ANALYSIS & OPINION

Brilliance in diode laser design

A project to increase the brilliance of direct diode laser systems, BRIDLE (Brilliant Industrial Diode Lasers), has been completed after 42 months. Dr Jens Biesenbach, CTO at project coordinator Dilas Diodenlaser, explains the technology behind the new diode laser architecture, work that could see diode lasers competing with fibre lasers in material processing

The tailored bar diode laser concept is an efficient way to fabricate a laser source that is high-brightness, scalable in power and economic to produce. It combines the advantages of both the single emitter and bar-based diode lasers: reliability, high power per element, efficient cooling, compactness, and handling of several emitters in a monolithic chip at one time throughout production.

It is now technically feasible to produce a diode laser for cutting applications – project partner Fraunhofer Institute for Laser Technology ILT has demonstrated laser cutting of stainless steel 1.4301 (2.5mm) with 800W of power and a cutting speed of 20mm/s (figure 1). Fraunhofer ILT also showed high-brightness diode laser welding with a spot size of 200µm (figure 2), while Riwalas demonstrated laser cladding, laying down a wear out resistant coating (figure 3).

Figure 1: Laser cutting of stainless steel 1.4301 (2.5mm) with 800W and cutting speed of 20mm/s. Courtesy of Fraunhofer ILT.

Figure 1: Laser cutting of stainless steel 1.4301 (2.5mm) with 800W and cutting speed of 20mm/s. Courtesy of Fraunhofer ILT.

Figure 2 (left): High-brightness laser welding with tailored bar source (spot size 200µm). Courtesy of Fraunhofer ILT.Figure 3 (right): Additive manufacturing with laser cladding (wear out resistant coating). Courtesy of Riwalas.

Figure 2 (top): High-brightness laser welding with tailored bar source (spot size 200µm). Courtesy of Fraunhofer ILT. Figure 3 (bottom): Additive manufacturing with laser cladding (wear out resistant coating). Courtesy of Riwalas.

The heart of Dilas’ concept, regarding its beam characteristics, is a tailored diode laser bar. By means of standard fast- and slow-axis collimation lenses and optically stacking of the emission lines of several bars in fast-axis, the resulting beam can be coupled into a 200µm fibre with a numerical aperture (NA) of 0.22. The ease with which the output of tailored diode laser bars can be shaped optically makes the concept inexpensive compared to complex micro-optics for bars.

Another main design aspect is the planar setup. This allows fully automated production steps that are high quality and cost-efficient. The bars are inspected, mounted to sub-mounts with micron-precision, tested and then sorted depending on wavelength and performance parameters. Multiple bars are mounted on a single base plate (figure 4), similar to an electronic circuit board. Active alignment of the micro-optics results in very high yield rates and best possible electro optical efficiency.

Figure 4: Concept of the base plate with tailored bars.

Figure 4: Concept of the base plate with tailored bars.

Scalable tailored bars for different fibre laser pumping and direct applications

As the design is modular in power and features, power levels up to 1,200W at 20mm x mrad with one pumping wavelength are possible. Up to 4kW with 25mm x mrad can be provided by using multiple wavelengths for direct diode applications.

Those modules are used for end-pumping fibre lasers, as well as for materials processing such as thin sheet metal welding and additive manufacturing. With its tailored bar platform, Dilas has shipped more than 30,000 units of pump modules for kilowatt-class fibre laser pumping.

In addition to the series of standard modules, a visible red module has been made at 638nm, producing 40W in 400µm ø, NA 0.2 for display and projection applications.

The next generation of commercial lasers need lightweight and compact pumping diodes, as do airborne lasers. The tailored bar concept has already been realised with a weight-to-power ratio of less than 1gr/W at power levels up to 600W.

To achieve high power and beam quality, combined with a small footprint, a compact and stackable electro-optical building block (a macro cooler) was introduced. In this way, an efficiency of greater than 55 per cent can be reached, because of reduced thermal resistance and cooling with industrial (non-deionised) water. In addition, the tailored bar concept, including the optical beam shaping, has been proven to withstand harsh treatment such as shocks and vibrations.

Industry requirements for many welding applications ask for up to 4kW. Densely stacked 2D arrangements of macro-coolers generate small 1kW power units. An optimised beam quality of less than 25mm x mrad for a single-wavelength sub-module was achieved by using spatial and polarisation multiplexing in an automated alignment procedure. Then, by combining several sub-modules (figure 5) with different wavelengths, the overlaped light can be coupled into a 400µm ø fibre with NA 0.12, corresponding to 25mm x mrad.

Figure 5: From left: Modified base plate (macro cooler) for tailored bars with FAC and SAC lensing; arrangement of eight stacked base plates; and integrated with beam shaping optics, providing a both-axis collimated beam at single wavelength.

Figure 5: From left: Modified base plate (macro cooler) for tailored bars with FAC and SAC lensing; arrangement of eight stacked base plates; and integrated with beam shaping optics, providing a both-axis collimated beam at single wavelength.

An output power of 4.1kW was achieved with 48 per cent electro-optical efficiency at a moderate current level of 35A, which is close to the maximum efficiency. Since the set-up is suitable for up to 50A, there is still headroom for higher output power for the next generation of tailored bars. With a laser system based on such an engine, metal welding can be performed as shown in figure 6.

High-quality laser welding not only allows optimised welding seams and smooth welding processes (figure 7), it also reduces costs and time requirements for reworking parts, as the welding procedure is almost spatter free. The high efficiency, the small footprint and the easy integration into existing production lines ensure low costs of ownership.

Figure 6: Welding curve for stainless steel (1.4301), also indicating the cross section of the welding seam at a speed of 2m/min.Figure 7: Example of thin metal welding.

Figure 6 (top): Welding curve for stainless steel (1.4301), also indicating the cross section of the welding seam at a speed of 2m/min. Figure 7 (bottom): Example of thin metal welding.

High-brightness diode lasers for 3D printing and cutting

Volume markets like cutting or remote welding were not commercially accessible for the diode laser in the past. The growing 3D printing market (figures 8 and 9) requires very small focus sizes for high resolution selective laser melting (SLM). For these applications a beam quality of less than 10mm x mrad is necessary. By using only 50µm-wide emitting structures instead of 100µm, as for the standard tailored bars, the brightness is improved and the base unit provides about 180W with a beam parameter product (BPP) of less than 10mm x mrad (100µm diameter, NA of <0.2).

For further power scaling, besides broad wavelength multiplexing with wavelength spacing of about 30nm, which is limited to five to six available wavelengths, dense wavelength multiplexing could allow power scaling by a factor of more than 30 times, while maintaining the beam quality of the single unit. A demonstrator, based on three different free-beam modules, was set up. Each module has been wavelength-stabilised with volume holographic gratings (VHGs) at 972nm, 976nm and 980nm with a line width less than 0.5nm (90 per cent). With off-the-shelf optics and no further optimisation, an output power of 400W and an electro-optical efficiency of greater than 40 per cent has been demonstrated. Thus, it is safe to assume that, with optimised optical components, this setup is capable of achieving 500W with an electro-optical efficiency of greater than 45 per cent.

Since internally the whole beam path of this dense wavelength multiplexing setup is linearly polarised, the route for 1kW modules with 10mm x mrad is pretty clear. The inclusion of more wavelengths in the broad available standard bands will allow multi-kilowatt class diode laser systems to be built.

The economy will define the success of the diode laser in the domain of fibre lasers, but the smart concept of the automated tailored bar design can play an important role in the race to market.

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