Macro-channel cooled, high power, fiber coupled diode lasers exceeding 1.2kW of output power

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1 Macro-channel coole, high power, fiber couple ioe lasers exceeing 1.2kW of output power Tobias Koenning* a, Kim Alegria a, Zoulan Wang a, Armin Segref a, Dean Stapleton a, Wilhelm Faßbener b, Marco Flament b, Karsten Rotter b, Axel Noeske b, Jens Biesenbach b a DILAS Dioe Laser Inc., 97 South Rita Roa, Suite 15, Tucson AZ 85747; b DILAS Dioenlaser GmbH, Galileo-Galilei-Strasse 1, Mainz, Germany ABSTRACT We report on a new series of fiber couple ioe laser moules exceeing 1.2kW of single wavelength optical power from a 4um /.2NA fiber. The units are constructe from passively coole laser bars as oppose to other comparably powere, commercially available moules that use micro-channel heat-sinks. Micro-channel heat sinks require cooling water to meet emaning specifications an are therefore prone to failures ue to contamination an increase the overall cost to operate an maintain the laser. Dilas new series of high power fiber couple ioe lasers are esigne to eliminate micro channel coolers an their associate failure mechanisms. Low-smile solering processes were evelope to maximize the brightness available from each ioe laser bar. The ioe laser brightness is optimally conserve using Dilas recently evelope propriety laser bar stacking geometry an optics. A total of 24 bars are couple into a single fiber core using a polarization multiplexing scheme. The moular esign permits further power scaling through wavelength multiplexing. Other customer critical features such as inustrial grae fibers, pilot beams, fiber interlocks an power monitoring are stanar features on these moules. The optical esign an the beam parameter calculations will be presente to explain the inherit esign trae offs. Results for single an ual wavelengths moules will be presente. Keywors: High power ioe laser, fiber coupling, materials processing 1. INTRODUCTION Two applications rive the evelopment of high power fiber couple ioe lasers. The materials processing inustry has manate the nee for more efficient laser systems with higher beam quality an longer life [1]. While lower powere ioe lasers are well establishe in solering an plastics weling applications, soli state, fiber, an CO 2 lasers ominate the inustrial weling market. Dioe lasers are physically smaller than other lasers, an their initial capital cost is not as large as it might be for traitional weling lasers because ioe lasers have fewer system components [2]. While the beam quality achieve toay oes not allow the use of ioe lasers for key-hole moe weling, the higher absorption of Aluminum at 8nm compare to CO2 an N:YAG lasers combine with the generally higher system efficiency makes the ioe laser an attractive tool [3]. Another avantage of the ioe laser is the capability of burr-free weling of stainless steel. With the beam quality achieve here, the lasers can be use in combination with scanners in applications like contour an quasi-simultaneous weling. Besies materials processing applications, the eman for stronger pump sources for both soli state an fiber lasers with ever better beam quality rises quickly. The single wavelength version presente here is well suite for this task an multiple systems can be combine by use of fiber pump combiners or ifferent free-space configurations epening on the pumping application. While ual wavelength pumping is not common toay, research is being conucte in this area. When fiber-coupling ioe lasers, two key factors etermine the outcome of the final prouct. The first is the source esign. The brightness of the source sets the bounary conitions an the final result can never be better than the source. The secon major challenge is to maintain the brightness of the source through all steps of the optical esign. Other esign requirements inclue lifetime, efficiency, an extra features like safety interlocks, power monitors, an pilot * t.koenning@ilas-inc.com; phone +1 (52) ; fax +1 (52) ;

2 beams. While the laser shown here is base on stanar chip material, all other aspects escribe above have been aresse uring the esign of the laser an will be presente here. 2.1 Heat sink 2. DESIGN Multiple esign requirements for the new Dilas kilowatt-class fiber couple ioe lasers mae it necessary to come up with a heat sink solution that has not been use for fiber couple moules before. While most existing fiber couple packages at Dilas are base on CS-type heat sinks, these heat sinks require about an inch-by-inch area per bar. For moules exceeing 1KW of optical power this leas to package imensions that are not esirable. Besies that, the thermal resistance of CS-type heat sinks has its limitations with ever increasing power capabilities of moern laser bars. Micro-channel coolers meet the size goal an have a low thermal resistance compare to CS-type heat sinks but ahere to other isavantages. For one, the cooling water has to be tightly controlle an small particle filters are require to maintain inustrial grae life times. Even with perfect water conitions the flow insie the micro channels causes wear an therefore the lifetime of a laser ioe is in most cases limite by the lifetime of the heat sink [4]. To achieve long term stability in copper, a flow rate of 2 m/s shoul not be exceee. Otherwise, erosion will increase the thermal resistance of the cooler over time an limit the life time of the evice. Another issue with micro-channel coolers is the amount of smile that becomes inherent when mounting the semiconuctor bar to the heat sink. In general it is easier to achieve low smile on a soli heat sink compare to a micro-channel coole heat sink where micro channels are locate only a few hunre micron below the laser bar. In orer to combine the avantages of both technologies, Dilas uses a macro-channel coole heat sink. By flowing the cooling water through the heat sink of each laser bar, a low thermal resistance of about.68 K/W is achieve. At the same time material thicknesses an water channels are esigne large enough to guarantee stable mounting of the laser bar with low smile an prevent any problems cause by particles in the cooling water. In aition the heat sink is esigne in a way that protrues forwar in the propagation irection of the laser. This esign as one axis for the heat flow an thus reuces the overall thermal resistance of the heat sink. As shown in Figure 1 (right) the heat sink is open to the bottom, which allows an efficient application of protective coatings to prevent corrosion. Thermal Resistance [K/W] Passively coole Macro-channel "Micro-channel" Passively coole - fit Macro-channel - fit "Micro-channel - fit" Flow Rate [l/h] Figure 1: Thermal resistance measurement of various heat sink types for one type of 976nm chip material (left); macro-channel coole heat sink (right) 2.2 Spatial beam combining by rotating prism Currently, two general methos of spatial beam combining for high power ioe lasers are well establishe. The first metho is to simply stack heat sinks on top of each other. While this metho works well for micro channel coole heat sinks, the stacking pitch is etermine by the thickness of the heat sink an typically not suitable for passively coole

3 heat sinks. Here, the typical metho of spatial beam combining is the use of mechanical steps in the base plate of the laser in combination with a 9 egree reirection of all beams by means of mirrors. While this metho is well establishe, it has two isavantages. The first isavantage is the fact that steps have to be machine into the base plate of the laser. For passively coole laser bars this base plate acts as a cooling interface an surface roughness is critical in orer to achieve goo thermal contact between each laser bar an the base plate. Steps a weight an make it more ifficult to achieve a low surface roughness compare to an even surface an increase overall machining cost. If water is use to cool the base plate, channel structures become more complex or the thermal resistance may iffer between bars ue to ifferent material thicknesses cause by these steps. This may result in a broaer spectrum of the combine beam ue to a varying rift in center wavelength between bars epening on the position of each bar on the step structure. Figure 2: Common methos for spatial beam combining: step-an-mirror (left); vertical stacks (right) A secon isavantage of the step structure is the mechanical tolerance of the step an the inflexibility of the esign. When trying to maintain the brightness of all laser bars through the optical train, it is crucial that beams are stacke as ense as possible in orer to achieve the lowest beam parameter prouct possible. Mechanical steps inherit tolerances that make it very ifficult to achieve this goal. Typically, a step size is chosen that minimizes clipping but as a compromise leaves some gaps between stacke beams to compensate for tolerances which leas to an increase BPP of the combine beam. Also, mechanical steps cannot be ajuste easily to vary the pitch. In praxis a variable pitch is very much esire as it not only allows trying ifferent optical esigns in a prototyping phase, but it also allows an active ajustment of the pitch epening on the actual beam. Especially when stacking beams in fast axis, the beam size per bar in some istance of the laser bar is highly epenent on the smile of each laser bar. Dilas has evelope a new metho of spatial beam combining that overcomes both of these isavantages. By aing a secon reflection to the beam path, one egree of freeom is gaine. The previously escribe metho with one mirror an mechanical steps allows controlling the far fiel pointing in the horizontal an vertical axis an the lateral position of the beam in the horizontal axis. The lateral position of the beam in vertical axis an thus the pitch is fixe by the mechanical step size. Dilas new rotating prism technology reirects the beam into the vertical axis prior to reflecting it by 9 egrees in the horizontal axis. In aition to a reirection of 9 egrees, this leas to a rotation of each beam by 9 egrees an beams are no longer stacke on top of each other vertically but arrange next to each other horizontally (Figure 3). As a result, all laser bars can be arrange on a flat mechanical plate which allows for even cooling, simplifie cooling channels an easy polishing of the plate for low surface roughness if require. In aition to these mechanical benefits, the stacking pitch can now be ajuste by a lateral movement of the rotating prism in the stacking irection (propagation irection of the incient beam). At the same time the lateral position of the beam in the vertical axis can be controlle by a left an right movement with respect to the incient beam. Pivoting the prism aroun two axes allows for a pointing correction of the beam both in the horizontal an vertical irection. These four egrees of freeom, compare to only three for the steps an mirror metho, allow stacking all beams accurately with full control over the lateral position an pointing of

4 each beam. As a result, beams can be stacke more ensely with the same power throughput compare to previously use methos an thus the beam quality of the laser bar can be better maintaine through the process of spatial beam combining. Rotate beam Incient beam Figure 3: New metho for spatial beam combining by rotating prism. Single prism an beam path (left); three bars spatially combine by rotating prisms (right) 2.3 Optical esign Any fiber couple ioe laser moule epens on the brightness of the laser bars that are use. While this brightness efines the limitations of what can be achieve, the goal for the optical concept is to maintain this brightness in the best possible way through the optical train an all the way into the fiber. The laser bar use is a 19 emitter bar with a fill factor of 2%. Each emitter is 1um wie an the center to center spacing between emitters is 5um. The fast axis ivergence was measure as 46 egrees (95% power inclue) an the axis ivergence is 7 egrees. Both, fast an axes are collimate using micro lenses with focal lengths of 6um an 4.2mm respectively. After axis collimation, a monolithic polarization beam combiner is use to reuce the beam with for each bar from 1mm to 5mm. Each twelve bars are then optically stacke in fast axis irection by use of the rotating prisms escribe above. Two moules of twelve laser bars each are arrange next to each other. The beams from both 12-bar moules are expane in axis to achieve a more symmetric beam an are spatially combine in fast axis irection. This results in a combine beam of 24 bars in fast axis irection, an half a bar (or 9.5 emitters) in axis irection. This combine beam is then focuse into the fiber. When fiber coupling high power ioe laser bars, the beam quality in axis is typically the limiting factor. For smaller fiber core sizes (i.e. 1um an 2um) various beam shaping methos exist to equalize the beam parameter prouct (BPP) in both orthogonal axes. However, when coupling a laser bar with the above escribe properties into a 4um core fiber with.2na, it is sufficient to use polarization beam combining to effectively reuce the near fiel beam with of the bar from 1mm to 5mm, without affecting the far fiel ivergence. In orer to maximize the power that can be couple into such fiber, multiple bars are stacke in fast axis irection until the BPP of the combine beam matches the BPP of fiber. = 5µ m = 4.2mm emitter, 12 f SAC mra θ (1) The beam with in axis after micro optics an polarization beam combiner is given by the pitch of the axis lens array, times the number of elements, ivie by 2 when taking the polarization beam combiner into account. 2. 5mm (2)

5 This leas to a beam parameter prouct in axis of BPP = = 3mm mra θ. (3) In fast axis similar calculations cannot be performe as easily as smile has a big influence on the beam quality. The FAC lenses are specifie to elivery >85% of the optical power within 3mra (full angle). Results have shown that with some selection of lenses an well controlle smile 4mra full with 95% percent power inclue can be achieve consistently. While the beam height at its waist in fast axis is only about 65um (full with), beams are optically stacke some istance away from the FAC lens an in orer to keep the efficiency high an average pitch of.9mm seems aequate. These numbers yiel a beam parameter prouct in fast axis for n stacke bars of BPP fast = n mra.45mm = n. 9mm mra 2. (4) A iagonal beam parameter prouct to combine fast an axis an to compare with the rotationally symmetric BPP of the fiber is efine by [5] as BPP total = BPP fast + BPP. With a BPP for the fiber of BPP fiber couple into the fiber can be estimate to =.2NA.2mm 4mm mra the maximum number of bars that can be BPPfiber = BPPfast + BPP = n.9mm mra + 3mra n 11 (5) However, this calculation assumes an even intensity istribution within a rectangular shape for both the near fiel an the far fiel istribution of each laser bar. This is not true an corners contain less power compare to the center. Base on previous experience an simulation results a total number of bars of 24 is selecte. While this is more than twice the amount than estimate above, this oes not mean that the expecte power loss is >5%. The combine beam parameter prouct of fast an axis for 24 bars is BPP total, 24 = 24.9mm mra + 3mm mra = 51. 6mm mra. (6) This is about 25% higher than the acceptance of the fiber when assuming even intensity istributions. Optical simulations show that the power loss for a real beam with its unerlying intensity istribution is still within acceptable bouns. However, the system esign is not loss-less at this point an losses are accepte to achieve a higher brightness at the penalty of efficiency. In orer to allow the use of spherical lenses for focusing, a telescope is use in axis. As shown by [5] there is an optimum magnification for this telescope in situations like this that minimizes the total BPP. In orer to minimize the total BPP the ratios of beam iameter an ivergence in fast an axis respectively have to be equal. fast Θ fast = (7) Θ With magnification. = M, magnifie an Θ = M Θ, magnifie equation 7 can be use to calculate the optimum

6 M = fast Θ Θ fast = 1.8mm 12mra 2.5mm 2mra = 5.9 (8) A computer optimize pair of spherical lenses is use to couple the now almost symmetric beam into the fiber. A water coole aperture in front of the focusing optics ensures that no light with an NA greater than.2 can reach the fiber KW unit base on single ioe wavelength 3. RESULTS Lasers with ifferent configurations have been built. While the first prototype was base on a 4um.2NA fiber, moules with 8um core fibers an.12 NA optics have been built for use with inustrial laser processing heas. Either an Optoskan QBH or Q5 (compatible to LLK-B) fiber connector may be chosen. Both configurations have been teste. Figure 4 shows a maximum power of about 1,25W an an electro optical efficiency of about 4% at the highest power level teste. The peak wavelength was measure at 976.nm with a spectral with of 2.8nm FWHM (4.1nm 9% power inclue). This result makes this laser not only useful in materials processing application but qualifies as a pump source for soli state an fiber lasers. With imensions of 25 x 23 x 86 mm 3 the laser hea can easily be integrate into a 19 rack-mountable box like the Dilas Compact Series. So far the weight has not been optimize. While the laser is not particularly heavy for a laser of this power level, large areas of material can be eliminate without affecting stability. Alternative material choices coul also be use to reuce weight. Power [W] Efficiency [%] Power Efficiency Current [A] Intensity Wavelength [nm] Figure 4: Results of single wavelength moule. Power an efficiency (left); wavelength (right) 3.2 2KW unit base on wavelength multiplexing A prototype unit has been built that uses two moifie 1KW moules that operate at ifferent wavelengths. For the first unit, ioe wavelengths of 915nm an 976nm have been selecte. A water coole QBH fiber was use, which is ieal for high power applications ue to its internal water cooling. A maximum of about 22W of optical power was achieve out of the fiber. The electro-optical efficiency at the highest operating power was above 4%. With two wavelength peaks which are separate by about 6nm, this unit is not esigne as a pump source but as an inustrial tool for materials processing. With the ability to a more wavelengths, this laser type can be scale up in power to achieve multiple kilo-watts of optical power within a beam parameter prouct of 4 mm mra.

7 25 2 Power [W] Power Efficiency Efficiency [%] Current [A] Figure 5: Results of ual-wavelength moule. Power an efficiency (left); picture of first prototype (right) 4. SUMMARY Challenges in esigning a high power, high brightness fiber couple laser have been iscusse. A new semi-active heatsink that oes not rely on micro channels but elivers efficient cooling at a small footprint has been shown. A big challenge in esigning a high brightness system is to maintain the brightness of the laser bars through the optical train. Often beam quality gets lost when optically stacking multiple laser bars. A new technology has been presente that allows for an active alignment of the stacking pitch for each element. This not only makes the system extremely flexible in a prototype phase, but also allows maximizing brightness while monitoring efficiency. The optical esign has been escribe an trae-offs between highest efficiency an highest brightness have been shown. Results have been shown for ifferent laser heas that have been built. While all lasers of this series inclue temperature sensors, fiber interlocks, an pilot beams, ifferent options are available for the fiber connector type an NA. Options with Optskan QBH an Q5 (compatible to Trumpf LLK-B) fiber connectors are available. Fibers with either 4um core an an NA of.2 or 8um core an incoupling optics for.12na can be use. For a single ioe wavelength, about 1,25W have been measure at the fiber output. The wavelength was measure at 976.nm with a spectral with of 2.8nm FWHM (4.1nm 9% power inclue) an electro optical efficiency of >4% which makes this unit an ieal pump source for soli state or fiber lasers. Other wavelengths are available upon request. A prototype unit with two ioe wavelengths at 915nm an 976nm has been built base on moifie versions of the 24- bar moule escribe above. 22W at the fiber output have been achieve at 4% electro optical efficiency. Moving forwar it is possible to combine more than 2 wavelengths to increase the total output power to multiple kw while maintaining a beam parameter prouct of 4 mm mra. REFERENCES [1] H. Schlüter, C. Tillkorn, U. Bonna, G. Charache, J. Hostetler, T. Li, C. Miester, R. Roff, T. Vethake, C. Schnitzler; Dense Spatial Multiplexing Enables High Brightness Multi-kW Dioe Laser Systems ; Proc. SPIE Vol. 614,(26). [2] S. Venkat; Weling with ioe lasers ; fabricator.com,(october 14, 28). [3] K. Howar, S. Lawson, Y. Zhou; Weling Aluminum Sheet Using a High-Power Dioe Laser ; Weling Journal, (May 26); pp

8 [4] W. Horn; High Power Dioe Lasers for Inustrial Applications ; ICALEO,(27). [5] Z. Wang, A. Segref, T. Koenning, R. Paney; Fiber couple Dioe Laser Beam Parameter Prouct Calculation an Rules for optimize Design ; SPIE LASE ,(211).

9 Copyright 211 Society of Photo-Optical Instrumentation Engineers. Speaker: Tobias P. Koenning, DILAS Dioe Laser, Inc. Paper Title: Macro-Channel Coole, High Power, Fiber Couple Dioe Lasers Exceeing 1.2kW of Output Power Session 3: Mounting an Cooling High Power Laser Dioes Paper: of Conference 7918 Date: Sunay, 23 January 211 Time: 3:3 PM - 5:1 PM Copyright 211, Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be mae for personal use only. Systematic reprouction an istribution, uplication of any material in this paper for a fee or for commercial purposes, or moification of the content of the paper are prohibite.