Martin Behringer

More brilliance from high-power laser diodes

The introduction of high power diode laser systems in industry has boosted the interest in these devices for a wide range of applications. Besides printing and soldering, cutting and deep penetration welding are becoming more important. An overview about the developments, an update on todays high power laser activities and an outlook will be given, what characteristics laser bars will have to fulfil in the near future. For higher brightness, laser bars with lower fill factors, monolithic integrated laser junctions and tapered laser designs were investigated. High power diode laser (HPDL) bars with 25% - 50% fill factor were operated between 40 W and 80 W and lifetimes up to 100 000 hours could be extrapolated. Tapered laser bars with 50W output power and high wall plug efficiencies were developed. Wavelength multiplexing and polarisation coupling were used in order to reach multi-kilo-Watt diode laser emission. Examples for applications will be given.

New developments on high-efficiency infrared and InGaAlP light-emitting diodes at OSRAM Opto Semiconductors

We present our latest results on developments of infrared and red light emitting diodes. Both chiptypes are based on the Thinfilm technology. For infrared the brightness has been raised by 25% with respect to former products in a package with standard silicon casting, corresponding to a brightness increase of 33% for the bare chip. In a lab package a wallplug efficiency of more than 72% at a wavelength of 850nm could be reached. For red InGaAlP LEDs we could demonstrate a light output in excess of 200lm/W and a brightness of 133lm at a typical operating current of 350mA.

High-brightness high-power kW system with tapered diode laser bars

We report on a diode laser system, which is based on tapered diode laser bars and provides a combination of high power and high beam quality comparable to high power lamp pumped solid-state-rod lasers. Until now diode laser systems with output powers in the kW-range are based on broad area diode lasers. However, the output of these kilowatt laser systems usually is characterized by a strongly asymmetric beam profile, which is a consequence of the asymmetric beam parameter product (BPP) of broad area diode lasers with regard to the slow- and the fast-axis direction. Apparently the output of such a laser system can not be coupled efficiently into a fiber, which is required for a variety of applications. The symmetrization of the BPP of such a laser system requires complicated and expensive beam shaping systems. In contrast tapered diode laser bars allow the design of high power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Power scaling is realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. The total output power of the tapered diode laser system was 3230 W at a current of 75 A. Fiber coupling yielded 2380 W at 75 A for a fiber with a core diameter of 800 μm (NA 0.22) and 1650 W at 60 A for a 600 μm fiber (NA 0.22), respectively. Focusing with an objective with a focal length of 62 mm led to a beam diameter of 0.52 mm in the focal plane. Taking into account the total power of 2380 W behind the fiber the resulting intensity in the focal plane was 1.1 MW/cm2.

High-power diode lasers: technology and application in Europe

The application field of high power semiconductor lasers is growing rapidly and covers e.g. solid state laser pumping, metal and plastic welding, hard and soft soldering, suface treatment and others. Preferably those applications are attractive, which do not require extremely high beam quality. We have investigated high power diode-laser bars from 808 nm to 980 nm. The scope of this presentation is on focusability and beam quality. For better beam shaping structures with reduced fill factor of 25% to 30% were developed. They were operated in continuous wave operation at power levels of up to 55 W. Tests indicate extrapolated lifetimes of more than 100,000 hours at 40 W at 980 nm cw and about 10,000 hours at 45 W - 50 W at 940 nm and 808 nm. Monolithically stacked NonostacksR were investigated. Operation up to 100°C with excellent lifetimes could be demonstrated. New concepts and applications for low mode number high power diode lasers like tapered laser bars are presented. Examples for various current areas of interest in European research facilities will be given.

High-brightness highly reliable InGaAlAs/GaAs laser bars with reduced fill factor and 60% efficiency

We have investigated high-power diode laser bars from 808 nm to 980 nm. The presentation is focussed upon the development of suitable laser bars for improved beam quality at increased output power. For better beam shaping structures with reduced fill factor of 30% were developed. They were operated in continuous wave mode at power levels of up to 60W. Moreover industrial applications require lifetimes of more than 10,000 hours. We present data yielding an extrapolated lifetime of up to 100,000 hours at 40W with 60% wall-plug efficiency at 980nm cw.

High-power IR laser in SMT package

Laser dies in an optical power range of 1-3 Watts are widely assembled in popular TO- packages. TO-packages suffer from high thermal resistance and limited output power. Bad thermal contact between circuit boards and TO-devices can cause overheating of laser chips, significantly reducing the operating life time. We developed a compact high heat-load SMT package for an optical power up to 7 Watts in CW operation with good life time results. The new package for high power laser chips combines highly efficient heat dissipation with Surface-mount technology. A Direct-Bonded-Copper (DBC) substrate acts as a base plate for the laser chip and heat sink. The attached frame is used for electrical contacting and acts as beam reflector where the laser light is reflected at a 45° mirror. In the application the DBC base plate of the SMT-Laser is directly soldered to a Metal-Core-PCB by reflow soldering. The overall thermal resistance from laser chip to the bottom of a MC-PCB was measured as low as 2.5 K/W. The device placement process can be operated by modern high-speed mounting equipment. The direct link between device and MC-PCB allows CW laser operation up to 6-7 watts at wavelengths of 808nm to 940nm without facing any overheating symptom like thermal roll over. The device is suitable for CW and QCW operation. In pulsed operation short rise and fall times of <2ns have been demonstrated. New application fields like infrared illumination for sensing purposes in the automotive industry and 3D imaging systems could be opened by this new technology.

Uniformity tests of individual segments of interband cascade diode laser Nanostacks

We investigate optoelectronic properties of monolithically stacked diode lasers, so-called Nanostack® devices that include two nominally identical waveguide segments separated by a specially designed tunnel junction. Near-field optical microscopy provides straightforward and separate access to the properties of both optically active segments. Device emission, namely electroluminescence and lasing, as well as photoluminescence and photocurrent data, are recorded with high spatial resolution and consistently interpreted. We find reduced laser emission from the laser segment that is situated closer to the substrate. We show that this is not caused by thermal effects but most likely due to a larger trap concentration within or in the vicinity of the quantum wells of this laser segment. Furthermore, we show that in the unbiased devices the potential gradient in this segment is significantly larger than in the one close to the heat sink. In addition it is shown that the coupling between both waveguides is margi...

Highly reliable and efficient laser bars and cost efficient packaging

High power diode lasers convince by their very efficient conversion of electrical into optical energy. Besides high efficiencies and record absolute power levels, reliability in all possible operation modes and cost become increasingly important. We present diode laser bars in the 940nm range with wall plug efficiencies of about 65% at an emission power of 100W and with excellent reliability. The test had been performed on a stack with 5 bars at an output power of 100W per 1cm bar and after about 4000hrs test time, lifetimes of more than 40 000hour were estimated. The efficiency of these bars was at the beginning and at the end of this test about 65%. Operation modes between cw operation and q-cw (200μsec pulses) were evaluated and it will be shown, that pulses in the range of 1Hz are the hardest conditions, which can cause catastrophic failures. Using submounts with matched thermal expansion coefficient, this failure was prevented and lifetimes similar to cw-operation were reached. In order to reduce costs of laser power, we developed a laser package that offers high power at good reliability and provides a collimated beam for about 5

High-power AlGaInAs/GaAs microstack laser bars

/W, as a cost target in mass production conditions. This was achieved by using packaging concepts that were developed for high power semiconductor devices. These results will further enhance the applicability of diode lasers in industrial application.

Near-infrared power LED for emerging security and defence applications

The maximum useful optical power of laser bars is limited due to thermal and lifetime constraints to typical values of 50 W/cm cw or 120 W/cm qcw. A promising new approach is the so-called microstack laser in which several laseractive areas are integrated vertically in the same monolithic structure. In order to drive these structures in series with high efficiency low-resistance tunnel-junctions have to be realized. By optimizing the MOVPE growth process tunnel- junctions with a specific differential resistivity of 2.5 X 10-4 (Omega) cm2 could be obtained, which are suitable for the monolithic inter-connection of laser structures.