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 catastrophic optical mirror damage (COMD) effect is analyzed for 808 nm emitting diode lasers in single-pulse
operation. During each single pulse, both nearfield of the laser emission and thermal image of the laser facet are
monitored with cameras being sensitive in the respective spectral regions. A temporal resolution in the μs-range is
achieved. The COMD is unambiguously related to the occurrence of a 'thermal flash' detected by thermal imaging. A
one-by-one correlation between emission nearfield, 'thermal flash', thermal runaway, and structural damage is observed.
As a consequence of the single-pulse-excitation technique, the propagation of 'dark bands' as observed in photo- or
cathodoluminescence maps in the plane of the active region from the front facet is halted after the first pulse. Because of
the rapidness of the thermal runaway, we propose the single-pulse technique for testing the facet stability and the
intentional preparation of early stages of COMD; even for diode lasers that regularly fail by other mechanisms.
For novel devices such as quantum dot lasers, the usual thermal characterization using temperature induced wavelength
shift is ineffective due to weak thermal shift of the inhomogeneously broadened gain-peak. This calls for new thermal
characterization techniques for such devices. To this end we have analyzed bulk thermal properties of broad area
quantum dot lasers theoretically, and have experimentally verified these calculations using the novel technique of microthermography.
InGaAs/GaAs 950 nm emitting, 50 μm wide and 1.5 mm long, large optical cavity quantum dot lasers
were used for the study. Our two-dimensional steady-state model self-consistently includes current spreading and
distributed heat sources in the device and using finite element method reproduces high resolution temperature maps in
the transverse cross section of the diode laser. A HgCdTe based thermocamera with detection spectral range 3.5-6.0 μm
was employed for micro-thermography measurements. Its microscope with 6x magnification has a nominal spatial
resolution of 4 μm/pixel for full frame images of 384×288 pixels. A ray tracing technique was used to model the
propagation of thermal radiation inside the transparent laser die which in turn links calculated and experimentally
derived temperature distributions. Excellent agreement was achieved which verifies the model-calculation and the
thermal radiation propagation scheme inherent in the experimental approach. This result provides a novel means for
determining reliable bulk temperature data from quantum dot lasers.
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.
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
patterns.
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.
Thermal imaging is demonstrated as an attractive alternative for standard temperature measurements in diode lasers. It
allows for the determination of time resolved temperature distributions in arbitrary materials of laser devices. Because of
the partial mid-infrared transparency of the semiconductor materials involved, several issues complicate the thermal
imaging approach. We analyze these detrimental effects for the case of GaAs based high-power diode lasers and
demonstrate how to circumvent them. This leads to a deeper insight into the composite thermal emission signal from
diode lasers and eventually to an accurate determination of absolute temperatures of semiconductor diode lasers.
Within the project TRUST a total of about 600 actively cooled high power laser diode bars is analyzed. These devices
are packaged by various project partners and by applying different packaging technologies. A number of analytical tools
is applied to the devices, among others strain profiling by photocurrent spectroscopy. We present selected results such as
the evolution of packaging-induced strains when advancing the technology from indium- to AuSn-soldering. The
thermal properties of all devices are screened before the aging experiments by using thermal imaging. This involves
monitoring of complete thermal profiles along each bar as well as the identification of "hot" emitters. These statistics
turns out to be batch-specific and sensitive to the soldering technology used.
The degradation behavior of broad-area laser diodes and bars emitting at 650 nm under constant power operation is
investigated. In addition to the increase in operation current the temperature of the laser facets was monitored using
Raman spectroscopy. The formation of defects was studied using photocurrent spectroscopy while cathodoluminescence
provided insight into the position of extended non-radiative defects at different stages of degradation. Although the facet
does not show any visible alteration even for failed devices, its immediate vicinity appears to be the starting point of the
observed gradual degradation effects. At the same time the local facet temperature is increased. The observed aging
behavior is compared to the known degradation scenarios for devices emitting at 808 nm. In both cases there is a clear
correlation between packaging-induced strain and observed degradation effects as demonstrated by the results obtained
for bars. For the red devices a correlation between optical load and facet temperature exists which proves that here facet
heating is indeed caused by re-absorption processes. Furthermore, the gradual degradation process is not accompanied
by the creation of dark bands along 100 directions as observed earlier for 808 nm devices. The observed gradual
degradation of the 650 nm devices is primarily accompanied by the formation of deep-level point defects, followed by
the creation of macroscopic areas of reduced luminescence intensity. Packaging induced strains become important when
gradual bar degradation is monitored at early stages.
With the widespread use of laser diodes in modern industry there has been an increasing demand for high optical output power devices with good beam quality and, ideally, low production and packaging costs. Reliability and long lifetime are essential requirements since they determine the extent to which such sources will be utilised. The devices of interest here are arrays of parabolic bow-tie lasers which have been specially designed to achieve high power with high brightness without the need for re-growth or sophisticated device fabrication. This paper presents a comparative study on laser diode arrays to investigate the effects of scaling and device geometry on device operation, including degradation and ageing. Temperature profiles at the array facets have been obtained using a thermal imaging system. The HgCdTe-based detector operates in the 1-5.5μm wavelength range. The results obtained indicate a smaller increase in temperature (2-5°C) in uncoupled arrays with respect to phase-coherent arrays and a considerable increase in temperature with increasing number of elements in the array. Such considerations are essential to properly manage thermal dissipation and improve the operational characteristics of such devices.
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