Wavelength stabilized multi-kw diode laser systems

Wavelength stabilized multi-kw diode laser systems Bernd Köhler *, Andreas Unger, Tobias Kindervater, Simon Drovs, Paul Wolf, Ralf Hubrich, Anna Beczkowiak, Stefan Auch, Holger Müntz, Jens Biesenbach DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz-Hechtsheim, Germany ABSTRACT We report on wavelength stabilized high-power diode laser systems with enhanced spectral brightness by means of Volume Holographic Gratings. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser systems is an important method to increase the efficiency of diode pumped solid-state lasers. It also enables power scaling by dense wavelength multiplexing. To ensure a wide locking range and efficient wavelength stabilization the parameters of the Volume Holographic Grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the Volume Holographic Grating, the reflectivity of the diode laser bar as well as its angular and spectral emission characteristics. In this paper we present detailed data on wavelength stabilized diode laser systems with and without fiber coupling in the spectral range from 634 nm up to 1533 nm. The maximum output power of 2.7 kw was measured for a fiber coupled system (1000 µm, NA 0.22), which was stabilized at a wavelength of 969 nm with a spectral width of only 0.6 nm (90% value). Another example is a narrow linewidth diode laser stack, which was stabilized at a wavelength of 1533 nm with a spectral bandwidth below 1 nm and an output power of 835 W. Keywords : High power diode laser, wavelength stabilization, Volume Holographic Grating, broad area diode laser, fiber coupling, spectral beam combining 1 INTRODUCTION High-power diode laser systems are well established laser sources for a variety of applications including materials processing and solid state laser pumping. The main advantages of such systems are high wall-plug efficiency, high optical power, reliability, long lifetime, relatively low investment costs and a small footprint. However, besides these numerous advantages, one drawback of high-power diode laser systems is their relatively poor spectral brightness. Typical broad area diode laser bars have a large spectral width of about 3 to 6 nm and the peak wavelength drifts with driving current and temperature. The rapid progress in the fiber laser area has increased the demand for efficient diode pump lasers. For Ytterbium (Yb)- fiber lasers around 1080 nm normally fiber coupled diode laser systems at 915, 940 and 980 nm are used as pump sources. Especially the pump region at 980 nm is important because of the high absorption coefficient in combination with a small absorption bandwidth. To ensure stable and efficient pumping over the whole operating range, it is helpful to control the spectrum of the pump diodes in such a way that the spectral bandwidth of the laser diode is always consistent with the absorption bandwidth of the active laser medium. For Thin Disk Yb:YAG lasers it is beneficial to pump the zero-phonon line at 969 nm to improve beam quality and opticaloptical efficiency because of a smaller quantum defect compared to standard 940 nm pump wavelength 1. Another example which requires a narrow spectral bandwidth for pumping is Nd:YVO 4 at 888 nm, which is advantageous because of its isotropic absorption region with equal absorption coefficients in both polarization directions and the reduced quantum defect compared to the pump region at 808 nm 2. * b.koehler@dilas.de, tel. +49 (0)6131 9226 133; fax +49 (0)6131 9226 255; www.dilas.de

One of the most demanding application with regard to spectral linewidth is optical pumping of alkali vapor lasers (e.g. rubidium or cesium) which requires a linewidth of about 10 GHz. For these demands spectral control of a diode laser pump source is absolutely mandatory for efficient pumping 3,4,5. In addition to enabling efficient pump sources for solid state lasers wavelength stabilization of high-power diode laser systems also enables power scaling by dense wavelength multiplexing. In recent years the brightness of diode laser bars has been significantly improved mainly by increasing the output power per emitter and by reducing the slow-axis divergence. The development led to the design of new types of diode laser bars with reduced number of emitters and increased pitch between the emitters. These minibars have advantages compared to the traditional 10 mm broad diode laser bars 6. Further brightness enhancement of diode laser systems is achieved by polarization coupling and wavelength multiplexing. Polarization coupling is limited to a factor of 2, whereas wavelength multiplexing is only limited by the number n of available wavelengths. As a matter of course, power scaling by wavelength multiplexing is achieved at the cost of spectral brightness. Wavelength multiplexing with standard broadband diode laser sources and wavelength couplers based on dielectric coatings requires a spectral distance of about 30 nm. Using diode laser sources with stabilized narrow emission spectra and Volume Holographic Gratings as combination elements the spectral distance can be significantly reduced down to 3 nm 7. As a consequence, the number of diode laser bars that can be multiplexed for a given spectral range increases, resulting in an enhancement of brightness. In the next section we will describe some general aspects of wavelength stabilization. 2 GENERAL ASPECTS OF WAVELENGTH STABILIZATION Different methods have been investigated in the past for improving the spectral brightness of broad area diode laser bars. These approaches can be divided into internal and external solutions. For internal solutions the wavelength stabilizing structure is integrated into the diode laser bar itself, whereas for external solutions separate bulk elements with integrated Bragg grating are used for wavelength stabilization. An example for a diode laser bar with internal wavelength stabilization is a distributed feedback diode laser (DFB) where the grating for selective spectral feedback is integrated in the structure of the active region of the laser bar itself. With such a device the wavelength shift with temperature is reduced down to about 0.08 nm/k and in addition the spectral bandwidth is reduced to less than 1 nm 8. It is evident that the fabrication process of such a DFB-diode laser is more complex leading to an increase in costs. Another drawback is the reduced efficiency of a DFB-diode laser, when compared to a standard broad area diode laser bar. In contrast to this internal approach wavelength stabilization by external components has also been investigated. One example for an external wavelength stabilizing element is a thick volume grating based on a photo-thermo-refractive (PTR) inorganic glass 9. Recording of highly efficient Bragg gratings in such photosensitive glass is achieved by periodic variation of the refractive index by UV exposure. Such volume diffractive gratings are commercially available from different vendors with slightly different nomenclatures, like Volume Bragg Grating (VBG) 10, Volume Holographic Grating (VHG) 11 or Reflecting Bragg Grating (RBG) 12. In contrast to the internal solution no modification of the chip structure is required for external wavelength stabilization, i.e. that standard diode laser bars can be used for wavelength stabilization with external Volume Holographic Gratings. This is an important advantage of the external solution. Furthermore, external stabilization leads to a further reduction in temperature drift and spectral bandwidth, when compared to the internal solution. The temperature drift can be reduced down to about 0.01 nm/k and the spectral bandwidth to less than 0.3 nm. However, one important disadvantage of external components is the requirement for sensitive and high-precision alignment of the VHG. A typical setup for a diode laser bar with external stabilization is shown in Fig. 1. Because of the angular sensitivity of the VHG it is advantageous to reduce the divergence of the diode laser bar especially in the fast-axis direction by collimating the beam with a fast-axis collimating lens (FAC). This will significantly increase the optical feedback by the VHG. Collimation of the beam in the slow-axis with a slow-axis collimating lens (SAC) is not mandatory. The VHG is positioned directly behind the FAC. The table in Fig. 1 shows typical alignment tolerances that are required for efficient wavelength stabilization.

typical tolerances for rotation x-axis y-axis z-axis ± 0.5 mrad ± 10 mrad ± 10 mrad Fig. 1: Typical setup for a wavelength stabilized diode laser bar with a VHG positioned directly behind the fast-axis collimating lens (FAC). The table shows typical alignment tolerances with respect to the shown setup. For efficient and stable operation of wavelength stabilization all relevant parameters have to be controlled carefully. The parameters of the diode laser bar include the reflectivity of the AR-coating of the output facet, the emitter structure, the cavity length, the smile, the angular emission characteristics and the mounting technology, which has an influence on the wavelength drift with current and temperature. The properties of a VHG are optimized by adapting the refractive index modulation, the spatial frequency and the thickness. These three independent parameters define the Bragg angle, the diffraction efficiency and the spectral and angular selectivity of the grating. In principal, for each configuration these VHG parameters have to be optimized separately. However, based on experience a value for the VHG reflectivity of about 15% is a good starting point for most common diode laser bars. A VHG with a higher reflectivity will increase the locking range at the cost of a higher power loss. This means that optimization of wavelength stabilization will always be a trade-off between locking range and power loss. Furthermore it is important to notice that the optimum reflectivity also depends on the demands of the application. For some applications the VHG has to be optimized for a large locking range, whereas for other applications low losses for fixed operating conditions could be requested. One means to overcome the sensitivity for smile is the integration of the grating structure into the FAC itself 13. Such an element is more insensitive to smile and misalignment. Due to the large angular divergence of the uncollimated beam and the small angular selectivity of the grating only a small part of the beam is reflected back into the diode laser cavity. In the case of misalignment or smile another part of the beam will be reflected to provide feedback. In contrast, for an ideal two component setup with good collimation and no smile nearly all light reflected from the VHG is coupled back into the diode laser cavity. On the other hand this implies that for efficient wavelength locking a significant increase of the reflectivity of the VHG-FAC to about 70% is required. A further advantage of a FAC with integrated VHG is that only one single element has to be handled and aligned. One disadvantage of a VHG-FAC is the relatively low refractive index of the PTR-material, which is typically based on silica (n=1.45). FACs are usually fabricated with high refractive index material like S-TiH53 or N-LAF21. By using low refractive index material, a smaller radius of curvature is required for the same focal length which is disadvantageous with respect to lens aberrations for high NA operation. 3 RESULTS FOR DIFFERENT CONFIGURATIONS In this section we will present different examples for wavelength stabilized diode laser units. In principle, wavelength stabilization is possible for all configurations and operation modes. This includes single diode laser bars, vertical and horizontal diode laser stacks, fiber coupled modules and complete turn-key systems. Operation mode can be continuous wave or pulsed mode (QCW, quasi-continuous wave). 3.1 Single diode laser bars The first example is a single diode laser bar with fast-axis collimation in the red spectral range mounted on a passively cooled heat sink. Wavelength stabilization is achieved by an external VHG at a central wavelength of 633.7 nm. Fig. 2 shows the output power as a function of the operating current with and without wavelength stabilization (left part) and the corresponding spectra (right part). The maximum output power with stabilization is 4 W at an operating current of 8.5 A

and a temperature of 20 C. The peak wavelength of the stabilized spectrum is 633.7 nm with a spectral bandwidth of less than 0.3 nm (90% power content value), which is significantly less compared to 1.5 nm without wavelength stabilization. The power vs. current characteristics shows that the lasing threshold with stabilization is reduced, which is typical for stabilized diodes because of additional feedback by the external grating. By adding optical elements for collimation and beam shaping fiber coupling of the diode laser bar into a 400 µm fiber with numerical aperture of 0.22 is possible. Fig. 2: Power vs. current curve of a wavelength stabilized diode laser bar with an external VHG at 633.7 nm (left diagram). The right diagram shows the corresponding spectra with and without wavelength stabilization. For efficient feedback it is advantageous to use a diode bar with collimation in one or both axes and insert the VHG after the collimating optics. However, for some applications, like side-pumped solid-state lasers, diode laser bars without collimation are used. For sufficient feedback the reflectivity of the VHG has to be increased significantly compared to the operation with a collimated beam. Such a setup is comparable to a FAC with integrated VHG where efficiencies of about 70% have to be used (sect. 2). The advantage of a setup without collimating optics is that alignment of the VHG is not critical, but distance to the facet should be minimized. Fig. 3 shows the performance of a wavelength stabilized diode laser bar without collimation. The diode laser bar is operated in QCW-mode with 1.3 % duty cycle (260 µs pulse width, 50 Hz repetition rate) at a temperature of 20 C. Maximum output power is 243 W at a current of 250 A with a corresponding efficiency of 52%. The right part of Fig. 3 shows the spectrum of the stabilized bar at a peak wavelength of 808.3 nm with a spectral width of less than 1 nm (90% value). Fig. 3: Power vs. current curve of an uncollimated wavelength stabilized diode laser bar with an external VHG at 808.3 nm (left diagram). The right diagram shows the corresponding spectra with and without wavelength stabilization.

3.2 Diode laser stacks One approach for scaling the output power of diode laser units is dense packaging of multiple diode laser bars on heat sinks next to each other (horizontal stack) or on top of each other (vertical stack). A typical setup of a vertical stack is shown in Fig. 4. The vertical stack consists of 30 diode laser bars mounted on actively cooled micro-channel heat sinks. Each bar is individually collimated in fast-axis direction and wavelength stabilized by an external VHG. The total output power is 3375 W at an operating current of 110 A. Overall efficiency with stabilization is above 60 %. The right diagram of Fig. 4 shows the stabilized spectrum at a peak wavelength of 884.7 nm and a temperature of 25 C. Although 30 individual spectra are combined the total width of the spectrum is below 0.7 nm (90% value). To ensure such small bandwidths even for large stacks the variation of the VHG parameters has to be very low. That is even more important when multiple stacks are combined in one setup. In sum, we built 4 different 30-bar stacks with a very small variation of of only ± 0.25 nm for the stack central wavelength (884.7 nm up to 885.2 nm). Fig. 4: Power vs. current curve of a wavelength stabilized 30-bar vertical diode laser stack with external VHGs at 884.7 nm (left diagram). The right diagram shows the corresponding spectra with and without wavelength stabilization. A similar setup of a diode laser stack at a different wavelength of 1533 nm is shown in Fig. 5. The vertical stack consists of 42 diode laser bars mounted on actively cooled micro-channel heat sinks. Each bar is individually collimated in both axes (FAC + SAC) and wavelength stabilized by an external VHG. The total output power is 835 W at an operating current of 60 A. Overall efficiency with stabilization is above 29 %. The right diagram of Fig. 5 shows the stabilized spectrum at a peak wavelength of 1533 nm and a temperature of 25 C. The total width of the spectrum is below 0.9 nm (90% value). Fig. 5: Power vs. current curve of a wavelength stabilized 42-bar vertical diode laser stack with external VHGs at 1533 nm (left diagram). The right diagram shows the corresponding spectrum.

3.3 Fiber coupled units In the last few years we developed a modular diode laser concept which is based on a standard building block for a variety of lasers with different output powers and beam qualities 14. According to the modular design principle the baseplates easily can be combined to scale output power, which is realized optically by spatial and / or polarization multiplexing. The advantage of a common baseplate as basic building block for the modular system is that the baseplate can be produced in high volume. The production process for the baseplate is highly automated which leads to a cost-efficient and reliable building block with high repeatability regarding optical properties. 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 for the whole baseplate. Another important design aspect is that the cooling strategy allows the use of industrial water for the bottom-cooled baseplate. The modular concept is schematically shown in Fig. 6. Starting with a one-plate unit with up to 300 W output power for a 200 µm fiber (NA 0.22) we end up with a laser system consisting of 8 baseplates resulting in 2.2 kw output power for a fiber diameter of 400 µm (NA 0.22) at one single wavelength (without wavelength stabilization).. Fig. 6: Schematic drawing of modular diode laser concept based on one common baseplate. By adding an external VHG for wavelength stabilization of a single plate unit up to 284 W are achieved for a 200 µm NA0.22 fiber at an operating current of 40 A with an overall efficiency of 50%. The wavelength is centered at 976 nm with a spectral bandwidth below 0.5 nm (90% value). Data are shown in Fig. 7 (left part) in combination with results from a long term test (right part). The parameters for the long term test are 284 W output power, 40 A current and 20 C temperature. The total runtime shown in the diagram is 3900 h, which indicates a lifetime of > 20000 h, when end of lifetime is defined by 20% power decrease. We have built more than 50 of such units with a mean value of the peak wavelength of 975.8 nm and a standard deviation of only ± 0.35 nm. Maximum deviation from the peak wavelength is only ± 0.55 nm and mean values for the linewidth are 0.85 nm (90% value) and 0.35 nm (FWHM), respectively.

Fig. 7: Power vs. current curve of a wavelength stabilized single plate unit with an external VHG at 976 nm (left diagram). The right diagram shows data of a 3900 h long term test at a current of 40 A and the corresponding spectrum. As mentioned before power scaling is realized by combining several base plates to one common laser unit. Fig. 8 shows the result for a laser unit with four base plates coupled into a 200 µm fiber with NA 0.22. At an operating current of 40 A a maximum output power of 726 W is achieved with wavelength stabilization and 785 W without wavelength stabilization. The corresponding efficiencies are 40% and 44%, respectively. The right part of Fig. 8 shows the corresponding spectra. The center wavelength of the stabilized spectrum is at 976.6 nm with a spectral width of only 0.7 nm (90% value), which is a significant reduction compared to the spectral width of 5.6 nm without spectral stabilization. Further power scaling will be achieved by power scaling of the base plate itself and will lead to 1 kw output power for the four-plate unit in the near future. Fig. 8: Power vs. current curve of a four plate unit with and without external wavelength stabilization at 976 nm (left diagram). The right diagram shows the corresponding spectra with and without wavelength stabilization. 3.4 Multi-kW fiber coupled systems The examples in the previous section were based on a modular concept, which uses the tailored bar concept in combination with a baseplate cooled with industrial water. A more compact setup can be realized with DI-water cooled vertical diode laser stacks as described in Sect. 3.2. We have developed a modular platform based on diode laser stacks with standard 10 mm broad diode laser bars, which is suitable for fiber coupling into a 1000 µm NA 0.22 fiber. Fig. 9 shows the result for a unit which is wavelength stabilized at a central wavelength of 976.4 nm. The maximum output power is 2.3 kw at an operating current of 65 A with a corresponding efficiency of 46%. The peak wavelength is centered at 976.4 nm with a spectral bandwidth below 0.6 nm (90% value). The diode laser module which is schematically shown in the left part of Fig. 9 can optionally be integrated into a stand-alone 19-inch mounting rack (right part of Fig. 9).

Fig. 9: Power vs. current curve of a 1000 µm NA 0.22 fiber coupled laser unit based on vertical diode laser stacks with external wavelength stabilization at 976.4 nm (left diagram). The right diagram shows the corresponding spectrum. By changing the central wavelength of the VHG the stabilized wavelength easily can be shifted to 969 nm, which is the important zero-phonon pump wavelength for Thin Disk Yb:YAG lasers. Fig. 10 shows the result for such a unit which is wavelength stabilized at a central wavelength of 968.9 nm. The maximum output power is 2.7 kw at an operating current of 75 A with a corresponding efficiency of 48 %. The peak wavelength is centered at 968.9 nm with a spectral bandwidth below 0.6 nm (90% value). Fig. 10: Power vs. current curve of a 1000 µm NA 0.22 fiber coupled laser unit based on vertical diode laser stacks with external wavelength stabilization at 968.9 nm (left diagram). The right diagram shows the corresponding spectrum.

4 SUMMARY AND OUTLOOK In conclusion, we have demonstrated efficient and stable wavelength locking for a couple of different configurations. Wavelength stabilization was realized for single bar modules, diode laser stacks and fiber coupled modules with fiber core diameters from 200 µm up to 1000 µm (NA 0.22). We have shown wavelength stabilization for a broad spectral range with different wavelengths from 634 nm to 1533 nm. The maximum output power of a wavelength stabilized fiber coupled system was 2.7 kw out of a 1000 µm fiber (NA 0.22). The center wavelength of this unit was 968.9 nm with a spectral bandwidth of only 0.6 nm (90% value). In summary, we have pointed out that high-power diode laser modules with enhanced spectral brightness are very attractive devices for more efficient pumping of solid-state lasers with a narrow absorption bandwidth. In addition, such wavelength stabilized devices are important for further scaling the brightness of diode laser systems. In the next few years a further increase in brightness of diode laser systems towards a BPP below 10 mm mrad with multi-kw output power is expected. Dense wavelength multiplexing with wavelength stabilized systems will help to realize these high brightness diode laser modules. We already have demonstrated a polarized output power of 410 W out of a 100 µm NA 0.2 fiber by using such a dense wavelength multiplexing approach 15. Combining several of these units will lead to the goal of a multi-kw diode laser source with a BPP below 10 mm mrad. ACKNOWLEDGEMENTS A part of this work was sponsored by the German Bundesministerium für Bildung und Forschung (BMBF) within the German National Funding Initiative Integrated optical components for High-Power Laser Sources (INLAS). REFERENCES 1. B. Weichelt et. al.; Enhanced performance of thin-disk lasers by pumping into the zero-phonon line ; Optics Letters Vol. 37, pp. 3045 (2012) 2. L. McDonagh et. al.; High-efficiency 60 W TEM 00 Nd:YVO 4 oscillator pumped at 888 nm ; Optics Letters Vol. 31, pp. 3297 (2006) 3. A. Gourevitch et. al.; Continuous wave, 30 W laser-diode bar with 10 GHz linewidth for Rb laser pumping ; Optics Letters Vol. 33, pp. 702 (2008) 4. T. Koenning et. al.; Narrow line diode laser stacks for DPAL pumping ; Proc. SPIE Vol. 8962, 89620F (2014) 5. H. Kissel et. al.; High-power diode laser pumps for alkali lasers (DPAL) ; Proc. SPIE Vol. 8241, 82410Q (2012) 6. M. Haag et. al.; Novel high-brightness fiber coupled diode laser device ; Proc. SPIE Vol. 6456, 6456-28 (2007) 7. C. Wessling et. al.; Dense wavelength multiplexing for a high power diode laser ; Proc. SPIE Vol. 6104, 6104-21 (2006) 8. P. Crump et. al.; Reliable operation of 976nm High Power DFB Broad Area Diode Lasers with over 60% Power Conversion Efficiency ; Proc. SPIE Vol. 7953, 79531G (2011) 9. G.B. Venus et. al.; High-brightness narrow-line laser diode source with volume Bragg-grating feedback ; Proc. SPIE Vol. 5711, pp. 166 (2005) 10. B.L. Volodin et. al.; Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings ; Optics Letters Vol. 29, pp. 1891 (2004) 11. C. Moser et. al.; Filters to Bragg About ; Photonics Spectra, pp. 82 (June 2005) 12. J. Lumeau et. al.; Tunable narrowband filter based on a combination of Fabry Perot etalon and volume Bragg grating ; Optics Letters Vol. 31, pp. 2417 (2006) 13. C. Schnitzler et. al.; Wavelength Stabilization of HPDL Array Fast-Axis Collimation Optic with integrated VHG ; Proc. SPIE Vol. 6456, 6456-12 (2007) 14. B. Köhler et. al.; Scalable high-power and high-brightness fiber coupled diode laser devices ; Proc. SPIE Vol. 8241, 8241-08 (2012) 15. A. Unger et. al.; Tailored bar concepts for 10mm-mrad fiber coupled modules scalable to kw-class direct diode lasers ; submitted to SPIE Conference 9348, paper 9348-08 (2015)