Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the
optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the
performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress
and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature
independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation.
Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output
power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong
dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes
on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently
high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative
defect recombination by electro-optical measurements, such confirming that low defect densities are essential
for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved
photoluminescence measurements yields potential fluctuations in the order of E<sub>0</sub>=57meV for our blue laser diodes. To
get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally
measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on
microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current
dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green
lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we
demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN.
The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V
of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C.
We present analytical methods to study the physical mechanisms of the optical output stability of blue InGaN laser
diodes in short and long-term stress experiments. Lasers of three ridge widths were stressed under constant current
condition at a constant current density. The output power of a 1.8μm ridge laser decreased down to 40% within 15h
mainly due to an increase in threshold current. Broader ridges showed a more stable output power. The decline in output
power was related to changes in quantum efficiency and longer carrier lifetimes after stress. A simple recombination
model was fitted to the measurements indicating no increase in non-radiative recombination centers. Instead the longer
carrier lifetimes could be well explained with a decrease in carrier density due to an additional current spreading. These
results were confirmed by changes in the sub-threshold wavelength shift before and after aging.
Laser projection arising as a new application in the consumer market has been the driving force for OSRAM Opto
Semiconductors to develop a frequency doubled semiconductor laser and the production technology necessary to make
the complexity of an advanced laser system affordable. Optically pumped frequency doubled semiconductor lasers
provide an ideal platform to serve the laser projection application. Based on this scalable technology, we developed a 50
mW green laser comprising all the properties that can be expected from a high performance laser: Excellent beam quality
and low noise, high speed modulation, good efficiency and long life time. More than that, the package is very compact
(<0.4 cm<sup>3</sup>) and may be operated passively cooled at up to 60°C. Managing lasing wavelength and controlling phase
matching conditions have been major design considerations. We will describe the key characteristics of the green laser,
and will also present results from reliability testing and production monitoring.
Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the
optical output power and wall plug efficiency. We report improvements of the performance of true blue single mode
InGaN laser at 450nm with output power of more than 200mW in cw operation for temperatures between 20°C and
60°C. We succeeded in temperature independent high wall plug efficiency of 15-18% for stable output power levels
from 100 to 200mW with estimated life times >4000h in cw operation. Furthermore, we present pioneering studies on
long green InGaN laser diodes. The technological challenge is to achieve InGaN-quantum wells with sufficiently high
material quality for lasing. We investigate the density of non radiative defects by electro-optical measurements confirming
that low defect densities are essential for stimulated emission. Laser operation at 516nm with more than 50mW output
power in cw operation is demonstrated in combination with a high wall plug efficiency of 2.7%.
True blue lasers with wavelengths of ~450 nm are of great interest for full color laser projection. These kind of
applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide
temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature
behavior of 60mW InGaN lasers in a range of -10 °C to 100 °C.
The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency
of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost
temperature independent injection efficiency in the temperature range that is of interest for most commercial
applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic
temperature T<sub>0</sub> of about 141K for our devices emitting at 445nm. This increasing threshold current density can be
explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the
threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated
by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold
current resulting in a nearly constant high wall plug efficiency of 13% between -10°C and 100°C.
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.
Red, green and blue semiconductor lasers are of great interest for full color laser projection. Mobile applications require
low power consumption and very small laser devices. InGaN lasers are the best choice for the blue color in applications
with output power requirements below 100mW: (1) they have much higher wall plug efficiencies than conventional blue
frequency doubled diode pumped solid state lasers and (2) they are more compact than semiconductor IR lasers with
subsequent second harmonic generation.
We present blue InGaN lasers with high efficiency at a power consumption of several 100mW. Excellent epitaxial
quality permits low internal losses. Threshold current densities and slope efficiencies are further optimized by improving
the facet coating. The laser threshold current is as low as 25mA and the slope efficiency reaches 1W/A. We present a
wall plug efficiency of 15% at output power levels of 60mW.
The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell
heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting
single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are
responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by
growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light
emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane
grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE
measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the
internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature
and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously.
Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly
influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the
radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency.
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.
The use of oxide confined VCSELs in datacom applications is demonstrated. The devices exhibit low threshold currents of approximately 3 mA and low electrical series resistance of about 50 (Omega) . The emission wavelength is in the 850 nm range. Life times of the devices are several million hours under normal operating conditions. VCSEL arrays are employed in a high performance parallel optical link called PAROLI<SUP>TM</SUP>. This optical ink provides 12 parallel channels with a total bandwidth exceeding 12 Gbit/s. The VCSELs optimized for the parallel optical link show excellent threshold current uniformity between channels of < 50 (mu) A. The array life time drops compared to a single device, but is still larger than 1 million hours.
Intersubband excitations play an important role for ultrafast carrier dynamics in quasi-two-dimensional semiconductors and for device applications. We present a study of the ultrafast coherent and incoherent dynamics of intersubband excitations in a pure electron plasma by means of femtosecond spectroscopy in the mid-infrared. The different relaxation processes following intersubband excitation of electrons in GaInAs/AllnAs quantum wells are observed in real-time and the relevant microscopic scattering mechanisms are identified. We find a decay of coherent intersubband polarizations on a time scale of several hundreds of femtoseconds which is governed by electron-electron scattering. Electrons excited to the n equals 2 conduction subband undergo intersubband scattering to the n equals 1 subband by emission of longitudinal optical phonons with characteristic time constants of 1 ps. This is followed by thermalization of the backscattered electrons on a similar time scale, involving both electron-electron and electron-phonon scattering. Eventually, the hot electron distribution cools down to lattice temperature within about 50 ps.