Tailored bar concepts for 10 mm-mrad fiber coupled modules scalable to kw-class direct diode lasers

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1 Tailored bar concepts for 1 mm-mrad fiber coupled modules scalable to kw-class direct diode lasers Andreas Unger*, Ross Uthoff, Michael Stoiber, Thomas Brand, Heiko Kissel, Bernd Köhler, Jens Biesenbach DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 1, Mainz-Hechtsheim, Germany ABSTRACT In this paper, laser modules based on newly developed tailored bars are presented. The modules allow efficient fiber coupling of more than 32 W into 1 mm-mrad or 16 W into 6 mm-mrad at one single wavelength. For further power scaling dense wavelength coupling concepts are presented which enable kw-class lasers with a beam quality of 1 mm-mrad. Keywords: High brightness diode laser, direct diode applications, fiber coupling 1. INTRODUCTION Direct diode lasers for materials processing in the kw power range are a highly active field of research and will potentially replace fiber lasers in applications like remote welding and sheet metal cutting. For these applications, beam parameter products well below 2 mm-mrad are required. Over the last years the tailored bar concept for efficient fiber coupling into 2 mm-mrad together with the tooling for fully automated mass production of laser modules for pumping applications was developed at DILAS. In this paper, laser modules based on newly developed tailored bars are presented. The modules allow efficient fiber coupling of more than 32 W into 1 mm-mrad or 16 W into 6 mm-mrad at one single wavelength. For further power scaling, dense wavelength coupling concepts are presented which enable kw-class lasers with a beam quality of 1 mm-mrad. 2.1 The T-Bar concept 2. MODULAR LASER CONCEPT The modular concept has already been described in previous publications 1,2. The basic subunit is a tailored minibar, with pitch, emitter size and number of emitters chosen such that a desired beam quality in slow-axis direction can be realized without using sophisticated beam shaping optics. The optical design concept is based on fast-axis collimator (FAC) and slow-axis collimator (SAC) lenses followed by only one additional focusing optic for efficient coupling into a fiber with 2 µm core and a numerical aperture of.22. The lateral structure of the tailored minibar is defined by 5 emitters with an emitter width of 1 µm spaced by a pitch of 1 µm. The advantage of such a low fill factor bar is a reduced thermal crosstalk between the emitters. The thermal simulation for the low fill factor bar shows negligible thermal crosstalk between the emitters which increases reliability and potential output power per emitter. The next step of the modular concept is to arrange seven tailored minibars on one baseplate in combination with FAC and SAC for each bar. In addition, mirrors are implemented on the baseplate to build an optical stack. All optical components are mounted automatically to ensure very high reproducibility and an efficient production process. As a result, pointing errors are minimized which is important for beam quality with regard to fiber coupling or wavelength stabilization, which is possible by using only one volume holographic grating (VHG) for the whole baseplate. Another important design aspect is that the cooling strategy allows the use of industrial water for the bottom-cooled baseplate. Continuous improvement of the laser bar and assembly techniques leads to fiber laser pump sources with 27W from a 2µm NA.22 fiber with over 55% power conversion efficiency. The single base plate is used for various beam sources accommodating up to 8 of such plates and delivering up to 2kW of laser power 2 (Fig 1).

2 Power (W) (%) Fig. 1: Schematic drawing of modular diode laser concept based on one common baseplate. T-Bars with improved brightness Since the T-bar is designed to couple to 2mm mrad without beam transformation coupling to smaller fibers again would need beam transformation optics or leads to coupling efficiency losses and increased optics cost. To keep the advantages of the concept, it is desirable to design an improved bar which keeps the same diode pitch as the original bar. In order to improve the brightness and enable coupling to 1µm NA.2 fiber, the fill factor of the new bar was lowered to 5%. Figure 2 shows the LI curve of the resulting bar. The bar is designed to deliver 3W of laser power with a beam parameter product of 7.5mm mrad and more than 6% power conversion efficiency. It can serve as a direct replacement of the conventional T-bar without changing optics or the base plate design Fig. 2: LI curve of the tailored bar with improved brightness 2.2 Performance of improved base plate Figure 3 shows the LIV curve of the improved base plate and a caustic scan (based on the second moment method) of the plate at a working current of 3A. From the base plate, 185W with a conversion efficiency of 55% at 3A is reached. The

3 Power 1µm NA.2 fiber (W) Power (W) beam parameter product in the slow axis was measured to 1 mm mrad. The detoriation of the BPP from bar to base plate is caused by edge effects of the slow axis collimation lens array % 56% % 48% 44% 4% Fig. 3: Left: LI curve of the base plate with 7 bars. Right: corresponding caustic scan based on the second moment method 2.3 Beam sources based on the improved base plate With the improved base plate, a fiber coupled beam source with coupling to 1µm NA.2 was built. Figure 4 shows the resulting LI curve. At the working point of 3A, 15W of output power with a conversion efficiency of 5% was reached. The LI curve shows a pronounced s-shape which can be explained with the bar characteristics as a function of current: at low currents a widening of the emitter is observed. Since the emitter is imaged onto the fiber, a larger spot on the fiber and low coupling efficiency results. With increasing current the emitter becomes smaller and coupling efficiency improves. The emitter pitch on the bar together with the focal length of the SA collimation array determines the acceptance angle of the slow axis divergence. With increasing current the slow axis divergence increases and when the acceptance angle of the SA collimation is reached side spots with increasing power appear and lower the overall coupling efficiency. This effect is seen as a rollover in the LI curve starting at around 3A. 2 6% % 4% 3% 2% 1% % Fig. 4: LI curve of the fiber coupled beam source with one base plate and coupling to a 1µm NA.2 fiber Since the output of the base plate is polarized, two base plates can be coupled via polarization coupling leading to over 3W from a 1µm NA.2 fiber. Figure 5. shows the resulting LIV curve.

4 Relative Signal Power (W) Z 4 6,% 3 45,% 2 3,% 1 15,%,% Fig. 5: LI curve of the fiber coupled beam source with two base plates and coupling to a 1µm NA.2 fiber 3. DENSE WAVELENGTH COUPLING OF WAVELENGTH STABILIZED BASE PLATES To further improve brightness, wavelength coupling can be employed. To achieve very high brightness and output powers in the kw range, standard wavelength coupling with 2-4nm wavelength pitch coupling is not sufficient. For these standard wavelength coupling schemes, the epitaxy of the laser bar material is adjusted to the correct wavelength and multiple base plates with spectral linewidth up to 6nm are coupled and the number of wavelength coupled base plates is limited to 4-5 in the 9xx nm band. To achieve higher brightness levels, a smaller channel spacing between the different wavelengths is needed which also makes spectral stabilization necessary. For coupling multiple base plates at a central wavelength of 976nm, a wavelength spacing of 4nm was choosen. A demonstrator setup with three baseplates was built. In these setup 3 plates are wavelength stabilized with VBGs at 972nm 976nm and 98nm (Fig. 6). The linewidth of the modules was smaller than.5nm. These three plates were than coupled with commercially available dichroic mirrors with an edge steepness of ~1nm. Angle tuning of the dichroic mirrors was used to achieve optimum coupling efficiency by using only one type of mirror. Figure 6 shows the Zemax simulation of the resulting setup. 1 Spectrum, C, C, C,9,8,7,6,5, ,3,2, Wavelength (nm) X Y Fig. 6. Left: spectra of the three wavelength stabilized base plates Right: Zemax simulation of the wavelength coupling prototype Figure 7 shows the output power after coupling of the three plates into a 1µm NA.2 fiber. An output power of 41W at 35A was achieved. Since all of the optics for this setup were stock optics, performance of the system was not ideal and ~4% power conversion efficiency was reached. With optimized optics we expect to achieve 5W of output power with >45% efficiency. This three plate setup still has a linear polarized output beam so the output power can be doubled to 1kW

5 Power (W) at a single central wavelength. Conventional coarse wavelength coupling can then be used to achieve several kw of output power from a 1µm fiber Fig. 7: LI curve of the fiber coupled prototype with three wavelength coupled base plates and coupling to a 1µm NA.2 fiber 4. SUMMARY In this paper, the development of new laser sources base on tailored bars with improved brightness was reported. These bars feature a beam parameter product of 7.5 mm mrad in the slow axis at an output power of 3W. Based on these bars, a base plate with 185W and 55% conversion efficiency was developed, which can be efficiently coupled to 1mm mrad fibers. Fiber coupled sources with 15W from one base plate and 3W from two base plates which are polarization coupled were presented. Based on dense wavelength coupling of three VBG-stabilized base plates with a spectral spacing of 4nm, 4W of output power from a 1µm NA.2 fiber was achieved. Polarization coupling and optimization of the coupling will lead to a beam source with 1kW of output power from 1µm NA.2 at a single central wavelength. Further wavelength coupling opens up the route to multi-kw sources based on the tailored bar concept with 1mm mrad beam quality. ACKNOWLEDGEMENT A part of this work was supported with funding from the Brilliant Industrial Diode laser (BRIDLE Project Number ) project under the 7 th Framework Programme (212-NMP-ICT-FoF) of the European Commission. 5. REFERENCES 1. Haag, M., Köhler, B., Biesenbach, J., et al., "Novel high-brightness fiber coupled diode laser device", Proceedings of SPIE Vol. 6456, 6456T (27) 2. B. Köhler et. al.; Scalable high-power and high-brightness fiber coupled diode laser devices ; Proc. SPIE Vol. 8241, (212)