New characterization methods for semiconductor lasers

摘要

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.