Stefan Hengesbach

Brightness and average power as driver for advancements in diode lasers and their applications

Spatial and spectral emission characteristics and efficiency of high-power diode laser (HPDL) based pump sources enable and define the performance of the fundamental solid state laser concepts like disk, fiber and slab lasers. HPDL are also established as a versatile tool for direct materials processing substituting other laser types like CO2 lasers and lamp pumped solid state lasers and are starting to substitute even some of the diode pumped solid state lasers. Both, pumping and direct applications will benefit from the further improvement of the brightness and control of the output spectrum of HPDL. While edge emitting diodes are already established, a new generation of vertical emitting diode lasers (VCSELs) made significant progress and provides easy scalable output power in the kW range. Beneficial properties are simplified beam shaping, flexible control of the temporal and spatial emission, compact design and low current operation. Other characteristics like efficiency and brightness of VCSELs are still lagging behind the edge emitter performance. Examples of direct applications like surface treatment, soldering, welding, additive manufacturing, cutting and their requirements on the HPDL performance are presented. Furthermore, an overview on process requirements and available as well as perspective performance of laser sources is derived.

Dense Wavelength Multiplexing and 35 µm-Fiber Coupling of Wavelength Chirped High Power Diode Laser Bars

We present a flexible, modular and cross-talk free concept for dense wavelength beam combining of HPDL by use of ultra-steep dielectric filters. Five emitters of one mini bar are coupled into a 35 µm fiber. Experimental results of internally and externally stabilized HPDL are presented.

100 W / 100 μm passively cooled, fiber coupled diode laser at 976 nm based on multiple 100 μm single emitters

We developed a high brightness fiber coupled diode laser module based on single diode lasers providing more than 60 Watts output power from a 100 micron fiber at the optimum fiber laser pump wavelength of 976 nm. The advantage of using multiple single emitters on a submount compared to using bars or mini bars is the direct fiber coupling by use of optical stacking and the fact that no beam transformation is needed. We achieved best brightness with a high fill factor, optical efficiency of more then 80% and wall-plug efficiency of more then 40%. The use of single emitters on a submount also extends the life span due to reduced failure (xn vs. x) per device (n individual emitters vs. n emitters on a bar (mini array)). Low drive current enables modulation.

Characterisation of ultra-narrow band filters for diode laser systems based on dense wavelength division multiplexing

We present a measurement platform to characterise combining elements used for dense wavelength division multiplexing, such as dielectric edge filters or Volume Bragg Gratings (notch filters). This platform enables the characterisation of the spectral and angular selectivity, the homogeneity of reflectance or transmittance and temperature-induced spectral shift of the band edge. The determination of the diffraction efficiency, the reflectance and the transmittance is carried out for both polarisations with high accuracy.

Simultaneous frequency stabilization, wavelength multiplexing and improvement of beam quality using a self-optimizing external cavity diode laser.

The combination of a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) and a subsequent feedback mirror forms an efficient single-stage frequency stabilization and multiplexing system. The laser emission of the connected diode laser sources is automatically forced to amplify the wavelengths with the maximum feedback and the minimum losses, respectively. Therefore, different transversal modes of a single emitter may be linked to different wavelength regions that fulfill the Bragg conditions of the VBGs with high efficiency. We demonstrate a multiplexing system with a channel spacing of 1.5 nm and an optical efficiency of 86%, up to 90% being feasible with modifications. The emission bandwidth of the diode laser single emitters is reduced to <300  pm. Due to transversal mode selection, the lateral beam quality factor M2 of the single emitters decreases by a factor of 1.4 on average, compared to the free running sources.

Frequency stabilisation of single emitters and the effect on the beam quality

We present the improvement of the lateral beam quality due to frequency stabilisation in a Dense Wavelength Division Multiplexing setup using Volume Bragg Gratings. The spectral width of each frequency stabilised channel amounts to less than 300 pm and an overall multiplexing efficiency of 86 % has been measured. The near and far field intensity distributions consist of discrete modes and were observed with a post positioned beam analysis module. When operated with frequency stabilisation, the lateral beam quality factor M2 of the single emitters decreased by a factor of 1.4 on average compared to the free running source.

Comparison of edge-emitter and vertical emitter based diode lasers for materials processing

Laser systems based on edge-emitting diodes are established in materials processing applications like soldering and welding and show the potential to substitute solid state and fiber lasers in cutting and deep penetration applications. Another type of diode lasers based on vertical emitters has made significant progress in the past few years. Power scaling to multi-kW level and a significant improvement in efficiency have been achieved. In this publication, the fundamental properties of both laser types are compared and the process regimes that can be addressed with state of the art commercial lasers based on edge-emitters and vertical emitters are presented.Laser systems based on edge-emitting diodes are established in materials processing applications like soldering and welding and show the potential to substitute solid state and fiber lasers in cutting and deep penetration applications. Another type of diode lasers based on vertical emitters has made significant progress in the past few years. Power scaling to multi-kW level and a significant improvement in efficiency have been achieved. In this publication, the fundamental properties of both laser types are compared and the process regimes that can be addressed with state of the art commercial lasers based on edge-emitters and vertical emitters are presented.

High Power Fiber Coupled Modules Based on Tapered Diode-Lasers at 975 nm

With the technological progress of tapered diode lasers brightness and output power of fiber coupled modules can be improved. Tapered diodes bear the potential to achieve high coupling efficiencies in multimode as well as single mode fibers. Within the BRIGHTER Project of the European Union several modules are designed to exploit this potential. The optical systems, the mechanical design and the experimental results of these modules will be presented. A design for a telecom pump module with a coupled power of 50 W in a 50 μm fiber with an NA of 0.22 at 975 nm will be presented. 16 collimated tapered single emitters aligned in four groups of four emitters are combined by mirrors and a polarizing beam splitter and coupled into the fiber. As a variant of this module four emitters are fiber coupled to achieve a power of 12 W of a 50 μm fiber with a NA of 0.13. A single mode fiber coupled module with a maximum output power of 1 W will be presented. Based on a tapered DFB Laser with a wavelength of 1060 nm it serves as a free space communication module. In another application this module is utilized as pump source for second harmonic generation. Equipped with a 975 nm tapered laser diode this module serves as a powerful pump source for Raman amplification.