InGaN lasers in the blue and green wavelength range have opened a wide variety of applications in the past years, which all require unique properties of the employed laser chips. In this paper we will show design and process developments for various InGaN laser designs, each optimized for its specific application. For applications which are very sensitive to energy consumption, like mobile AR/VR devices, we investigated InGaN laser chips with resonator lengths as short as 50 μm. To achieve this, we developed an etched facets technology to overcome the challenges of scribing and breaking for facet generation for such short resonator lengths. The etched facets of these devices are coated on-wafer with a dielectric mirror to achieve the desired reflectivity. Depending on the reflectivity chosen, these devices show ultra-low threshold currents below 3mA and output powers above 50 mW. Combined with a flip-chip design with both contacts on one side, such chips can be integrated into silicon wafer-based beam combiners to generate RBG PIC chips for VR/AR laser projection. For high power applications, we will present data of laser bars. Bars emitting at 430 nm achieved 100 W of continuouswave output power per bar and conversion efficiencies of 50%. Together with bars emitting at 450 nm, that were shown in previous publications, wavelength-multiplexing for materials-processing systems can be realized yielding blue laser light sources with multiple kilowatts of output powers.
Progress in several fields has enabled the use of LiDAR sensing for a multitude of applications like autonomous driving, pre-crash sensors, gesture recognition, and environmental monitoring. All the applications demand challenging specifications of the sensing system components to achieve the required performance parameters detection range, angular resolution, eye-safety, and several others. In this work, we report on recent advances in our pulsed edge-emitting IR laser diodes, which can be used as laser light sources for scanning-beam and flash-mode time-of-flight LiDAR systems. We developed a technique to reduce the temperature-induced emission wavelength shift in our monolithically stacked epitaxial waveguides from 22nm to only 2.8nm over a heatsink temperature range from 25°C to 120°C, which is the crucial temperature range for many systems. Within this 95K range our Fabry-Perot edge-emitters feature a wavelength shift below the 7nm usually achieved in DFB type edge-emitters and VCSELs. There is no power penalty for the wavelength stabilization. We also demonstrate output power scaling by about 60% by increasing the number of waveguide stages in the stacked epitaxy structure from 3 to 5. This results in a short-pulse peak output power of 260W at 50A from a single device with an emission wavelength of 910nm and a near field width of about 220μm. Finally, we discuss the performance improvements of devices with 900μm and 1200μm long resonators compared to standard 600μm resonators. The demonstrated advances of the pulsed edge-emitting laser light sources enable various system improvements and widespread adoption of LiDAR sensing in many applications.
Blue high-power semiconductor lasers have increased greatly in performance over the recent decade enabling new application fields from high brightness projection up to materials processing beyond 1000W output power systems. Base for best system performance is optimal chip design and reliability of the semiconductor device. In this paper chip design optimization of blue high-power semiconductor laser bars will be shown: In contrast to IR laser bars with high lateral emitter fill factors beyond 50%, optimum design with maximum output power and efficiency for GaN laser bars is currently at very low fill factors in the range of 10%. Laser bar designs ranging from 5% fill factor up to 12.5% fill factor were fabricated and investigated. Additionally, two different emitter pitches with 200μm and 400μm were compared. The design with an emitter width of 30μm and a pitch of 400μm resulted in overall best performance. Additionally, lifetime investigations of single emitters in TO-packages will be discussed. The laser diodes were tested up to 5000h duration at different conditions in operating temperatures ranging from 64°C to 96°C and output power up to 3.5W. Dominating degradation mechanism is wear-out which is accelerated by optical output power and additional thermal activation. Extrapolation of the test results in combination with an acceleration model points towards a median lifetime of up to 65.000h for 25°C operation.
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-power near-Infrared LED (IRED) are gaining more and more interest in a large variety of commercial, industrial
and military applications.
IRED are based on InAlGaAs semiconductor structures which cover a spectral range of 780 nm to 1100 nm. This
wavelength range is supposed to be not visible to the human eye. But, depending on the radiant intensity and wavelength,
a reddish glow is still evident. Therefore, in covert applications longer wavelength of 940 nm or even higher are
preferred due to the much lower sensitivity of the human eye compared to 850 nm. On the other hand at around 850 nm
the spectral sensitivity of CMOS or CCD cameras or other silicon based photo detectors is at its maximum. We present
the latest developments in high power IRED in the quest for more than 1 W from a single 1mm2 die.
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.
Laser Diode Arrays continue to gain momentum as versatile, cost effective, reliable solution for a wide variety of existing and emerging illumination and pumping applications. In order to meet these growing demands, designers find themselves faced with three challenges: reducing system size, improving user serviceability, and managing cost. We developed a compact laser package platform that offers high output power, good reliability, and different beam collimation options. Both active cooling and passive cooling is possible with this new packaging concept. It has the footprint of the TO263 package and is based on packaging concepts that were developed for high power semiconductor devices and high volume opto semiconductor products like Light Emitting Diodes. High efficiency and high power laser bars are critical to various pumping and material processing applications. Wavelength multiplexing is an option to increase output power from laser systems. Typical wavelengths used are 808nm, 940nm and 980nm. We discuss the results of wavelength multiplexing of 880nm high power lasers.
We present a novel hybrid light emitting device design based on a standard InAlGaAs/GaAs high-power laser diode array chip as a pump source and a narrow-gap PbSe-layer as active optical material. Maximum cw output powers of more than 1.1 mW and slope efficiencies of 0.4 mW/A are obtained at 25 °C. The external power efficiency amounts to 3.5×10-2 %. The emission wavelength is 4.2 μm, with a half width of 770 nm (50 meV). Details about the optimization of the emitter material and device design are discussed as well.
Martin Behringer, Soenke Tautz, Wolfgang Pammer, Klaus Friepes, Ulrich Steegmueller, Marc Philippens, Josip Maric, Harald Koenig, Johann Luft, Gerhard Herrmann, Alexis Schmitt
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$/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.
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.
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.
Martin Behringer, Marc Philippens, W. Teich, Alexis Schmitt, Stefan Morgott, Joerg Heerlein, Gerhard Herrmann, Johann Luft, G. Seibold, Jens Biesenbach, Thomas Brand, Marcel Marchiano
KEYWORDS: Semiconductor lasers, Diodes, Polarization, Multiplexing, High power diode lasers, Collimation, High power lasers, Laser cutting, Laser development, Printing
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 today's 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.
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.
We have investigated the degradation behavior of high power diode-laser bars at 80 nm with single quantum and double quantum well structures in continuous wave operation. The 1 cm bars have a fill factor of 50%. Laser diodes with different resonator lengths from 300 micrometers to 2000 micrometers have been investigated. Different bars were mounted on actively cooled submounts and operated at comparable current densities and heat load.
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.
Solid state laser pumping is an important application for high power semiconductor lasers. The higher electro-optical conversion efficiency (50%), and narrower spectral emission (2 - 5 nm) of laser diodes allow for more efficient pumping compared to flash lamps, and consequently lead to superior thermal and optical properties of the solid state laser. A further performance increase and thus price reduction, especially for pulsed and qcw operation, is conceivable using a promising approach proposed almost 20 years ago which we dubbed 'micro-stack lasers.' It consists in vertically integrating multiple active laser junctions in one -- structure using degenerately doped tunnel junctions to electrically connect the intermediate reverse junctions. By stacking 2 to 4 emitters in this way, the output power of semiconductor lasers could theoretically be increased by a factor of 2 to 4 as the reliable output power is mainly limited by the power density at the laser facets. Modern growth technology can provide the necessary 10 to 20 micrometer thick high quality epitaxial layers. The limiting factors rather originate from the additional electrical and thermal resistance, and from current spreading towards the deeper junctions.
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