High-brightness high-power kw-system with tapered diode laser bars

Transcription

1 High-brightness high-power kw-system with tapered diode laser bars Bernd Kö hler *a, Jens Biesenbach a, Thomas Brand a, Matthias Haag a, Sandra Huke a, Axel Noeske a, Gabriele Seibold a, Martin Behringer b, Johann Luft b a DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, Mainz-Hechtsheim, Germany; b Osram Opto Semiconductors GmbH & Co, Wernerwerkstr. 2, Regensburg, Germany ABSTRACT We report on a diode laser system, which is based on tapered diode laser bars and provides a combination of high power and high beam quality comparable to high power lamp pumped solid-state-rod lasers. Until now diode laser systems with output powers in the kw-range are based on broad area diode lasers. However, the output of these kilowatt laser systems usually is characterized by a strongly asymmetric beam profile, which is a consequence of the asymmetric beam parameter product () (a) of broad area diode lasers with regard to the - and the fast-axis direction. Apparently the output of such a laser system can not be coupled efficiently into a fiber, which is required for a variety of applications. The symmetrization of the of such a laser system requires complicated and expensive beam shaping systems. In contrast tapered diode laser bars allow the design of high power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Power scaling is realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. The total output power of the tapered diode laser system was 3230 W at a current of 75 A. Fiber coupling yielded 2380 W at 75 A for a fiber with a core diameter of 800 µm (NA 0.22) and 1650 W at 60 A for a 600 µm fiber (NA 0.22), respectively. Focusing with an objective with a focal length of 62 mm led to a beam diameter of 0.52 mm in the focal plane. Taking into account the total power of 2380 W behind the fiber the resulting intensity in the focal plane was 1.1 MW/cm 2. Keywords: High power diode laser, tapered diode laser, high brightness, beam quality, diode laser stack, fiber coupling, incoherent coupling 1. INTRODUCTION The main benefits of high power diode laser systems are a high wall-plug efficiency, high optical power, reliability, long lifetime, relatively low investment costs and a small footprint. As a result of these advantages high power diode laser systems have gained substantial interest for a variety of applications including materials processing and solid state laser pumping 1. However, besides these numerous advantages, the major drawback of high power diode laser systems is their poor beam quality. Therefore high power diode laser systems are mainly used for applications where extremely high beam quality is not absolutely necessary. At the moment the beam quality of commercially available high power diode lasers with multi-kw output power is limited to about 100 mm mrad. Compared to lamp pumped solid state lasers with a below 30 mm mrad, it becomes evident that high power diode lasers based on broad area multimode emitters are not able to replace lamp pumped solid state laser systems at present. Since the poor beam quality is a basic feature of a broad area diode laser bar, an improvement of the beam quality can only be achieved by a modification of the lateral chip structure 2. One approach to enhance the lateral beam quality is a diode laser bar with a tapered resonator geometry 3. The main benefit of tapered diode lasers is the combination of high output power and high beam quality. Tapered diode laser bars provide about an order of magnitude higher brightness than conventional broad area multimode semiconductor lasers 4. This allows the design of high power laser systems with a symmetric beam profile without the necessity of using complicated beam shaping systems. * b.koehler@dilas.de, tel. +49 (0) ; fax +49 (0) ; (a) defined as w q /2 (half beam waist diameter w 0 times half far field divergence angle θ /2 ) = 0

2 2. TAPERED DIODE LASER BARS AND STACKS The ouptut beam of a common broad area diode laser bar is characterized by a highly asymmetric profile with regard to beam dimension and divergence angle. Typical values for the source width are 10 mm for the lateral direction in the plane of the pn-transition (-axis) and 1 µm for the vertical direction (fast-axis), respectively. The beam divergence angles are typically 35 in fast-direction and 5-7 in -direction, respectively (a). Consequently the resulting beam parameter products are highly asymmetric. Whereas the beam quality in the fast-direction is about 1 mm mrad and thus nearly diffraction limited, the beam quality in the -direction is in the region of 500 mm mrad, which is far beyond the diffraction limit. Enhancement of the beam quality in the -direction can be achieved by means of a cylindrical lens array for individual collimation of the single emitters in the -axis (SAC-array). Improvement of beam quality by means of a SAC-array is a consequence of an enhancement of the filling factor F : d em F = (1) d Pitch The maximum theoretical enhancement factor strongly depends on the emitter structure of the diode laser bar and is defined by the inverse ratio 1/F of emitter size d em and emitter pitch d Pitch. For typical broad area diode laser bars the filling factor is between 20% and 50% and thus the maximum enhancement of beam quality is limited to a factor of 2-5. It should be emphasized that the of the individual emitter is unchanged. The improvement of the of the diode bar is solely a consequence of filling factor enhancement. 2.1 Basic properties of tapered diode laser bars The vertical structure (fast-axis) of tapered diode laser bars is similar to that of common broad area diode laser bars. The significant difference between tapered diode lasers and broad area diode lasers becomes apparent when regarding the lateral structure (cf. Fig. 1). Fig. 1 also shows the internal structure of a single emitter of the tapered diode bar structure. In contrast to a broad area multimode emitter, the single emitter of a tapered diode laser bar consists of a small ridge waveguide followed by a tapered amplifier section. The high beam quality is defined by the ridge waveguide and the high output power is provided by the tapered section, while maintaining the beam quality of the ridge waveguide. Typical data for the diode parameters are summarized in Table 1 and Table 2. Table 1: Typical data for the single emitter of a tapered diode laser bar. power [W] output facet d emitter [µ m] source width d ridge [µ m] Single emitter of a tapered diode bar l ridge [mm] l taper [mm] refractive Index n d astigmatism (»l taper /n) [mm] beam divergence q / 2 [ ] M 2 single emitter single emitter [mm mrad] One important consequence of the tapered structure is the difference of source position for the fast- and the -axis. Whereas the source position of the fast-axis is on the output facet of the diode bar, the source position of the -axis is located inside the diode bar at the transition between the ridge waveguide and the tapered part. This astigmatism defines the characteristics of the microoptics, particularly with regard to the position and the focal length of the -axis collimation lens. According to Equ. (1) the filling factor of a tapered diode laser is given by F d ridge / d Pitch and is significantly smaller, when compared to a common broad area diode laser bar. However, it should be mentioned that the beam divergence of a tapered diode laser bar in -direction usually exceeds the -axis divergence of a common broad area diode laser bar. (a) the divergence angles are defined as half far field divergence angle (1/e 2 - value)

3 The data in Table 2 show that the of a single bar without SAC-array is nearly independent from the diode bar type. The difference in beam quality in the -direction becomes apparent when a SAC-array is added to the diode bar for filling factor enhancement in the -direction. Practical experience shows that filling factors of about 0.9 are feasible, which is close to the theoretical value of 1. For the data of Table 2 the beam quality of the tapered diode laser bar in the -direction is improved by a factor of 15 when compared to broad area type A and by a factor of 9 when compared to broad area type B. Fig. 2 shows a simulation of the beam divergence after adding microoptics to the different diode bar types. The difference in the after filling factor enhancement by means of a SAC-array results in significant different beam divergences behind the SAC. Table 2: Typical data for diode laser bars with different lateral structures. diode structure bar width [mm] number of emitters emitter width [µ m] beam divergence q / 2 [ ] single emitter [mm mrad] M 2 single emitter pitch [µ m] filling factor [%] diode bar [mm mrad] diode bar optimized filling factor [mm mrad] broad area type A broad area type B tapered structure tapered diode laser bar broad area diode laser bar n n d emitter pitch d emitter pitch tapered diode laser single emitter d ridge emitter source l ridge virtual emitter source θ n l tapered structure d astigmatism emitter source fast output facet q 2 d emitter Fig. 1: Lateral structure of a tapered diode laser bar (top left) compared to a broad area diode laser bar (top right). Typical data for the different diode laser bar structures are summarized in Table 2. Typical data for a single emitter (bottom) of a tapered diode laser bar are summarized in Table 1.

4 The beam quality of the diode laser bar in the fast-direction is nearly diffraction limited. However adding a FAC (fast axis collimation lens) to the diode bar for fast-axis collimation leads to a slight degradation of the in the fast-axis. The extent of the degradation strongly depends on the quality of the FAC with regard to lens aberration, misalignment and smile. Typical values for the in the fast-direction are 1-2 mm mrad. The total of the diode laser bar is given by: tot 2 2 symmetrization = + fast fi 2 fast (2) where and fast are the beam parameter products of the diode laser bar without symmetrization of the. Perfect symmetrization of the diode laser bar will lead to a significant reduction of the total as shown on the right side of Equ. (2). From Equ. (2) it becomes evident that an improvement of the beam quality in -direction by a factor of n only leads to an improvement of the total beam quality by a factor of n. Fig. 2: Simulation of -axis divergence for different types of diode laser bars with microoptical elements for beam shaping. Typical -axis divergences are 90 mrad for the diode structure 200/400 with FAC, 40 mrad for diode structure 150/500 with FAC/SAC and 3 mrad for the tapered diode structure with FAC and SAC. The divergence in fast-axis behind the FAC is in the range of 1-3 mrad in each case. 2.2 Tapered diode laser stacks Stacking diode laser bars on top of each other is common practice for power scaling of diode laser systems but with the drawback that the output power can only be increased at the cost of the beam quality in the fast-axis direction. The approximately scales with the height of the stack because the divergence in fast-axis direction is primarily not influenced by stacking. As a consequence of the increase in the fast-direction the asymmetry between the of the fast- and the -direction is reduced. However for common broad area diode lasers and typical numbers of diode bars per stack the resulting is still asymmetric with regard to and fast-axis direction. For efficient coupling of the diode laser light into optical fibers a symmetric is mandatory. Therefore subsequent symmetrization and beam shaping elements are necessary in order to obtain a symmetric for the whole stack. In contrast, symmetrization of a stack with tapered diode lasers (with FAC and SAC-array) is simply realized by choosing the appropriate number of diodes per stack with the additional benefit that also the divergences of - and

5 fast-axis are almost equal. Therefore no subsequent beam shaping optic is necessary. In Table 3 typical parameters for a symmetrized stack of tapered diode lasers are summarized. Taking into account different incoherent multiplexing setups, a diode laser system that is based on tapered diode laser stacks can provide output powers in the multi-kw range with a better than 30 mm mrad. This combination of high output power and high beam quality shows that a tapered diode laser system has the potential to replace lamp pumped solid state lasers. Table 3: Typical data for a stack of tapered diode lasers. The calculations are based on a residual fast-axis divergence of 2 mrad behind the FAC, an output power of 40 W per bar, a filling factor of 90% in -direction and an optical loss of 6 % for FAC and SAC. M 2 single emitter Tapered diode laser stack with FAC and SAC stack stack fast [mm mrad] [mm mrad] number of diode bars in stack total stack [mm mrad] (a) total power [W] Characterization of tapered diode laser bars and stacks As decribed in the previous section the beam quality and therefore the coupling efficiency strongly depends on the characteristics of the individual tapered diode laser bar. Until now high power tapered diode laser bars with well-defined and reproducible properties are not commercially available. In order to realize an efficient system consisting of many single diode bars, a detailed characterization of the individual diode bars is inevitable. Therefore each individual diode laser bar was analyzed with regard to power, center wavelength and spectral width. For some random samples we also examined astigmatism, virtual emitter size and polarization state. These investigations revealed remarkable differences of the characteristic parameters for different wavelengths. For instance the average output power per bar at a current of 78 A was nm, nm, nm and nm. The average spectral bandwidth was nm, nm, nm and nm. Furthermore significant differences were observed even for diode laser bars with the same design wavelength. In order to maintain the beam quality of individual laser bars the microoptics for collimating the and the fast-axis were mounted after stacking the single diodes on top of each other. Afterwards each individual stack was characterized with regard to power, beam quality and fiber coupling efficiency. In the low-current regime slightly above treshold current the coupling efficiency was approximately 80 % for all wavelengths. However, increasing the current led to a wavelength dependent decrease in coupling efficiency. The results are summarized in Table 4. Table 4: Typical coupling efficiencies of tapered diode laser stacks at different wavelengths. The efficiencies are given for low current slightly above threshold and a typical operating current of 70 A, respectively. The fiber core diameters were 600 µm and 800 µm, respectively (numerical aperture 0.22). Coupling efficiency at 808 nm 880 nm 940 nm 980 nm 600 µm fiber NA 0.22 low power (current) 80 % (20 A) 73 % (40 A) 77 % (10 A) 77 % (10 A) high 70 A 57 % 62 % 36 % 54 % 800 µm fiber NA 0.22 low power (current) 82 % (20 A) 83 % (40 A) 80 % (10 A) 81 % (10 A) high 70 A 67 % 75 % 46 % 61 % (a) 2 2 total defined as : + fast

6 The decrease of coupling efficiency with increasing current was further investigated by measuring the beam quality of the stacks with an appropriate beam diagnostic device (a). Fig. 3 shows the beam profile of an 808 nm stack for two different currents of 20 A (left diagram) and 70 A (right diagram). The beam diameter of the -direction was enlarged from 1.4 mm up to 3.6 mm, whereas the beam diameter in the fast-direction was nearly constant in the current range of A. The of the -direction rised from 39 mm mrad at 20 A up to 104 mm mrad at a current of 70 A. Fig. 3: Beam profile measurements of a 808 nm tapered diode laser stack in the focal plane of a lens with focal length f =100 mm. The measurements were performed for two different currents of 20 A (left side) and 70 A (right side). The remarkable difference between the measured and the theoretical value of 16 mm mrad (cf. Table 3) indicates that the properties of the tapered diode laser bar are not stable, but change as a function of current and temperature. The worsening of beam quality in the -direction with increasing current is primarily ascribed to a variation of position, size and divergence angle of the virtual source of the -axis. This is supported by measurements performed by Knitsch et. al. 5. Detailed analysis of the current dependent astigmatism yielded a shift of about 1 µm per ampere. Furthermore the measured M 2 -value of the single tapered diode emitter was in the range of 6-8. The theoretical value of 16 mm mrad for the in the -axis direction is based on a single emitter beam quality of M 2 =2. Furthermore the source position can vary for different emitters on an individual tapered diode laser bar. As a result beam collimation of the individual emitters (with SAC-array) is not optimal and therefore the overall beam quality is downgraded. On the other hand an imperfect SAC-array can also worsen the beam quality if the focal length of the SACarray is not constant for all segments. Finally these effects lead to an increased spot size on the workpiece or a reduced coupling efficiency into an optical fiber. 2.4 Simulation of fiber coupling efficiency To determine the impact of different parameters on the beam quality of the tapered diode laser bar we performed some raytracing simulations. The basic parameters of the simulation were a -axis divergence of 6, a beam quality of M 2 =2 for the single emitter (corresponding to an emitter size of 11.4 µm in -direction), an astigmatism of 600 µm and a fast-axis divergence angle of 30. The theoretical of the stack is 23 mm mrad (cf. Table 3), which should allow efficient fiber coupling into an optical fiber with a core diameter of 300 µm and a numerical aperture of The results of the simulation are shown in Fig. 4. Each single diagram in Fig. 4 shows the variation of one parameter, whereas the remaining parameters are constant. As expected, the simulation shows a very good fiber coupling efficiency of nearly 90% for the basic parameters. Increasing the -axis divergence or the emitter size results in a rapid decrease of fiber coupling efficiency. This is evident because the of the -axis directly scales with the -axis divergence and emitter size. Taking into account the source shift of about 1 µm per ampere (cf. Sect 2.3), the current dependent astigmatism will result in an astigmatism of about 70 µm. The simulation shows that this will lead to a fall in coupling efficiency from 90 % to about 70 % - 80 %. However, the experimental results yielded much lower coupling efficiencies even for fibers with a larger core diameter. Therefore the simulations indicate that astigmatism is not the only reason for the decrease in coupling efficiency. The fast-axis divergence does not directly affect the beam quality of the stack. However, a high fast-axis divergence makes high demands on the quality of the FAC, because the numerical aperture of the FAC has to match the divergence of the fast-direction. Imperfect collimation in the fast-axis direction due to lens abberations of the FAC will also lead to a reduction in fiber coupling efficiency. (a) Primes FocusMonitor (rotating pinhole);

7 efficiency [%] efficiency [%] axis divergence half angle [ ] emitter size direction [µ m] astigmatism [µ m] fast-axis divergence half angle [ ] Fig. 4: Simulation of coupling efficiency of a tapered diode laser stack into an optical fiber [cf. text]. 3. LAYOUT AND DESIGN OF THE LASER SYSTEM Similarly to conventional high power laser systems based on broad area diode bars, the basic unit for the high power diode laser system composed of tapered diode lasers is a stack, where the individual tapered diode laser bars are arranged on top of each other. Each single diode bar is provided with appropriate microoptics for collimating both the fast- and the -axis, respectively. Power scaling is realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and finally wavelength multiplexing. After all the beam is coupled into an optical fiber by means of a suitable focusing optics. As described in Sect. 2.2 symmetrization of the laser beam is simply realized by selecting the appropriate stack height. Consequently, in contrast to a high power laser system based on broad area diode bars, after power scaling no subsequent beam symmetrization is necessary. The schematical setup of the laser is shown in Fig. 5. According to Fig. 5 the system is based on the combination of a total of 16 individual stacks. For each wavelength first of all two stacks are coupled into the same optical path via spatial multiplexing. Subsequently the two stack pairs are coupled via polarization multiplexing. Since all individual bars have the same polarization it is necessary to rotate the polarization of two stacks with an appropriate half-wave plate. Finally the wavelength modules each with four stacks are coupled via wavelength multiplexing with suitable edge filters.

8 It should be noted that polarization and wavelength coupling of tapered diode lasers make high demands on the thin film coupling devices, because the spectral width of tapered diode lasers (typically > 5 nm) is significantly larger than the spectral width of conventional broad area diode lasers. The spectral width has an important impact on the coupling efficiency, especially if the wavelength difference of the concerned beams is small. Fig. 5: Schematical setup of the high-power, high-brightness tapered diode laser system. The decrease of beam quality with increasing current has to be considered when designing the mechanical setup of the laser system. As a consequence with increasing current some part of the beam will be cut off by components inside the device, which will finally lead to a loss in total output power and to a heating of the components. On the other hand this truncation is desirable for fiber coupling because it reduces the exposure of the fiber. Therefore apertures inside the device have to be designed carefully to minimize heating of the fiber. The mechanical setup of the laser unit is shown in Fig. 6. The geometrical dimensions of the head are 350 x 270 x 280 mm (length x width x height), its total weight is 29 kg including the fiber coupling unit, that is apparent on the right side of Fig. 6. Taking into account the fiber coupling unit the length is increased to 660 mm. Fig. 6: Mechanical setup of the high-power, high-brightness tapered diode laser system with fiber coupling unit.

9 4. CHARACTERIZATION OF THE LASER SYSTEM The system was characterized in detail with regard to beam quality, total output power and fiber coupling efficiency. The results are summarized in this section. 4.1 Laser system without fiber Fig. 7 shows the total output power as a function of diode current. The maximum output power was 3230 W at a current of 75 A. It should be noted that the current for the 940 nm module was limited to 60 A, because of the deterioration in beam quality for higher currents, which was in particular observed for the 940 nm module. This limitation led to the small deviation of the power vs. current characteristics from linear above 60 A. A measurement of the beam quality of the laser head is shown in Fig. 8 and Fig. 9 for an operating current of 40 A. Focusing the beam with an objective with a focal length of 98 mm resulted in a beam diameter of 0.65 mm in the fast direction and 0.95 mm in the -direction, respectively. Taking into account the total power of A the resulting intensity in the focal plane was 265 kw/cm 2. The beam quality is characterized by a of 49 mm mrad for the fast-direction and 90 mm mrad for the direction, respectively (cf. Fig. 8). Increasing the current to 70 A will raise the power by a factor of 2.5, but the Fig. 7: Output power as a function of operating current. intensity will not increase by the same factor due to the broadening of the spot size for higher current. beam radius [mm] direction fast-direction M 2 = 300 (90 mm*mrad) M 2 = 163 fast (49 mm*mrad) position z [mm] Fig. 8: Beam quality measurement of the laser system for an operating current of 40 A. The focal length of the objective was 98 mm.

10 Fig. 9: Beam profile of the laser head without fiber for an operating current of 40 A in the focal plane at z = 9 mm (cf. Fig. 8). 4.2 Results for fiber coupling with 600 µ m and 800 µ m NA0.22 fiber The results for fiber coupling are shown in Fig. 10 for two optical fibers with core diameters of 800 µm (left diagram) and 600 µm (right diagram), respectively. The numerical aperture of both fibers was Maximum output power was A for the 800 µm fiber and A for the 600 µm fiber. The corresponding efficiencies were 72 % and 68 %, respectively. It should be noted that for the 600 µm fiber the current of the 940 nm module was limited to 50 A (cf. Sect. 4.1). Fig. 11 shows a measurement of beam quality behind the 800 µm fiber and at a current of 70 A. Focusing with an objective with a focal length of 62 mm yielded a beam diameter of 0.52 mm in the focal plane. Taking into account the total power of 2380 W after the fiber the resulting intensity at the focal plane was 1.1 MW/cm 2. Fig. 10: Output power and coupling efficiency for a 800 µm fiber (left) and a 600 µm fiber (right), both with a numerical aperture of 0.22.

11 Fig. 11: Beam profile in the focal plane of an objective with a focal length of 62 mm for an operating current of 75 A. The fiber had a core diameter of 800 µm and a numerical aperture of SUMMARY AND CONCLUSION We have demonstrated a high-brightness diode laser system based on tapered diode laser bars with multi-kw output power. Intensities above 1 MW/cm 2 allow the system to be used for deep penetration welding applications and other applications, which require high power densities, as proven by experiments whithin the national German research project MDS 6. It should be noted that the system described did not use the full theoretical capacity of tapered diode laser bars with regard to beam quality. This was due to the drawbacks mentioned in the sections above. The main problem was the decrease in lateral beam quality with increasing current. In the future the design and fabrication of tapered diode laser bars has to be improved in order to build reliable systems with reproducible properties. Systems based on tapered diode lasers have the capability of a in the range of mm mrad with output powers in the multi-kw range. The results show that high power tapered diode laser systems are well on the way to replace lamp pump solid state lasers in the near future. ACKNOWLEDGEMENTS A part of this work was sponsored by the German Bundesministerium für Bildung und Forschung (BMBF) within the project Modular Diode Laser Beam Tools (MDS) 6. The authors gratefully acknowledge Osram Opto Semiconductors for the supply of tapered diode laser bars at 808 nm, 940 nm and 980 nm. We also would like to thank the Fraunhofer Institute for Applied Solid-State Physics (IAF) * for the supply of tapered diode laser bars at 880 nm. * mikulla@iaf.fhg.de; Fraunhofer Institute for Applied Solid-State Physics (IAF), Tullastrasse 72, Freiburg, Germany; phone / ; fax /

12 REFERENCES 1. F. Bachmann; Chances and Limitations of High Power Diode Lasers ; Proc. SPIE Vol. 5336, p.95 (2004) 2. A. Schmitt, M. Behringer, G. Herrmann, M. Philippens, J. Herrlein, J. Luft; High-brightness highly-reliable InGaAlAs/GaAs laser bars with reduced fill factor and 60% efficiency ; Proc. SPIE Vol. 4973, p.1 (2003) 3. Kintzer et al.; High-power, strained-layer amplifiers and lasers with tapered gain regions ; IEEE Photon. Tech. Lett., Vol. 5, (1993) 4. M. Mikulla; Tapered high-power, high-brightness diode lasers: Design and Performance ; in High power diode lasers ; Ed. Roland Diehl, Springer Topics in Applied Physics Vol. 78; p (2000) 5. A. Knitsch et. al.; Characterization of tapered diode laser bars for the use in high power diode laser systems ; Proc. SPIE Vol. 5336, p.56 (2004) 6. F. Bachmann; Application adapted diode laser systems A result of the German national research project MDS ; Proc. SPIE Vol. 4973, p.68 (2003)