermal
resistance as well as an optical system for a highly efficient fibre coupling. With this system it is possible to couple 25
Watt continuous wave power from a single laser diode bar on a passive heat sink into a fibre with 200 μm core
diameter.
The basis for this performance is a heat sink with an exceptionally low thermal resistance. Several new features are
introduced to reach a low overall gradient between the laser diode temperature and the ambient temperature. In addition,
it does geometrically fit to the layout of the optical design. Shape and aspect ratio of both heat sink and housing of the
laser system are matched to each other. Another feature is the use of hard-soldered or pressed bars to achieve a thermo-mechanically
stable performance. The long-term thermal characteristic was tested. The operation temperature comes to
saturation after about 30 minutes. Therefore it can be used for continuous wave operation at 25 Watt output power. At a
quasi continuous operation at 70 percent duty cycle a peak power of 30 Watt out of the fibre is possible.
From this technology results a compact fibre coupled laser system what is simple to drive compared with current high
power laser systems, because there is no need to control the operating temperature. This gives way for more compact
driver solutions. Fields of application are laser marking systems and material processing, where a simple driver system
is requested. Also medical applications need this requirement and a compact cooling too so that mobile integrated
solutions become possible. Further developments allow multiple laser diode systems for specific industrial applications
demanding more power. Our measurements show the potential for direct air-cooled laser systems with 100 Watt power
out of the fibre.
We present new developed high power diode laser modules which are performing at outstanding brightness and their
applications. The combination of recently designed laser diode bars on passive heat sinks and optimized micro-optics
results to laser modules up to 50W out of a 100μm fibre with a 0.22 NA at one single wavelength based on broad area
laser bars (BALB) and up to 50W out of 50μm fibre with a 0.22 NA based on single-mode emitter array laser (SEAL)
bars. The fibre coupled systems are based on diode lasers with a collimated beam of superior beam data, namely < 10
mm x 10 mm beam diameter (FW1/e2) and < 2mrad x 2mrad divergence (FW1/e2). Such free beam diode lasers deliver
30 W or 60 W output power.
The applications for such laser diode modules varies from direct marking, cutting and welding of metals and other
materials up to pumping of fibre lasers and amplifiers. Marking speed with up to 30mm/s on stainless steel was
observed with 20W laser power and 50&mgr;m fibre with a conventional marking setup. Cutting speed of about 1m/min of
0.2mm Kovar sheet was shown with a diode laser module with 50W laser power from a 100&mgr;m fibre.
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