The industry of laser marking, direct application and solid state laser pumping requires highly reliable and highly
efficient laser diodes. In general, all applications demand improved brightness and temperature stability, and this by
decreasing costs per watt. Instead of increasing the cavity length, we demonstrate in this paper an increase of power
with standard cavity length with a clear focus of cost reduction and high efficiency. Improvements in the semiconductor
material and packaging enable higher power and higher operation temperature. This technology raised the efficiency by
6 % of 808 nm bar with 50 % filling factor and a resonator length of 1.5 mm.
Now, passively cooled diode lasers have reached nearly the performance of actively cooled ones. With this new design
new fiber coupling modules with high brightness and high operation temperature for air cooled systems can be
achieved.
High power semiconductor lasers are commonly used as efficient pump sources for solid state lasers or multiplexing
applications. Common wavelengths are 808nm e.g. for pumping of Nd:YAG and 9xx nm for pumping of disk or fiber
lasers. Together with these wavelengths 880nm can be used as 3rd or 4th wavelength for multiplexing in direct material
processing lasers. These industrial lasers are typically operating with some kW laser output power. For scaling to higher
powers up to several kW, management of waste energy and power supply is gaining more and more importance. High
efficient and reliable diode sources are vital to build systems with very good overall performance. The German
framework project "BRILASI" had the target to develop basic technologies of next generation brilliant high power diode
lasers for industrial applications.
In this paper we present laser bars which combine industrial standards with highest efficiencies at 808, 880, 940 and
980nm and power range above 100W/bar. Room temperature efficiencies of 70% were demonstrated at wavelengths
above 900nm and power levels of 130W. For 808nm, we reached efficiencies up to 62% at 20°C. For high temperature
operation, we will show laser structures of 808nm optimized for 50°C.
Improvements of laser diode bar efficiency and mounting technology have boosted output powers of passively cooled
diode lasers beyond the 100W cw limit. After an introduction about reliablity statements and reliability assessment, the
performance increase by technology improvements is documented in current-step failure discrimination tests. Electro-optical
parameters of improved diode lasers are subsequently presented in detail as well as the results of lifetime tests at
different powers and in different operation modes - steady-state and repetitive/intermittent ("hard pulse") cw operation.
KEYWORDS: Semiconductor lasers, Reliability, Diodes, Resonators, High power lasers, Pulsed laser operation, Continuous wave operation, Electro optics, Near field optics
We report present advantages of high power 9xxnm diode laser bars for pumping of disc laser and especially for
pumping fibre lasers and amplifiers.
The strong demand for reduce system costs needs to have a good compromise in improved diode laser power, conversion
efficiency, reliability and beam quality leading to simplified system designs. Basis of the new generation for the 9xxnm
laser diode bars at JENOPTIK Diode Lab is a low loss wave guide AlGaAs - structure with low vertical far field angle of
27° (FWHM). Recently we demonstrate an output power in excess of 500W in CW operation from a diode laser bar with
50% filling factor and 3.0mm cavity length. This record was possible due to high power conversion efficiency of >68 %,
optimised facet coating technology and an excellent active cooling. New results on conductive cooled high brightness
laser bars of 20% filling factor with special emphasis to the needs of high efficiency fibre coupling will be presented.
Lifetime tests under long pulse conditions have demonstrated a very high reliability for 120 W laser bars with 50 %
filling factor and for 60 W laser bars with 20 % and 30 % filling factor.
The new packaging technology from JENOPTIK Laserdiode GmbH and the new chip technology from JENOPTIK
Diode Lab GmbH increases the output power, the quality and durability of new broad area lasers.
Tests with different pulse widths and duty cycles have been conducted. A maximum linear power density of
213mW/&mgr;m has been found for 808nm and 980nm laser, limited by thermal rollover. The tests were performed for duty
cycles from 0.1% to 5% and pulse widths of 50&mgr;s and 100&mgr;m. Over 32W output power was reached for 150&mgr;m emitter
at a 0.1 % duty cycle and 50&mgr;s pulse length. With the new diode laser technology 10mm bars with a 44% filling factor
were produced. These laser bars, mounted on micro channel coolers, reached a maximum output power of 1000W. To
our knowledge this is the highest power reported up to now for 980nm material with 100&mgr;s pulses and 0.1% duty cycle.
In this paper we report on quasi-continuous-wave (q-cw) operation of monolithically stacked laser diode bars.
Monolithically stacked laser diode bars consist of more than one laser diode grown on top of each other. In between
every two laser diodes a tunnel junction is included to ensure proper current injection to all lasers.
In comparison to a standard laser operated at the same optical power level, the monolithic laser stack has a significantly
reduced optical mirror load. Furthermore the required current is reduced drastically, which has positive consequences
on both laser lifetime and diode driver costs. If one otherwise compares a monolithic integrated laser bar stack with a
setup of three separate standard laser bars, the monolithic laser bar stack is characterized by very low costs per watt as
well as high brilliance.
By using monolithically stacked laser diode bars we were able to exceed an optical power of 500 W in q-cw mode and
are moving to even higher output power levels. Typical wavelengths are in the range between 800 and 1000 nm.
This paper is mainly dedicated to a short-time scale reliability study of different packages applied to the same type of laser diode bars: indium and gold-tin packaged laser bars are operated in cw hard-pulse mode with increasing currents until their destruction. The destruction currents serve as guide values for long-time aging tests that should be performed at lower currents. Gold-tin packaged diode lasers turn out to have clearly higher destruction currents in hard-pulse mode. This result is underlined by long-time aging tests at appropriate currents.
KEYWORDS: Semiconductor lasers, Heatsinks, Copper, Resistance, High power lasers, High power diode lasers, Laser marking, Oxides, Patents, Laser systems engineering
In order to achieve a thermally stable diode laser system based on high power diode laser bars, actively cooled heatsinks in form of micro channel heat sinks (MCHS) are used to face the power loss density of 106 W/m2 while requiring a minimum device volume. At identical junction temperature, passively cooled diode lasers are usually lower in power and the device volume is much higher due to the heat flux spreading design of passive heatsinks.
However, as a matter of principle, the cooling with MCHS sinks requires a sealing between the heat sink itself and the system around. This sealing is usually achieved by o-rings, what can never avoid the transfer of vapor from the cooling system into the vicinity of the diode laser. Extreme requirements on availability, which lead to corresponding lifetime requirements, like in telecom applications, already require passively cooled diode lasers without any water in the inner system boundaries.
For applications not requiring the extreme compact design volume of actively cooled diode lasers but requiring extreme lifetime or a minimum outlay on the periphery, we started looking into passively cooled diode laser stacks.
To achieve a minimized temperature rise in the junction, we already developed a new copper-based heat sink, spreading the power loss in an optimized manner.
Based on this heatsink, we started developing a heat exchanger with a low thermal resistance while keeping the water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on a low ambient temperature with a minimum of periphery requirements.
The design of a novel mounting substrate for high power diode laser bars is presented. This substrate is combining the high thermal conductivity of diamond with the property of being able to adjust its coefficient of thermal expansion (CTE) to that of the laser material GaAs. Such a unique feature has become possible by attributing to the hard material diamond an artificial ductility by laser cutting of stress relieving openings in the diamond substrates. Combining two of these substrates in a sandwich with a middle layer of copper, one is able to realize a desired CTE just by choosing the right copper layer thickness.
Based on the results of 3D-FEM calculations, some of these diamond-copper-diamond substrates have been produced with different copper thicknesses. The technique of electronic speckle pattern interferometry (ESPI) has been employed to measure the average CTE of these substrates. For diamond thicknesses of 0,3 mm, a copper foil thickness of 0,05 mm enabled a CTE-match with GaAs. A nearly stress free state in the laser bars mounted on these substrates has been demonstrated by photocurrent spectroscopy.
In order to achieve a thermally stable diode laser system based on high power diode laser bars, micro channel heat sinks are used to face the dissipated power with a density of 106 W/m2. Passively cooled diode lasers are either lower in power or facing higher junction temperatures. As a matter of principle the cooling with micro channel heat sinks requires a sealing between the heat sink itself and the system around. The leakage of this sealing, normally achieved by O-rings, can be reduced but never avoided. Sensible systems and extreme lifetime requirements, like in the telecom applications, already require passively cooled diode lasers with no water in the inner system boundaries. To achieve a minimized temperature shift in the junction, we developed a new copper based heat sink, spreading the dissipated heat in an optimised manner. Based on this, our further research shows that the higher temperature shift in a passive submount compared with active ones can be tolerated for a system, if the heat resistance to the external water heat exchanger is minimized.
For applications either with or without the requirement of a thermo electric cooling element (TEC), we developed a technical solution for a heat exchanger, to keep water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on an regular ambient temperature and a minimum of junction temperature mismatch.
We report on the potential of the photocurrent technique as analytical tool for diode laser testing. The physics involved into the generation of photocurrents as well as experimental requirements for detecting them are highlighted. Based on a number of practical examples, we demonstrate how knowledge about the photoelectrical properties of diode lasers can help to learn about stress and defects within packaged devices or how non-perfect device fabrication may be discovered. These results are discussed in conjunction with device reliability issues.
High power diode laser bars and stacks are of great interest in industrial applications due to their high electro-optical efficiency, their small type of construction and maintenance free operation. With highly sophisticated beam shaping optics diode lasers can be used as pumping sources for solid state and fiber lasers and direct for material processing, e.g. welding, soldering and marking metals. We have developed different fiber coupled diode laser systems with output power up to greater than 2 kW cw into a spot 0 1.0 mm (power density greater than 250 kW/cm2) and systems with output power 170 W cw into a spot 0 0.38 mm (power density about 150 kW/cm2). The 2 kW system operates with a 0 1.5 mm fiber (N.A. 0.32) and consists of polarization and wavelength coupled stacks with an overall electro-optical efficiency of 23%. The smaller system operates either with a 0 0.6 mm (N.A. 0.22) or 0 0.4 mm (N.A. 0.33) fiber and consists of a single stack. Polarization and wavelength coupling will be realized in future. The overall electro-optical efficiency is about 27%.
Thermally induced strain caused by device packaging is studied in high-power semiconductor laser arrays by a novel non-invasive technique. Measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of quantum-confined optical transitions in the optical active region. These shifts by up to 10 meV serve as a measure for strain and are compared with model calculations. We demonstrate that different packaging techniques cause different packaging-induced strains. We also show that the packaging-induced strain portion, which gets transmitted through the solder material, differs for different packaging technologies. An intentionally strain- reduced packaging technique is shown to transmit about one quarter of the potential packaging-induced strain towards the optical active layer, whereas another packaging technique, which provides highly reliable 'single-chip' devices is found to transmit about half of the potential amount. Spatially resolved measurements demonstrate strain gradients within the devices. Also temporal strain evolution is monitored. We show that 'the burn-in' is accompanied by strain accumulation whereas for long-term operation strain relaxation occurs.
KEYWORDS: Semiconductor lasers, Control systems, Temperature metrology, Resistance, Cooling systems, Liquids, Beam controllers, High power diode lasers, Sensors, Diodes
In this paper investigations of temperature control of micro channel cooled high power diode lasers are presented. After a short motivation the theoretical background of a complete liquid cooling system is introduced and the simulation results are compared to the behavior of a real-world system. Because of the nonlinear dependence of the thermal resistance on the water flux modern control algorithms are required to achieve a sufficient control quality and robustness. Their design and characterization are presented as well. Based on the results of diode laser temperature control the potentials to realize a highly efficient wavelength control are discussed. To support future system integration and to realize the controller with minimal hardware expenditure an application specific integrated circuit (ASIC) was developed.
High-power diode lasers such as `cm-bar arrays' are important for many applications. The `p-side down packaging', i.e. the direct mounting of the epitaxial layer sequence on a heat spreader ensures sufficient thermal properties, however, in such a geometry, additional mechanical strain of the active region represents a central issue, affecting both the laser parameter as well as lifetime and reliability of the device. Thermally induced strain caused by device packaging is studied in high-power semiconductor laser arrays by a novel non-invasive technique. Photocurrent measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of all allowed optical transitions in the active region. These shifts serve as a measure for strain and are compared with model calculations. Depending on the specific heat spreader materials we find compressive or tensile mounting induced strain contributions. For a given packaging architecture, about one quarter of the mounting induced strain is transferred to the quantum well region of the device. Spatially resolved measurements allow to measure lateral strain gradients in the devices. Using this data for calibration we show that polarization resolved electroluminescence scans can be used as convenient measure for strain homogeneity test also in quantum-well devices.
KEYWORDS: Semiconductor lasers, High power diode lasers, High power lasers, Diodes, Heatsinks, Resistance, Copper, Collimation, Materials processing, Solid state lasers
High power laser diodes and especially high power laser diode modules made enormous progress in the last few years. Different aspects of high power laser diodes are treated starting from general description of high power laser diodes and their mounting techniques, characterizing the electro- optical behavior of single laser bars and finally presenting beamshaping optics for the collimation of large modules. The later technique allows for symmetrical focal spots in the kilowatt range with a beam quality of about 170 mm*mrad. Different aspects of current applications of high power laser diodes are presented.
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