Haiyan An

Advances in high power and high brightness laser bars with enhanced reliability

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.

Packaging of high-power bars for optical pumping and direct applications

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.

Methods for slow axis beam quality improvement of high power broad area diode lasers

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.

Bar efficiency and beam quality for coarse wavelength multiplexing

We will present results from optical power conversion efficiencies as well as near field and far field study of CB packaged bars in the 900-1060 nm wavelength region for coarse wavelength multiplexing application.

Industrial high-power diode lasers: reliability, power, and brightness

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 parameters. 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 simultaneously. 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.

Unveiling laser diode "fossil" and the dynamic analysis for heliotropic growth of catastrophic optical damage in high power laser diodes.

Taking advantage of robust facet passivation, we unveil a laser “fossil” buried within a broad area laser diode (LD) cavity when the LD was damaged by applying a high current. For the first time, novel physical phenomena have been observed at these dramatically elevated energy densities within the nanoscale LD waveguide. The observation of the laser “fossil” is interpreted with different mechanisms, including: the origination of bulk catastrophic optical damage (COD) due to locally high energy densities, heliotropic COD growth, solid-liquid-gas phase transformations, strong longitudinal phonon cooling effect on the molten COD wave front, and the formation of patterns due to laser lateral modes. For the first time the COD propagation is analyzed temporally by an acoustic phonon bouncing model and the COD velocity is extrapolated to be exponentially decreasing from more than 800 μm/μs to a few μm/μs within a 20 μs time period as the energy density dissipates.

Advancements in high-power high-brightness laser bars and single emitters for pumping and direct diode application

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.

Methods for Slow Axis Beam Quality Improvements Leading to High Brightness Multi-kW Direct Diode Laser Systems

We will present methods we used to further improve the slow axis BPP without efficiency penalty. Results of performance for multi-KW direct diode laser system application with this technology will be presented as well.

Wavelength stabilization of high power laser systems using volume holographic gratings

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.

High brightness laser systems incorporating advanced laser bars

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.