BONN, GERMANY--A major German collaborative program reached its first milestone this year. The initial three-year phase of Laser 2000, which ends this summer, has invested DM 100 million ($57 million) in research and development projects involving all aspects of high-power diode and diode-pumped solid-state lasers. More than half the funding was provided by the German government through the Federal Ministry for Science and Technology.
Partners in Laser 2000—of which there are more than 30—include major industrial companies such as Siemens and research groups in German universities. The commercial partners are already making good use of the research, and the group has just announced plans to extend the program into a second three-year phase.
According to the Association of German Equipment Manufacturers (VDMA), German industrial laser manufacturers have a 40% share of the world market. The desire to retain this market lead has driven the program to develop high-power technologies specifically for industrial applications. "The German laser industry has managed to get a strong foothold in the world market for conventional systems," says Laser 2000 coordinator Roland Diehl, of the Fraunhofer Institute for Solid State Physics (Freiburg). "To prevent erosion of their competitive position and to step into novel fields of laser applications, the companies have not only to respond to the challenge of a novel technology but also to actively shape it," says Diehl.
The Laser 2000 program has three focal points. The first includes crystal growth, epitaxy, and device patterning. A source of prime-quality gallium arsenide (GaAs) will be operational in Germany soon as a direct result of Laser 2000. The second focal point addresses the problems of heat sinking, soldering, and mounting lasers and has included growth of diamond material for heat sinks. Improvements in this area lead to improved reliability and lifetime of products. Micro-optic beam shaping and delivery systems have also been developed.
Design of diode-pumped solid-state lasers has been studied for the third area, and the leading product emerging from this part of the program is the disk laser, subsequently licensed to Advanced Photonic Systems (APS; Berlin).
Also as part of the Laser 2000 collaboration, Siemens (Regensburg) has been actively enhancing the properties of its high-power lasers. Improvements include, in particular, the efficiency, reliability (lifetime), and usable optical power density of the lasers. Outputs from 940-nm gallium indium arsenide/aluminum gallium arsenide/ gallium arsenide (GaInAs/AlGaAs/GaAs) and 808-nm AlGaInAs/AlGaAs/GaAs devices—1 W from 200-µm-diameter single emitters and 25 W from 10-mm bars mounted by Laser 2000 partner Dilas (Mainz)—are impressive, adding fuel to the aluminum-free materials debate (see Fig. 1 on p. 47).
Laser efficiencies of more than 50% have been obtained, with lifetimes exceeding 50,000 hours for the 940-nm devices and 20,000 hours for the 808-nm devices. By combining the beams in a fiberoptic delivery system, a power density of 50 kW/cm2 has been obtained in a numerical aperture of 0.22. Such high powers are being exploited by Dilas in material processing, with a system producing 800 W already demonstrated.
Diode-pumped lasers
German companies and research groups are also developing new diode-pumped solid-state lasers, which still offer superior beam quality as compared to diode lasers. The most recent results within the Laser 2000 program—from the Laser Zentrum, Hannover—show continuous wave (CW) output powers of almost 2 kW from a Nd:YAG rod-based system (see Fig. 2).
The diode-pumped disk laser, originally developed at the University of Stuttgart, offers an exciting way forward into higher-power systems. A crystal disk with a typical thickness of 0.3 mm and diameter of 7 mm is mounted onto an actively cooled copper heat sink that provides axial cooling. Disks can be stacked to provide more power. The maximum power obtained so far in the laboratory is more than 250 W CW with a 50% conversion efficiency.
Last June at Laser 97 in Munich, APS showed a prototype system. The initial products deliver an output power of 10 W at 1030 nm. The first lasers will be delivered to customers in the fall. The APS laser uses a Yb:YAG crystal disk, which offers a theoretical quantum efficiency of 91% compared to 76% with Nd:YAG. In addition, Yb:YAG has a broad absorption band at 940 nm, ideally suited for pumping with InGaAs diode lasers. Ytterbium-doped YAG also has no concentration quenching up to at least 20% doping level, and there are no losses due to excited-state absorption or upconversion. At cooling temperatures between 0° and 20°C an efficiency of 45% was achieved. Reducing the temperature to -74°C produced an efficiency of 64% at 50 W.
Bremen-based Bremlas has produced demonstration models of modulated microchip laser arrays (MLARs) emitting in the blue/green spectral region. Some models produce blue light by direct frequency doubling of diode arrays with potassium niobate, using a cavity enhancement around the diodes and crystal. This device produces up to 10 mW per channel in the array. Diode pumping of a laser crystal such as Nd:SLB with subsequent intracavity frequency doubling produces up to 40 mW of green light per channel in the array. Self-frequency-doubling materials have also been assessed. In all cases a multichannel modulator is used at the output to provide modulated light.
Bridget Marx | Contributing Editor, UK
Bridget Marx was Contributing Editor, UK for Laser Focus World.