Fiber coupled diode laser of high spectral and spatial beam quality with kw class output power

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1 Fiber coupled diode laser of high spectral and spatial beam quality with kw class output power Christian Wessling, Martin Traub, Dieter Hoffmann Fraunhofer Institute for Laser Technology, Aachen, Germany ABSTRACT High optical output power in the multi-kw range from a fiber coupled diode laser can reach the beam quality of lamp pumped solid state lasers. Direct diode laser application as deep penetration metal welding becomes feasible. Polarization and wavelength multiplexing are established techniques to scale the optical power of diode lasers at almost constant beam quality. By use of volume diffraction gratings in an external cavity laser it is possible to constrict the spectral bandwidth of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the wavelength multiplexing of diode laser beams at small distance of the center wavelengths can be realized. The development of a fiber coupled diode laser is presented. The set up consists of twelve modules which serve for an average optical power of 1.5 kw. Each module utilizes dense wavelength multiplexing of two diode laser bars with a center wavelength spacing of 3 nm. The diode laser bars are wavelength stabilized at center wavelengths of 98 nm, 911 nm, 975 nm and 978 nm. The spectral bandwidth of all diode laser bars is within 1 nm in the full power range. Stable operation at an average power of 136 W at 98/911 nm and 115 W at 975/978 nm with a wavelength shift less than.1 nm is achieved by the modules. Further coarse wavelength and polarization multiplexing and beam transformation enable fiber coupling to a 6 µm fiber with a numerical aperture of.175 (95% power inclusion). Keywords: wavelength stabilization, beam combining, dense wavelength multiplexing, volume diffraction grating, external cavity, high power diode laser, fiber coupling 1. INTRODUCTION Increasing the optical power of diode lasers to a level of one or more kilowatt at high beam quality for fiber coupling to small diameters less than 6 µm is a question of the optical output power and beam quality of single diode lasers and their concentrated arrangement (e.g. diode laser stacks). Apart from improving the diode laser chip performance beam combining methods as polarization and wavelength multiplexing are used. State of the art for power scaling and retaining beam quality is the wavelength multiplexing by dielectric filters. Limitations on spectral beam combining of high power diode lasers arise from the thermal wavelength shift within the operating range up to 1 nm, the tolerance of the emission wavelength of ±3 (5) nm (customized), the typical spectral width of 3 to 5 nm and the wavelength shift due to aging of the laser diodes. Regarding the slope of dielectric filters for polarized light in combination with the typical spectral emission of the diode lasers the center wavelength spacing is limited to a range of 2 to 3 nm. As the typical emission wavelengths of commercially available high power diode lasers are 88 nm, 88 nm, 915 nm, 94 nm and 98 nm, mostly optimized for pumping solid state lasers, the coarse multiplexing in this context enables a maximum fivefold beam combination. To break with these limitations a new approach based on volume diffraction gratings is investigated. Volume diffraction gratings feature high angular and spectral selectivity, diffraction efficiency up to 98% and high thermal stability [3,4,5]. Adapted to an external cavity diode laser, volume diffraction gratings provide for a narrow optical feedback and by this stabilize the emission spectrum of the diode laser. Wavelength stabilization by volume diffraction grating leads to a reduction of the spectral width of high power diode lasers by a factor of 1 and reduction of the thermal driven wavelength shift by a factor of at least 3. Narrowing the line width and stabilizing the center wavelength of the high power diode laser arrays allows a center wavelength spacing for spectral beam combining down to 3 nm (dense wavelength multiplexing). The performance of wavelength stabilization of 24 diode laser arrays is presented in section 2. The results from 12 twowavelength multiplexing units are described in section 3. In section 4 the modular concept of the diode laser is specified. The overall performance of the fiber coupled diode laser is summarized in section 5. High-Power Diode Laser Technology and Applications V, edited by Mark S. Zediker, Proc. of SPIE Vol. 6456, , (27) X/7/$18 doi: / Proc. of SPIE Vol

2 2. WAVELENGTH STABILIZATION OF HIGH POWER DIODE LASER BARS Narrow and stable emission from the diode laser is obligatory for dense wavelength multiplexing. The requirements are a spectral width smaller than the wavelength selectivity of the grating for spectral beam combining. Further the thermal induced wavelength shift of the diode laser of typically 1 nm within the operating range must be reduced to a level below.1 nm. The performance of wavelength stabilization and twofold dense wavelength multiplexing has been demonstrated in [1] with 4 W diode laser bars at 935 nm and 938 nm. High power diode laser bars with antireflection coated front facet (R <.5%) provide wavelength stabilization without side lobes in the full power range. The antireflection coating serves for a locking range of the diode laser larger than 2 nm. By this the thermal induced wavelength shift of the free running diode laser which is defined by the thermal coefficient of the diode material of typically.25 to.3 nm/k is replaced by the thermal coefficient of the grating material of about.5 to.1 nm/k in stabilized operation. Even if the thermal coefficient of the volume diffraction indicates a reduction of the wavelength shift by at least a factor of 3 the residual wavelength shift is still critical with respect to the dense wavelength multiplexing approach. Thus the acceptable limit for temperature rise of the volume diffraction grating that is heated by absorption of laser radiation is 1 K. Therefore the gratings used for wavelength stabilization in the following are mounted on water cooled heat sinks. The heat sinks limit the temperature rise due to absorption in the grating by the passing laser beam as well as absorption of stray light in the mount which in term (if not cooled) would heat the grating. Figure 1 represents the free running and stabilized emission spectra of an 8 W diode laser bar with.5% AR-coating on the front facet and volume grating peak diffraction efficiency of 17.5 %. The wavelength stabilization with volume diffraction gratings in an external cavity diode laser with AR-coated diode laser bars enables stable and narrow emission in the full operating range. The stabilized spectral width with volume diffraction gratings of 1.5 mm thickness is between.15 and.25 nm (FWHM). The spectral integrated intensity reaches 95% power inclusion within a spectral width of.6 to 1. nm. Figure 2 shows the representative light-current characteristic of a free running and wavelength stabilized diode laser bar. Due to the low feedback from the AR-coated front facet of the diode laser the free running threshold current is about 2 A. With the narrow lined feedback from the grating the threshold current is reduced, in this case in Figure 2 to 9.6 A. The wavelength stabilization is performed on two types of diode laser bars which differ from the emission wavelength, the optical output power and the filling factor. One type emitting at 91 nm has a filling factor of 2% and a nominal optical output power of 9 W at 9 A. The other bars emitting at 976 nm have a filling factor of 3% and a nominal optical output power of 7 W at 9 A. The diode lasers are collimated in fast axis by a cylindrical micro lens (FAC) and in slow axis by a micro lens array (SAC). Note that the beam divergence in slow axis of the collimated array with 2% filling factor is about 3.4 (95% power inclusion). In relation the slow axis beam divergence of the array with 3% filling factor is up to 5 (95% power inclusion). In principle the lower filling factor Normalized Intensity [a.u.] 1E ,2 a),2 b) Optical Power [W] c).8 nm.2 nm d) nm P 2 free running P 1 stabilized 4 1 Slope,9 W/A 2 I S = 9,6 A Strom [A] Figure 2: Free running and stabilized light-current characteristic of a 7 W diode laser bar with.5% ARcoated front facet (978 nm peak wavelength). Intensity [a.u.] 1,1 1 1E-3 Figure 1: Emission spectra of a diode laser bar with.5% ARcoating on facet and volume grating efficiency of 17.5% at optical output power of 79 W. a) Stabilized spectral width λ FWHM =.2 nm b) Free running c) 98% of the optical output power within.8 nm d) Spectral side lobe suppression ratio > 25 db Spectral integrated intensity [a.u.] Ratio P 1 /P [%] Proc. of SPIE Vol

3 of the 2% array in comparison to 3% with equal beam divergence from the uncollimated bar causes a larger beam quality by a factor of 1.5 in reference to the 1 mm bar with the slow axis collimation micro lens array. The fast axis beam divergence is less than.46 (95% power inclusion) depending on the smile of the diode laser bar and the quality of the FAC (focal length.9 mm). The smile values range from ±.2 µm to ±.86 µm (average ±.37 µm) for the 975 nm/978 nm bars and from ±.19 µm to ±.48 µm (average ±.32 µm) for the 98 nm/911 nm bars. The light-current characteristics of 24 single wavelength stabilized and fast and slow axis collimated diode laser bars reach an average slope efficiency of 9% up to 95% of standard bars with FAC and SAC. The average slope efficiency from twelve wavelength stabilized diode laser bars at 975 nm/978 nm is.86 W/A starting from a threshold current of 8.6 A. The average slope efficiency from twelve wavelength stabilized diode laser bars at 98 nm/911 nm is.93 W/A starting from a threshold current of 7 A. The spectra of all 24 stabilized diode laser bars demonstrate the required constant peak wavelength and narrow emission in the full operating range. 3. DENSE WAVELENGTH MULTIPLEXING OF HIGH POWER DIODE LASER BARS The reduction of the center wavelength spacing for spectral beam combining to the dimension of some nanometers brings about the decrease of the tolerable beam divergence to the dimension of some milliradiant. As the superimposition of the following high power diode laser bars is concerned the beam propagation angles have to be a tenth of the fast axis beam divergence half angle (< 2.5 mrad) in order to maintain the beam quality. In the context of the spectral beam combining with volume diffraction gratings from the Bragg matching condition λ(t) = 2n(T)Λ(T) cos(θ) can be seen that the beam incident angle θ and the wavelength λ(t) is linked to the temperature-dependent grating period Λ(T) and index of refraction n(t). The uniformity of the wavelength λ(t) is defined by the wavelength stabilization as described in section 2. Variation of the grating period and index of refraction due to an increase in temperature of the grating for a constant wavelength result in the deviation of the Bragg matching incident angle θ. In addition to the incident angle the angular distribution of the beam is to be matched to the angular selectivity of the grating. Figure 3 gives an example of a Gaussian intensity distribution of a fast axis collimated diode laser bar with beam divergence of 2.5 mrad and the angular selectivity of a volume diffraction grating with a theoretical peak diffraction efficiency of 99.9%. The displayed beam divergence angle exceeds the grating selectivity defined by the half width at first zero of the diffraction efficiency distribution. In this case either the mismatch of the beam divergence to the grating selectivity or the deviation of the incident angle causes diffraction losses as can be seen by the convolution of a Gaussian intensity distribution of a fast axis collimated diode laser with the angular selective diffraction efficiency of a volume diffraction gratings in Figure 4. The diagram shows the diffraction efficiency as a function of the beam divergence angle and the (Bragg matching) in-plane incident angle. For illustration the grating has a theoretical peak diffraction efficiency of 99.9% and an angular selectivity of 1.85 mrad (half width at first zero). Note first that the diffraction efficiency Diffraction efficiency [-],2 5,95,9 5,75,7 5,55, Divergence angle [mrad] VBG δθ HWFZ = 1,85 mrad Gaussian beam θ = 2.5 mrad Figure 3: Gaussian intensity distribution of a fast axis collimated diode laser bar and the angular selectivity of volume diffraction grating with a theoretical peak diffraction efficiency of 99.9%. Diffraction efficiency [-] Standard-FAC Angular distribution [-],5 1,5 2, 2,5 3, 3,5 4, 4,5 Divergence angle [mrad] BC 15 BC 3 BC 45 λ = 976 nm d = 1,5 mm n = 7e-4 Figure 4: Convolution of a Gaussian intensity distribution of a fast axis collimated diode laser (1/e² beam divergence half angle) with the angular selectivity of a volume diffraction grating (99.9% peak diffraction efficiency) in dependence on the incidence angle. Proc. of SPIE Vol

4 decreases rapidly with the beam divergence and the inplane incident angle. The beam divergence angles of fast axis collimated 1 mm diode laser bars with.9 mm focal length of the FAC is assigned in the diagram. The variance of the divergence angle from 1 mrad to 4.5 mrad for constant focal length is a matter of the smile and initial fast axis divergence of the diode laser bar as well as the quality of the micro lens. As can be seen in the diagram moderate diffraction losses less than 7% arise if the inplane incident angle is below 15 and the residual beam divergence angle is smaller than 2 mrad. Of course, the diffraction limited beam divergence of about.7 mrad (FAC:.9 mm focal length) would be sufficient but is not maintained with respect to a collimated diode laser bar. Not only the in plane angular selectivity of the grating is close to the beam divergence of fast axis collimated diode laser bars but also the out-of plane selectivity to the slow axis beam divergence. The significant difference is that the in-plane selectivity scales in some mrad whereas the out-of plane selectivity has dimension of a few degrees. Both values can be reached from fast and slow axis collimated diode laser bars but with high requirements on collimation, smile and beam divergence. One example of the optical power and efficiency of a dense wavelength multiplexing unit is given in Figure 5. The non constant but slightly modulated diffraction efficiency curve shows the temperature influence on the grating parameter as described above. The gratings used for the diode laser have a peak diffraction efficiency of at least 85% (unlike the theoretical value of 99% in Figure 3 and Figure 4). The angular selectivity is about 5 mrad (FWHM) in-plane and about 4.6 (FWe -2 M) out-of plane. From six DWM units at 98 nm/911 nm an average diffraction efficiency of 84.7% is obtained. As a result of the lower beam quality particularly with regard to the slow axis beam divergence of the 975 nm/978 nm DWM units the diffraction efficiency is just 64.3%. By comparative measurement using the same diffraction gratings but with diode laser beams with a slow axis beam divergence of 2 (95% power inclusion) the diffraction efficiency is increased to more than 8%. Transmission losses at the volume diffraction gratings are caused by Fresnel reflexion at the surfaces (<.5%), absorption (<<.5%) and scattering (< 2.5%). Both types of wavelength stabilized diodes have average transmission efficiency at the diffraction gratings of 97.1%. Thus the transmission losses are not predominantly determined by the angular distribution of the laser beam but by the material parameters. 4. MODULAR CONCEPT FOR A FIBER COUPLED DIODE LASER BASED ON WAVELENGTH STABILIZED DIODE LASER BARS Due to the required low beam divergence angle in fast and slow axis the dense wavelength multiplexing is performed by diode laser bars which are collimated in both directions. The spectral beam combining of every single beam from each collimated diode laser bar implies the modular set up of dense wavelength multiplexing (DWM) units which can be treated as a single diode laser source in the following. Twelve dense wavelength multiplexing units are set up for a 1 kw laser. One type utilizes the stabilized diode laser bars at 98 nm and 911 nm, the other at 975 nm and 978 nm. Although in this design the diode laser bars are mounted on micro channel heat sinks larger conduction cooled packages (CCP) could be used with respect to a reduced optical output power of these devices. The DWM units are separately exchangeable to serve for precise adjustment of the volume diffraction grating for spectral beam combining. Blocks of three DWM units are geometrical stacked in fast axis. Further polarization and coarse wavelength multiplexing by dielectric filters (center wavelength spacing of 64 nm) is used for power scaling that result in a total power of 1.5 kw from all DWM units. By use of a pair of step mirrors the unequally distributed beam quality in both directions from the threefold geometrical stacked diode laser beams is assimilated. The rectangular shaped beam aperture with the large slow axis divergence related to the minor dimension and the low fast axis beam divergence related to major dimension from the step mirror is imaged to a square on the fiber end with a diameter of 6 µm and a numerical aperture of.22. The housing of the diode laser has the dimension 7 x 55 x 22 mm³ (L x W x H). Optical Power [W] Efficiency diffracted transmitted multiplexed 1 Optical power diffracted 2 transmitted multiplexed Current [A] Figure 5: Optical power and efficiency of a dense wavelength multiplexing unit of the transmitted (911 nm) and diffracted (98 nm) diode laser beams at the volume diffraction grating Efficiency [%] Proc. of SPIE Vol

5 5. EXPERIMENTAL RESULTS FROM THE FIBER COUPELD DIODE LASER The following performance of the fiber coupled diode laser is measured in continuous operation at the fiber end. The spectral beam quality is outlined in the emission spectra in Figure 6 and Figure 7 corresponding to the two coarse wavelength ranges. Intensity [a.u.] log Intensity [a.u.],2 1, Ge Spektrum bei 976 nm 97,88 nm 91,95 nm 1E Intensity [a.u.] log Intensity [a.u.],2 1, Ge Spektrum bei 976 nm 974,79 nm 977,84 nm 1E Figure 7: Emission spectrum from 96 nm to 912 nm with stabilized peak wavelengths at nm and nm of fiber coupled diode laser with an optical output power of 57 W (related light-current characteristic in figure 8). Figure 6: Emission spectrum from 973 nm to 98 nm with stabilized peak wavelengths at nm and nm of fiber coupled diode laser with an optical output power of 382 W (related light-current characteristic in figure 8). Optical Power 952 W 1 98/911 nm und 975/978 nm Power ratio[-],95,9 5 NA (95%) =,175 rad Optical Power [W] /911 nm 975/978 nm,75,14,16,18,2,22 Numerical Aperture [rad] Current [A] Figure 8: Power inclusion within the numerical aperture at the fiber exit. Figure 9: Light-Current characteristics of the diode laser measured at the fiber exit. All diode lasers from the twelve DWM units are stabilized within a spectral width at full width at half maximum less than.25 nm. There residual wavelength shift of each peak wavelength is less than.1 nm in the full power range. The spectral attenuation off all DWM units is about 2 db. Figure 8 illustrates the power inclusion within the numerical aperture of the fiber for the maximum optical output power of 952 W. The light-current characteristic of the diode laser in Figure 9 measured at the fiber exit shows an almost linear increase of the optical output power with the injection current up to 952 W at 9 A for both coarse wavelength ranges. The power contribution of 382 W from the 975 nm/978 nm DWM units is less than 57 W from the 98 nm/911 nm DWM units. The major reason is the reduced diffraction efficiency caused by the larger slow axis beam divergence angle as described in section 3. The second reason is the reduced stabilized output power from the diode laser bars. The third Proc. of SPIE Vol

6 reason is a larger beam divergence angle in fast axis at high injection current level greater than 6 A which causes additional losses at the fiber coupling of about 1% what is indicated by the small deviation from linear characteristic. The fiber coupling efficiency of the 98 nm/911 nm DWM units is about 98%. 6. CONCLUSION AND OUTLOOK The twofold dense wavelengths multiplexing with volume diffraction gratings of two types of high power diode laser bars has been demonstrated. The multiplexing efficiency is determined by the convolution of the spectral and angular selectivity of the grating and the beam divergence of the diode laser. Both the fast and slow axis beam divergence from a collimated diode laser bar is close to the angular selectivity of the grating. Concerning the wavelength stabilization all DWM units demonstrate a stable and narrow emission spectrum in the full operating range. The power loss due to wavelength stabilization is in the range from 5% and 1%. The volume diffraction gratings for spectral beam combining utilized in this laser feature an average transmission efficiency of 97.1% and maximum diffraction efficiency with a collimated diode laser bar of 87.4% is obtained. The low diffraction efficiency of the DWM units at 975 nm/978 nm is mainly caused by the mismatch of the slow axis beam divergence (> 2 ) to the out of plane angular selectivity of the volume diffraction grating. The fiber coupled total power is 952 W from an industrial fiber optic cable of 6 µm diameter and a power inclusion of 95% within a numerical aperture of.175. Due to the spatial beam quality the diode laser will find application in direct metal welding. Further the high spectral beam quality allows for efficient pumping of Yb: YAG fiber lasers. A perspective of the dense wavelength multiplexing technique is given in Figure 1. Starting from the current wavelength multiplexing performance the cumulative efficiency for multiple beam combination is proposed. Regarding the present state of the art a tenfold dense wavelengths multiplexing would yield in a cumulative efficiency of more than 7%. Further improvement should be achieved by adapting both the beam characteristic of the diode laser and the angular selectivity distribution of the volume diffraction gratings. In addition the enlargement of the peak diffraction efficiency of homogenous and large aperture gratings to values exceeding 95% give reason for the major curve in Figure 1. X-fold power scaling by enlargement the wavelength number for spectral beam combining will lead to diode lasers that reach the brilliance of lamp pumped solid state lasers with optical output powers of several kilowatt. ACKNOWLEDGEMENT Part of this work was sponsored by the German government, Federal Ministry of Education under contract no. 13N853. REFERENCES,2 η Transmission =,97 η Transmission =,99,1 η η Diffraction = 5 Diffraction = 4 η Diffraction =, C. Wessling, M. Traub, D. Hoffmann, R. Poprawe: Dense wavelength multiplexing for a high power diode laser. In: High-Power Diode Laser Technology and Applications IV, edited by Mark S. Zediker, Proc. of SPIE 614 (26) Venus, Armen Sevian, and Leonid Glebov: Spectral stabilization of high efficiency diode bars by external Bragg resonator. Proceedings of Solid State and Diode Lasers Technical Review. Los Angeles 25, P Christophe Moser, Gregory Steckman, Filters to Bragg about - Volume holographic gratings offer distinct filter qualities, Photonic Spectra, June 25 Cumulative efficiency [-],9,7,5,3 Status quo 976 nm η Stabilization =,95 Status quo 91 nm η Stabilization =,93 η Transmission =,97 Goal Number of beam sources [-] η Stabilization =,96 Figure 1: Cumulative multiplexing efficiency as a function of beam source number: Status quo at 976 nm and 91 nm and estimated performance by improved laser beam and grating parameter. Proc. of SPIE Vol

7 4. G.Venus, A.Sevian, V.Smirnov, L.Glebov. Invited Paper: High-Brightness Narrow-Line Laser Diode Source with Volume Bragg-Grating Feedback" High-Power Diode Laser Technology and Applications III. Ed.: M. Zediker. Proceedings of SPIE 5711 (25) Volodin, B. L.; Dolgy, S. V.; Melnik, E. Performance enhancement of high-power laser diodes and arrays by use of volume Bragg grating technology, High-power diode laser technology and applications III; Zediker, Mark S., Proceedings of SPIE 5711 (SPIE, Bellingham, WA, 25) 6. Volodin, B. L.; Dolgy, S. V.; Melnik, E., E. Downs, J. Shaw and V. S. Ban: Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings, Optic Letters, Vol. 29, No. 16, Glebov, L. Glebova, V. Smirnov, M. Dubinskii. L. Merkle, S. Papernov, and A. Schmid: Laser Damage Resistance of Photo-Thermo-Refractive Glass Bragg Gratings, 17th Annual Solid State and Diode Laser Technology Review, SSDLTR-24 Technical Digest, Poster-4, Albuquerque, NM, June I.Ciapurin, V.Smirnov, G.Venus, L.Glebova, E.Rotari, and L.Glebov, High-Power Laser Beam Control by PTR Bragg Gratings, 24th Annual Conference on Lasers and Electro-Optics, CLEO/IQES and PhAST Technical Digest, Paper Code CTuP51, San Francisco, CA, May J.W. Goodman, Introduction to Fourier Optics, Chapter 9, McGraw Hill, 2 nd edition, Singapore 1996 Proc. of SPIE Vol

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