This paper focuses on the kinetics of Cr⁴⁺ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr⁴⁺ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr³⁺ to Cr⁴⁺. The activation energy (E_a) of Cr³⁺ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr³⁺ to Cr⁴⁺ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr⁴⁺ ions exchange positions with the Al³⁺ ions, switching from “A” to “D” sites, is faster than Cr³⁺ to Cr⁴⁺ oxidation. In the temperature range of 900–1300 °C, the reaction enthalpy of Al³⁺/Cr⁴⁺ ion exchange between octahedral “A” and tetrahedral “D” lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.

A methodological approach for advanced facet temperature characterization of operating diode lasers is presented. It relies on the concerted application of micro-Raman spectroscopy and thermoreflectance mapping. The latter technique allows for fast facet mapping, whereas the Raman data provide the temperature calibration. Residual effects, e.g., caused by the different reflectances of the materials involved into the laser structure, are discussed. Since both techniques provide rather complementary information, their concerted application appears as an effective tool for advanced device inspection.

An asymmetric heterostructure design has been proposed to meet high-power laser diode (LD) requirements, such as a high catastrophic optical damage threshold, a low internal loss, low thermal and electrical resistances and a low vertical beam divergence. The asymmetry has been designed to shift the optical field in heterostructure waveguide toward the n-side, where losses are lower than those at the p-side. The main features of the design are: 1) a thin anti-guiding layer inserted between the “active” [containing quantum well (QW)] and the passive waveguides to elongate field penetration toward the n-side and to increase degree of freedom in design; and 2) reduced p-cladding layer thickness (thanks to the shift of the optical field toward the n-side) aimed at reducing the diode resistances due to shortened QW to heatsink distance. The characteristics of pulsed and CW operation of high-power LDs based on the asymmetric GaAsP/AlGaAs/GaAs heterostructure are presented. A lower temperature increase of the active region of asymmetric LDs compared with equivalent symmetric LDs has been evidenced by time-resolved spectra and thermovision characteristics.

The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed. Both thermal imaging and the analysis of the thermal wavelength tuning behavior are employed to extract the device temperature as a function of time. A steady-state thermal situation is established with rise times of about 10 and $60\mathrm{ms}$ for active and passive cooling, respectively. The latter number, however, is substantially increased by the particular properties of the external heat sink. Such results are confirmed by model calculations based on the finite element method.

Imaging thermography in the $3\text{–}5\mu \mathrm{m}$ wavelength range is applied to the analysis of thermal properties of high-power diode lasers. We investigate these devices by inspecting their front facets as well as their active regions along the resonator. The latter is done through top windows within the substrate. Raw data are found to be mostly interfered by thermal radiation traveling through the substrate, which is transparent for infrared light. Substracting this contribution and recalibration allows for obtaining realistic temperature profiles along laser structures. Facet heating is analyzed complementary by micro-Raman spectroscopy. We show how hot spots at the front facet, in the substrate, or even in the active region within the substrate are discovered. Our approach paves the way for an advanced methodology of device screening.