Opto-thermal simulation model for optimizing the thermal response of the optical properties of Ce:YAG single-crystal phosphors

As laser diodes replace LEDs in the remote phosphor configuration, a new class of lighting solutions is emerging. Laser-excited remote phosphor (LERP) systems, have advanced characteristics compared to their LED-based counterparts. The focus of this study is on Ce:YAG single-crystals that exhibit outstanding thermal stability and are therefore ideal for LERP applications. Although the quantum efficiency of these materials is stable for most operating temperatures of lighting systems, even for temperatures below the thermal quenching threshold the absorption and emission spectra of the phosphor are temperature-dependent. Furthermore, the ability to calculate the temperature profile of the phosphor layer is a prerequisite for the development of an effective thermal management system. For these reasons, a multi-physics simulation framework has been developed that couples the optical and thermal effects. This is achieved here by using the ray tracing software OpticStudio in conjunction with F.E.M. heat transfer calculations performed in ANSYS. In addition to the temperature-dependent quantum efficiency considered in the current state-of-the-art simulation schemes, temperature-dependent models for the mean free path for absorption and emission spectrum have been established. For modeling purposes the single-crystal Ce:YAG phosphor can be considered a bulk diffuser; the incident light is either down-converted and homogeneously scattered or refracted. A built-in bulk scattering model has been customized to simulate this behavior. After a ray tracing is performed, the absorbed flux is calculated from the ray data generated. The resulting heat generation rates are computed in such a way that they can be directly applied as element-wise loads to solve the steady-state heat equation. The experimental validation of the model has shown that the simulated and measured temperature profiles are in good accordance and will be presented here. As a consequence, the thermal response of the phosphor’s optical properties can be taken into account and optimized.