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Stable and compact mounting scheme for > 1kW QCW diode laser stacks at 940nm C.Fiebig, B. Eppich, W. Pittroff G. Erbert Ferdinand-Braun-Institut für Höchstfrequenztechnik, Berlin, Germany ABSTRACT For the pumping of solid state lasers with high peak power pulses up to the TW range QCW diode laser stacks with pulse lengths between 200µs and 2ms are used. To realize long-term stable pump modules we already presented high power, high brightness 100W QCW diode laser bars [1] having a lateral aperture of 1.7mm only, a length of 4mm and a vertical divergence of 14° FWHM. Based on these we have developed a mounting scheme for stacks with > 1kW output power using these new kind of diode lasers. Due to the geometric dimensions of the chip we successfully realized a stack with a passive cooling scheme on both sides. Furthermore, we only used expansion matched materials such as CuW and Al2O3 ceramics, as well as AuSn solder processes for fixing the parts together. As a result the stack is very insensitive against environmental influences. Due to the small vertical divergence we were able to use fast axis collimators with large focal lengths, which relax the lens adjustment tolerances. At the conference we will present results for diode laser stacks with an output power of more than 1kW at duty-cycles up to 10% and an efficiency of about 50%. The beam parameter product for such diode laser stacks result in < 50mm·mrad for the vertical direction and in < 75mm·mrad for the lateral direction. These beam parameter values enable the coupling of the pump module to an optical fiber having a 1.2mm core diameter and a NA of 0.22. Furthermore, the low vertical fill factor of the stack radiation allows the combination of two stacks by beam deflection mirrors without significantly degrading beam quality, hence doubling the power coupled into the same fiber. Keywords: QCW, diode laser stack, conductively cooled, passively cooled, fiber coupling, expansion matched

1. INTRODUCTION High power diode laser stacks are required in nearly all fields of industry, medicine and research, including space applications. Due to the availability of high output power in QCW operation diode laser stacks become more and more important for the pumping of solid state lasers with high peak power pulses up to the TW range. Typically the laser chips for stacks have a chip width of 10mm and a short resonator length. This results in a large thermal power density that needs dissipation, which requires micro channel coolers to achieve high output powers. The challenge for such cooling systems is to optimize these complex and not expansion matched packaging and to reduce the effect of corrosion of the copper based micro channel coolers [2]. Several approaches have been presented to improve theses cooling systems to achieve a more reliable and stable device. Besides mechanical improvements of the micro channels and approaches for an optimized water chemistry [2] there are several approaches to replace the copper coolers by expansion matched coolers [3], combined packaging systems or macro channels [4]. The disadvantage of the reduced cooling effect is being compensated by using longer resonators. In the case of QCW diode laser stacks the cooling system can be kept more simple because of the reduced generation of heat. Thus, conductively cooled systems can be an alternative solution. Using diode laser stacks for fiber coupled pump modules there is a particular need in a low beam parameter product (BPP) to reduce the costs and to improve the efficiency of fiber coupling. Using 10mm laser bars for such a laser stack is difficult because special beam shaping systems are required to reduce the BPP caused by the large aperture. The simultaneous challenges of simple conductive cooling as well as a simple fiber coupling scheme can be solved by using laser chips with a reduced aperture, thus keeping the power at a similar level like that of commonly used 10mm bars [5]. The power loss caused by the reduced laser bar width is typically compensated by using a higher filling factor and a longer resonator. High-Power Diode Laser Technology and Applications VI, edited by Mark S. Zediker, Proc. of SPIE Vol. 6876, 68760J, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.762720

Proc. of SPIE Vol. 6876 68760J-1

In this paper we present a simple, stable and compact mounting scheme for conductively cooled QCW diode laser stacks using such kind of laser chips. The design and mounting scheme of our expansion matched diode laser stacks is described in section II. We also present a simple passive cooling system for these kind of stacks. In section III we show the measurement results concerning the L-I-V characteristic, spectral behavior and beam propagation. The stability of our expansion matched diode laser stack is shown and we will describe a system for a simple and low-loss fiber coupling scheme. Finally, the results are summarized in section IV and we will give a short outlook on our further work in section V.

2. DIODE LASER STACK DESIGN 2.1 Principal design of the QCW diode laser stack The basic idea of the QCW diode laser stack: -

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In previous proceedings [1] we already demonstrated 940nm diode lasers for Yb:YAG laser systems with more than 100W QCW output power, which had an aperture width being 5 to 10 times smaller than commonly used 10mm bars. We presented diode laser bars with 13 emitters and an overall aperture of about 1.7mm (filling factor about 60%), a length of 4mm and a vertical far field of 15° FWHM. Based on these we developed a mounting scheme for stacks with > 1kW output power using these new kinds of diode lasers. The 4mm long chips allow an optimal cooling homogeneity to both sides of the cavity. Thus, we can omit expensive, expansion matched micro channel coolers that are commonly used for stack designs. We are able to solder the chips directly onto thick expansion matched CuW carriers enabling an efficient conductive cooling. Further cooling is achieved by additional expansion matched macro channel coolers that are soldered on each side of the stack. The dimensions of the used subassemblies are shown in Fig. 1. Taking into account the thicknesses of the bonding support and the solder the vertical pitch between the stack subassemblies is 3mm. Static and dynamic thermal simulations for such CuW have been carried out. If we assume an average heating of 1W per emitter we achieve a simulated thermal resistance of about 1.85 K/W. In the case of the dynamic simulation we assume a pulsed heating of 7W per emitter. Figure 2 shows the temperature distribution of the CuW assembly after 1ms. Due to the temperature gradient between the inner and outer emitters (Fig. 2) we expect an additional spectral broadening by the thermal chirp of about 4nm. Figure 3 shows the increase of the maximum chip temperature versus the pulse length.

Fig. 1 Dimensional sketch of CuW subassembly and total height of stack

Due to the fact that we choose only expansion matched materials for our design we are able to achieve a high stability and reliability of the stack. The small vertical far field angle of the laser chips allows us to use collimation lenses with a large focal length. Therefore, the divergence behind these lenses is significantly reduced and the adjustment tolerances are less critical. Finally the small aperture results in a lower lateral BPP compared to commonly used 10mm bars. This allows us to develop a low-cost coupling scheme into a multimode fiber (1.2mm NA 0.2) without expensive beam


shaping. In order to produce a stack with an output power of more than 1kW and to keep the conditions for the fiber coupling a QCW diode laser stack with 12 layers is reasonable. The overall vertical pitch of such a stack is 33mm.

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Fig. 3 Dynamic simulation of CuW assembly. Temperature rise of the chip versus the time. 7W/emitter is estimated.

2.2 Mounting Scheme The diode laser chips and the Al2O3 bonding supports are soldered directly with AuSn onto the CuW carriers by using a ζ-phase process [6]. In order to achieve a good thermal contact resistance a sufficient surface quality is required. This optimization process has been carried out by our CuW manufacturer. The advantage of using the ζ-phase is its high melting temperature of T > 500°C. Therefore the following high temperature soldering processes are uncritical for the stability of the diode laser subassemblies. For stacking these parts to 12 layers we developed a special mounting tool with high accuracy distance spacers. Between the CuW subassemblies AuSn preforms are fixed onto the Al2O3 bonding supports. Figure 4 shows a subassembly and a soldered diode laser stack. The right hand side of Figure 4 shows that the deviation from the vertical pitch after the soldering process is smaller than +-12µm.


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2.3 Cooling system For an efficient cooling of the stacks to both sides simple built-on water cooled, ceramic heat sinks are used. These guarantee a homogeneous cooling along the cavity. The heat sinks are also expansion matched and were soldered on each side by using SnAg preforms. Finally the diode laser stack is fixed in a package that has connectors for the power and the water supply. The cooling system was manufactured by a subcontractor. 2.4 CTE parameters The design of the stack is based upon expansion matched materials. The CTE parameters of the used materials and subassemblies are shown in Tab. 1. Diode laser chip

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Table 1 CTE parameters of the materials used


3. RESULTS 3.1 L-I-V and spectral characteristic of QCW diode laser stack The diode laser stacks are designed to achieve output powers larger than 1kW and an energy per pulse up to 1J. Figure 5 shows the L-I-V characteristic of a stack measured for a pulse width of 1ms and a duty cycle of 2% (solid line). The stack has a threshold current of 14A, a slope of about 10.5W/A and maximum efficiency of about 45%. The maximum output power at 130A is about 1.2kW. The stack voltage at this current is about 21.8V. A comparison between measurements for 2% duty cycle up to 10% duty cycle (solid with circle) concerning L-I characteristic is also plotted in Figure 5. In order to achieve a comparable output power characteristic for higher duty cycles it is necessary to decrease the cooling temperature from 22°C for 2% dc to 6°C for 10% dc. The maximum output power at 130A changes from 1175W for 2% dc to 1135W for 10% dc. The inset in Figure 5 also shows the spectral characteristics of such a diode laser stack at 1kW as comparison of both duty cycles. The peak wavelengths are about 940nm and the spectral widths are about 7nm (95% of power).

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3.2 Beam parameter of QCW diode laser stack The fast axes of the individual stack levels are collimated with fast axis collimating (FAC) lenses by fixing the FAC bottom tabs with UV adhesive on the CuW subassembly. We used a Hexapod for adjusting the FAC. The measurement of the vertical divergence is done by using the following optical setup (Fig. 6). The vertical divergence of the individual bars as well as the whole stack can be calculated by dividing the measured width (CCD camera) of the vertical far field distribution by the focal length of the focusing lens L2. The lateral divergence is obtained by using a similar setup with a CCD camera and a diffuser panel.

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All beam parameters were measured in quasi-cw operation (1ms, 20 Hz) at an output power of 1kW. Fig. 7 shows the typical vertical and lateral far field distributions of our collimated diode laser stacks. The plot also shows the integrated power density. The overall divergence of the stacks in vertical direction is about 5mrad and the lateral divergence is about 170mrad (measured values apply to 95% of power). This corresponds to a BPP of the diode laser stack of < 50 mm mrad and < 75 mm mrad in vertical and lateral direction, respectively.

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3.3 Stability test of mounting scheme We verified the stability of the diode laser stack mounting scheme using temperature environment tests, where the temperature was looped between –40°C and +85°C for a hundred times. In a first set of measurements we tested if there is an effect on the L-I-V characteristic and the spectral properties (Fig.8) of an uncollimated diode laser stack. In a second series of measurements we analyzed if there is an effect on the BPP of a collimated diode laser stack. In order to obtain the BPP we measured the divergence of the whole stack and the relative far field position in vertical direction of each bar. As to be seen in the plots, both temperature environment test series show no effect concerning the diode laser stack characteristics, neither in the L-I-V characteristic nor in the beam parameter of the stack and the individual bars respectively. The results demonstrate that our mounting scheme with the exclusive use of expansion matched materials and AuSn solder guarantees a high stability and reliability of the stack. Furthermore the stability of the beam parameter guarantees very good conditions for a stable fiber coupling scheme.

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3.4 Fiber Coupling Due to the small beam parameter, in contrast to typical stacks also in lateral direction, we are able to design a simple low-loss scheme for coupling into a multimode fiber with a core diameter of 1.2mm and a numerical aperture of 0.22. If we assume approximately Gaussian shaped near field and far field distributions, then beam waist and BPP of the stack should not exceed 0.92mm and 340mrad, respectively (95% stack power), in order to keep the coupling losses below 2%. This corresponds to a required beam propagation factor M² of < 260. Figure 9 shows the simulated optical setup and the beam propagation of such a simple fiber coupling scheme by using commercially available cylindrical lenses (lenses are not corrected). The beam propagation is simulated with the simulation software ZEMAX. The single emitters were approximated as Gauss-Shell-emitters having the well-known geometric dimension [1] and the measured far field distributions. Taking into account the uncorrected lenses the simulated beam propagation factor for such a diode laser stack behind the optical system is about 215 in lateral and 170 in vertical direction. Due to the aforementioned estimation a low-loss coupling should be possible. The ZEMAX simulation of the fiber coupling shows losses of about 2% that are probably caused by aberrations. Finally we compare the simulated beam propagation by opposing a QCW diode laser caustic to the ZEMAX simulation (Fig. 10). The measured images are in good agreement with the simulations. Due to the aberration of the uncorrected lenses a theoretic maximum coupling efficiency of 98% should be feasible by using the presented optical coupling scheme.

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Fig. 9 Simulated optical fiber coupling scheme (Zemax); geometric optical beam propagation in side view (top) and top view (bottom), uncorrected lenses (Thorlabs): L1 f =152mm LJ1895L1; L2 f =76.3mm LJ1703L2; L3 f =51.9mm LJ1728L2

Fig. 10 Comparison of the simulated (top) and the measured (bottom) beam profiles near the beam waist.


4. SUMMARY We show a conductively cooled approach to realize stable and compact 1kW diode laser stacks. The presented mounting scheme utilizes only expansion matched materials, safeguarding the stability and reliability of the device. The reduction of the diode laser bar aperture results in a lateral beam parameter product of below 75mm mrad. Taking into account the vertical BPP of below 50mm mrad the stack can be easily applied for a fiber coupling system. Finally, we show such a simple and cost effective fiber coupling scheme where a coupling efficiency of 98% should be feasible.

5. OUTLOOK 5.1 Vertical collimation with a FAC-Array The next step will be to achieve a simplification of the separate collimation of each stack level by using a FAC-Array. The vertical far field angle of only 15° FWHM allows us to use extremely large focal length. Optical simulations show that the totalized mismatch between the diode laser stack and these FAC-Array has to be lower than 10µm. Currently the vertical pitch tolerances are +- 12µm. By optimizing our mounting tools we want to reduce these tolerances to a value lower than 5µm. In that case the vertical pitch of the individual FAC-Array lenses has to be below 5µm. In order to get more flexibility for the overall tolerances a further increase of the pitch between the bars or a decrease of the fast axis angle of the bars below 15° FWHM might be beneficial.

Fig. 11 Left: Individual FAC for each stack layer. Mounting and lens tolerances are uncritical. right: FAC-Array. Pitch and lens tolerances are critical. Improvement of mounting scheme is necessary.


5.2 Enhancing brightness via beam combining Due to the vertical distance between adjacent laser bars the vertical fill factor of the radiation is below 50%. This allows for a comp-shaped beam combination of two or more stack elements e.g. by a pile of glass plates with reflective side face as given in Figure 12 or similar schemes. Since this incoherent coupling will neither increase the divergence nor the near field extension. Combining two stack elements should only slightly increase the beam propagation factor compared to a single stack but double the output power resulting in nearly doubled brightness.

Fig. 12 Example of a comb-shaped beam combining using two stacks.

REFERCENCE 1

C. Fiebig, G. Erbert, W. Pittroff, H. Wenzel, A. Maaßdorf, S. Einfeldt and G. Tränkle, “High power, high brightness 100 W QCW diode laser at 940 nm” Proceedings of the SPIE, Volume 6456, pp. 64560K (2007) 2 Haake,John M. and Faircloth, Brian O.,"Requirements for long-life microchannel coolers for direct diode laser systems," Proceedings of the SPIE, Volume 5711, pp. 121-131 (2005) 3 D. Lorenzen and F. Dorsch, „Mounting substrate and heat sink for high-power diode laser bars,“ US Patent US 6,535,533 B2 (H01 S 3/04), 2000. 4 M. Leers, K. Boucke, Ch. Scholz and T. Westphalen, „Next generation of cooling approaches for diode laser bars,“ Proc. SPIE, vol. 6456, pp. 1A-1 – 1A-10, 2007 5 N. Lichtenstein, Y. Manz, J. Müller, J. Troger, S. Pawlik, A. Thies, S. Weiß, R. Baettig, C. Harder, „High Brightness Laser Diode Bars“, Proceedings of SPIE Vol. 6104 (2006) 6 W. Pittroff, G. Erbert, G. Beister, F. Bugge, A. Klein, A. Knauer, J. Maege, P. Ressel, J. Sebastian, R. Staske and G. Tränkle, „Mounting of high power laser Diodes on boron nitride heat sinks using an optimized AuSn metallurgy“, IEEETrans. Adv. Packag., vol. 24, no. 4, pp. 434-441, 2001
