KEYWORDS: Semiconductor lasers, Diodes, Fiber lasers, High power lasers, Laser systems engineering, Reliability, Laser development, Disk lasers, Fiber couplers, Packaging
We have continuously optimized high fill factor bar and packaging design to increase power and efficiency for thin disc
laser system pump application. On the other hand, low fill factor bars packaged on the same direct copper bonded (DCB)
cooling platform are used to build multi-kilowatt direct diode laser systems. We have also optimized the single emitter
designs for fiber laser pump applications. In this paper, we will give an overview of our recent advances in high power
high brightness laser bars and single emitters for pumping and direct diode application. We will present 300W bar
development results for our next generation thin disk laser pump source. We will also show recent improvements on
slow axis beam quality of low fill factor bar and its application on performance improvement of 4-5 kW TruDiode laser
system with BPP of 30 mm*mrad from a 600 μm fiber. Performance and reliability results of single emitter for multiemitter
fiber laser pump source will be presented as well.
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.
KEYWORDS: Laser systems engineering, High power lasers, Disk lasers, Reflectivity, Volume holography, Holography, Pulsed laser operation, Semiconductor lasers, Absorption, Diodes
We present our latest experimental results in wavelength stabilization of high power laser diode systems by using Volume Holographic (Bragg) Gratings. Such systems are used as optical pumps to increase the efficiency and brightness of Thin Disk Lasers. To achieve a wide locking range from threshold until maximum operation current (for example from 30A to 250A), careful control of laser system alignment is necessary to ensure effective feedback and locking, without using strong gratings which could reduce laser efficiency. For this purpose, we use wavefront correction optics to compensate for laser bar smile and Fast Axis Collimation pointing errors. We reduce the pointing errors from ~ 1 mrad to an average under 0.1 mrad across the bar and across the entire stack. Time resolved spectra are used to investigate the dynamic locking behavior with the goal of achieving a locking speed comparable to the rise time of the current (100 μs). Experimental results for multi-kW laser systems are presented, both in CW and soft pulsed operation modes.
KEYWORDS: Waveguides, Semiconductor lasers, Thermal effects, Broad area laser diodes, Refractive index, Laser systems engineering, Near field, High power lasers, Fiber coupled lasers, Optical filters
For high brightness direct diode laser systems, it is of fundamental importance to improve the slow axis beam quality of the incorporated laser diodes regardless what beam combining technology is applied. To further advance our products in terms of increased brightness at a high power level, we must optimize the slow axis beam quality despite the far field blooming at high current levels. The later is caused predominantly by the built-in index step in combination with the thermal lens effect. Most of the methods for beam quality improvements reported in publications sacrifice the device efficiency and reliable output power. In order to improve the beam quality as well as maintain the efficiency and reliable output power, we investigated methods of influencing local heat generation to reduce the thermal gradient across the slow axis direction, optimizing the built-in index step and discriminating high order modes. Based on our findings, we have combined different methods in our new device design. Subsequently, the beam parameter product (BPP) of a 10% fill factor bar has improved by approximately 30% at 7 W/emitter without efficiency penalty. This technology has enabled fiber coupled high brightness multi-kilowatt direct diode laser systems. In this paper, we will elaborate on the methods used as well as the results achieved.
KEYWORDS: Semiconductor lasers, Laser systems engineering, High power lasers, Disk lasers, Fiber couplers, Laser development, Laser applications, Collimation, Photonics, Solid state lasers
The performance of high power and high brightness systems has been developing and is developing fast. In the multi kW regime both very high spatial and spectral brightness systems are emerging. Also diode laser pumped and direct diode lasers are becoming the standard laser sources for many applications. The pump sources for thin Disk Laser systems at TRUMPF Photonics enabled by high power and efficiency laser bars are becoming a well established standard in the industry with over two thousand 8 kW Disk Laser pumps installed in TruDisk systems at the customer site. These systems have proven to be a robust and reliable industrial tool. A further increase in power and efficiency of the bar can be easily used to scale the TruDisk output power without major changes in the pump source design. This publication will highlight advanced laser systems in the multi kW range for both direct application and solid state laser pumping using specifically tailored diode laser bars for high spatial and/or high spectral brightness. Results using wavelength stabilization techniques suitable for high power CW laser system applications will be presented. These high power and high brightness diode laser systems, fiber coupled or in free space configuration, depending on application or customer need, typically operate in the range of 900 to 1070 nm wavelength.
The advances in laser-diode technology have enabled high efficiency direct diode base modules to emerge as a building block for industrial high power laser systems. Consequently, these systems have been implemented with advance robust, higher-brightness and reliable laser sources for material processing application. Here at the company, we use low-fill factor bars to build fiber-coupled and passively cooled modules, which form the foundation for “TruDiode,” the series of TRUMPF direct diode laser systems that can perform in the multi-kilowatt arena with high beam quality. However, higher reliable output power, additional efficiency and greater slow axis beam quality of the high power laser bars are necessary to further increase the brightness and reduce the cost of the systems. In order to improve the slow axis beam quality, we have optimized the bar epitaxial structures as well as the lateral design. The detailed near field and far field studies of the slow axis for each individual emitters on the bar provide us with information about the dependency of beam quality as a function of the drive current. Based on these study results for direct diode application, we have optimized the high brightness bar designs at 900-1070nm wavelengths. In addition, high power and high efficiency laser bars with high fill factors have been used to build the pump sources for thin disc laser systems at TRUMPF Photonics. For better system performances with lower costs, we have further optimized bar designs for this application. In this paper, we will give an overview of our recent advances in high power and brightness laser bars with enhanced reliability. We will exhibit beam quality study, polarization and reliability test results of our laser bars in the 900-1070nm wavelengths region for coarse wavelength multiplexing. Finally, we will also present the performance and reliability results of the 200W bar, which will be used for our next generation thin disk laser pump source.
This paper describes an innovative, high throughput manufacturing test system for testing high power laser-diode stacks. These stacks are based on a single high power bar building block, which can be stacked either vertically or horizontally to deliver extremely high output power (>3kW) from a compact package which can range from a single bar to over 25 bars in one package. Testing these various form-factors presents many challenges in high-volume manufacturing e.g.
repeated changes of tooling and set-up to accommodate mixture of configurations. The automated test system described in this paper can accommodate any configuration of multi-bar stacks to test critical optical characteristics (LIV, Optical Spectrum Characteristics, Optical Power, Optical Divergence, water flow rate, water pressure etc.). Key to the automated station is a custom designed integrating sphere and universal stack holder with automated water flow
configuration. The automated test system significantly improves the throughput by decreasing the test time by 50% (compared to manual testing). Individual bars comprising stack have different spectrum and the custom designed integrating sphere enables accurate spectrum analysis (centroid wavelength, FWHM) of the combined spectrum, as well as accurate power measurement.
High power diode lasers have demonstrated reliable output power of more than 50W per diode far beyond 10,000 hours. Record output power of more than 300W per diode laser bar has been reported. The improved reliability of the semiconductor material demands a review of the performance of the actively water cooled heatsink with regards to the expected lifetime. Results from corrosion tests at various water conditions for durations of more than 13,000 hours predict an extended usage of water-cooled heatsink beyond 20,000 hours without significant performance change.
A compact, reliable semiconductor laser source for materials processing, medical and pumping applications is described. This industrial laser source relies on a combination of technologies that have matured in recent years. In particular, effective means of stacking and imaging monolithic semiconductor laser arrays (a.k.a., bars), together with advances in the design and manufacture of the bars, have enabled the production of robust sources at market-competitive costs. Semiconductor lasers are presently the only lasers known that combine an efficiency of about 50% with compact size and high reliability. Currently the maximum demonstrated output power of a 10-mm-wide semiconductor laser bar exceeds the 260 W level when assembled on an actively cooled heat sink. (The rated power is in the range of 50 to 100 W.) Power levels in the kW range can be reached by stacking such devices. The requirements on the stacking technique and the optic assembly to achieve high brightness are discussed. Optics for beam collimation in fast and slow axis are compared. An example for an optical setup to use in materials processing will be shown. Spot sizes as low as 0.4 mm X 1.2 mm at a numerical aperture of 0.3 and output power of 1 kW are demonstrated. This results in a power density of more than 200 kW/cm2. A setup for further increase in brightness by wavelength and polarization coupling will be outlined. For incoherent coupling of multiple beams into a single core optical fiber, a sophisticated beam-shaping device is needed to homogenize the beam quality of stacked semiconductor lasers.
Monolithic linear arrays of diode lasers, also known as diode laser bars, are the basic elements for most high-power laser applications such as solid-state laser pumping or material processing. Cylindrical microlenses used as fast- axis collimators for 10-mm diode bars require very high angles of aperture (up to 100 degree FW1/e2, equivalent to a numerical aperture of approx. 0.8) to capture most of the emitted laser power. For the efficient longitudinal pumping of laser rods, or the narrow focusing of the diode laser radiation (fiber coupling, material processing), high- quality microlenses with small lens aberrations are necessary to avoid power losses and beam quality degradation. A technique for coupling the output of high- power diode laser bars into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with core diameter down to 400 micrometers . This technique comprises two micro step-mirrors for beam shaping. The overall efficiency from one diode-laser bar to fiber is 71% with 20 W cm laser power through the fiber. Coupling of 12 diode laser bars and power of 200 W out of a fiber with core diameter of 0.8 mm and NA equals 0.2 is achievable with this technique.
Basis of the developments discussed in the presentation are 10 mm GaAs diode laser bars mounted on copper micro channel heat sinks. Optimizing the micro channel heat sinks leads to decreased thermal resistance and decreased pressure drop. In the presentation the steps to ten times reduced pressure drop and optical power output of the diode lasers of over 100 Watts will be described.
Basis of the developments discussed in the presentation are 10 mm GaAs diode laser bars mounted on copper micro channel heat sinks. Optimizing the micro channel heat sinks leads to decreased thermal resistance and decreased pressure drop. In the presentation the steps to ten times reduced pressure drop and optical power output of the diode lasers of over 100 Watts will be described.
Nonlinear optical devices, such as harmonic generators, provide a means of extending the frequency range of available laser sources. In this paper we report on a novel concept for scaling the output power of frequency doubled Nd:YAG lasers. Conceptually, the laser consists of multi oscillators, which are placed in a common resonator. The frequency doubler is an etalon shaped nonlinear crystal with dichroitic coatings. The second harmonic generated in different oscillators is coaxially superpositioned.
Decreasing film thicknesses and sizes of microstructures require an ultraprecise removal of the material and a reduction of the heat-affected zone. For these applications picosecond laser pulses seem to offer new challenges. Because of the short pulse length higher power densities can be reached and rapid heating can possibly lead to an earlier evaporation of the material and to a reduction of the molten zone which resolidifies after the end of the laser pulse at the edges of the processed area. The removal of different materials was investigated using laser pulses with a pulse length of 40 ps produced by a diode-pumped mode-locked Nd:YAG-laser in combination with a regenerative amplifier. The laser radiation was focused to a 7 micrometers spot diameter, yielding power densities up to 5 X 1012 W/cm2. Pump and probe investigations were used to study the interaction of intense ultrashort laser beams with matter. By this technique ultrashort processes with time resolution determined by the pulse length of pump and probe pulses can be photographed. The measurements allow a detailed characterization of the material removal including melting, vaporization and fast resolidification and the feedback of the surrounding atmosphere to the processed microstructures. The single-shot removal threshold fluence and the removal rate per pulse for 40 ps laser pulses and a wavelength of 1064 nm were determined for Si3N4-ceramics. Different materials like metals, semiconductors and ceramics were microstructured by picosecond laser radiation yielding structural dimensions smaller than 20 micrometers .
We developed a compact fiber-coupled high-power diode-laser unit with optical output power up to 40 W cw, coupled into a multimode fiber with 600 micrometers core diameter and NA 0.22. This diode-laser unit is suitable for pumping solid-sate or fiber lasers as well as for material processing. Essential part is a novel beam-shaping system with compact size, high flexibility and low alignment requirements, which uses a pair of micro step-mirrors. The whole unit fits into a housing of approximately 110 X 100 X 90 mm.
Monolithic linear arrays of diode lasers, also known as diode laser bars, are the basic elements for most high-power laser applications such as solid-state laser pumping or material processing. Cylindrical microlenses used as fast- axis collimators for 10-mm diode bars require very high angles of aperture (up to 100 degree FW1/e2, equivalent to a numerical aperture of approx. 0.8) to capture most of the emitted laser power. For the efficient longitudinal pumping of laser rods, or the narrow focusing of the diode laser radiation (fiber coupling, material processing), high- quality microlenses with small lens aberrations are necessary to avoid power losses and beam quality degradation. In this paper, lens design considerations are presented together with alignment tolerances and the variation of the collimated beam parameters. Measuring methods for the lens characterization are discussed. Manufacturing techniques are summarized, and the performance of microlenses is measured by the beam width and divergence of the collimated diode laser beam.
A technique for coupling the radiation of a high-power diode laser bar into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with 400 micrometers core diameter. The principal item of the fiber-coupling system is a pair of micro step-mirrors--a novel design for beam shaping. The overall efficiency from diode-laser to fiber is 71% with 20 W cw laser power through the fiber. Polarization and wavelength multiplexing renders the system scaleable to higher output power which makes it highly suitable for material processing and pumping of lasers.
Developments concerning high-power diode lasers, optical systems for beam shaping and superposition and complete systems for direct materials processing are discussed, along with examples of their industrial applications.
Picosecond laser pulses were used for microstructuring metals, ceramics and diamond. Using 40 ps laser pulses line patterns with a minimum line width in the range of 7 micrometer were formed in diamond without destroying the crystallinity. In Si3N4-ceramics holes were drilled with diameters smaller than 6 micrometer. Ultrathin copper and gold films of 1 micrometer thickness were removed from fused silica and silicon (100) without influencing the substrates. The ablation process was investigated by high-speed photography and pump-&- probe-measurements with a time resolution of 40 picoseconds. The measurements allow a detailed characterization of the melting, vaporization and solidification processes. Shock waves in the ambient atmosphere were detected and the energy in the shock wave was determined.
The expansion of the laser induced plasma during material processing of Cu, Au, Si, and Si3N4 with ultrashort laser pulses is investigated using streak photography and high speed photography using delayed 35 ps laser pulses. Pulses of a diode-pumped, modelocked Nd:YAG laser which are amplified by a regenerative amplifier having pulse durations of 35 ps and a maximal energy of 0.1 mJ are used. Experiments are performed using both the fundamental wavelength of 1064 nm and by frequency doubling the wavelength of 532 nm. The onset of plasma formation is during the leading edge of the pulse. The spatial expansion of the plasma corresponds to the self-similar motion of a spherical plane wave in a gas. Independent of the processed material in ambient atmosphere the energy content of the shock wave is about one fifth of the optical energy of the laser pulse.
The paper summarizes activities of the two Fraunhofer-Institutes ILT and IPT concerning the development of high-power laser-diode stacks and their direct industrial applications. With microchannel coolers in copper technology and ultra-precision machined micro-optics a stack of 330 - 400 W total power with a maximum intensity of the focused beam of 2 104 W/cm2 has been built and tested in first applications. By further improvements of the lens-fabrication and -alignment technology as well as increase of the number of stacked diodes an output power in the kW-range and intensities up to about 105 W/cm2 shall be achieved in the near future. Applications of such laser sources in surface technology, in the processing of plastics, in laser-assisted machining and in brazing are discussed.
This paper describes an application of the diamond turning technology for the manufacturing of cylindrical micro lenses. A lens doublet consisting of an aspherical PMMA lens mounted on a sapphire plate was designed. Various surface curvatures are transferable to a crooked stripe of optical material like polymethylmethacrylat. A diamond tool cuts the circular ring at its outer periphery. The approximately 80 cm long, circular lens stripe relaxes after taking off from the chuck to the original straight condition. Microlenses of any length are gotten by separating the long lens stripe to individual lenses. For a quality inspection the surface was examined by a laser interferometer measurement and the collimation performance of the doublet was tested. The measured surface roughness is better than 12 nm RMS. To apply a 10 mm diode bar a lens with 12 X 1,5 mm2 entry aperture was used in the collimating tests. Experimental results show collimating rates for high performance diode laser applications of 70% in a divergence angle of about 4 mrad.
The growing significance of laser technology in industrial manufacturing is also observed in case of plastic industry. Laser cutting and marking are established processes. Laser beam welding is successfully practiced in processes like joining foils or winding reinforced prepregs. Laser radiation and its significant advantages of contactless and local heating could even be an alternative to conventional welding processes using heating elements, vibration or ultrasonic waves as energy sources. Developments in the field of laser diodes increase the interest in laser technology for material processing because in the near future they will represent an inexpensive energy source.
A high average output power of a solid state laser is achieved by means of a temporal multiplexing method. It combines several laser beams and conserves the beam quality of each beam in the combined laser beam. The multiplexing method is demonstrated for three laser beams delivering a total laser power of 3 kW. Two concepts for the laser and multiplexer setup are realized. The first aims at an optimized beam quality whereas the second takes the demands of a flexible work tool into account. Both systems are using optical fibers for the beam delivery to the workstation. The requirements for delivering high power laser beams by optical fibers are discussed. Applications in laser beam welding of aluminum and stainless steel are shown.
Volker Krause, Hans-Georg Treusch, Peter Loosen, T. Kimpel, Jens Biesenbach, Arnd Koesters, F. Robert, H. Oestreicher, Marcel Marchiano, Bernhard DeOdorico
This paper describes as an alternative to the silicon technology the production, assembly, and performance of microchannel coolers in copper technology. A realization technique for these copper coolers is described for one specific cooler geometry with a stacked design of 5 microstructured copper plates. These 5 layers are bonded together by diffusion welding at a temperature near 800 degree(s)C. This bonding process allows the fabrication of 3 X 8 coolers in one step in which the 5 structured copper plates of a size of 100 X 150 mm2 are bonded together. The structuring procedure is currently done by etching and laser machining. Microchannels with a width of 60 - 100 micrometers and a depth of 300 micrometers are used in the experiments as microstructures for better comparability to already existing theoretical and experimental results. A thermal resistance of 0.44 K/W of these copper coolers is demonstrated for a device with a 10 mm laser diode bar mounted on the front edge of the cooler. Experimental results for this configuration reach an average laser diode power of 71.5 Watts from one single bar with a footprint of 0.6 X 10 mm2. The experimental results are compared to 3-D heat flow calculations.
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