Progress is presented on ongoing research and development into ultra-high power and efficiency bars that achieve significantly higher useful optical output power and higher brightness than are currently commercially available. In previous work (2017), the authors reported on bars that deliver over 1 kW continuous wave (cw) diode laser power, when cooled using 15°C water. Our current studies are focused on increasing the usable output power (power within a targeted beam angle), which is essential for real world industrial applications. These ongoing studies have enabled the first demonstration of 500 W cw output power from a 10 mm x 6 mm laser diode bar with a lateral far field angle of only 8°. In efforts to further improve brightness, we also present our latest progress on high power SMEBs (Single Mode Emitter Bars). These emitters operate in a close to diffraction limited optical mode (M² < 1.5, laterally and vertically). This new technology enables a significant increase in Diode Laser brightness. We demonstrate in excess of 55% electro optical efficiency at > 200 W cw laser bar power for SMEBs.
The advance of high power semiconductor diode laser technology is driven by the rapidly growing industrial laser market, with such high power solid state laser systems requiring ever more reliable diode sources with higher brightness and efficiency at lower cost. In this paper we report simulation and experimental data demonstrating most recent progress in high brightness semiconductor laser bars for industrial applications. The advancements are in three principle areas: vertical laser chip epitaxy design, lateral laser chip current injection control, and chip cooling technology. With such improvements, we demonstrate disk laser pump laser bars with output power over 250W with 60% efficiency at the operating current. Ion implantation was investigated for improved current confinement. Initial lifetime tests show excellent reliability. For direct diode applications <1 um smile and >96% polarization are additional requirements. Double sided cooling deploying hard solder and optimized laser design enable single emitter performance also for high fill factor bars and allow further power scaling to more than 350W with 65% peak efficiency with less than 8 degrees slow axis divergence and high polarization.
de lasers are key components in material processing laser systems. While mostly used as pump sources for solid state or fiber lasers, direct diode laser systems using dense wavelength multiplexing have come on the market in recent years. These systems are realized with broad area lasers typically, resulting in beam quality inferior to disk or fiber lasers. We will present recent results of highly efficient ridge waveguide (RW) lasers, developed for dense-wavelength-beamcombining (DWBC) laser systems expecting beam qualities comparable to solid state laser systems and higher power conversion efficiencies (PCE).<p> </p> The newly developed RW lasers are based on vertical structures with an extreme double asymmetric large optical cavity. Besides a low vertical divergence these structures are suitable for RW-lasers with (10 μm) broad ridges, emitting in a single mode with a good beam quality. The large stripe width enables a lateral divergence below 10° (95 % power content) and a high PCE by a comparably low series resistance. We present results of single emitters and small test arrays under different external feedback conditions. Single emitters can be tuned from 950 nm to 975 nm and reach 1 W optical power with more than 55 % PCE and a beam quality of M<sup>2</sup> < 2 over the full wavelength range. The spectral width is below 30 pm FWHM. 5 emitter arrays were stabilized using the same setup. Up to now we reached 3 W optical power, limited by power supply, with 5 narrow spectral lines.
For the water vapour DIAL “WALES” the wavelength regions around 935 nm, 942 nm and 944 nm have been identified as the most suitable wavelength ranges. These wavelengths can be obtained using opticalparametric-oscillators (OPOs), stimulated Raman shifters and the Ti-Sapphire laser but none of these systems could deliver all the needed parameters like beam quality, efficiency, pulse length and energy yet. Also these systems are comparably big and heavy making them less suitable for a satellite based application. <p> </p>A fourth possibility to achieve these wavelength ranges is to shift the quasi-3-level laser lines (938 nm and 946 nm) of the Nd:YAG laser by replacing aluminium and yttrium by other rare earth elements. Changes of the host lattice characteristics lead to a shift of the upper and lower laser levels. <p> </p>These modified crystals are summarized under the name of "Mixed Garnet" crystals. Only the Mixed Garnet lasers can be pumped directly with diode laser and use a direct approach to generate the required laser pulses without frequency conversion. Therefore no additional non-linear crystals or special pump lasers are needed and a higher electric to optical efficiency is expected as well as single frequency operation using spectral tuning elements like etalons. <p> </p>In a first phase such mixed garnet crystals had been grown and characterised. The outcome was the selection of the gadolinium-scandium garnet for the most suitable laser crystal. During a second phase the complete laser system with output energy about 18 mJ in single 20 ns pulses and up to 8 mJ in free running mode with a combined pulse width of 250 μs at 942 nm have been demonstrated. <p> </p>The results of the first laser operation and the achieved performance parameter are reported.
Progress will be presented on ongoing research into the development of ultra-high power and efficiency bars achieving significantly higher output power, conversion efficiency and brightness than currently commercially available. We combine advanced InAlGaAs/GaAs-based epitaxial structures and novel lateral designs, new materials and superior cooling architectures to enable improved performance. Specifically, we present progress in kilowatt-class 10-mm diode laser bars, where recent studies have demonstrated 880 W continuous wave output power from a 10 mm x 4 mm laser diode bar at 850 A of electrical current and 15°C water temperature. This laser achieves < 60% electro-optical efficiency at 880 W CW output power.
Diode lasers are now basic pump sources of crystal, glass fiber and other solid state lasers. Progress in the performance of all these lasers is related. Examples of recently developed diode pumped lasers and Raman frequency converters are described for applications in materials processing, Lidar and medical surgery.
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.
High power semiconductor lasers, single emitters and bars are developing fast.
During the last decade key parameters of diode lasers, such as beam quality, power, spatial and
spectral brightness, efficiency as well as reliability have been greatly improved. However, often
only individual parameters have been optimized, accepting an adverse effect in the other key
For demanding industrial applications in most cases it is not sufficient to achieve a record value in
one of the parameters, on the contrary it is necessary to optimize all the mentioned parameters
To be able to achieve this objective it is highly advantageous to have insight in the whole process
chain, from epitaxial device structure design and growth, wafer processing, mounting, heat sink
design, product development and finally the customer needs your final product has to fulfill.
In this publication an overview of recent advances in industrial diode lasers at TRUMPF will be
highlighted enabling advanced applications for both high end pump sources as well as highest
brightness direct diode.
Until now, diode laser concepts were used in applications in the multi-kilowatt range, in which
actively cooled diode bars were used and combined via stacking. Per diode stack, laser outputs of
just over a kilowatt can be achieved. If outputs of several kilowatts are to be achieved, the radiation
of several stacks must be combined. A multi-kilowatt laser with industrially useful beam quality can
only be realized through appropriate procedures such as wavelength or spatial combining. The
beam quality of the coupled stacks corresponds to the quality of the individual stacks. If the beam
quality of such systems is pushed to the limit of the feasible, this reduces the efficiency of the total
Today, such conventional fiber delivered diode lasers with a beam quality of about 100 mm*mrad
achieve an outstanding efficiency of about 40%, but can only be used for laser soldering or other
surface processing. With conventional diode lasers, if the beam quality is improved to about 40
mm*mrad, the efficiency falls to about 32%. In order to tap all the efficiency advantages of direct
application of diode lasers and further improve the beam quality, TRUMPF has implemented a new
concept in its direct diode lasers of the TruDiode series. The basis of this concept is the use of a
fiber-coupled diode module with previously unachieved technical output characteristics. Another
major advantage of the approach is that the diodes are passively cooled. This paper will highlight
those characteristics, as well as provide technical details of the TruDiode series, extremely low
running cost and associated application fields.
Diode-pumped solid-state lasers are gaining acceptance as the desired laser source for materials processing as well as a
host of new applications that are expanding rapidly. Because of this, the performance, stability and lifetime of the diode-pump
source face unprecedented scrutiny. Increasing the lifetime of the diode, while increasing power, remains a
primary focus of the industry. One lifetime limiting issue is that of a voltage potential in the water cooling channels
which can cause cooler degradation and lower efficiency over time. Studies have been carried out that explore different
cooling approaches based on passive schemes where insulation layers are present to shield the voltage from the water
channels. However, with the introduction of insulation layers, a reduction of the deployable power from that of
microchannel coolers is seen. This report explores the effects of passive cooling approaches on the power and
divergence of 1 cm AuSn/CuW mounted bars with fill factors ranging from 10% to 50%. It is shown that a 150 W array
can be realized on a passive cooler and multiplexed to give a 1600 W stack. Thermal modeling is presented along with
life-test data for passively cooled devices.
Water vapour absorption wavelengths have been directly generated by diode pumped Nd:YGG crystals emitting at 935 nm and with Nd:GSAG crystals emitting at 942 nm in cw and pulsed operation. In addition the 1064 nm fundamental wavelength from Nd:YAG pump lasers with pulse lengths of 10 or 20 ns was shifted using Stimulated Raman Scattering (SRS) or Ti:Sapphire (TiSa) lasers. The potential of Nd:GSAG, Nd:YGG, SRS and TiSa laser systems is compared for future incorporation into a satellite based Lidar system. High output energies are possible by recent advances of fiber coupled diode sources allowing pulsed longitudinal pumping of Q-switched solid state lasers.
For weather forecast, especially for civil protection from high-impact weather events, measuring the three-dimensional
distribution of water vapour by DIAL techniques is a fundamental concern. Especially for development and evaluation of
atmospheric models, knowledge of water vapour distribution is important. Suitable wavelengths for a water vapour
DIAL are e.g. around 943 nm. This region can be reached with well established technologies such as the optical
parametric oscillator (OPO) and the Ti:Sapphire laser. But these systems suffer from low efficiency and complex set-up.
In contrast the Nd:GSAG laser presented here can be directly pumped with 808 nm laser diodes. This supports the
realisation of an efficient and compact laser system. Different oscillator and amplifier setups working at 943 nm were
realised. An output energy of >17 mJ in a 100 ns pulse with 10 Hz repetition rate was demonstrated. In a MOPA system
a double pass gain of 1.5 and an output energy of >18 mJ was achieved. The Nd:GSAG laser oscillator was successfully
injection seeded with DFB laser diode from FBH-Berlin. Also the gain cross section in a Nd:GSAG laser crystal from
941-944 nm was measured. The FWHM of the homogeneous line is 2 nm with a peak stimulated emission cross section
of 4.0•10<sup>-20</sup> cm<sup>2</sup> at 942.7 nm.
The three-dimensional measurement of the global water vapor distribution in the atmosphere considerably improves the reliability of the weather forecast and climate modeling. A spaceborne Differential Absorption Lidar (DIAL) is able to per-form this task by use of suitable absorption lines of the broad absorption spectrum of water vapor. Because no interference with the absorption of other molecules exists, the range of 935/936 nm, 942/943 nm are the most preferred wavelength ranges for a water vapor DIAL. The challenge is to develop a dedicated efficient high power laser source emitting at these wavelengths. The comparison between frequency converters based on stimulated Raman scattering (SRS) and Ti:Sapphire and the directly generated Mixed Garnet laser shows the favorable properties of each concept and helps to evaluate the most suitable concept. Development of Raman frequency converters for high pulse energies concentrates on linear resonator de-signs and seeding using the Raman material as a direct amplifier based on Raman four-wave-mixing. In addition a seeded and frequency stabilized pulsed Ti:Sapphire laser system with output pulses up to 22 mJ injection-seeded at the water vapor absorption line at 935.684 nm with a spectral purity up to 99.9 % has been developed. Direct generation of the wavelengths 935/936 nm and 942/943 nm required for water vapor detection is possible with diode-pumped, Nd-doped YGG- and GSAG-crystals. First experiments resulted in pulse energies of 18 mJ in Q-switched and 86 mJ in free-running operation at 942 nm wavelength.