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.
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.
Deep insight into thermal effects in the broad-area lasers is the main condition of obtaining the improved devices. We present the analytical solution of the two-dimensional, stationary heat conduction equation yielding the temperature profile in the laser cross-section in plane parallel to the mirrors. Our approach allows for considering various heating mechanisms and assessing their contribution to the total temperature of the device.
The laser diodes and laser bars with InGaAlAs/GaAs active region are attractive as high power devices operating at around 808 nm. The quaternary InGaAlAs active region seems to have distinctive advantages over the standard GaAs quantum well construction. The most important of them is that quantum wells, required to achieve desired wavelength can be wider, which provides better carrier confinement. Another advantage is better thermal conductivity of InGaAlAs as comparing to GaAs. We have modeled single and double quantum well separate confinement heterostructure lasers with various cavity lengths. The well thickness and indium content in the active region were optimized to obtain 808 nm wavelength with acceptable threshold current density. Numerical simulation based on the selfconsistent solution of drift diffusion equations, Schrödinger equation and photon rate equation has been used to optimize the high power lasers design. In this work we have used commercial simulation package PICS3D developed by Crosslight Soft. Inc.
Threshold current and differential quatnum efficiency of broad contact lasers with optically asymmetric mirrors is discussed with the purpose to reveal factors essential for optimization of the power efficiency of such lasers.