Starting with a seminal paper by Forbes , orthogonal polynomials have received considerable interest as descriptors of
lens shapes for <i>imaging</i> optics. However, there is little information on the application of orthogonal polynomials in the
field of <i>non-imaging</i> optics. Here, we consider fundamental cases related to LED primary and secondary optics. To make
it most realistic, we avoid many of the simplifications of non-imaging theory and consider the full complexity of LED
optics. In this framework, the benefits of orthogonal polynomial surface description for LED optics are evaluated in
comparison to a surface description by widely used monomials.
Recent years have seen a surge in LED-based automotive headlamps including a variety of lighting functions like lowbeam,
high-beam, day-time running light as well as fog-light. Many of those lighting functions have been realized by
designs that statically provide specific illumination patterns. In contrast, existing adaptive designs rely on either moving
shutters or electronically-complex matrix sources.
In this paper, alternative options will be explored for an automotive headlamp that combines low-beam and high-beam
out of a single LED.
The light source comprises two rows of chips arranged on a common carrier resulting in a compact LED. At the same
time, electronic complexity is reduced by driving just the two rows independently.
Primary optics collects the emission of the two closely-spaced chip rows and simultaneously provides a way to separate
respective contributions. The subsequent secondary optics is based on facetted reflector shapes to realize low-beam and
Efficiency, tolerances, system size, and cross talk will be evaluated for different primary optics based on refraction,
reflection as well as TIR.
Good primary optics for LEDs are crucial for applications that work technically and economically. But what is
"good"? For various optical design patterns, we look at the complex interplay between optics, manufacturing,
tolerancing, lifetime and cost.
A two-dimensional self-consistent laser model has been used for the simulation of the facet heating of red emitting
AlGaInP lasers. It solves in the steady-state the complete semiconductor optoelectronic and thermal equations in the
epitaxial and longitudinal directions and takes into account the population of different conduction band valleys. The
model considers the possibility of two independent mechanisms contributing to the facet heating: recombination at
surface traps and optical absorption at the facet. The simulation parameters have been calibrated by comparison with
measurements of the temperature dependence of the threshold current and slope efficiency of broad-area lasers. Facet
temperature has been measured by micro-Raman spectrometry in devices with standard and non absorbing mirrors
evidencing an effective decrease of the facet heating due to the non absorbing mirrors. A good agreement between
experimental values and calculations is obtained for both devices when a certain amount of surface traps and optical
absorption is assumed. A simulation analysis of the effect of non absorbing mirrors in the reduction of facet heating in
terms of temperature, carrier density, material gain and Shockly-Read-Hall recombination rate profiles is provided.
The microscopic processes accompanying the catastrophic optical damage process in semiconductor lasers are discussed.
For 808 and 650 nm edge-emitting broad-area devices relevant parameters such as surface recombination velocities, bulk
and front facet temperatures are determined and discussed. Facet temperatures vs. laser output and temperature profiles
across laser stripes reveal a strong correlation to near-field intensity and degradation signatures. Furthermore, the
dynamics of the fast catastrophic optical damage process is analyzed by simultaneous high-speed infrared thermal and
optical imaging of the emitter stripe. The process is revealed as fast and spatially confined. It is connected with a
pronounced impulsive temperature flash detected by a thermocamera.
Semiconductor disk lasers have attracted a lot of interest in the last few years due to high output power combined
with good beam quality and possible wavelength engineering. One of the disadvantages is the need for external
optical pumping by edge-emitting semiconductor lasers that increase packaging effort and cost. Therefore,
semiconductor disk lasers with monolithically integrated pump lasers would be of high interest. We report on
a novel design and experimental realization to monolithically integrate pump lasers with a semiconductor disk
laser in a one-step epitaxial design. By careful design of integrated pump lasers and stacking sequence, it is
possible to efficiently excite vertical emitter areas with different mesa sizes. First results are shown at 1060 nm
emission wavelength with high output power out of mesa diameters of 100 μm to 400 μm. The devices can be
conveniently characterized on a wafer level using dry-etched pump laser facets. In pulsed operation 1.7W out of
a 100 μm diameter mesa and 2.5W out of a 200 μm diameter mesa are demonstrated. Additionally, more than
0.6W in cw operation using a 400 μm structure were achieved. In summary, an innovative approach for truly
monolithic integration of a semiconductor disk laser with pump lasers has been pioneered.
An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB)
microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind
catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field
μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start
from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects
are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD
dependence on temperature and power was analyzed in detail by
micro-Raman spectroscopy and real-time thermal
imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated
photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached
in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been
established. This understanding of the semiconductor physics behind COD is a key element for further improvement in
output power of AlGaInP diode lasers.
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.
We demonstrate 0.7W cw output power at 520nm from an intracavity frequency doubled optically pumped semiconductor disk laser at room temperature. High beam quality and optical conversion efficiency of 10% has been achieved.
Optically-pumped semiconductor disk lasers offer high output power
in combination with good beam quality. By optimizing epitaxial
quality as well as thermal resistance, we have demonstrated more than 8W of continuous-wave, room-temperature emission at 1000nm. These high power-levels are tied to high optical-conversion efficiencies of more than 40%. Whereas available wavelengths for solid-state disk lasers are restricted to a set of atomic transitions, semiconductor disk lasers can be conveniently tailored to meet almost any wavelength. Building upon the high-power results at 1000nm, we have extended the emission range towards 900nm as well as 1100nm. Two prominent examples are devices realized at 920nm and 1040nm, in each case demonstrating several Watts of laser output.
High efficiency, high power and excellent beam quality has been
achieved in optically-pumped semiconductor disc lasers (OPS-disc
laser) emitting at 1000nm. Minimizing the thermal resistance
between active region and heat-sink, more than 5.5W of continuous
wave (cw) output has been obtained at room-temperature. Even more
remarkable, the laser characteristics corresponding to this power
display differential efficiencies of better than 50% and
optical conversion efficiencies of better than 40%. This
combination of high power and high efficiency represents the best
reported values so far. As such, a highly efficient beam converter
has been realized, transforming low-brightness optical pump power
into high-brightness laser emission.
We present a comparison of experimental and microscopically based model results for optically pumped vertical external cavity surface emitting semiconductor lasers. The quantum well gain model is based on a quantitative ab-initio approach that allows calculation of a complex material susceptibility dependence on the wavelength, carrier density and lattice temperature. The gain model is coupled to the macroscopic thermal transport, spatially resolved in both the radial and longitudinal directions, with temperature and carrier density dependent pump absorption. The radial distribution of the refractive index and gain due to temperature variation are computed. Thermal managment issues, highlighted by the experimental data, are discussed. Experimental results indicate a critical dependence of the input power, at which thermal roll-over occurs, on the thermal resistance of the device. This requires minimization of the substrate thickness and optimization of the design and placement of the heatsink. Dependence of the model results on the radiative and non-radiative carrier recombination lifetimes and cavity losses are evaluated.