More brilliance from high-power laser diodes

Modern high power diode laser systems supply an output power of up to 3 kW, which is sufficient for their use in material processing. By building up a low cost 3D-laser assembly consisting of a diode laser and a common industry robot, combined with an off-line programming system and a flexible sensor interface, it was possible to make use of the high power diode laser’s characteristic attributes. This system is already qualified for different manufacturing methods like laser beam welding, hardening and remelting. Detailed information about the required system technique and working results for each manufacturing process are given as follows.Modern high power diode laser systems supply an output power of up to 3 kW, which is sufficient for their use in material processing. By building up a low cost 3D-laser assembly consisting of a diode laser and a common industry robot, combined with an off-line programming system and a flexible sensor interface, it was possible to make use of the high power diode laser’s characteristic attributes. This system is already qualified for different manufacturing methods like laser beam welding, hardening and remelting. Detailed information about the required system technique and working results for each manufacturing process are given as follows.

In this work we report on a novel optical design for beam shaping and focalization of high-power diode laser bars. The goals of our study are: the increase the optical throughput of the beam shaping device with respect to standard solutions and either to enhance the irradiance on a target or to inject the laser beam into a smaller fibre than with respect to beam shaping system based on plane surfaces. The high power diode laser bars pose serious difficulties in their optical handling due to their strong difference between the two transverse axes, which induce a strong astigmatic and asymmetric output radiation. As is well known, the beam quality is very different in the two axes called slow axis and fast axis, and in particular the slow axis is composed by the superposition of several multimodal sources. The beam quality in this axis is very low (its etendue may exceed 2000 mm mrad). On the other hand, the fast axis has a very high beam quality, near diffraction limited, although with very high divergence (30°-50°). The common solution for the application of the laser radiation is a fast axis aspheric micro lens in front of the emitters, in order to achieve its collimation. Typical values of the fast axis collimated beam are 0.7mm and less than 6mrad. However, the so obtained collimated beam is poorly focusable with a standard lens, and a few methods were proposed to overcome the problem. The more relevant solutions include: the stepped mirror technique, the plane parallel mirrors pair, micro prisms array and confocal micro lens array. Each of these techniques is based on the equalization of the beam parameter product by the subdivision of the beam in the slow axis and its reshaping. For all these techniques the efficiency spans from 50% to 70%. The best focalization results allow the coupling in a fibre of 400μm diameter, with NA-0.22. The aim of this work is the design and the realization of a new device, that is considered as target the following aspects: 1) the maximum optical efficiency in the beam shaping process, 2) the optimal equalization of the beam parameter product for the two axes, 3) the use of few optical elements and 4) a very compact size. These goals are addressed by a scheme that splits the collimated beam from the laser diode into different portions while the length of the optical paths of each sub element is kept constant, and by the subsequent use of short focal length aspheric lenses for the focalization of the transformed beam. Each sub-beam is deflected by a couple of plane parallel mirrors, whose normal is directed to equalize the BPP without any mutual shadowing. An optimal solution can be easily envisaged for a laser source of common size of 0.7 x 10 mm. The condition on equal optical path length has the noticeable property of placing the virtual position of the individual portions into which the original beam is split at the same distance with respect to target. Thanks to this, their subsequent focusing is unaffected by the axial displacement of the common solution by the stepped mirrors. In fact, to correct this effect, this latter technique requires the use of a prism pair, involving complexity, size enlargement and higher costs. In this work both an extensive ray tracing and optical analysis is presented as well as the experimental characterization of an experimental model. Moreover, we also report on the technique for the realization of th tilted-face plane mirrors of which is composed our beam shaping device. The scheme of beam shaping here reported can be extended to higher power beam by means of the technique of the beam combination by polarization coupling or that of the optical beam compression. Examples of theses developments are discussed in the paper, and experimental results presented. The most direct applications of the class of optical devices here reported are the high power diode laser direct application in material processing or manufacturing, the coupling into multimode optical fiber of the diode laser radiation as well as the fiber laser end pumping.

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A. Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

Temperature continues to be an issue in the reliability of high power laser diodes. Any effort, therefore, made to understand the dynamics of temperature on the performance of laser diodes is important. This is because it serves as a catalyst for the generation of nonradiative recombination centers, which ultimately kills laser diodes. In this paper, the convolution of simulation results was done to compare it with experimental results, which revealed more details that hitherto, could not have been captured by experiments alone. The inability of experiments to capture these details is due to the limited spatial resolution of the charged-coupled device (CCD) camera, which is approximately 30 µm much larger than the thickness of the quantum well active region used in the experiment. The convolution results showed a further smile-shaped profile within the four groups of array emitters, which in the experimental results, were considered as one emitter due to the limited spatial resolution of the CCD camera. The use of convolution to determine more details was investigated, due to the dominant effect of temperature found across the output power distribution of high power semiconductor laser diode bars.

High power diode lasers have become very attractive as an intelligent tool for direct material processing as well as pump sources for miniaturized solid state lasers and fiber lasers since optical beam transformation systems generate the required beam circularization from the originally highly asymmetric brightness of the output beam. This presentation focuses on reliable microassembling processes to realize micro-optical beam transformation systems for high power diode laser bars and stacks for the efficient coupling of the beam into optical fibers at reasonable costs. The modular semi automatic micro assembly set up realized consists of a six axes alignment system with an accuracy in the submicron range for the three spatial areas as well as microrad for the three rotational axes, a semi-automatic glue dispenser as well as respective grippers and magazines which are necessary for the handling of the micro optical components. The established automatic processes for the mounting of the fast axis collimation lenses are of particular interest for the assembly of e.g. fiber coupled modules of diode laser stacks whereas more than 20 fast-axis collimations are necessary.

High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time. The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.

High power diode laser bars and stacks are of great interest in industrial applications due to their high electro-optical efficiency, their small type of construction and maintenance free operation. With highly sophisticated beam shaping optics diode lasers can be used as pumping sources for solid state and fiber lasers and direct for material processing, e.g. welding, soldering and marking metals. We have developed different fiber coupled diode laser systems with output power up to greater than 2 kW cw into a spot 0 1.0 mm (power density greater than 250 kW/cm<SUP>2</SUP>) and systems with output power 170 W cw into a spot 0 0.38 mm (power density about 150 kW/cm<SUP>2</SUP>). The 2 kW system operates with a 0 1.5 mm fiber (N.A. 0.32) and consists of polarization and wavelength coupled stacks with an overall electro-optical efficiency of 23%. The smaller system operates either with a 0 0.6 mm (N.A. 0.22) or 0 0.4 mm (N.A. 0.33) fiber and consists of a single stack. Polarization and wavelength coupling will be realized in future. The overall electro-optical efficiency is about 27%.

Quantification of thermoreflectance temperature measurements in high-power semiconductor devices—lasers and laser bars

In order to achieve a thermally stable diode laser system based on high power diode laser bars, micro channel heat sinks are used to face the dissipated power with a density of 10⁶ W/m². Passively cooled diode lasers are either lower in power or facing higher junction temperatures. As a matter of principle the cooling with micro channel heat sinks requires a sealing between the heat sink itself and the system around. The leakage of this sealing, normally achieved by O-rings, can be reduced but never avoided. Sensible systems and extreme lifetime requirements, like in the telecom applications, already require passively cooled diode lasers with no water in the inner system boundaries. To achieve a minimized temperature shift in the junction, we developed a new copper based heat sink, spreading the dissipated heat in an optimised manner. Based on this, our further research shows that the higher temperature shift in a passive submount compared with active ones can be tolerated for a system, if the heat resistance to the external water heat exchanger is minimized. For applications either with or without the requirement of a thermo electric cooling element (TEC), we developed a technical solution for a heat exchanger, to keep water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on an regular ambient temperature and a minimum of junction temperature mismatch.

A beam shaping technique for a high power diode laser bar is presented. This optical design reshapes the highly asymmetric and astigmatic radiation of a high power diode laser bar, equalizing the beam quality in the orthogonal axes without loss of brightness. The optical paths of each sub-beam are equal, realizing the conservation of the beam quality in a system just a third of the size of previous solutions with similar performances. We present the results of a simulation and the production of a prototype for focusing the beam of a QCW bar to a spot size of about 300 µm with an NA of 0.28. The method for the realization of the optical elements is explained in detail.

High Power Diode Laser Bars serve as the foundation for many high-power laser systems used in manufacturing and research. Efficient heat removal is essential for reducing cost per watt by increasing available output power or extending the effective lifetime of these devices. In this paper we will introduce a novel method for electrically isolated water impingement cooling of high-power diode laser bars operating in the 975nm region. We will present results from 58 emitter diode laser bars operating in the 975nm region with CW output power levels exceeding 500W and a thermal resistivity of less than 0.12 K/W.

New technology regarding manufacturing of micro cooling systems and selective laser melting was presented at Photonics West 2005; at Photonics West 2006 the first non-corrosive active micro coolers manufactured by this procedure were introduced to the auditorium. This presentation shows the latest results regarding active micro cooling systems for high power diode laser with a matched CTE. After the first non corrosive micro coolers passed the tests regarding cooling performance, there were still two tasks which had to be accomplished: First one was the homogeneity of cooling. In order to achieve the homogeneity, the design of the supporting structure in front of the micro cooling structures was modified. Measurements of cooling performance and the range of the wavelength along the laser bar showed that with the modified structure a homogeneous inflow was achieved as well as homogeneous cooling along the laser bar. The presentation highlights the measurements and results. Second point of development was the quest for a material combination with an adequate thermal conductivity for keeping the cooling performance, but with a coefficient of thermal expansion that matches with the CTE of diode laser bar material. After testing several materials fitting to these requirements, only one material was left, which was capable to machine workable micro coolers with the SLM procedure. The presentation contains the first results of the micro coolers with a matched CTE regarding thermal resistance, CTE and packaging with gold - tin solder.

This paper presents the model to estimate the reliability of high power diode laser system based on failure rate of each diode. The diodes' reliability is estimated from multi-cell life test with accelerated optical power and junction temperature. The system is then considered as a k-out-of-n:G load sharing system. The hypo-exponential distribution is applied to model the system level reliability. Two solutions to hypo-exponential model are introduced to compute the reliability at expected time and two examples are presented to illustrate the model. The numerical examples verify that the scaling and squaring method is more stable than exact algorithm considering numerical instability. Estimating with the hypo-exponential model, the single emitter diode laser system shows reliability higher than 0.999 at the lifetime required by customer.

Diode Lasers have been used for years in low power density applications. The lack of sufficient beam quality and life time in the range of a few thousand hours limited their applicability severely. However, in the past years the intrinsic beam quality as well as the lifetime was increased significantly. Single bars with beam parameter products around BPP=20 mm*mrad have been demonstrated [4]. Lifetimes of 30.000h (90 %) @100W cw operation are available [5]. Consequently the market increasingly accepts this type of lasers and new application domains are investigated.In this paper, existing high power diode lasers and new concepts for increasing power by incoherent superposition, wavelength multiplexing and stacking are summarized. As a result, a laser roadmap is derived. The status and perspectives of new direct diode laser applications are reviewed, especially single shot simultaneous processing.Diode Lasers have been used for years in low power density applications. The lack of sufficient beam quality and life time in the range of a few thousand hours limited their applicability severely. However, in the past years the intrinsic beam quality as well as the lifetime was increased significantly. Single bars with beam parameter products around BPP=20 mm*mrad have been demonstrated [4]. Lifetimes of 30.000h (90 %) @100W cw operation are available [5]. Consequently the market increasingly accepts this type of lasers and new application domains are investigated.In this paper, existing high power diode lasers and new concepts for increasing power by incoherent superposition, wavelength multiplexing and stacking are summarized. As a result, a laser roadmap is derived. The status and perspectives of new direct diode laser applications are reviewed, especially single shot simultaneous processing.

“Smile” in high power diode laser arrays is mainly caused by thermal stress generated during packaging processes and operation. By numerical modeling, a three-dimensional finite element model of a conduction-cooled packaged 60W high power diode laser array was established. Finite element method (FEM) was conducted to simulate and analyze the thermal and mechanical characteristic of the diode laser bar in different scenarios. According to the variation of thermal stress and displacement across the laser bar, the influence of temperature and driving current on “smile” was analyzed and discussed. Numerical simulation results show that “smile” will decrease with the increasing heatsink temperature and increase with the increasing driving current. In this paper, the corresponding quantitative relationships of smile with heatsink temperature and driving current were obtained. The work provides theoretical guidance for optimizing high power diode laser array design, packaging process and reducing “smile”.