We introduce a toolbox for modelling laser diode operation over a large temperature range, as is encountered in long-pulse hair removal, and in mobile applications such as LiDAR. Power-to-current characteristics and lifetime estimations are sought for arbitrary pulse patterns in quasi-continuous-wave (QCW) operation. Our model is based on (1) the Zth-representation of the package thermal transient, (2) a temperature-dependent family of diode characteristics replacing the insufficient T0,T1-approach, and (3) the assumption that the device lifetime depends on the maximum junction temperature. The simulated evolution of output power and temperature is experimentally verified. Using our model, we assess the influence of the package geometry on the diode temperature and on the efficiency of diode-pumped solid state lasers. We also re-assess lifetime data, and derive safe operating parameters for an arbitrary pulse length.
Laser stacks emitting short light pulses are ideally suited for medical and cosmetic applications. Developing enhanced, stable and reliable assembly processes, Jenoptik is reaching for higher energy densities and lower manufacturing costs. In this paper an improved technology for actively cooled QCW stacks is presented. Based on simulations and experimental data, the impacts on the laser stack performance are described and shown as power-current and thermal impedance plots. We show that the bar-to-bar pitch can be reduced from 1.7 mm to 1.2 mm without detrimental thermal effects for pulse durations up to 100 ms.
High-energy class laser systems operating at high average power are destined to serve fundamental research and commercial applications. System cost is becoming decisive, and JENOPTIK supports future developments with the new range of 500 W quasi-continuous wave (QCW) laser diode bars. In response to different strategies in implementing high-energy class laser systems, pump wavelengths of 880 nm and 940 nm are available. The higher power output per chip increases array irradiance and reduces the size of the optical system, lowering system cost. Reliability testing of the 880 nm laser diode bar has shown 1 Gshots at 500 W and 300 μs pulse duration, with insignificant degradation. Parallel operation in eight-bar diode stacks permits 4 kW pulse power operation. A new high-density QCW package is under development at JENOPTIK. Cost and reliability being the design criteria, the diode stacks are made by simultaneous soldering of submounts and insulating ceramic. The new QCW stack assembly technology permits an array irradiance of 12.5 kW/cm². We present the current state of the development, including laboratory data from prototypes using the new 500 W laser diode in dense packaging.
High-power quasi-CW laser bars are of great interest as pump sources of solid-state lasers generating high-energy ultrashort
pulses for high energy projects. These applications require a continuous improvement of the laser diodes for
reliable optical output powers and simultaneously high electrical-to-optical power efficiencies. An overview is presented
of recent progress at JENOPTIK in the development of commercial quasi-CW laser bars emitting around 880 nm and
940 nm optimized for peak performance.
At first, performances of 1.5 mm long laser bars with 75% fill-factor are presented. Both, 880 nm and 940 nm laser bars
deliver reliable power of 500 W with wall-plug-efficiencies (WPE) <55% within narrow beam divergence angles of 11°
and 45° in slow-axis and fast-axis directions, respectively. The reliability tests at 500 W powers under application quasi-
CW conditions are ongoing. Moreover, laser bars emitting at 880 nm tested under 100 μs current pulse duration deliver
1 kW output power at 0.9 kA with only a small degradation of the slope efficiency. The applications of 940 nm laser bars
require longer optical pulses and higher repetition rates (1-2 ms, ~10 Hz). In order to achieve output powers at the level
of 1 kW under such long pulse duration, heating of the laser active region has to be minimized. Power-voltage-current
characteristics of 4 mm long cavity bars with 50% fill-factor based on an optimized laser structure for strong carrier
confinement and low resistivity were measured. We report an output power of 0.8 kW at 0.8 A with <60% conversion
efficiency (52% WPE). By increasing the fill-factor of the bars a further improvement of the WPE at high currents is
A new high-power semiconductor laser diode module, emitting at 760 nm is introduced. This wavelength permits
optimum treatment results for fair skin individuals, as demonstrated by the use of Alexandrite lasers in dermatology.
Hair removal applications benefit from the industry-standard diode laser design utilizing highly efficient, portable and
light-weight construction. We show the performance of a tap-water-cooled encapsulated laser diode stack with a window
for use in dermatological hand-pieces. The stack design takes into account the pulse lengths required for selectivity in
heating the hair follicle vs. the skin. Super-long pulse durations place the hair removal laser between industry-standard
CW and QCW applications. The new 760 nm laser diode bars are 30% fill factor devices with 1.5 mm long resonator
cavities. At CW operation, these units provide 40 W of optical power at 43 A with wall-plug-efficiency greater than
50%. The maximum output power before COMD is 90 W. Lifetime measurements starting at 40 W show an optical
power loss of 20% after about 3000 h. The hair removal modules are available in 1x3, 1x8 and 2x8 bar configurations.
High-power laser bars and single emitters have proven as attractive light sources for many industrial applications such as direct material processing or as pump sources for solid state and fiber-lasers. There is also a great interest in quasi-CW laser bars for high-energy projects. These applications require a continuous improvement of laser diodes for reliable optical output powers, high electrical-to-optical efficiencies, brightness and costs. In this paper JENOPTIK presents an overview of recent research for highly efficient CW and quasi-CW laser devices emitting in a wide wavelength range between 880 nm and 1020 nm. The last research results concern the 9xx single emitters and laser arrays. The 9xx nm 12 W single emitters and 976 nm 55 W laser arrays have efficiencies above 65%. New life time tests for single emitter devices currently exceed 1300 hours of reliable operation at room temperature and over 1500 hours at 45°C. Because of the small far field distribution of the optical power, the high output power and the small near field the 55 W arrays show a brightness of 75 MW x cm-2sr-1 with 95% power content. The technology for new generation 940 nm high fill-factor bars has been currently extended to emission wavelengths of 976 nm and 1020 nm with excellent results: 200 W output power with 63% efficiency using passive cooling. The innovative design of the laser structure enables, moreover, the realization of 500 W 880 nm quasi-CW laser bars with wall-plug efficiencies of 55% and a narrow fast-axis divergence angle of 40° (95% power content).
This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need
to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride
nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best
results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and
the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride
nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.