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.
Infrared breath analysis is used in diagnostics of respiratory diseases, pulmonary function testing, and for metabolic studies. With selective and highly sensitive instruments exhaled trace gas concentrations can be related to specific diseases.
For many applications also a time resolution below 0.1s is needed. Frequently, performance is limited by the IR source. New developments offer solutions even for compact instruments. Different setups employing quantum cascade lasers (QCL), VCSELs, and a new optically pumped IR emitter are compared focusing on CO2 measurements as an example.
The implementation of more complex diode laser concepts also increases the demands for improved measurement technology and the need for new analytical tools. In particular concerning the thermal properties of novel high-power devices, there are several established experimental methods. Micro-Raman spectroscopy as well as reflectance techniques, such as photo- and thermo-reflectance measurements, provide information on facet temperatures, whereas emission wavelength shifts enable for the determination of averaged temperatures along the laser axis. Here we report on the successful application of a complementary technique, namely imaging thermography in the 1.5-5 μm wavelength range using a thermocamera, to diode laser analysis. The use of this known technique for the purpose of device analysis became possible due to the enormous technical progress achieved in the field of infrared imaging. We investigate high-power diode lasers and laser arrays by inspecting their front facets. We find raw data to be frequently contaminated by thermal radiation traveling through the substrate, which is transparent for infrared light. Subtraction of this contribution and re-calibration allows for the determination of realistic temperature profiles along laser structures, however, without spatially resolving the facet heating at the surface of the laser waveguide. Furthermore, we show how hot spots at the front facet can be pinpointed. Thus our approach also paves the way for an advanced methodology of device screening.
We demonstrate the applicability of imaging thermography for investigations of mechanisms associated with gradual degradation in diode lasers. The introduction of two spectral channels provides the means for separate observation of deep level luminescence and thermal radiation emitted according to Planck's law. In the near IR region we found the signal detected by the camera to be mainly affected by mid-gap deep-level luminescence. An intensity increase of the luminescence signal for an aged diode laser compared to an unaged device is noticed. It can be explained by an increase of deep level defect concentration during the aging. In the mid IR, we mainly encounter thermal radiation, which can be used for the analysis of the thermal properties of devices. In present work the thermal behavior of the device subjected to an aging of 3000 hours is analyzed. A significant increase of device temperature is noticed.
We report on novel evaluation methodology of high-power diode lasers that potentially will increase the reliability level of these devices. The study is carried out for wide-stripe, 808 nm diode lasers with low fast-axis beam divergence that base on a double-barrier single quantum well separate confinement heterostructure. The diodes are assembled in standard packages with base diameter &slasho; = 9 mm. Degradation of diode lasers is a result of the interaction between internal and external factors. Thus, insight into degradation mechanisms is only possible with a complex characterization of the devices. In our analysis we involved standard measurements such as current-voltage, light-current characterizations, as well as advanced methods such as high-resolution thermography. The latter one allows for investigations of thermal properties of diode lasers including fast temperature profiling and defect recognition. We discuss the usefulness of above techniques for screening purposes. Finally we present results of reliability tests of the diode lasers. A correlation between initial tests and lifetest results is shown.
Light emitting devices for the infrared spectral region are used in a lot of application fields. In the mid infrared (MIR) region, where a lot of gases show strong absorptions, the optical output power of inexpensive emitters in the relevant wavelength range is too low. An optically pumped emitter for the MIR region around 4 μm based on narrow gap semiconductors is demonstrated. The pumping takes place using inexpensive near-infrared (around 1 μm) high power continuous wave (cw) semiconductors laser. The radiation is converted by the narrow gap semiconductor into the MIR region as spontaneous emission. Molecular beam epitaxy (MBE) grown IV-VI lead chalcogenide-based compounds, especially PbSe, are applied for frequency conversion. The structural and optical quality of these thin film materials is characterized mainly by X-ray defraction measurements (XRD) and photo luminescence (PL) spectroscopy. For high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest. Due to the high pumping powers the device design-especially the thermal management of the active PbSe film-plays an important role. We will present a preparation technique for optically pumped, surface structured PbSe emitters in transmission geometry exploiting the transparency of the substrates and glues in the relevant wavelength region. The measured total emission power of the emitters exceeds 0.5 mW. Using an optimised design total emission powers up to 2 mW were achieved.
We present a novel hybrid light emitting device design based on a standard InAlGaAs/GaAs high-power laser diode array chip as a pump source and a narrow-gap PbSe-layer as active optical material. Maximum cw output powers of more than 1.1 mW and slope efficiencies of 0.4 mW/A are obtained at 25 °C. The external power efficiency amounts to 3.5×10-2 %. The emission wavelength is 4.2 μm, with a half width of 770 nm (50 meV). Details about the optimization of the emitter material and device design are discussed as well.
Thermal properties of 808 nm emitting high-power diode lasers are investigated by means of micro-thermography. A thermo-camera equipped with a 384x288 pixel HgCdTe-detector (cut off wavelength at 5.5 micron) and IR-micro-objective is used, which allows for thermal imaging with a spatial resolution of 5 μm. A novel methodological approach for data re-calibration for absolute temperature measurements is proposed. We present steady-state thermal distributions from broad-area devices. The remarkable agreement of this data with the results of modeling work has been reached. Cross-calibration of the micro-thermographic results is obtained by complementary micro-Raman data that give information about facet temperatures with a spatial resolution of about 1 micron. Transient thermal properties are monitored with a temporal resolution of 1.4 ms. Such thermal transients illustrate the heat flow trough the device after turning on the operation current. Special experiments are done in order to detect and localize hot spots at the facet and within the devices. Moreover, we show that the analysis of thermal images can be used as a recognition method of defects hidden inside the cavity, even if they are not detectable by visual inspection. These activities are paving the way towards a novel screening methodology.
We report on the development of epitaxial thin film materials for optical pumped light emitting devices in the wavelength range of 4-5 μm. The active layers are lead selenide (PbSe) thin films grown by molecular-beam epitaxy (MBE) on single crystalline, infrared transparent BaF2 substrates. The electrical properties of the layers were determined by van der Pauw Hall measurements. A dependency of the PL intensity on the dopant type and carrier concentration was found. To increase the output power, layers with antireflection coatings were grown and characterized by Fourier-transform infrared (FTIR) spectroscopy and photoluminescence (PL) measurements. A further possibility to increase the extraction efficiency is surface texturing. Infrared imaging and PL measurements at samples with different surface structures, prepared by wet chemical etching, are presented. To improve the heat dissipation, which is a problem of optical pumped devices due to the small efficiency and pump densities up to some kW/cm2, the BaF2 substrates were removed and the active layers were transferred to different heat sinks with significantly higher thermal conductivities. Afterwards the PL intensities were compared among each other.
An optically pumped emitter for the mid-infrared region around 4 µm based on narrow gap semiconductors is demonstrated. The pumping takes place in the near-infrared around 1 μm and the radiation is converted by the narrow ap semiconductor into the MIR region as spontaneous emission. IV-VI lead chalcogenide-based compounds, especially PbSe and III-V InAsSb-based quantum well systems are applied for frequency conversion. These materials are grown by MBE and characterized mainly by photo luminescence spectroscopy. For a high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest.
Transient thermal properties of actively cooled InAlGaAs-based high-power diode laser arrays, so-called 'cm-bars', are investigated for 'low-frequency' pulsed operation with a repetition rate of 0.1-10 Hz. Under these operation conditions the devices experience almost complete thermal cycles between the temperature of the heat sink and the operation temperature typical for continuous wave operation. We analyze the thermal tuning behavior that is governed by the thermal bandgap shift (85%) but substantially modified by pure pressure tuning (15%). During operation further modification arises from inhomogeneous temperature distribution along the 'cm-bar.' Our results offer the possibility specifying optimum operation condition of 'low-frequency' pulsed operation resulting in improved reliability.