Transient thermal analysis for VCSEL Diodes

Reliability of vertical-cavity surface-emitting laser (VCSEL) diodes is essential for e.g. LiDARs, which are used to enable autonomous driving. One crucial parameter for VCSELs is the junction temperature. Higher temperatures reduce the efficiency, shift the wavelength, and reduce the lifetime. The temperature itself depends, among others, on the thermal resistance of the thermal path responsible for heat dissipation. Typically, the thermal resistance of VCSELs is measured using the temperature depended shift of the peak wavelength. However, the sensitivity is very low, which reduces the accuracy, and the measurement duration is long, which inhibits time-resolved inspection. Instead, this paper introduces and verifies the thermal characterization of VCSELs by using the temperature dependence of the forward voltage of the VCSEL. This approach is used similar for LEDs. The method permits the omission of cost-intense spectrometers with high wavelength resolution, increases the accuracy and enables time resolved evaluation. Latter is required for transient thermal analysis (TTA) to measure e.g., the thermal impedance.